Experience

Publications

• Dispersion stabilization of silver nanoparticles in synthetic lung fluid studied under in situ conditions

  Nanotoxicology

  The dispersion stabilization of silver nanoparticles (AgNPs) in synthetic lung fluid was studied to interrogate the effects on colloidal stability due to the principal constituents of the fluid. The colloidal stability of 20 nm citrate-AgNPs dispersed in the presence of each constituent of the synthetic lung fluid (individually, the complete fluid, and without additives) was observed during titration of increasing sodium chloride concentration. A variety of complementary in situ measurement techniques were utilized, including dynamic light scattering, ultraviolet-visible absorption spectroscopy, atomic force microscopy, and small-angle X-ray scattering, which provided a collective set of information that enabled far better understanding of the dispersion behavior in the fluid than any one technique alone. It was observed that AgNPs continued to adsorb bovine serum albumin (BSA) protein from the synthetic lung fluid solution as the sodium chloride concentration increased, until a maximum BSA coating was achieved prior to reaching the physiological sodium chloride concentration of 154 mmol L−1. BSA was determined to be the constituent of the synthetic lung fluid that is required to provide colloidal stability at high salt loadings, though the phospholipid constituent exerts a subtle effect. Additionally, as AgNPs are a distinctly different class of nanoparticles apart from the carbon nanotubes and titanium dioxide nanoparticles initially reported to be dispersible using this fluid, this work also demonstrates the broad applicability of synthetic lung fluid in providing stable dispersions for engineered nanoparticles for use in biological assays.

• Dispersion stabilization of silver nanoparticles in synthetic lung fluid studied under in situ conditions

  Nanotoxicology

  The dispersion stabilization of silver nanoparticles (AgNPs) in synthetic lung fluid was studied to interrogate the effects on colloidal stability due to the principal constituents of the fluid. The colloidal stability of 20 nm citrate-AgNPs dispersed in the presence of each constituent of the synthetic lung fluid (individually, the complete fluid, and without additives) was observed during titration of increasing sodium chloride concentration. A variety of complementary in situ measurement techniques were utilized, including dynamic light scattering, ultraviolet-visible absorption spectroscopy, atomic force microscopy, and small-angle X-ray scattering, which provided a collective set of information that enabled far better understanding of the dispersion behavior in the fluid than any one technique alone. It was observed that AgNPs continued to adsorb bovine serum albumin (BSA) protein from the synthetic lung fluid solution as the sodium chloride concentration increased, until a maximum BSA coating was achieved prior to reaching the physiological sodium chloride concentration of 154 mmol L−1. BSA was determined to be the constituent of the synthetic lung fluid that is required to provide colloidal stability at high salt loadings, though the phospholipid constituent exerts a subtle effect. Additionally, as AgNPs are a distinctly different class of nanoparticles apart from the carbon nanotubes and titanium dioxide nanoparticles initially reported to be dispersible using this fluid, this work also demonstrates the broad applicability of synthetic lung fluid in providing stable dispersions for engineered nanoparticles for use in biological assays.

• Adsorption and Conformation of Serum Albumin Protein on Gold Nanoparticles Investigated Using Dimensional Measurements and in Situ Spectroscopic Methods

  Langmuir

  The adsorption and conformation of bovine serum albumin (BSA) on gold nanoparticles (AuNPs) were interrogated both qualitatively and quantitatively via complementary physicochemical characterization methods. Dynamic light scattering (DLS), asymmetric-flow field flow fractionation (AFFF), fluorescence spectrometry, and attenuated total reflectance- Fourier transform infrared (ATR-FTIR) spectroscopy were combined to characterize BSA-AuNP conjugates under fluid conditions, while conjugates in the aerosol state were characterized by electrospray-differential mobility analysis (ES-DMA). The presence of unbound BSA molecules interferes with DLS analysis of the conjugates, particularly as the AuNP size decreases (i.e., below 30 nm in diameter). Under conditions where the γ value is high, where γ is defined as the ratio of scattering intensity by AuNPs to the scattering intensity by unbound BSA, DLS size results are consistent with results obtained after fractionation by AFFF. Additionally, the AuNP hydrodynamic size exhibits a greater proportional increase due to BSA conjugation at pH values below 2.5 compared with less acidic pH values (3.4-7.3), corresponding with the reversibly denatured (E or F form) conformation of BSA below pH 2.5. Over the pH range from 3.4 to 7.3, the hydrodynamic size of the conjugate is nearly constant, suggesting conformational stability over this range. Because of the difference in the measurement environment, a larger increase of AuNP size is observed following BSA conjugation when measured in the wet state (i.e., by DLS and AFFF) compared to the dry state (by ES-DMA).

• Dispersion stabilization of silver nanoparticles in synthetic lung fluid studied under in situ conditions

  Nanotoxicology

  The dispersion stabilization of silver nanoparticles (AgNPs) in synthetic lung fluid was studied to interrogate the effects on colloidal stability due to the principal constituents of the fluid. The colloidal stability of 20 nm citrate-AgNPs dispersed in the presence of each constituent of the synthetic lung fluid (individually, the complete fluid, and without additives) was observed during titration of increasing sodium chloride concentration. A variety of complementary in situ measurement techniques were utilized, including dynamic light scattering, ultraviolet-visible absorption spectroscopy, atomic force microscopy, and small-angle X-ray scattering, which provided a collective set of information that enabled far better understanding of the dispersion behavior in the fluid than any one technique alone. It was observed that AgNPs continued to adsorb bovine serum albumin (BSA) protein from the synthetic lung fluid solution as the sodium chloride concentration increased, until a maximum BSA coating was achieved prior to reaching the physiological sodium chloride concentration of 154 mmol L−1. BSA was determined to be the constituent of the synthetic lung fluid that is required to provide colloidal stability at high salt loadings, though the phospholipid constituent exerts a subtle effect. Additionally, as AgNPs are a distinctly different class of nanoparticles apart from the carbon nanotubes and titanium dioxide nanoparticles initially reported to be dispersible using this fluid, this work also demonstrates the broad applicability of synthetic lung fluid in providing stable dispersions for engineered nanoparticles for use in biological assays.

• Adsorption and Conformation of Serum Albumin Protein on Gold Nanoparticles Investigated Using Dimensional Measurements and in Situ Spectroscopic Methods

  Langmuir

  The adsorption and conformation of bovine serum albumin (BSA) on gold nanoparticles (AuNPs) were interrogated both qualitatively and quantitatively via complementary physicochemical characterization methods. Dynamic light scattering (DLS), asymmetric-flow field flow fractionation (AFFF), fluorescence spectrometry, and attenuated total reflectance- Fourier transform infrared (ATR-FTIR) spectroscopy were combined to characterize BSA-AuNP conjugates under fluid conditions, while conjugates in the aerosol state were characterized by electrospray-differential mobility analysis (ES-DMA). The presence of unbound BSA molecules interferes with DLS analysis of the conjugates, particularly as the AuNP size decreases (i.e., below 30 nm in diameter). Under conditions where the γ value is high, where γ is defined as the ratio of scattering intensity by AuNPs to the scattering intensity by unbound BSA, DLS size results are consistent with results obtained after fractionation by AFFF. Additionally, the AuNP hydrodynamic size exhibits a greater proportional increase due to BSA conjugation at pH values below 2.5 compared with less acidic pH values (3.4-7.3), corresponding with the reversibly denatured (E or F form) conformation of BSA below pH 2.5. Over the pH range from 3.4 to 7.3, the hydrodynamic size of the conjugate is nearly constant, suggesting conformational stability over this range. Because of the difference in the measurement environment, a larger increase of AuNP size is observed following BSA conjugation when measured in the wet state (i.e., by DLS and AFFF) compared to the dry state (by ES-DMA).

• Disinfection action of electrostatic versus steric-stabilized silver nanoparticles on E. coli under different water chemistries

  Colloids and Surfaces B: Biointerfaces

  The capping layer stabilizing silver nanoparticles (AgNPs) affects its aggregation, dissolution, and net disinfection action, especially under conditions of varying water composition, such as, pH, ionic strength and organic matter content. Herein, we correlate the silver ion (Ag+) release and reactive oxygen species (ROS) generation rates for AgNPs of varying functionalization to their net disinfection coefficient on Escherichia coli, under conditions of differing water chemistries. For electrostatically stabilized citrate-capped AgNPs, the rate of ROS generation, as measured using a fluorescent dye, is found to dominate over that of Ag+ release, especially for smaller sized AgNP suspensions (∼10 nm) at low pH (∼6.2). For these AgNPs, the ROS disinfection mechanism is confirmed to dominate net disinfection action, as measured by the live/dead assay, especially at low levels of organic matter. Steric stabilization of AgNPs by protein or starch-capped layers enables disinfection through reducing AgNP aggregation and promoting silver dissolution over ROS generation. We suggest the involvement of protons and dissolved oxygen in causing the independent formation of Ag+ and ROS, regardless of the AgNP capping layer. While protein-capping layers effectively stabilize AgNPs, the generated ROS is likely dissipated by interference with the bulky capping layer, whereas the interference is lower with citrate-capping layers. Steric stabilization of AgNPs enables disinfection within a wide range of water chemistries, whereas effective disinfection can occur under electrostatic stabilization, only at low NaCl (<1 mmol/L) and organic matter (<5 mg/L) levels.

• Dispersion stabilization of silver nanoparticles in synthetic lung fluid studied under in situ conditions

  Nanotoxicology

  The dispersion stabilization of silver nanoparticles (AgNPs) in synthetic lung fluid was studied to interrogate the effects on colloidal stability due to the principal constituents of the fluid. The colloidal stability of 20 nm citrate-AgNPs dispersed in the presence of each constituent of the synthetic lung fluid (individually, the complete fluid, and without additives) was observed during titration of increasing sodium chloride concentration. A variety of complementary in situ measurement techniques were utilized, including dynamic light scattering, ultraviolet-visible absorption spectroscopy, atomic force microscopy, and small-angle X-ray scattering, which provided a collective set of information that enabled far better understanding of the dispersion behavior in the fluid than any one technique alone. It was observed that AgNPs continued to adsorb bovine serum albumin (BSA) protein from the synthetic lung fluid solution as the sodium chloride concentration increased, until a maximum BSA coating was achieved prior to reaching the physiological sodium chloride concentration of 154 mmol L−1. BSA was determined to be the constituent of the synthetic lung fluid that is required to provide colloidal stability at high salt loadings, though the phospholipid constituent exerts a subtle effect. Additionally, as AgNPs are a distinctly different class of nanoparticles apart from the carbon nanotubes and titanium dioxide nanoparticles initially reported to be dispersible using this fluid, this work also demonstrates the broad applicability of synthetic lung fluid in providing stable dispersions for engineered nanoparticles for use in biological assays.

• Adsorption and Conformation of Serum Albumin Protein on Gold Nanoparticles Investigated Using Dimensional Measurements and in Situ Spectroscopic Methods

  Langmuir

  The adsorption and conformation of bovine serum albumin (BSA) on gold nanoparticles (AuNPs) were interrogated both qualitatively and quantitatively via complementary physicochemical characterization methods. Dynamic light scattering (DLS), asymmetric-flow field flow fractionation (AFFF), fluorescence spectrometry, and attenuated total reflectance- Fourier transform infrared (ATR-FTIR) spectroscopy were combined to characterize BSA-AuNP conjugates under fluid conditions, while conjugates in the aerosol state were characterized by electrospray-differential mobility analysis (ES-DMA). The presence of unbound BSA molecules interferes with DLS analysis of the conjugates, particularly as the AuNP size decreases (i.e., below 30 nm in diameter). Under conditions where the γ value is high, where γ is defined as the ratio of scattering intensity by AuNPs to the scattering intensity by unbound BSA, DLS size results are consistent with results obtained after fractionation by AFFF. Additionally, the AuNP hydrodynamic size exhibits a greater proportional increase due to BSA conjugation at pH values below 2.5 compared with less acidic pH values (3.4-7.3), corresponding with the reversibly denatured (E or F form) conformation of BSA below pH 2.5. Over the pH range from 3.4 to 7.3, the hydrodynamic size of the conjugate is nearly constant, suggesting conformational stability over this range. Because of the difference in the measurement environment, a larger increase of AuNP size is observed following BSA conjugation when measured in the wet state (i.e., by DLS and AFFF) compared to the dry state (by ES-DMA).

• Disinfection action of electrostatic versus steric-stabilized silver nanoparticles on E. coli under different water chemistries

  Colloids and Surfaces B: Biointerfaces

  The capping layer stabilizing silver nanoparticles (AgNPs) affects its aggregation, dissolution, and net disinfection action, especially under conditions of varying water composition, such as, pH, ionic strength and organic matter content. Herein, we correlate the silver ion (Ag+) release and reactive oxygen species (ROS) generation rates for AgNPs of varying functionalization to their net disinfection coefficient on Escherichia coli, under conditions of differing water chemistries. For electrostatically stabilized citrate-capped AgNPs, the rate of ROS generation, as measured using a fluorescent dye, is found to dominate over that of Ag+ release, especially for smaller sized AgNP suspensions (∼10 nm) at low pH (∼6.2). For these AgNPs, the ROS disinfection mechanism is confirmed to dominate net disinfection action, as measured by the live/dead assay, especially at low levels of organic matter. Steric stabilization of AgNPs by protein or starch-capped layers enables disinfection through reducing AgNP aggregation and promoting silver dissolution over ROS generation. We suggest the involvement of protons and dissolved oxygen in causing the independent formation of Ag+ and ROS, regardless of the AgNP capping layer. While protein-capping layers effectively stabilize AgNPs, the generated ROS is likely dissipated by interference with the bulky capping layer, whereas the interference is lower with citrate-capping layers. Steric stabilization of AgNPs enables disinfection within a wide range of water chemistries, whereas effective disinfection can occur under electrostatic stabilization, only at low NaCl (<1 mmol/L) and organic matter (<5 mg/L) levels.

• Tiered guidance for risk-informed environmental health and safety testing of nanotechnologies

  Journal of Nanoparticle Research

• Dispersion stabilization of silver nanoparticles in synthetic lung fluid studied under in situ conditions

  Nanotoxicology

  The dispersion stabilization of silver nanoparticles (AgNPs) in synthetic lung fluid was studied to interrogate the effects on colloidal stability due to the principal constituents of the fluid. The colloidal stability of 20 nm citrate-AgNPs dispersed in the presence of each constituent of the synthetic lung fluid (individually, the complete fluid, and without additives) was observed during titration of increasing sodium chloride concentration. A variety of complementary in situ measurement techniques were utilized, including dynamic light scattering, ultraviolet-visible absorption spectroscopy, atomic force microscopy, and small-angle X-ray scattering, which provided a collective set of information that enabled far better understanding of the dispersion behavior in the fluid than any one technique alone. It was observed that AgNPs continued to adsorb bovine serum albumin (BSA) protein from the synthetic lung fluid solution as the sodium chloride concentration increased, until a maximum BSA coating was achieved prior to reaching the physiological sodium chloride concentration of 154 mmol L−1. BSA was determined to be the constituent of the synthetic lung fluid that is required to provide colloidal stability at high salt loadings, though the phospholipid constituent exerts a subtle effect. Additionally, as AgNPs are a distinctly different class of nanoparticles apart from the carbon nanotubes and titanium dioxide nanoparticles initially reported to be dispersible using this fluid, this work also demonstrates the broad applicability of synthetic lung fluid in providing stable dispersions for engineered nanoparticles for use in biological assays.

• Adsorption and Conformation of Serum Albumin Protein on Gold Nanoparticles Investigated Using Dimensional Measurements and in Situ Spectroscopic Methods

  Langmuir

  The adsorption and conformation of bovine serum albumin (BSA) on gold nanoparticles (AuNPs) were interrogated both qualitatively and quantitatively via complementary physicochemical characterization methods. Dynamic light scattering (DLS), asymmetric-flow field flow fractionation (AFFF), fluorescence spectrometry, and attenuated total reflectance- Fourier transform infrared (ATR-FTIR) spectroscopy were combined to characterize BSA-AuNP conjugates under fluid conditions, while conjugates in the aerosol state were characterized by electrospray-differential mobility analysis (ES-DMA). The presence of unbound BSA molecules interferes with DLS analysis of the conjugates, particularly as the AuNP size decreases (i.e., below 30 nm in diameter). Under conditions where the γ value is high, where γ is defined as the ratio of scattering intensity by AuNPs to the scattering intensity by unbound BSA, DLS size results are consistent with results obtained after fractionation by AFFF. Additionally, the AuNP hydrodynamic size exhibits a greater proportional increase due to BSA conjugation at pH values below 2.5 compared with less acidic pH values (3.4-7.3), corresponding with the reversibly denatured (E or F form) conformation of BSA below pH 2.5. Over the pH range from 3.4 to 7.3, the hydrodynamic size of the conjugate is nearly constant, suggesting conformational stability over this range. Because of the difference in the measurement environment, a larger increase of AuNP size is observed following BSA conjugation when measured in the wet state (i.e., by DLS and AFFF) compared to the dry state (by ES-DMA).

• Disinfection action of electrostatic versus steric-stabilized silver nanoparticles on E. coli under different water chemistries

  Colloids and Surfaces B: Biointerfaces

  The capping layer stabilizing silver nanoparticles (AgNPs) affects its aggregation, dissolution, and net disinfection action, especially under conditions of varying water composition, such as, pH, ionic strength and organic matter content. Herein, we correlate the silver ion (Ag+) release and reactive oxygen species (ROS) generation rates for AgNPs of varying functionalization to their net disinfection coefficient on Escherichia coli, under conditions of differing water chemistries. For electrostatically stabilized citrate-capped AgNPs, the rate of ROS generation, as measured using a fluorescent dye, is found to dominate over that of Ag+ release, especially for smaller sized AgNP suspensions (∼10 nm) at low pH (∼6.2). For these AgNPs, the ROS disinfection mechanism is confirmed to dominate net disinfection action, as measured by the live/dead assay, especially at low levels of organic matter. Steric stabilization of AgNPs by protein or starch-capped layers enables disinfection through reducing AgNP aggregation and promoting silver dissolution over ROS generation. We suggest the involvement of protons and dissolved oxygen in causing the independent formation of Ag+ and ROS, regardless of the AgNP capping layer. While protein-capping layers effectively stabilize AgNPs, the generated ROS is likely dissipated by interference with the bulky capping layer, whereas the interference is lower with citrate-capping layers. Steric stabilization of AgNPs enables disinfection within a wide range of water chemistries, whereas effective disinfection can occur under electrostatic stabilization, only at low NaCl (<1 mmol/L) and organic matter (<5 mg/L) levels.

• Tiered guidance for risk-informed environmental health and safety testing of nanotechnologies

  Journal of Nanoparticle Research

• Unexpected Changes in Functionality and Surface Coverage for Au Nanoparticle PEI Conjugates: Implications for Stability and Efficacy in Biological Systems

  Langmuir

• Dispersion stabilization of silver nanoparticles in synthetic lung fluid studied under in situ conditions

  Nanotoxicology

  The dispersion stabilization of silver nanoparticles (AgNPs) in synthetic lung fluid was studied to interrogate the effects on colloidal stability due to the principal constituents of the fluid. The colloidal stability of 20 nm citrate-AgNPs dispersed in the presence of each constituent of the synthetic lung fluid (individually, the complete fluid, and without additives) was observed during titration of increasing sodium chloride concentration. A variety of complementary in situ measurement techniques were utilized, including dynamic light scattering, ultraviolet-visible absorption spectroscopy, atomic force microscopy, and small-angle X-ray scattering, which provided a collective set of information that enabled far better understanding of the dispersion behavior in the fluid than any one technique alone. It was observed that AgNPs continued to adsorb bovine serum albumin (BSA) protein from the synthetic lung fluid solution as the sodium chloride concentration increased, until a maximum BSA coating was achieved prior to reaching the physiological sodium chloride concentration of 154 mmol L−1. BSA was determined to be the constituent of the synthetic lung fluid that is required to provide colloidal stability at high salt loadings, though the phospholipid constituent exerts a subtle effect. Additionally, as AgNPs are a distinctly different class of nanoparticles apart from the carbon nanotubes and titanium dioxide nanoparticles initially reported to be dispersible using this fluid, this work also demonstrates the broad applicability of synthetic lung fluid in providing stable dispersions for engineered nanoparticles for use in biological assays.

• Adsorption and Conformation of Serum Albumin Protein on Gold Nanoparticles Investigated Using Dimensional Measurements and in Situ Spectroscopic Methods

  Langmuir

  The adsorption and conformation of bovine serum albumin (BSA) on gold nanoparticles (AuNPs) were interrogated both qualitatively and quantitatively via complementary physicochemical characterization methods. Dynamic light scattering (DLS), asymmetric-flow field flow fractionation (AFFF), fluorescence spectrometry, and attenuated total reflectance- Fourier transform infrared (ATR-FTIR) spectroscopy were combined to characterize BSA-AuNP conjugates under fluid conditions, while conjugates in the aerosol state were characterized by electrospray-differential mobility analysis (ES-DMA). The presence of unbound BSA molecules interferes with DLS analysis of the conjugates, particularly as the AuNP size decreases (i.e., below 30 nm in diameter). Under conditions where the γ value is high, where γ is defined as the ratio of scattering intensity by AuNPs to the scattering intensity by unbound BSA, DLS size results are consistent with results obtained after fractionation by AFFF. Additionally, the AuNP hydrodynamic size exhibits a greater proportional increase due to BSA conjugation at pH values below 2.5 compared with less acidic pH values (3.4-7.3), corresponding with the reversibly denatured (E or F form) conformation of BSA below pH 2.5. Over the pH range from 3.4 to 7.3, the hydrodynamic size of the conjugate is nearly constant, suggesting conformational stability over this range. Because of the difference in the measurement environment, a larger increase of AuNP size is observed following BSA conjugation when measured in the wet state (i.e., by DLS and AFFF) compared to the dry state (by ES-DMA).

• Disinfection action of electrostatic versus steric-stabilized silver nanoparticles on E. coli under different water chemistries

  Colloids and Surfaces B: Biointerfaces

  The capping layer stabilizing silver nanoparticles (AgNPs) affects its aggregation, dissolution, and net disinfection action, especially under conditions of varying water composition, such as, pH, ionic strength and organic matter content. Herein, we correlate the silver ion (Ag+) release and reactive oxygen species (ROS) generation rates for AgNPs of varying functionalization to their net disinfection coefficient on Escherichia coli, under conditions of differing water chemistries. For electrostatically stabilized citrate-capped AgNPs, the rate of ROS generation, as measured using a fluorescent dye, is found to dominate over that of Ag+ release, especially for smaller sized AgNP suspensions (∼10 nm) at low pH (∼6.2). For these AgNPs, the ROS disinfection mechanism is confirmed to dominate net disinfection action, as measured by the live/dead assay, especially at low levels of organic matter. Steric stabilization of AgNPs by protein or starch-capped layers enables disinfection through reducing AgNP aggregation and promoting silver dissolution over ROS generation. We suggest the involvement of protons and dissolved oxygen in causing the independent formation of Ag+ and ROS, regardless of the AgNP capping layer. While protein-capping layers effectively stabilize AgNPs, the generated ROS is likely dissipated by interference with the bulky capping layer, whereas the interference is lower with citrate-capping layers. Steric stabilization of AgNPs enables disinfection within a wide range of water chemistries, whereas effective disinfection can occur under electrostatic stabilization, only at low NaCl (<1 mmol/L) and organic matter (<5 mg/L) levels.

• Tiered guidance for risk-informed environmental health and safety testing of nanotechnologies

  Journal of Nanoparticle Research

• Unexpected Changes in Functionality and Surface Coverage for Au Nanoparticle PEI Conjugates: Implications for Stability and Efficacy in Biological Systems

  Langmuir

• UV-induced photochemical transformations of citrate capped silver nanoparticle suspensions

  Journal of Nanoparticle Research

  Due to the increasing use of silver nanoparticles (AgNPs) in consumer products, it is essential to understand how variables, such as light exposure, may change the physical and chemical characteristics of AgNP suspensions. To this end, the effect of 300 nm ultraviolet (UV) light on (20, 40, 60 and 80) nm citrate-capped AgNP suspensions has been investigated. As a consequence of irradiation, the initial yellow hue of the AgNP suspensions is transformed towards a near colorless solution due to the loss of the surface plasmon resonance (SPR) absorbance. The decrease in SPR absorbance followed a first-order decay process for all particle sizes with a rate constant that increased linearly with the AgNP specific surface area and non-linearly with light intensity. The rate of loss of the SPR absorbance decreased with increasing citrate concentration, suggesting a surface-mediated transformation. Absorbance, atomic force microscopy, and dynamic light scattering results all indicated that AgNP photolysis was accompanied by a diameter decrease and occasional aggregation. Furthermore, in situ transmission electron microscopy imaging using a specialized liquid cell also showed a decrease in the particle size and the formation of a core–shell structure in UV-exposed AgNPs. X-ray photoelectron spectroscopy analysis suggested that this shell consisted of oxidized silver. The SPR in UV-exposed AgNP suspensions could be regenerated by addition of a strong reducing agent (NaBH4), supporting the idea that oxidized silver is present after photolysis. Evidence for UV-enhanced dissolution and the production of silver ions was obtained with the Donnan membrane technique. This study reveals that the physico-chemical properties of aqueous AgNP suspensions will change significantly upon exposure to UV light, with implications for environmental health and safety risk assessments.

• Dispersion stabilization of silver nanoparticles in synthetic lung fluid studied under in situ conditions

  Nanotoxicology

  The dispersion stabilization of silver nanoparticles (AgNPs) in synthetic lung fluid was studied to interrogate the effects on colloidal stability due to the principal constituents of the fluid. The colloidal stability of 20 nm citrate-AgNPs dispersed in the presence of each constituent of the synthetic lung fluid (individually, the complete fluid, and without additives) was observed during titration of increasing sodium chloride concentration. A variety of complementary in situ measurement techniques were utilized, including dynamic light scattering, ultraviolet-visible absorption spectroscopy, atomic force microscopy, and small-angle X-ray scattering, which provided a collective set of information that enabled far better understanding of the dispersion behavior in the fluid than any one technique alone. It was observed that AgNPs continued to adsorb bovine serum albumin (BSA) protein from the synthetic lung fluid solution as the sodium chloride concentration increased, until a maximum BSA coating was achieved prior to reaching the physiological sodium chloride concentration of 154 mmol L−1. BSA was determined to be the constituent of the synthetic lung fluid that is required to provide colloidal stability at high salt loadings, though the phospholipid constituent exerts a subtle effect. Additionally, as AgNPs are a distinctly different class of nanoparticles apart from the carbon nanotubes and titanium dioxide nanoparticles initially reported to be dispersible using this fluid, this work also demonstrates the broad applicability of synthetic lung fluid in providing stable dispersions for engineered nanoparticles for use in biological assays.

• Adsorption and Conformation of Serum Albumin Protein on Gold Nanoparticles Investigated Using Dimensional Measurements and in Situ Spectroscopic Methods

  Langmuir

  The adsorption and conformation of bovine serum albumin (BSA) on gold nanoparticles (AuNPs) were interrogated both qualitatively and quantitatively via complementary physicochemical characterization methods. Dynamic light scattering (DLS), asymmetric-flow field flow fractionation (AFFF), fluorescence spectrometry, and attenuated total reflectance- Fourier transform infrared (ATR-FTIR) spectroscopy were combined to characterize BSA-AuNP conjugates under fluid conditions, while conjugates in the aerosol state were characterized by electrospray-differential mobility analysis (ES-DMA). The presence of unbound BSA molecules interferes with DLS analysis of the conjugates, particularly as the AuNP size decreases (i.e., below 30 nm in diameter). Under conditions where the γ value is high, where γ is defined as the ratio of scattering intensity by AuNPs to the scattering intensity by unbound BSA, DLS size results are consistent with results obtained after fractionation by AFFF. Additionally, the AuNP hydrodynamic size exhibits a greater proportional increase due to BSA conjugation at pH values below 2.5 compared with less acidic pH values (3.4-7.3), corresponding with the reversibly denatured (E or F form) conformation of BSA below pH 2.5. Over the pH range from 3.4 to 7.3, the hydrodynamic size of the conjugate is nearly constant, suggesting conformational stability over this range. Because of the difference in the measurement environment, a larger increase of AuNP size is observed following BSA conjugation when measured in the wet state (i.e., by DLS and AFFF) compared to the dry state (by ES-DMA).

• Disinfection action of electrostatic versus steric-stabilized silver nanoparticles on E. coli under different water chemistries

  Colloids and Surfaces B: Biointerfaces

  The capping layer stabilizing silver nanoparticles (AgNPs) affects its aggregation, dissolution, and net disinfection action, especially under conditions of varying water composition, such as, pH, ionic strength and organic matter content. Herein, we correlate the silver ion (Ag+) release and reactive oxygen species (ROS) generation rates for AgNPs of varying functionalization to their net disinfection coefficient on Escherichia coli, under conditions of differing water chemistries. For electrostatically stabilized citrate-capped AgNPs, the rate of ROS generation, as measured using a fluorescent dye, is found to dominate over that of Ag+ release, especially for smaller sized AgNP suspensions (∼10 nm) at low pH (∼6.2). For these AgNPs, the ROS disinfection mechanism is confirmed to dominate net disinfection action, as measured by the live/dead assay, especially at low levels of organic matter. Steric stabilization of AgNPs by protein or starch-capped layers enables disinfection through reducing AgNP aggregation and promoting silver dissolution over ROS generation. We suggest the involvement of protons and dissolved oxygen in causing the independent formation of Ag+ and ROS, regardless of the AgNP capping layer. While protein-capping layers effectively stabilize AgNPs, the generated ROS is likely dissipated by interference with the bulky capping layer, whereas the interference is lower with citrate-capping layers. Steric stabilization of AgNPs enables disinfection within a wide range of water chemistries, whereas effective disinfection can occur under electrostatic stabilization, only at low NaCl (<1 mmol/L) and organic matter (<5 mg/L) levels.

• Tiered guidance for risk-informed environmental health and safety testing of nanotechnologies

  Journal of Nanoparticle Research

• Unexpected Changes in Functionality and Surface Coverage for Au Nanoparticle PEI Conjugates: Implications for Stability and Efficacy in Biological Systems

  Langmuir

• UV-induced photochemical transformations of citrate capped silver nanoparticle suspensions

  Journal of Nanoparticle Research

  Due to the increasing use of silver nanoparticles (AgNPs) in consumer products, it is essential to understand how variables, such as light exposure, may change the physical and chemical characteristics of AgNP suspensions. To this end, the effect of 300 nm ultraviolet (UV) light on (20, 40, 60 and 80) nm citrate-capped AgNP suspensions has been investigated. As a consequence of irradiation, the initial yellow hue of the AgNP suspensions is transformed towards a near colorless solution due to the loss of the surface plasmon resonance (SPR) absorbance. The decrease in SPR absorbance followed a first-order decay process for all particle sizes with a rate constant that increased linearly with the AgNP specific surface area and non-linearly with light intensity. The rate of loss of the SPR absorbance decreased with increasing citrate concentration, suggesting a surface-mediated transformation. Absorbance, atomic force microscopy, and dynamic light scattering results all indicated that AgNP photolysis was accompanied by a diameter decrease and occasional aggregation. Furthermore, in situ transmission electron microscopy imaging using a specialized liquid cell also showed a decrease in the particle size and the formation of a core–shell structure in UV-exposed AgNPs. X-ray photoelectron spectroscopy analysis suggested that this shell consisted of oxidized silver. The SPR in UV-exposed AgNP suspensions could be regenerated by addition of a strong reducing agent (NaBH4), supporting the idea that oxidized silver is present after photolysis. Evidence for UV-enhanced dissolution and the production of silver ions was obtained with the Donnan membrane technique. This study reveals that the physico-chemical properties of aqueous AgNP suspensions will change significantly upon exposure to UV light, with implications for environmental health and safety risk assessments.

• SP 1200-8: Preparation of silver nanoparticle loaded cotton threads to facilitate measurement development for textile applications

  NIST Technical Series Publication

  FOREWORD This NIST Special Publication (SP) is one in a series of NIST SPs that address research needs articulated in the National Nanotechnology Initiative (NNI) Environmental, Health, and Safety Research Strategy published in 2011 [1]. This Strategy identified a Nanomaterial Measurement Infrastructure (NMI) as essential for science-based risk assessment and risk management of nanotechnology-enabled products as pertaining to human health, exposure, and the environment. NIST was identified as the lead federal agency in the NMI core research area of the Strategy. This research area includes development of measurement tools for the detection and characterization of engineered nanomaterials in nanotechnology-enabled products. Textiles containing silver nanoparticles, the focus of this SP, are consumer products of great interest for nanotechnology environmental, health, and safety (nano-EHS) research and for product regulation. The protocol presented in this SP describes a strategy for developing a validation known to contain silver nanoparticles loaded onto the surface of a cotton thread. These validation threads can be used to test the capabilities of various analytical methods to characterize silver nanoparticles in textiles.

• Dispersion stabilization of silver nanoparticles in synthetic lung fluid studied under in situ conditions

  Nanotoxicology

  The dispersion stabilization of silver nanoparticles (AgNPs) in synthetic lung fluid was studied to interrogate the effects on colloidal stability due to the principal constituents of the fluid. The colloidal stability of 20 nm citrate-AgNPs dispersed in the presence of each constituent of the synthetic lung fluid (individually, the complete fluid, and without additives) was observed during titration of increasing sodium chloride concentration. A variety of complementary in situ measurement techniques were utilized, including dynamic light scattering, ultraviolet-visible absorption spectroscopy, atomic force microscopy, and small-angle X-ray scattering, which provided a collective set of information that enabled far better understanding of the dispersion behavior in the fluid than any one technique alone. It was observed that AgNPs continued to adsorb bovine serum albumin (BSA) protein from the synthetic lung fluid solution as the sodium chloride concentration increased, until a maximum BSA coating was achieved prior to reaching the physiological sodium chloride concentration of 154 mmol L−1. BSA was determined to be the constituent of the synthetic lung fluid that is required to provide colloidal stability at high salt loadings, though the phospholipid constituent exerts a subtle effect. Additionally, as AgNPs are a distinctly different class of nanoparticles apart from the carbon nanotubes and titanium dioxide nanoparticles initially reported to be dispersible using this fluid, this work also demonstrates the broad applicability of synthetic lung fluid in providing stable dispersions for engineered nanoparticles for use in biological assays.

• Adsorption and Conformation of Serum Albumin Protein on Gold Nanoparticles Investigated Using Dimensional Measurements and in Situ Spectroscopic Methods

  Langmuir

  The adsorption and conformation of bovine serum albumin (BSA) on gold nanoparticles (AuNPs) were interrogated both qualitatively and quantitatively via complementary physicochemical characterization methods. Dynamic light scattering (DLS), asymmetric-flow field flow fractionation (AFFF), fluorescence spectrometry, and attenuated total reflectance- Fourier transform infrared (ATR-FTIR) spectroscopy were combined to characterize BSA-AuNP conjugates under fluid conditions, while conjugates in the aerosol state were characterized by electrospray-differential mobility analysis (ES-DMA). The presence of unbound BSA molecules interferes with DLS analysis of the conjugates, particularly as the AuNP size decreases (i.e., below 30 nm in diameter). Under conditions where the γ value is high, where γ is defined as the ratio of scattering intensity by AuNPs to the scattering intensity by unbound BSA, DLS size results are consistent with results obtained after fractionation by AFFF. Additionally, the AuNP hydrodynamic size exhibits a greater proportional increase due to BSA conjugation at pH values below 2.5 compared with less acidic pH values (3.4-7.3), corresponding with the reversibly denatured (E or F form) conformation of BSA below pH 2.5. Over the pH range from 3.4 to 7.3, the hydrodynamic size of the conjugate is nearly constant, suggesting conformational stability over this range. Because of the difference in the measurement environment, a larger increase of AuNP size is observed following BSA conjugation when measured in the wet state (i.e., by DLS and AFFF) compared to the dry state (by ES-DMA).

• Disinfection action of electrostatic versus steric-stabilized silver nanoparticles on E. coli under different water chemistries

  Colloids and Surfaces B: Biointerfaces

  The capping layer stabilizing silver nanoparticles (AgNPs) affects its aggregation, dissolution, and net disinfection action, especially under conditions of varying water composition, such as, pH, ionic strength and organic matter content. Herein, we correlate the silver ion (Ag+) release and reactive oxygen species (ROS) generation rates for AgNPs of varying functionalization to their net disinfection coefficient on Escherichia coli, under conditions of differing water chemistries. For electrostatically stabilized citrate-capped AgNPs, the rate of ROS generation, as measured using a fluorescent dye, is found to dominate over that of Ag+ release, especially for smaller sized AgNP suspensions (∼10 nm) at low pH (∼6.2). For these AgNPs, the ROS disinfection mechanism is confirmed to dominate net disinfection action, as measured by the live/dead assay, especially at low levels of organic matter. Steric stabilization of AgNPs by protein or starch-capped layers enables disinfection through reducing AgNP aggregation and promoting silver dissolution over ROS generation. We suggest the involvement of protons and dissolved oxygen in causing the independent formation of Ag+ and ROS, regardless of the AgNP capping layer. While protein-capping layers effectively stabilize AgNPs, the generated ROS is likely dissipated by interference with the bulky capping layer, whereas the interference is lower with citrate-capping layers. Steric stabilization of AgNPs enables disinfection within a wide range of water chemistries, whereas effective disinfection can occur under electrostatic stabilization, only at low NaCl (<1 mmol/L) and organic matter (<5 mg/L) levels.

• Tiered guidance for risk-informed environmental health and safety testing of nanotechnologies

  Journal of Nanoparticle Research

• Unexpected Changes in Functionality and Surface Coverage for Au Nanoparticle PEI Conjugates: Implications for Stability and Efficacy in Biological Systems

  Langmuir

• UV-induced photochemical transformations of citrate capped silver nanoparticle suspensions

  Journal of Nanoparticle Research

  Due to the increasing use of silver nanoparticles (AgNPs) in consumer products, it is essential to understand how variables, such as light exposure, may change the physical and chemical characteristics of AgNP suspensions. To this end, the effect of 300 nm ultraviolet (UV) light on (20, 40, 60 and 80) nm citrate-capped AgNP suspensions has been investigated. As a consequence of irradiation, the initial yellow hue of the AgNP suspensions is transformed towards a near colorless solution due to the loss of the surface plasmon resonance (SPR) absorbance. The decrease in SPR absorbance followed a first-order decay process for all particle sizes with a rate constant that increased linearly with the AgNP specific surface area and non-linearly with light intensity. The rate of loss of the SPR absorbance decreased with increasing citrate concentration, suggesting a surface-mediated transformation. Absorbance, atomic force microscopy, and dynamic light scattering results all indicated that AgNP photolysis was accompanied by a diameter decrease and occasional aggregation. Furthermore, in situ transmission electron microscopy imaging using a specialized liquid cell also showed a decrease in the particle size and the formation of a core–shell structure in UV-exposed AgNPs. X-ray photoelectron spectroscopy analysis suggested that this shell consisted of oxidized silver. The SPR in UV-exposed AgNP suspensions could be regenerated by addition of a strong reducing agent (NaBH4), supporting the idea that oxidized silver is present after photolysis. Evidence for UV-enhanced dissolution and the production of silver ions was obtained with the Donnan membrane technique. This study reveals that the physico-chemical properties of aqueous AgNP suspensions will change significantly upon exposure to UV light, with implications for environmental health and safety risk assessments.

• SP 1200-8: Preparation of silver nanoparticle loaded cotton threads to facilitate measurement development for textile applications

  NIST Technical Series Publication

  FOREWORD This NIST Special Publication (SP) is one in a series of NIST SPs that address research needs articulated in the National Nanotechnology Initiative (NNI) Environmental, Health, and Safety Research Strategy published in 2011 [1]. This Strategy identified a Nanomaterial Measurement Infrastructure (NMI) as essential for science-based risk assessment and risk management of nanotechnology-enabled products as pertaining to human health, exposure, and the environment. NIST was identified as the lead federal agency in the NMI core research area of the Strategy. This research area includes development of measurement tools for the detection and characterization of engineered nanomaterials in nanotechnology-enabled products. Textiles containing silver nanoparticles, the focus of this SP, are consumer products of great interest for nanotechnology environmental, health, and safety (nano-EHS) research and for product regulation. The protocol presented in this SP describes a strategy for developing a validation known to contain silver nanoparticles loaded onto the surface of a cotton thread. These validation threads can be used to test the capabilities of various analytical methods to characterize silver nanoparticles in textiles.

• Gold nanorod separation and characterization by asymmetric-flow field flow fractionation with UV–Vis detection

  Analytical Bioanalytical Chemistry

  The application of asymmetric-flow field flow fractionation (A4F) for low aspect ratio gold nanorod (GNR) fractionation and characterization was comprehensively investigated. We report on two novel aspects of this application. The first addresses the analytical challenge involved in the fractionation of positively charged nanoparticles by A4F, due to the interaction that exists between the negatively charged native membrane and the analyte. We show that the mobile phase composition is a critical parameter for controlling fractionation and mitigating the membrane-analyte interaction. A mixture of ammonium nitrate and cetyl trimethyl ammonium bromide at different molar ratios enables separation of GNRs with high recovery. The second aspect is the demonstration of shape-based separation of GNRs in A4F normal mode elution (i.e., Brownian mode). We show that the elution of GNRs is due both to aspect ratio and a steric-entropic contribution for GNRs with the same diameter. This latter effect can be explained by their orientation vector inside the A4F channel. Our experimental results demonstrate the relevance of the theory described by Beckett and Giddings for non-spherical fractionation (Beckett and Giddings, J Colloid and Interface Sci 186(1):53–59, 1997). However, it is shown that this theory has its limit in the case of complex GNR mixtures, and that shape (i.e., aspect ratio) is the principal material parameter controlling elution of GNRs in A4F; the apparent translational diffusion coefficient of GNRs increases with aspect ratio. Finally, the performance of the methodology developed in this work is evaluated by the fractionation and characterization of individual components from a mixture of GNR aspect ratios.

• Dispersion stabilization of silver nanoparticles in synthetic lung fluid studied under in situ conditions

  Nanotoxicology

  The dispersion stabilization of silver nanoparticles (AgNPs) in synthetic lung fluid was studied to interrogate the effects on colloidal stability due to the principal constituents of the fluid. The colloidal stability of 20 nm citrate-AgNPs dispersed in the presence of each constituent of the synthetic lung fluid (individually, the complete fluid, and without additives) was observed during titration of increasing sodium chloride concentration. A variety of complementary in situ measurement techniques were utilized, including dynamic light scattering, ultraviolet-visible absorption spectroscopy, atomic force microscopy, and small-angle X-ray scattering, which provided a collective set of information that enabled far better understanding of the dispersion behavior in the fluid than any one technique alone. It was observed that AgNPs continued to adsorb bovine serum albumin (BSA) protein from the synthetic lung fluid solution as the sodium chloride concentration increased, until a maximum BSA coating was achieved prior to reaching the physiological sodium chloride concentration of 154 mmol L−1. BSA was determined to be the constituent of the synthetic lung fluid that is required to provide colloidal stability at high salt loadings, though the phospholipid constituent exerts a subtle effect. Additionally, as AgNPs are a distinctly different class of nanoparticles apart from the carbon nanotubes and titanium dioxide nanoparticles initially reported to be dispersible using this fluid, this work also demonstrates the broad applicability of synthetic lung fluid in providing stable dispersions for engineered nanoparticles for use in biological assays.

• Adsorption and Conformation of Serum Albumin Protein on Gold Nanoparticles Investigated Using Dimensional Measurements and in Situ Spectroscopic Methods

  Langmuir

  The adsorption and conformation of bovine serum albumin (BSA) on gold nanoparticles (AuNPs) were interrogated both qualitatively and quantitatively via complementary physicochemical characterization methods. Dynamic light scattering (DLS), asymmetric-flow field flow fractionation (AFFF), fluorescence spectrometry, and attenuated total reflectance- Fourier transform infrared (ATR-FTIR) spectroscopy were combined to characterize BSA-AuNP conjugates under fluid conditions, while conjugates in the aerosol state were characterized by electrospray-differential mobility analysis (ES-DMA). The presence of unbound BSA molecules interferes with DLS analysis of the conjugates, particularly as the AuNP size decreases (i.e., below 30 nm in diameter). Under conditions where the γ value is high, where γ is defined as the ratio of scattering intensity by AuNPs to the scattering intensity by unbound BSA, DLS size results are consistent with results obtained after fractionation by AFFF. Additionally, the AuNP hydrodynamic size exhibits a greater proportional increase due to BSA conjugation at pH values below 2.5 compared with less acidic pH values (3.4-7.3), corresponding with the reversibly denatured (E or F form) conformation of BSA below pH 2.5. Over the pH range from 3.4 to 7.3, the hydrodynamic size of the conjugate is nearly constant, suggesting conformational stability over this range. Because of the difference in the measurement environment, a larger increase of AuNP size is observed following BSA conjugation when measured in the wet state (i.e., by DLS and AFFF) compared to the dry state (by ES-DMA).

• Disinfection action of electrostatic versus steric-stabilized silver nanoparticles on E. coli under different water chemistries

  Colloids and Surfaces B: Biointerfaces

  The capping layer stabilizing silver nanoparticles (AgNPs) affects its aggregation, dissolution, and net disinfection action, especially under conditions of varying water composition, such as, pH, ionic strength and organic matter content. Herein, we correlate the silver ion (Ag+) release and reactive oxygen species (ROS) generation rates for AgNPs of varying functionalization to their net disinfection coefficient on Escherichia coli, under conditions of differing water chemistries. For electrostatically stabilized citrate-capped AgNPs, the rate of ROS generation, as measured using a fluorescent dye, is found to dominate over that of Ag+ release, especially for smaller sized AgNP suspensions (∼10 nm) at low pH (∼6.2). For these AgNPs, the ROS disinfection mechanism is confirmed to dominate net disinfection action, as measured by the live/dead assay, especially at low levels of organic matter. Steric stabilization of AgNPs by protein or starch-capped layers enables disinfection through reducing AgNP aggregation and promoting silver dissolution over ROS generation. We suggest the involvement of protons and dissolved oxygen in causing the independent formation of Ag+ and ROS, regardless of the AgNP capping layer. While protein-capping layers effectively stabilize AgNPs, the generated ROS is likely dissipated by interference with the bulky capping layer, whereas the interference is lower with citrate-capping layers. Steric stabilization of AgNPs enables disinfection within a wide range of water chemistries, whereas effective disinfection can occur under electrostatic stabilization, only at low NaCl (<1 mmol/L) and organic matter (<5 mg/L) levels.

• Tiered guidance for risk-informed environmental health and safety testing of nanotechnologies

  Journal of Nanoparticle Research

• Unexpected Changes in Functionality and Surface Coverage for Au Nanoparticle PEI Conjugates: Implications for Stability and Efficacy in Biological Systems

  Langmuir

• UV-induced photochemical transformations of citrate capped silver nanoparticle suspensions

  Journal of Nanoparticle Research

  Due to the increasing use of silver nanoparticles (AgNPs) in consumer products, it is essential to understand how variables, such as light exposure, may change the physical and chemical characteristics of AgNP suspensions. To this end, the effect of 300 nm ultraviolet (UV) light on (20, 40, 60 and 80) nm citrate-capped AgNP suspensions has been investigated. As a consequence of irradiation, the initial yellow hue of the AgNP suspensions is transformed towards a near colorless solution due to the loss of the surface plasmon resonance (SPR) absorbance. The decrease in SPR absorbance followed a first-order decay process for all particle sizes with a rate constant that increased linearly with the AgNP specific surface area and non-linearly with light intensity. The rate of loss of the SPR absorbance decreased with increasing citrate concentration, suggesting a surface-mediated transformation. Absorbance, atomic force microscopy, and dynamic light scattering results all indicated that AgNP photolysis was accompanied by a diameter decrease and occasional aggregation. Furthermore, in situ transmission electron microscopy imaging using a specialized liquid cell also showed a decrease in the particle size and the formation of a core–shell structure in UV-exposed AgNPs. X-ray photoelectron spectroscopy analysis suggested that this shell consisted of oxidized silver. The SPR in UV-exposed AgNP suspensions could be regenerated by addition of a strong reducing agent (NaBH4), supporting the idea that oxidized silver is present after photolysis. Evidence for UV-enhanced dissolution and the production of silver ions was obtained with the Donnan membrane technique. This study reveals that the physico-chemical properties of aqueous AgNP suspensions will change significantly upon exposure to UV light, with implications for environmental health and safety risk assessments.

• SP 1200-8: Preparation of silver nanoparticle loaded cotton threads to facilitate measurement development for textile applications

  NIST Technical Series Publication

  FOREWORD This NIST Special Publication (SP) is one in a series of NIST SPs that address research needs articulated in the National Nanotechnology Initiative (NNI) Environmental, Health, and Safety Research Strategy published in 2011 [1]. This Strategy identified a Nanomaterial Measurement Infrastructure (NMI) as essential for science-based risk assessment and risk management of nanotechnology-enabled products as pertaining to human health, exposure, and the environment. NIST was identified as the lead federal agency in the NMI core research area of the Strategy. This research area includes development of measurement tools for the detection and characterization of engineered nanomaterials in nanotechnology-enabled products. Textiles containing silver nanoparticles, the focus of this SP, are consumer products of great interest for nanotechnology environmental, health, and safety (nano-EHS) research and for product regulation. The protocol presented in this SP describes a strategy for developing a validation known to contain silver nanoparticles loaded onto the surface of a cotton thread. These validation threads can be used to test the capabilities of various analytical methods to characterize silver nanoparticles in textiles.

• Gold nanorod separation and characterization by asymmetric-flow field flow fractionation with UV–Vis detection

  Analytical Bioanalytical Chemistry

  The application of asymmetric-flow field flow fractionation (A4F) for low aspect ratio gold nanorod (GNR) fractionation and characterization was comprehensively investigated. We report on two novel aspects of this application. The first addresses the analytical challenge involved in the fractionation of positively charged nanoparticles by A4F, due to the interaction that exists between the negatively charged native membrane and the analyte. We show that the mobile phase composition is a critical parameter for controlling fractionation and mitigating the membrane-analyte interaction. A mixture of ammonium nitrate and cetyl trimethyl ammonium bromide at different molar ratios enables separation of GNRs with high recovery. The second aspect is the demonstration of shape-based separation of GNRs in A4F normal mode elution (i.e., Brownian mode). We show that the elution of GNRs is due both to aspect ratio and a steric-entropic contribution for GNRs with the same diameter. This latter effect can be explained by their orientation vector inside the A4F channel. Our experimental results demonstrate the relevance of the theory described by Beckett and Giddings for non-spherical fractionation (Beckett and Giddings, J Colloid and Interface Sci 186(1):53–59, 1997). However, it is shown that this theory has its limit in the case of complex GNR mixtures, and that shape (i.e., aspect ratio) is the principal material parameter controlling elution of GNRs in A4F; the apparent translational diffusion coefficient of GNRs increases with aspect ratio. Finally, the performance of the methodology developed in this work is evaluated by the fractionation and characterization of individual components from a mixture of GNR aspect ratios.

• In-situ UV/Vis, SAXS and TEM study of single phase Au Nanoparticle Growth

  Chemistry of Materials

  Given the diverse scientific and technological applications of gold nanoparticles (Au NPs), understanding the impact of macromolecular additives on the distribution of size, shape, and composition is crucial to ensure reproducibility and lower production cost. In situ measurement of the evolution of these distributions challenges current techniques; however, it is critical for in-line manufacturing controls. Using mild Au(I) reduction by tert-butylamine-borane in toluene, the utility and limitations of SAXS, UV/Vis spectroscopy, and TEM are considered by comparing the mean nanoparticle size, size distribution, and relative number density. Individually, these techniques are insufficient to follow these parameters through the initial process of nucleation and growth; either providing insufficient information on the number of particles (UV/Vis), introducing artifacts (TEM), or not providing a unique solution for the shape of the distribution (SAXS). However, when used in conjunction, especially SAXS calibrated with TEM of reaction aliquots, the time evolution of these parameters can be quantified. For the single-phase Au NP synthesis with Au(I) and mild reductants, four distinct experimental regions are revealed that entail two growth mechanisms, which complement previous discussions of single-mode growth kinetics. The kinetics of Au(I):borane reactions are dependent on the Au precursor/reductant ratio and the thiol capping agent length, when reductant concentration is low. The final reaction products exhibit an LSW size distribution.

• Dispersion stabilization of silver nanoparticles in synthetic lung fluid studied under in situ conditions

  Nanotoxicology

  The dispersion stabilization of silver nanoparticles (AgNPs) in synthetic lung fluid was studied to interrogate the effects on colloidal stability due to the principal constituents of the fluid. The colloidal stability of 20 nm citrate-AgNPs dispersed in the presence of each constituent of the synthetic lung fluid (individually, the complete fluid, and without additives) was observed during titration of increasing sodium chloride concentration. A variety of complementary in situ measurement techniques were utilized, including dynamic light scattering, ultraviolet-visible absorption spectroscopy, atomic force microscopy, and small-angle X-ray scattering, which provided a collective set of information that enabled far better understanding of the dispersion behavior in the fluid than any one technique alone. It was observed that AgNPs continued to adsorb bovine serum albumin (BSA) protein from the synthetic lung fluid solution as the sodium chloride concentration increased, until a maximum BSA coating was achieved prior to reaching the physiological sodium chloride concentration of 154 mmol L−1. BSA was determined to be the constituent of the synthetic lung fluid that is required to provide colloidal stability at high salt loadings, though the phospholipid constituent exerts a subtle effect. Additionally, as AgNPs are a distinctly different class of nanoparticles apart from the carbon nanotubes and titanium dioxide nanoparticles initially reported to be dispersible using this fluid, this work also demonstrates the broad applicability of synthetic lung fluid in providing stable dispersions for engineered nanoparticles for use in biological assays.

• Adsorption and Conformation of Serum Albumin Protein on Gold Nanoparticles Investigated Using Dimensional Measurements and in Situ Spectroscopic Methods

  Langmuir

  The adsorption and conformation of bovine serum albumin (BSA) on gold nanoparticles (AuNPs) were interrogated both qualitatively and quantitatively via complementary physicochemical characterization methods. Dynamic light scattering (DLS), asymmetric-flow field flow fractionation (AFFF), fluorescence spectrometry, and attenuated total reflectance- Fourier transform infrared (ATR-FTIR) spectroscopy were combined to characterize BSA-AuNP conjugates under fluid conditions, while conjugates in the aerosol state were characterized by electrospray-differential mobility analysis (ES-DMA). The presence of unbound BSA molecules interferes with DLS analysis of the conjugates, particularly as the AuNP size decreases (i.e., below 30 nm in diameter). Under conditions where the γ value is high, where γ is defined as the ratio of scattering intensity by AuNPs to the scattering intensity by unbound BSA, DLS size results are consistent with results obtained after fractionation by AFFF. Additionally, the AuNP hydrodynamic size exhibits a greater proportional increase due to BSA conjugation at pH values below 2.5 compared with less acidic pH values (3.4-7.3), corresponding with the reversibly denatured (E or F form) conformation of BSA below pH 2.5. Over the pH range from 3.4 to 7.3, the hydrodynamic size of the conjugate is nearly constant, suggesting conformational stability over this range. Because of the difference in the measurement environment, a larger increase of AuNP size is observed following BSA conjugation when measured in the wet state (i.e., by DLS and AFFF) compared to the dry state (by ES-DMA).

• Disinfection action of electrostatic versus steric-stabilized silver nanoparticles on E. coli under different water chemistries

  Colloids and Surfaces B: Biointerfaces

  The capping layer stabilizing silver nanoparticles (AgNPs) affects its aggregation, dissolution, and net disinfection action, especially under conditions of varying water composition, such as, pH, ionic strength and organic matter content. Herein, we correlate the silver ion (Ag+) release and reactive oxygen species (ROS) generation rates for AgNPs of varying functionalization to their net disinfection coefficient on Escherichia coli, under conditions of differing water chemistries. For electrostatically stabilized citrate-capped AgNPs, the rate of ROS generation, as measured using a fluorescent dye, is found to dominate over that of Ag+ release, especially for smaller sized AgNP suspensions (∼10 nm) at low pH (∼6.2). For these AgNPs, the ROS disinfection mechanism is confirmed to dominate net disinfection action, as measured by the live/dead assay, especially at low levels of organic matter. Steric stabilization of AgNPs by protein or starch-capped layers enables disinfection through reducing AgNP aggregation and promoting silver dissolution over ROS generation. We suggest the involvement of protons and dissolved oxygen in causing the independent formation of Ag+ and ROS, regardless of the AgNP capping layer. While protein-capping layers effectively stabilize AgNPs, the generated ROS is likely dissipated by interference with the bulky capping layer, whereas the interference is lower with citrate-capping layers. Steric stabilization of AgNPs enables disinfection within a wide range of water chemistries, whereas effective disinfection can occur under electrostatic stabilization, only at low NaCl (<1 mmol/L) and organic matter (<5 mg/L) levels.

• Tiered guidance for risk-informed environmental health and safety testing of nanotechnologies

  Journal of Nanoparticle Research

• Unexpected Changes in Functionality and Surface Coverage for Au Nanoparticle PEI Conjugates: Implications for Stability and Efficacy in Biological Systems

  Langmuir

• UV-induced photochemical transformations of citrate capped silver nanoparticle suspensions

  Journal of Nanoparticle Research

  Due to the increasing use of silver nanoparticles (AgNPs) in consumer products, it is essential to understand how variables, such as light exposure, may change the physical and chemical characteristics of AgNP suspensions. To this end, the effect of 300 nm ultraviolet (UV) light on (20, 40, 60 and 80) nm citrate-capped AgNP suspensions has been investigated. As a consequence of irradiation, the initial yellow hue of the AgNP suspensions is transformed towards a near colorless solution due to the loss of the surface plasmon resonance (SPR) absorbance. The decrease in SPR absorbance followed a first-order decay process for all particle sizes with a rate constant that increased linearly with the AgNP specific surface area and non-linearly with light intensity. The rate of loss of the SPR absorbance decreased with increasing citrate concentration, suggesting a surface-mediated transformation. Absorbance, atomic force microscopy, and dynamic light scattering results all indicated that AgNP photolysis was accompanied by a diameter decrease and occasional aggregation. Furthermore, in situ transmission electron microscopy imaging using a specialized liquid cell also showed a decrease in the particle size and the formation of a core–shell structure in UV-exposed AgNPs. X-ray photoelectron spectroscopy analysis suggested that this shell consisted of oxidized silver. The SPR in UV-exposed AgNP suspensions could be regenerated by addition of a strong reducing agent (NaBH4), supporting the idea that oxidized silver is present after photolysis. Evidence for UV-enhanced dissolution and the production of silver ions was obtained with the Donnan membrane technique. This study reveals that the physico-chemical properties of aqueous AgNP suspensions will change significantly upon exposure to UV light, with implications for environmental health and safety risk assessments.

• SP 1200-8: Preparation of silver nanoparticle loaded cotton threads to facilitate measurement development for textile applications

  NIST Technical Series Publication

  FOREWORD This NIST Special Publication (SP) is one in a series of NIST SPs that address research needs articulated in the National Nanotechnology Initiative (NNI) Environmental, Health, and Safety Research Strategy published in 2011 [1]. This Strategy identified a Nanomaterial Measurement Infrastructure (NMI) as essential for science-based risk assessment and risk management of nanotechnology-enabled products as pertaining to human health, exposure, and the environment. NIST was identified as the lead federal agency in the NMI core research area of the Strategy. This research area includes development of measurement tools for the detection and characterization of engineered nanomaterials in nanotechnology-enabled products. Textiles containing silver nanoparticles, the focus of this SP, are consumer products of great interest for nanotechnology environmental, health, and safety (nano-EHS) research and for product regulation. The protocol presented in this SP describes a strategy for developing a validation known to contain silver nanoparticles loaded onto the surface of a cotton thread. These validation threads can be used to test the capabilities of various analytical methods to characterize silver nanoparticles in textiles.

• Gold nanorod separation and characterization by asymmetric-flow field flow fractionation with UV–Vis detection

  Analytical Bioanalytical Chemistry

  The application of asymmetric-flow field flow fractionation (A4F) for low aspect ratio gold nanorod (GNR) fractionation and characterization was comprehensively investigated. We report on two novel aspects of this application. The first addresses the analytical challenge involved in the fractionation of positively charged nanoparticles by A4F, due to the interaction that exists between the negatively charged native membrane and the analyte. We show that the mobile phase composition is a critical parameter for controlling fractionation and mitigating the membrane-analyte interaction. A mixture of ammonium nitrate and cetyl trimethyl ammonium bromide at different molar ratios enables separation of GNRs with high recovery. The second aspect is the demonstration of shape-based separation of GNRs in A4F normal mode elution (i.e., Brownian mode). We show that the elution of GNRs is due both to aspect ratio and a steric-entropic contribution for GNRs with the same diameter. This latter effect can be explained by their orientation vector inside the A4F channel. Our experimental results demonstrate the relevance of the theory described by Beckett and Giddings for non-spherical fractionation (Beckett and Giddings, J Colloid and Interface Sci 186(1):53–59, 1997). However, it is shown that this theory has its limit in the case of complex GNR mixtures, and that shape (i.e., aspect ratio) is the principal material parameter controlling elution of GNRs in A4F; the apparent translational diffusion coefficient of GNRs increases with aspect ratio. Finally, the performance of the methodology developed in this work is evaluated by the fractionation and characterization of individual components from a mixture of GNR aspect ratios.

• In-situ UV/Vis, SAXS and TEM study of single phase Au Nanoparticle Growth

  Chemistry of Materials

  Given the diverse scientific and technological applications of gold nanoparticles (Au NPs), understanding the impact of macromolecular additives on the distribution of size, shape, and composition is crucial to ensure reproducibility and lower production cost. In situ measurement of the evolution of these distributions challenges current techniques; however, it is critical for in-line manufacturing controls. Using mild Au(I) reduction by tert-butylamine-borane in toluene, the utility and limitations of SAXS, UV/Vis spectroscopy, and TEM are considered by comparing the mean nanoparticle size, size distribution, and relative number density. Individually, these techniques are insufficient to follow these parameters through the initial process of nucleation and growth; either providing insufficient information on the number of particles (UV/Vis), introducing artifacts (TEM), or not providing a unique solution for the shape of the distribution (SAXS). However, when used in conjunction, especially SAXS calibrated with TEM of reaction aliquots, the time evolution of these parameters can be quantified. For the single-phase Au NP synthesis with Au(I) and mild reductants, four distinct experimental regions are revealed that entail two growth mechanisms, which complement previous discussions of single-mode growth kinetics. The kinetics of Au(I):borane reactions are dependent on the Au precursor/reductant ratio and the thiol capping agent length, when reductant concentration is low. The final reaction products exhibit an LSW size distribution.

• Challenges for physical characterization of silver nanoparticles under pristine and environmentally relevant conditions

  Journal of Environmental Monitoring

  The reported size distribution of silver nanoparticles (AgNPs) is strongly affected by the underlying measurement method, agglomeration state, and dispersion conditions. A selection of AgNP materials with vendor-reported diameters ranging from 1 nm to 100 nm, various size distributions, and biocompatible capping agents including citrate, starch and polyvinylpyrrolidone were studied. AgNPs were diluted with either deionized water, moderately hard reconstituted water, or moderately hard reconstituted water containing natural organic matter. Rigorous physico-chemical characterization by consensus methods and protocols where available enables an understanding of how the underlying measurement method impacts the reported size measurements, which in turn provides a more complete understanding of the state (size, size distribution, agglomeration, etc.) of the AgNPs with respect to the dispersion conditions. An approach to developing routine screening is also presented.

• Dispersion stabilization of silver nanoparticles in synthetic lung fluid studied under in situ conditions

  Nanotoxicology

  The dispersion stabilization of silver nanoparticles (AgNPs) in synthetic lung fluid was studied to interrogate the effects on colloidal stability due to the principal constituents of the fluid. The colloidal stability of 20 nm citrate-AgNPs dispersed in the presence of each constituent of the synthetic lung fluid (individually, the complete fluid, and without additives) was observed during titration of increasing sodium chloride concentration. A variety of complementary in situ measurement techniques were utilized, including dynamic light scattering, ultraviolet-visible absorption spectroscopy, atomic force microscopy, and small-angle X-ray scattering, which provided a collective set of information that enabled far better understanding of the dispersion behavior in the fluid than any one technique alone. It was observed that AgNPs continued to adsorb bovine serum albumin (BSA) protein from the synthetic lung fluid solution as the sodium chloride concentration increased, until a maximum BSA coating was achieved prior to reaching the physiological sodium chloride concentration of 154 mmol L−1. BSA was determined to be the constituent of the synthetic lung fluid that is required to provide colloidal stability at high salt loadings, though the phospholipid constituent exerts a subtle effect. Additionally, as AgNPs are a distinctly different class of nanoparticles apart from the carbon nanotubes and titanium dioxide nanoparticles initially reported to be dispersible using this fluid, this work also demonstrates the broad applicability of synthetic lung fluid in providing stable dispersions for engineered nanoparticles for use in biological assays.

• Adsorption and Conformation of Serum Albumin Protein on Gold Nanoparticles Investigated Using Dimensional Measurements and in Situ Spectroscopic Methods

  Langmuir

  The adsorption and conformation of bovine serum albumin (BSA) on gold nanoparticles (AuNPs) were interrogated both qualitatively and quantitatively via complementary physicochemical characterization methods. Dynamic light scattering (DLS), asymmetric-flow field flow fractionation (AFFF), fluorescence spectrometry, and attenuated total reflectance- Fourier transform infrared (ATR-FTIR) spectroscopy were combined to characterize BSA-AuNP conjugates under fluid conditions, while conjugates in the aerosol state were characterized by electrospray-differential mobility analysis (ES-DMA). The presence of unbound BSA molecules interferes with DLS analysis of the conjugates, particularly as the AuNP size decreases (i.e., below 30 nm in diameter). Under conditions where the γ value is high, where γ is defined as the ratio of scattering intensity by AuNPs to the scattering intensity by unbound BSA, DLS size results are consistent with results obtained after fractionation by AFFF. Additionally, the AuNP hydrodynamic size exhibits a greater proportional increase due to BSA conjugation at pH values below 2.5 compared with less acidic pH values (3.4-7.3), corresponding with the reversibly denatured (E or F form) conformation of BSA below pH 2.5. Over the pH range from 3.4 to 7.3, the hydrodynamic size of the conjugate is nearly constant, suggesting conformational stability over this range. Because of the difference in the measurement environment, a larger increase of AuNP size is observed following BSA conjugation when measured in the wet state (i.e., by DLS and AFFF) compared to the dry state (by ES-DMA).

• Disinfection action of electrostatic versus steric-stabilized silver nanoparticles on E. coli under different water chemistries

  Colloids and Surfaces B: Biointerfaces

  The capping layer stabilizing silver nanoparticles (AgNPs) affects its aggregation, dissolution, and net disinfection action, especially under conditions of varying water composition, such as, pH, ionic strength and organic matter content. Herein, we correlate the silver ion (Ag+) release and reactive oxygen species (ROS) generation rates for AgNPs of varying functionalization to their net disinfection coefficient on Escherichia coli, under conditions of differing water chemistries. For electrostatically stabilized citrate-capped AgNPs, the rate of ROS generation, as measured using a fluorescent dye, is found to dominate over that of Ag+ release, especially for smaller sized AgNP suspensions (∼10 nm) at low pH (∼6.2). For these AgNPs, the ROS disinfection mechanism is confirmed to dominate net disinfection action, as measured by the live/dead assay, especially at low levels of organic matter. Steric stabilization of AgNPs by protein or starch-capped layers enables disinfection through reducing AgNP aggregation and promoting silver dissolution over ROS generation. We suggest the involvement of protons and dissolved oxygen in causing the independent formation of Ag+ and ROS, regardless of the AgNP capping layer. While protein-capping layers effectively stabilize AgNPs, the generated ROS is likely dissipated by interference with the bulky capping layer, whereas the interference is lower with citrate-capping layers. Steric stabilization of AgNPs enables disinfection within a wide range of water chemistries, whereas effective disinfection can occur under electrostatic stabilization, only at low NaCl (<1 mmol/L) and organic matter (<5 mg/L) levels.

• Tiered guidance for risk-informed environmental health and safety testing of nanotechnologies

  Journal of Nanoparticle Research

• Unexpected Changes in Functionality and Surface Coverage for Au Nanoparticle PEI Conjugates: Implications for Stability and Efficacy in Biological Systems

  Langmuir

• UV-induced photochemical transformations of citrate capped silver nanoparticle suspensions

  Journal of Nanoparticle Research

  Due to the increasing use of silver nanoparticles (AgNPs) in consumer products, it is essential to understand how variables, such as light exposure, may change the physical and chemical characteristics of AgNP suspensions. To this end, the effect of 300 nm ultraviolet (UV) light on (20, 40, 60 and 80) nm citrate-capped AgNP suspensions has been investigated. As a consequence of irradiation, the initial yellow hue of the AgNP suspensions is transformed towards a near colorless solution due to the loss of the surface plasmon resonance (SPR) absorbance. The decrease in SPR absorbance followed a first-order decay process for all particle sizes with a rate constant that increased linearly with the AgNP specific surface area and non-linearly with light intensity. The rate of loss of the SPR absorbance decreased with increasing citrate concentration, suggesting a surface-mediated transformation. Absorbance, atomic force microscopy, and dynamic light scattering results all indicated that AgNP photolysis was accompanied by a diameter decrease and occasional aggregation. Furthermore, in situ transmission electron microscopy imaging using a specialized liquid cell also showed a decrease in the particle size and the formation of a core–shell structure in UV-exposed AgNPs. X-ray photoelectron spectroscopy analysis suggested that this shell consisted of oxidized silver. The SPR in UV-exposed AgNP suspensions could be regenerated by addition of a strong reducing agent (NaBH4), supporting the idea that oxidized silver is present after photolysis. Evidence for UV-enhanced dissolution and the production of silver ions was obtained with the Donnan membrane technique. This study reveals that the physico-chemical properties of aqueous AgNP suspensions will change significantly upon exposure to UV light, with implications for environmental health and safety risk assessments.

• SP 1200-8: Preparation of silver nanoparticle loaded cotton threads to facilitate measurement development for textile applications

  NIST Technical Series Publication

  FOREWORD This NIST Special Publication (SP) is one in a series of NIST SPs that address research needs articulated in the National Nanotechnology Initiative (NNI) Environmental, Health, and Safety Research Strategy published in 2011 [1]. This Strategy identified a Nanomaterial Measurement Infrastructure (NMI) as essential for science-based risk assessment and risk management of nanotechnology-enabled products as pertaining to human health, exposure, and the environment. NIST was identified as the lead federal agency in the NMI core research area of the Strategy. This research area includes development of measurement tools for the detection and characterization of engineered nanomaterials in nanotechnology-enabled products. Textiles containing silver nanoparticles, the focus of this SP, are consumer products of great interest for nanotechnology environmental, health, and safety (nano-EHS) research and for product regulation. The protocol presented in this SP describes a strategy for developing a validation known to contain silver nanoparticles loaded onto the surface of a cotton thread. These validation threads can be used to test the capabilities of various analytical methods to characterize silver nanoparticles in textiles.

• Gold nanorod separation and characterization by asymmetric-flow field flow fractionation with UV–Vis detection

  Analytical Bioanalytical Chemistry

  The application of asymmetric-flow field flow fractionation (A4F) for low aspect ratio gold nanorod (GNR) fractionation and characterization was comprehensively investigated. We report on two novel aspects of this application. The first addresses the analytical challenge involved in the fractionation of positively charged nanoparticles by A4F, due to the interaction that exists between the negatively charged native membrane and the analyte. We show that the mobile phase composition is a critical parameter for controlling fractionation and mitigating the membrane-analyte interaction. A mixture of ammonium nitrate and cetyl trimethyl ammonium bromide at different molar ratios enables separation of GNRs with high recovery. The second aspect is the demonstration of shape-based separation of GNRs in A4F normal mode elution (i.e., Brownian mode). We show that the elution of GNRs is due both to aspect ratio and a steric-entropic contribution for GNRs with the same diameter. This latter effect can be explained by their orientation vector inside the A4F channel. Our experimental results demonstrate the relevance of the theory described by Beckett and Giddings for non-spherical fractionation (Beckett and Giddings, J Colloid and Interface Sci 186(1):53–59, 1997). However, it is shown that this theory has its limit in the case of complex GNR mixtures, and that shape (i.e., aspect ratio) is the principal material parameter controlling elution of GNRs in A4F; the apparent translational diffusion coefficient of GNRs increases with aspect ratio. Finally, the performance of the methodology developed in this work is evaluated by the fractionation and characterization of individual components from a mixture of GNR aspect ratios.

• In-situ UV/Vis, SAXS and TEM study of single phase Au Nanoparticle Growth

  Chemistry of Materials

  Given the diverse scientific and technological applications of gold nanoparticles (Au NPs), understanding the impact of macromolecular additives on the distribution of size, shape, and composition is crucial to ensure reproducibility and lower production cost. In situ measurement of the evolution of these distributions challenges current techniques; however, it is critical for in-line manufacturing controls. Using mild Au(I) reduction by tert-butylamine-borane in toluene, the utility and limitations of SAXS, UV/Vis spectroscopy, and TEM are considered by comparing the mean nanoparticle size, size distribution, and relative number density. Individually, these techniques are insufficient to follow these parameters through the initial process of nucleation and growth; either providing insufficient information on the number of particles (UV/Vis), introducing artifacts (TEM), or not providing a unique solution for the shape of the distribution (SAXS). However, when used in conjunction, especially SAXS calibrated with TEM of reaction aliquots, the time evolution of these parameters can be quantified. For the single-phase Au NP synthesis with Au(I) and mild reductants, four distinct experimental regions are revealed that entail two growth mechanisms, which complement previous discussions of single-mode growth kinetics. The kinetics of Au(I):borane reactions are dependent on the Au precursor/reductant ratio and the thiol capping agent length, when reductant concentration is low. The final reaction products exhibit an LSW size distribution.

• Challenges for physical characterization of silver nanoparticles under pristine and environmentally relevant conditions

  Journal of Environmental Monitoring

  The reported size distribution of silver nanoparticles (AgNPs) is strongly affected by the underlying measurement method, agglomeration state, and dispersion conditions. A selection of AgNP materials with vendor-reported diameters ranging from 1 nm to 100 nm, various size distributions, and biocompatible capping agents including citrate, starch and polyvinylpyrrolidone were studied. AgNPs were diluted with either deionized water, moderately hard reconstituted water, or moderately hard reconstituted water containing natural organic matter. Rigorous physico-chemical characterization by consensus methods and protocols where available enables an understanding of how the underlying measurement method impacts the reported size measurements, which in turn provides a more complete understanding of the state (size, size distribution, agglomeration, etc.) of the AgNPs with respect to the dispersion conditions. An approach to developing routine screening is also presented.

• Competitive Adsorption of Thiolated Polyethylene Glycol and Mercaptopropionic Acid on Gold Nanoparticles Measured by Physical Characterization Methods

  Langmuir

  Competitive adsorption kinetics between thiolated polyethylene glycol (SH-PEG) and mercaptopropionic acid (MPA) on gold nanoparticles (Au-NPs) were studied using a prototype physical characterization approach that combines dynamic light scattering (DLS) and electrospray differential mobility analysis (ES-DMA). The change in hydrodynamic particle size (intensity average) due to the formation of SH-PEG coatings on Au-NPs was measured by DLS in both two-component (Au-NP þ MPA or Au-NP @ SH-PEG) and three-component (Au-NP @ MPA @ SH-PEG) systems. ES-DMA was employed to quantify the surface coverage of SH-PEG and establish a correlation between surface coverage and the change in particle size measured by DLS.Achange in the equilibrium binding constant for SH-PEG on Au-NPs at various concentrations of SH-PEG and MPA showed that the presence of MPA reduced the binding affinity of SH-PEG to the Au-NP surface. Kinetic studies showed that SH-PEG was desorbed from the Au-NP surface following a second-order desorption model after subsequently introducing MPA. The desorption rate constant of SH-PEG from the Au-NP surface by MPA displacement was strongly affected by the concentration of MPA and the excess SH-PEG in solution.

• Dispersion stabilization of silver nanoparticles in synthetic lung fluid studied under in situ conditions

  Nanotoxicology

  The dispersion stabilization of silver nanoparticles (AgNPs) in synthetic lung fluid was studied to interrogate the effects on colloidal stability due to the principal constituents of the fluid. The colloidal stability of 20 nm citrate-AgNPs dispersed in the presence of each constituent of the synthetic lung fluid (individually, the complete fluid, and without additives) was observed during titration of increasing sodium chloride concentration. A variety of complementary in situ measurement techniques were utilized, including dynamic light scattering, ultraviolet-visible absorption spectroscopy, atomic force microscopy, and small-angle X-ray scattering, which provided a collective set of information that enabled far better understanding of the dispersion behavior in the fluid than any one technique alone. It was observed that AgNPs continued to adsorb bovine serum albumin (BSA) protein from the synthetic lung fluid solution as the sodium chloride concentration increased, until a maximum BSA coating was achieved prior to reaching the physiological sodium chloride concentration of 154 mmol L−1. BSA was determined to be the constituent of the synthetic lung fluid that is required to provide colloidal stability at high salt loadings, though the phospholipid constituent exerts a subtle effect. Additionally, as AgNPs are a distinctly different class of nanoparticles apart from the carbon nanotubes and titanium dioxide nanoparticles initially reported to be dispersible using this fluid, this work also demonstrates the broad applicability of synthetic lung fluid in providing stable dispersions for engineered nanoparticles for use in biological assays.

• Adsorption and Conformation of Serum Albumin Protein on Gold Nanoparticles Investigated Using Dimensional Measurements and in Situ Spectroscopic Methods

  Langmuir

  The adsorption and conformation of bovine serum albumin (BSA) on gold nanoparticles (AuNPs) were interrogated both qualitatively and quantitatively via complementary physicochemical characterization methods. Dynamic light scattering (DLS), asymmetric-flow field flow fractionation (AFFF), fluorescence spectrometry, and attenuated total reflectance- Fourier transform infrared (ATR-FTIR) spectroscopy were combined to characterize BSA-AuNP conjugates under fluid conditions, while conjugates in the aerosol state were characterized by electrospray-differential mobility analysis (ES-DMA). The presence of unbound BSA molecules interferes with DLS analysis of the conjugates, particularly as the AuNP size decreases (i.e., below 30 nm in diameter). Under conditions where the γ value is high, where γ is defined as the ratio of scattering intensity by AuNPs to the scattering intensity by unbound BSA, DLS size results are consistent with results obtained after fractionation by AFFF. Additionally, the AuNP hydrodynamic size exhibits a greater proportional increase due to BSA conjugation at pH values below 2.5 compared with less acidic pH values (3.4-7.3), corresponding with the reversibly denatured (E or F form) conformation of BSA below pH 2.5. Over the pH range from 3.4 to 7.3, the hydrodynamic size of the conjugate is nearly constant, suggesting conformational stability over this range. Because of the difference in the measurement environment, a larger increase of AuNP size is observed following BSA conjugation when measured in the wet state (i.e., by DLS and AFFF) compared to the dry state (by ES-DMA).

• Disinfection action of electrostatic versus steric-stabilized silver nanoparticles on E. coli under different water chemistries

  Colloids and Surfaces B: Biointerfaces

  The capping layer stabilizing silver nanoparticles (AgNPs) affects its aggregation, dissolution, and net disinfection action, especially under conditions of varying water composition, such as, pH, ionic strength and organic matter content. Herein, we correlate the silver ion (Ag+) release and reactive oxygen species (ROS) generation rates for AgNPs of varying functionalization to their net disinfection coefficient on Escherichia coli, under conditions of differing water chemistries. For electrostatically stabilized citrate-capped AgNPs, the rate of ROS generation, as measured using a fluorescent dye, is found to dominate over that of Ag+ release, especially for smaller sized AgNP suspensions (∼10 nm) at low pH (∼6.2). For these AgNPs, the ROS disinfection mechanism is confirmed to dominate net disinfection action, as measured by the live/dead assay, especially at low levels of organic matter. Steric stabilization of AgNPs by protein or starch-capped layers enables disinfection through reducing AgNP aggregation and promoting silver dissolution over ROS generation. We suggest the involvement of protons and dissolved oxygen in causing the independent formation of Ag+ and ROS, regardless of the AgNP capping layer. While protein-capping layers effectively stabilize AgNPs, the generated ROS is likely dissipated by interference with the bulky capping layer, whereas the interference is lower with citrate-capping layers. Steric stabilization of AgNPs enables disinfection within a wide range of water chemistries, whereas effective disinfection can occur under electrostatic stabilization, only at low NaCl (<1 mmol/L) and organic matter (<5 mg/L) levels.

• Tiered guidance for risk-informed environmental health and safety testing of nanotechnologies

  Journal of Nanoparticle Research

• Unexpected Changes in Functionality and Surface Coverage for Au Nanoparticle PEI Conjugates: Implications for Stability and Efficacy in Biological Systems

  Langmuir

• UV-induced photochemical transformations of citrate capped silver nanoparticle suspensions

  Journal of Nanoparticle Research

  Due to the increasing use of silver nanoparticles (AgNPs) in consumer products, it is essential to understand how variables, such as light exposure, may change the physical and chemical characteristics of AgNP suspensions. To this end, the effect of 300 nm ultraviolet (UV) light on (20, 40, 60 and 80) nm citrate-capped AgNP suspensions has been investigated. As a consequence of irradiation, the initial yellow hue of the AgNP suspensions is transformed towards a near colorless solution due to the loss of the surface plasmon resonance (SPR) absorbance. The decrease in SPR absorbance followed a first-order decay process for all particle sizes with a rate constant that increased linearly with the AgNP specific surface area and non-linearly with light intensity. The rate of loss of the SPR absorbance decreased with increasing citrate concentration, suggesting a surface-mediated transformation. Absorbance, atomic force microscopy, and dynamic light scattering results all indicated that AgNP photolysis was accompanied by a diameter decrease and occasional aggregation. Furthermore, in situ transmission electron microscopy imaging using a specialized liquid cell also showed a decrease in the particle size and the formation of a core–shell structure in UV-exposed AgNPs. X-ray photoelectron spectroscopy analysis suggested that this shell consisted of oxidized silver. The SPR in UV-exposed AgNP suspensions could be regenerated by addition of a strong reducing agent (NaBH4), supporting the idea that oxidized silver is present after photolysis. Evidence for UV-enhanced dissolution and the production of silver ions was obtained with the Donnan membrane technique. This study reveals that the physico-chemical properties of aqueous AgNP suspensions will change significantly upon exposure to UV light, with implications for environmental health and safety risk assessments.

• SP 1200-8: Preparation of silver nanoparticle loaded cotton threads to facilitate measurement development for textile applications

  NIST Technical Series Publication

  FOREWORD This NIST Special Publication (SP) is one in a series of NIST SPs that address research needs articulated in the National Nanotechnology Initiative (NNI) Environmental, Health, and Safety Research Strategy published in 2011 [1]. This Strategy identified a Nanomaterial Measurement Infrastructure (NMI) as essential for science-based risk assessment and risk management of nanotechnology-enabled products as pertaining to human health, exposure, and the environment. NIST was identified as the lead federal agency in the NMI core research area of the Strategy. This research area includes development of measurement tools for the detection and characterization of engineered nanomaterials in nanotechnology-enabled products. Textiles containing silver nanoparticles, the focus of this SP, are consumer products of great interest for nanotechnology environmental, health, and safety (nano-EHS) research and for product regulation. The protocol presented in this SP describes a strategy for developing a validation known to contain silver nanoparticles loaded onto the surface of a cotton thread. These validation threads can be used to test the capabilities of various analytical methods to characterize silver nanoparticles in textiles.

• Gold nanorod separation and characterization by asymmetric-flow field flow fractionation with UV–Vis detection

  Analytical Bioanalytical Chemistry

  The application of asymmetric-flow field flow fractionation (A4F) for low aspect ratio gold nanorod (GNR) fractionation and characterization was comprehensively investigated. We report on two novel aspects of this application. The first addresses the analytical challenge involved in the fractionation of positively charged nanoparticles by A4F, due to the interaction that exists between the negatively charged native membrane and the analyte. We show that the mobile phase composition is a critical parameter for controlling fractionation and mitigating the membrane-analyte interaction. A mixture of ammonium nitrate and cetyl trimethyl ammonium bromide at different molar ratios enables separation of GNRs with high recovery. The second aspect is the demonstration of shape-based separation of GNRs in A4F normal mode elution (i.e., Brownian mode). We show that the elution of GNRs is due both to aspect ratio and a steric-entropic contribution for GNRs with the same diameter. This latter effect can be explained by their orientation vector inside the A4F channel. Our experimental results demonstrate the relevance of the theory described by Beckett and Giddings for non-spherical fractionation (Beckett and Giddings, J Colloid and Interface Sci 186(1):53–59, 1997). However, it is shown that this theory has its limit in the case of complex GNR mixtures, and that shape (i.e., aspect ratio) is the principal material parameter controlling elution of GNRs in A4F; the apparent translational diffusion coefficient of GNRs increases with aspect ratio. Finally, the performance of the methodology developed in this work is evaluated by the fractionation and characterization of individual components from a mixture of GNR aspect ratios.

• In-situ UV/Vis, SAXS and TEM study of single phase Au Nanoparticle Growth

  Chemistry of Materials

  Given the diverse scientific and technological applications of gold nanoparticles (Au NPs), understanding the impact of macromolecular additives on the distribution of size, shape, and composition is crucial to ensure reproducibility and lower production cost. In situ measurement of the evolution of these distributions challenges current techniques; however, it is critical for in-line manufacturing controls. Using mild Au(I) reduction by tert-butylamine-borane in toluene, the utility and limitations of SAXS, UV/Vis spectroscopy, and TEM are considered by comparing the mean nanoparticle size, size distribution, and relative number density. Individually, these techniques are insufficient to follow these parameters through the initial process of nucleation and growth; either providing insufficient information on the number of particles (UV/Vis), introducing artifacts (TEM), or not providing a unique solution for the shape of the distribution (SAXS). However, when used in conjunction, especially SAXS calibrated with TEM of reaction aliquots, the time evolution of these parameters can be quantified. For the single-phase Au NP synthesis with Au(I) and mild reductants, four distinct experimental regions are revealed that entail two growth mechanisms, which complement previous discussions of single-mode growth kinetics. The kinetics of Au(I):borane reactions are dependent on the Au precursor/reductant ratio and the thiol capping agent length, when reductant concentration is low. The final reaction products exhibit an LSW size distribution.

• Challenges for physical characterization of silver nanoparticles under pristine and environmentally relevant conditions

  Journal of Environmental Monitoring

  The reported size distribution of silver nanoparticles (AgNPs) is strongly affected by the underlying measurement method, agglomeration state, and dispersion conditions. A selection of AgNP materials with vendor-reported diameters ranging from 1 nm to 100 nm, various size distributions, and biocompatible capping agents including citrate, starch and polyvinylpyrrolidone were studied. AgNPs were diluted with either deionized water, moderately hard reconstituted water, or moderately hard reconstituted water containing natural organic matter. Rigorous physico-chemical characterization by consensus methods and protocols where available enables an understanding of how the underlying measurement method impacts the reported size measurements, which in turn provides a more complete understanding of the state (size, size distribution, agglomeration, etc.) of the AgNPs with respect to the dispersion conditions. An approach to developing routine screening is also presented.

• Competitive Adsorption of Thiolated Polyethylene Glycol and Mercaptopropionic Acid on Gold Nanoparticles Measured by Physical Characterization Methods

  Langmuir

  Competitive adsorption kinetics between thiolated polyethylene glycol (SH-PEG) and mercaptopropionic acid (MPA) on gold nanoparticles (Au-NPs) were studied using a prototype physical characterization approach that combines dynamic light scattering (DLS) and electrospray differential mobility analysis (ES-DMA). The change in hydrodynamic particle size (intensity average) due to the formation of SH-PEG coatings on Au-NPs was measured by DLS in both two-component (Au-NP þ MPA or Au-NP @ SH-PEG) and three-component (Au-NP @ MPA @ SH-PEG) systems. ES-DMA was employed to quantify the surface coverage of SH-PEG and establish a correlation between surface coverage and the change in particle size measured by DLS.Achange in the equilibrium binding constant for SH-PEG on Au-NPs at various concentrations of SH-PEG and MPA showed that the presence of MPA reduced the binding affinity of SH-PEG to the Au-NP surface. Kinetic studies showed that SH-PEG was desorbed from the Au-NP surface following a second-order desorption model after subsequently introducing MPA. The desorption rate constant of SH-PEG from the Au-NP surface by MPA displacement was strongly affected by the concentration of MPA and the excess SH-PEG in solution.

• Comparison of Electrical Properties of Viruses Studied by AC Capacitance Scanning Probe Microscopy

  Journal of the American Chemical Society

  Capacitances of five types of viruses, adenovirus type 5 (AV5), herpes simplex virus type 1 (HSV1), simian virus 40 (SV40), vaccinia (MVA), and cowpea mosaic virus (CPMV), were compared by AC capacitance scanning probe microscopy. This technique, using a Pt-coated AFM tip as an electrode to probe capacitance of materials between the tip and a bottom electrode, has been applied to study surface structures of semiconductors and polymers with nanometer spatial resolution; however, biological samples at the nanoscale have not been explored by this technique yet. Because most biological cells are poor conductors, this approach to probe electric properties of cells by capacitance is logical. This scanning probe technique showed that each virus has distinguishable and characteristic capacitance. A series of control experiments were carried out using mutant viruses to validate the origin of the characteristic capacitance responses for different viruses. A mutation on the capsid in HSV1 with green fluorescence proteins increased capacitance from 9 × 10-6 to 1 × 10-5 F/cm2 at the frequency of 104 Hz. Herpes simplex virus type 2 (HSV2) decreased capacitance when its envelope and glycoproteins were chemically extracted. These control experiments indicate that dielectric properties of capsid proteins and envelope glycoproteins significantly influence overall dielectric constants of viruses. Because those capsid proteins and glycoproteins are characteristic of the virus strain, this technique could be applied to detect and identify viruses at the single viron level using their distinct capacitance spectra as fingerprints without labeling.

• Dispersion stabilization of silver nanoparticles in synthetic lung fluid studied under in situ conditions

  Nanotoxicology

  The dispersion stabilization of silver nanoparticles (AgNPs) in synthetic lung fluid was studied to interrogate the effects on colloidal stability due to the principal constituents of the fluid. The colloidal stability of 20 nm citrate-AgNPs dispersed in the presence of each constituent of the synthetic lung fluid (individually, the complete fluid, and without additives) was observed during titration of increasing sodium chloride concentration. A variety of complementary in situ measurement techniques were utilized, including dynamic light scattering, ultraviolet-visible absorption spectroscopy, atomic force microscopy, and small-angle X-ray scattering, which provided a collective set of information that enabled far better understanding of the dispersion behavior in the fluid than any one technique alone. It was observed that AgNPs continued to adsorb bovine serum albumin (BSA) protein from the synthetic lung fluid solution as the sodium chloride concentration increased, until a maximum BSA coating was achieved prior to reaching the physiological sodium chloride concentration of 154 mmol L−1. BSA was determined to be the constituent of the synthetic lung fluid that is required to provide colloidal stability at high salt loadings, though the phospholipid constituent exerts a subtle effect. Additionally, as AgNPs are a distinctly different class of nanoparticles apart from the carbon nanotubes and titanium dioxide nanoparticles initially reported to be dispersible using this fluid, this work also demonstrates the broad applicability of synthetic lung fluid in providing stable dispersions for engineered nanoparticles for use in biological assays.

• Adsorption and Conformation of Serum Albumin Protein on Gold Nanoparticles Investigated Using Dimensional Measurements and in Situ Spectroscopic Methods

  Langmuir

  The adsorption and conformation of bovine serum albumin (BSA) on gold nanoparticles (AuNPs) were interrogated both qualitatively and quantitatively via complementary physicochemical characterization methods. Dynamic light scattering (DLS), asymmetric-flow field flow fractionation (AFFF), fluorescence spectrometry, and attenuated total reflectance- Fourier transform infrared (ATR-FTIR) spectroscopy were combined to characterize BSA-AuNP conjugates under fluid conditions, while conjugates in the aerosol state were characterized by electrospray-differential mobility analysis (ES-DMA). The presence of unbound BSA molecules interferes with DLS analysis of the conjugates, particularly as the AuNP size decreases (i.e., below 30 nm in diameter). Under conditions where the γ value is high, where γ is defined as the ratio of scattering intensity by AuNPs to the scattering intensity by unbound BSA, DLS size results are consistent with results obtained after fractionation by AFFF. Additionally, the AuNP hydrodynamic size exhibits a greater proportional increase due to BSA conjugation at pH values below 2.5 compared with less acidic pH values (3.4-7.3), corresponding with the reversibly denatured (E or F form) conformation of BSA below pH 2.5. Over the pH range from 3.4 to 7.3, the hydrodynamic size of the conjugate is nearly constant, suggesting conformational stability over this range. Because of the difference in the measurement environment, a larger increase of AuNP size is observed following BSA conjugation when measured in the wet state (i.e., by DLS and AFFF) compared to the dry state (by ES-DMA).

• Disinfection action of electrostatic versus steric-stabilized silver nanoparticles on E. coli under different water chemistries

  Colloids and Surfaces B: Biointerfaces

  The capping layer stabilizing silver nanoparticles (AgNPs) affects its aggregation, dissolution, and net disinfection action, especially under conditions of varying water composition, such as, pH, ionic strength and organic matter content. Herein, we correlate the silver ion (Ag+) release and reactive oxygen species (ROS) generation rates for AgNPs of varying functionalization to their net disinfection coefficient on Escherichia coli, under conditions of differing water chemistries. For electrostatically stabilized citrate-capped AgNPs, the rate of ROS generation, as measured using a fluorescent dye, is found to dominate over that of Ag+ release, especially for smaller sized AgNP suspensions (∼10 nm) at low pH (∼6.2). For these AgNPs, the ROS disinfection mechanism is confirmed to dominate net disinfection action, as measured by the live/dead assay, especially at low levels of organic matter. Steric stabilization of AgNPs by protein or starch-capped layers enables disinfection through reducing AgNP aggregation and promoting silver dissolution over ROS generation. We suggest the involvement of protons and dissolved oxygen in causing the independent formation of Ag+ and ROS, regardless of the AgNP capping layer. While protein-capping layers effectively stabilize AgNPs, the generated ROS is likely dissipated by interference with the bulky capping layer, whereas the interference is lower with citrate-capping layers. Steric stabilization of AgNPs enables disinfection within a wide range of water chemistries, whereas effective disinfection can occur under electrostatic stabilization, only at low NaCl (<1 mmol/L) and organic matter (<5 mg/L) levels.

• Tiered guidance for risk-informed environmental health and safety testing of nanotechnologies

  Journal of Nanoparticle Research

• Unexpected Changes in Functionality and Surface Coverage for Au Nanoparticle PEI Conjugates: Implications for Stability and Efficacy in Biological Systems

  Langmuir

• UV-induced photochemical transformations of citrate capped silver nanoparticle suspensions

  Journal of Nanoparticle Research

  Due to the increasing use of silver nanoparticles (AgNPs) in consumer products, it is essential to understand how variables, such as light exposure, may change the physical and chemical characteristics of AgNP suspensions. To this end, the effect of 300 nm ultraviolet (UV) light on (20, 40, 60 and 80) nm citrate-capped AgNP suspensions has been investigated. As a consequence of irradiation, the initial yellow hue of the AgNP suspensions is transformed towards a near colorless solution due to the loss of the surface plasmon resonance (SPR) absorbance. The decrease in SPR absorbance followed a first-order decay process for all particle sizes with a rate constant that increased linearly with the AgNP specific surface area and non-linearly with light intensity. The rate of loss of the SPR absorbance decreased with increasing citrate concentration, suggesting a surface-mediated transformation. Absorbance, atomic force microscopy, and dynamic light scattering results all indicated that AgNP photolysis was accompanied by a diameter decrease and occasional aggregation. Furthermore, in situ transmission electron microscopy imaging using a specialized liquid cell also showed a decrease in the particle size and the formation of a core–shell structure in UV-exposed AgNPs. X-ray photoelectron spectroscopy analysis suggested that this shell consisted of oxidized silver. The SPR in UV-exposed AgNP suspensions could be regenerated by addition of a strong reducing agent (NaBH4), supporting the idea that oxidized silver is present after photolysis. Evidence for UV-enhanced dissolution and the production of silver ions was obtained with the Donnan membrane technique. This study reveals that the physico-chemical properties of aqueous AgNP suspensions will change significantly upon exposure to UV light, with implications for environmental health and safety risk assessments.

• SP 1200-8: Preparation of silver nanoparticle loaded cotton threads to facilitate measurement development for textile applications

  NIST Technical Series Publication

  FOREWORD This NIST Special Publication (SP) is one in a series of NIST SPs that address research needs articulated in the National Nanotechnology Initiative (NNI) Environmental, Health, and Safety Research Strategy published in 2011 [1]. This Strategy identified a Nanomaterial Measurement Infrastructure (NMI) as essential for science-based risk assessment and risk management of nanotechnology-enabled products as pertaining to human health, exposure, and the environment. NIST was identified as the lead federal agency in the NMI core research area of the Strategy. This research area includes development of measurement tools for the detection and characterization of engineered nanomaterials in nanotechnology-enabled products. Textiles containing silver nanoparticles, the focus of this SP, are consumer products of great interest for nanotechnology environmental, health, and safety (nano-EHS) research and for product regulation. The protocol presented in this SP describes a strategy for developing a validation known to contain silver nanoparticles loaded onto the surface of a cotton thread. These validation threads can be used to test the capabilities of various analytical methods to characterize silver nanoparticles in textiles.

• Gold nanorod separation and characterization by asymmetric-flow field flow fractionation with UV–Vis detection

  Analytical Bioanalytical Chemistry

  The application of asymmetric-flow field flow fractionation (A4F) for low aspect ratio gold nanorod (GNR) fractionation and characterization was comprehensively investigated. We report on two novel aspects of this application. The first addresses the analytical challenge involved in the fractionation of positively charged nanoparticles by A4F, due to the interaction that exists between the negatively charged native membrane and the analyte. We show that the mobile phase composition is a critical parameter for controlling fractionation and mitigating the membrane-analyte interaction. A mixture of ammonium nitrate and cetyl trimethyl ammonium bromide at different molar ratios enables separation of GNRs with high recovery. The second aspect is the demonstration of shape-based separation of GNRs in A4F normal mode elution (i.e., Brownian mode). We show that the elution of GNRs is due both to aspect ratio and a steric-entropic contribution for GNRs with the same diameter. This latter effect can be explained by their orientation vector inside the A4F channel. Our experimental results demonstrate the relevance of the theory described by Beckett and Giddings for non-spherical fractionation (Beckett and Giddings, J Colloid and Interface Sci 186(1):53–59, 1997). However, it is shown that this theory has its limit in the case of complex GNR mixtures, and that shape (i.e., aspect ratio) is the principal material parameter controlling elution of GNRs in A4F; the apparent translational diffusion coefficient of GNRs increases with aspect ratio. Finally, the performance of the methodology developed in this work is evaluated by the fractionation and characterization of individual components from a mixture of GNR aspect ratios.

• In-situ UV/Vis, SAXS and TEM study of single phase Au Nanoparticle Growth

  Chemistry of Materials

  Given the diverse scientific and technological applications of gold nanoparticles (Au NPs), understanding the impact of macromolecular additives on the distribution of size, shape, and composition is crucial to ensure reproducibility and lower production cost. In situ measurement of the evolution of these distributions challenges current techniques; however, it is critical for in-line manufacturing controls. Using mild Au(I) reduction by tert-butylamine-borane in toluene, the utility and limitations of SAXS, UV/Vis spectroscopy, and TEM are considered by comparing the mean nanoparticle size, size distribution, and relative number density. Individually, these techniques are insufficient to follow these parameters through the initial process of nucleation and growth; either providing insufficient information on the number of particles (UV/Vis), introducing artifacts (TEM), or not providing a unique solution for the shape of the distribution (SAXS). However, when used in conjunction, especially SAXS calibrated with TEM of reaction aliquots, the time evolution of these parameters can be quantified. For the single-phase Au NP synthesis with Au(I) and mild reductants, four distinct experimental regions are revealed that entail two growth mechanisms, which complement previous discussions of single-mode growth kinetics. The kinetics of Au(I):borane reactions are dependent on the Au precursor/reductant ratio and the thiol capping agent length, when reductant concentration is low. The final reaction products exhibit an LSW size distribution.

• Challenges for physical characterization of silver nanoparticles under pristine and environmentally relevant conditions

  Journal of Environmental Monitoring

  The reported size distribution of silver nanoparticles (AgNPs) is strongly affected by the underlying measurement method, agglomeration state, and dispersion conditions. A selection of AgNP materials with vendor-reported diameters ranging from 1 nm to 100 nm, various size distributions, and biocompatible capping agents including citrate, starch and polyvinylpyrrolidone were studied. AgNPs were diluted with either deionized water, moderately hard reconstituted water, or moderately hard reconstituted water containing natural organic matter. Rigorous physico-chemical characterization by consensus methods and protocols where available enables an understanding of how the underlying measurement method impacts the reported size measurements, which in turn provides a more complete understanding of the state (size, size distribution, agglomeration, etc.) of the AgNPs with respect to the dispersion conditions. An approach to developing routine screening is also presented.

• Competitive Adsorption of Thiolated Polyethylene Glycol and Mercaptopropionic Acid on Gold Nanoparticles Measured by Physical Characterization Methods

  Langmuir

  Competitive adsorption kinetics between thiolated polyethylene glycol (SH-PEG) and mercaptopropionic acid (MPA) on gold nanoparticles (Au-NPs) were studied using a prototype physical characterization approach that combines dynamic light scattering (DLS) and electrospray differential mobility analysis (ES-DMA). The change in hydrodynamic particle size (intensity average) due to the formation of SH-PEG coatings on Au-NPs was measured by DLS in both two-component (Au-NP þ MPA or Au-NP @ SH-PEG) and three-component (Au-NP @ MPA @ SH-PEG) systems. ES-DMA was employed to quantify the surface coverage of SH-PEG and establish a correlation between surface coverage and the change in particle size measured by DLS.Achange in the equilibrium binding constant for SH-PEG on Au-NPs at various concentrations of SH-PEG and MPA showed that the presence of MPA reduced the binding affinity of SH-PEG to the Au-NP surface. Kinetic studies showed that SH-PEG was desorbed from the Au-NP surface following a second-order desorption model after subsequently introducing MPA. The desorption rate constant of SH-PEG from the Au-NP surface by MPA displacement was strongly affected by the concentration of MPA and the excess SH-PEG in solution.

• Comparison of Electrical Properties of Viruses Studied by AC Capacitance Scanning Probe Microscopy

  Journal of the American Chemical Society

  Capacitances of five types of viruses, adenovirus type 5 (AV5), herpes simplex virus type 1 (HSV1), simian virus 40 (SV40), vaccinia (MVA), and cowpea mosaic virus (CPMV), were compared by AC capacitance scanning probe microscopy. This technique, using a Pt-coated AFM tip as an electrode to probe capacitance of materials between the tip and a bottom electrode, has been applied to study surface structures of semiconductors and polymers with nanometer spatial resolution; however, biological samples at the nanoscale have not been explored by this technique yet. Because most biological cells are poor conductors, this approach to probe electric properties of cells by capacitance is logical. This scanning probe technique showed that each virus has distinguishable and characteristic capacitance. A series of control experiments were carried out using mutant viruses to validate the origin of the characteristic capacitance responses for different viruses. A mutation on the capsid in HSV1 with green fluorescence proteins increased capacitance from 9 × 10-6 to 1 × 10-5 F/cm2 at the frequency of 104 Hz. Herpes simplex virus type 2 (HSV2) decreased capacitance when its envelope and glycoproteins were chemically extracted. These control experiments indicate that dielectric properties of capsid proteins and envelope glycoproteins significantly influence overall dielectric constants of viruses. Because those capsid proteins and glycoproteins are characteristic of the virus strain, this technique could be applied to detect and identify viruses at the single viron level using their distinct capacitance spectra as fingerprints without labeling.

• Persistence of singly dispersed silver nanoparticles in natural freshwaters, synthetic seawater, and simulated estuarine waters.

  Science of the Total Environment

  This investigation focuses on predicting the persistence of citrate-capped 20nm AgNPs by measuring their colloidal stability in natural freshwaters and synthetic aquatic media. Ultraviolet-visible absorbance spectroscopy, dynamic light scattering, and atomic force microscopy were used to evaluate the colloidal stability of AgNPs in locally-obtained pond water, moderately hard reconstituted water alone or with natural organic matter (NOM), synthetic seawater, and also the individual chemicals most prevalent in seawater. Singly dispersed AgNPs in seawater and waters with greater than 20mmolL(-1) sodium chloride were unstable, with the optical absorbance approaching zero within the first ten hours of mixing. Agglomeration rates as a function of water chemistry and NOM are tested as a hypothesis to explain the rates of disappearance of singly dispersed AgNPs. Other samples, mostly those with lower salinity or NOM, maintained varying degrees of colloidal stability during time studies up to 48h. This indicates likelihood that some AgNPs will be stable long enough in freshwater to successfully enter estuarine or marine systems. These results should enable a more efficient design of nanoEHS risk assessment experiments by predicting the aquatic or soil compartments at greatest potential risk for accumulation of and exposure to citrate capped 20nm AgNPs.

• Dispersion stabilization of silver nanoparticles in synthetic lung fluid studied under in situ conditions

  Nanotoxicology

  The dispersion stabilization of silver nanoparticles (AgNPs) in synthetic lung fluid was studied to interrogate the effects on colloidal stability due to the principal constituents of the fluid. The colloidal stability of 20 nm citrate-AgNPs dispersed in the presence of each constituent of the synthetic lung fluid (individually, the complete fluid, and without additives) was observed during titration of increasing sodium chloride concentration. A variety of complementary in situ measurement techniques were utilized, including dynamic light scattering, ultraviolet-visible absorption spectroscopy, atomic force microscopy, and small-angle X-ray scattering, which provided a collective set of information that enabled far better understanding of the dispersion behavior in the fluid than any one technique alone. It was observed that AgNPs continued to adsorb bovine serum albumin (BSA) protein from the synthetic lung fluid solution as the sodium chloride concentration increased, until a maximum BSA coating was achieved prior to reaching the physiological sodium chloride concentration of 154 mmol L−1. BSA was determined to be the constituent of the synthetic lung fluid that is required to provide colloidal stability at high salt loadings, though the phospholipid constituent exerts a subtle effect. Additionally, as AgNPs are a distinctly different class of nanoparticles apart from the carbon nanotubes and titanium dioxide nanoparticles initially reported to be dispersible using this fluid, this work also demonstrates the broad applicability of synthetic lung fluid in providing stable dispersions for engineered nanoparticles for use in biological assays.

• Adsorption and Conformation of Serum Albumin Protein on Gold Nanoparticles Investigated Using Dimensional Measurements and in Situ Spectroscopic Methods

  Langmuir

  The adsorption and conformation of bovine serum albumin (BSA) on gold nanoparticles (AuNPs) were interrogated both qualitatively and quantitatively via complementary physicochemical characterization methods. Dynamic light scattering (DLS), asymmetric-flow field flow fractionation (AFFF), fluorescence spectrometry, and attenuated total reflectance- Fourier transform infrared (ATR-FTIR) spectroscopy were combined to characterize BSA-AuNP conjugates under fluid conditions, while conjugates in the aerosol state were characterized by electrospray-differential mobility analysis (ES-DMA). The presence of unbound BSA molecules interferes with DLS analysis of the conjugates, particularly as the AuNP size decreases (i.e., below 30 nm in diameter). Under conditions where the γ value is high, where γ is defined as the ratio of scattering intensity by AuNPs to the scattering intensity by unbound BSA, DLS size results are consistent with results obtained after fractionation by AFFF. Additionally, the AuNP hydrodynamic size exhibits a greater proportional increase due to BSA conjugation at pH values below 2.5 compared with less acidic pH values (3.4-7.3), corresponding with the reversibly denatured (E or F form) conformation of BSA below pH 2.5. Over the pH range from 3.4 to 7.3, the hydrodynamic size of the conjugate is nearly constant, suggesting conformational stability over this range. Because of the difference in the measurement environment, a larger increase of AuNP size is observed following BSA conjugation when measured in the wet state (i.e., by DLS and AFFF) compared to the dry state (by ES-DMA).

• Disinfection action of electrostatic versus steric-stabilized silver nanoparticles on E. coli under different water chemistries

  Colloids and Surfaces B: Biointerfaces

  The capping layer stabilizing silver nanoparticles (AgNPs) affects its aggregation, dissolution, and net disinfection action, especially under conditions of varying water composition, such as, pH, ionic strength and organic matter content. Herein, we correlate the silver ion (Ag+) release and reactive oxygen species (ROS) generation rates for AgNPs of varying functionalization to their net disinfection coefficient on Escherichia coli, under conditions of differing water chemistries. For electrostatically stabilized citrate-capped AgNPs, the rate of ROS generation, as measured using a fluorescent dye, is found to dominate over that of Ag+ release, especially for smaller sized AgNP suspensions (∼10 nm) at low pH (∼6.2). For these AgNPs, the ROS disinfection mechanism is confirmed to dominate net disinfection action, as measured by the live/dead assay, especially at low levels of organic matter. Steric stabilization of AgNPs by protein or starch-capped layers enables disinfection through reducing AgNP aggregation and promoting silver dissolution over ROS generation. We suggest the involvement of protons and dissolved oxygen in causing the independent formation of Ag+ and ROS, regardless of the AgNP capping layer. While protein-capping layers effectively stabilize AgNPs, the generated ROS is likely dissipated by interference with the bulky capping layer, whereas the interference is lower with citrate-capping layers. Steric stabilization of AgNPs enables disinfection within a wide range of water chemistries, whereas effective disinfection can occur under electrostatic stabilization, only at low NaCl (<1 mmol/L) and organic matter (<5 mg/L) levels.

• Tiered guidance for risk-informed environmental health and safety testing of nanotechnologies

  Journal of Nanoparticle Research

• Unexpected Changes in Functionality and Surface Coverage for Au Nanoparticle PEI Conjugates: Implications for Stability and Efficacy in Biological Systems

  Langmuir

• UV-induced photochemical transformations of citrate capped silver nanoparticle suspensions

  Journal of Nanoparticle Research

  Due to the increasing use of silver nanoparticles (AgNPs) in consumer products, it is essential to understand how variables, such as light exposure, may change the physical and chemical characteristics of AgNP suspensions. To this end, the effect of 300 nm ultraviolet (UV) light on (20, 40, 60 and 80) nm citrate-capped AgNP suspensions has been investigated. As a consequence of irradiation, the initial yellow hue of the AgNP suspensions is transformed towards a near colorless solution due to the loss of the surface plasmon resonance (SPR) absorbance. The decrease in SPR absorbance followed a first-order decay process for all particle sizes with a rate constant that increased linearly with the AgNP specific surface area and non-linearly with light intensity. The rate of loss of the SPR absorbance decreased with increasing citrate concentration, suggesting a surface-mediated transformation. Absorbance, atomic force microscopy, and dynamic light scattering results all indicated that AgNP photolysis was accompanied by a diameter decrease and occasional aggregation. Furthermore, in situ transmission electron microscopy imaging using a specialized liquid cell also showed a decrease in the particle size and the formation of a core–shell structure in UV-exposed AgNPs. X-ray photoelectron spectroscopy analysis suggested that this shell consisted of oxidized silver. The SPR in UV-exposed AgNP suspensions could be regenerated by addition of a strong reducing agent (NaBH4), supporting the idea that oxidized silver is present after photolysis. Evidence for UV-enhanced dissolution and the production of silver ions was obtained with the Donnan membrane technique. This study reveals that the physico-chemical properties of aqueous AgNP suspensions will change significantly upon exposure to UV light, with implications for environmental health and safety risk assessments.

• SP 1200-8: Preparation of silver nanoparticle loaded cotton threads to facilitate measurement development for textile applications

  NIST Technical Series Publication

  FOREWORD This NIST Special Publication (SP) is one in a series of NIST SPs that address research needs articulated in the National Nanotechnology Initiative (NNI) Environmental, Health, and Safety Research Strategy published in 2011 [1]. This Strategy identified a Nanomaterial Measurement Infrastructure (NMI) as essential for science-based risk assessment and risk management of nanotechnology-enabled products as pertaining to human health, exposure, and the environment. NIST was identified as the lead federal agency in the NMI core research area of the Strategy. This research area includes development of measurement tools for the detection and characterization of engineered nanomaterials in nanotechnology-enabled products. Textiles containing silver nanoparticles, the focus of this SP, are consumer products of great interest for nanotechnology environmental, health, and safety (nano-EHS) research and for product regulation. The protocol presented in this SP describes a strategy for developing a validation known to contain silver nanoparticles loaded onto the surface of a cotton thread. These validation threads can be used to test the capabilities of various analytical methods to characterize silver nanoparticles in textiles.

• Gold nanorod separation and characterization by asymmetric-flow field flow fractionation with UV–Vis detection

  Analytical Bioanalytical Chemistry

  The application of asymmetric-flow field flow fractionation (A4F) for low aspect ratio gold nanorod (GNR) fractionation and characterization was comprehensively investigated. We report on two novel aspects of this application. The first addresses the analytical challenge involved in the fractionation of positively charged nanoparticles by A4F, due to the interaction that exists between the negatively charged native membrane and the analyte. We show that the mobile phase composition is a critical parameter for controlling fractionation and mitigating the membrane-analyte interaction. A mixture of ammonium nitrate and cetyl trimethyl ammonium bromide at different molar ratios enables separation of GNRs with high recovery. The second aspect is the demonstration of shape-based separation of GNRs in A4F normal mode elution (i.e., Brownian mode). We show that the elution of GNRs is due both to aspect ratio and a steric-entropic contribution for GNRs with the same diameter. This latter effect can be explained by their orientation vector inside the A4F channel. Our experimental results demonstrate the relevance of the theory described by Beckett and Giddings for non-spherical fractionation (Beckett and Giddings, J Colloid and Interface Sci 186(1):53–59, 1997). However, it is shown that this theory has its limit in the case of complex GNR mixtures, and that shape (i.e., aspect ratio) is the principal material parameter controlling elution of GNRs in A4F; the apparent translational diffusion coefficient of GNRs increases with aspect ratio. Finally, the performance of the methodology developed in this work is evaluated by the fractionation and characterization of individual components from a mixture of GNR aspect ratios.

• In-situ UV/Vis, SAXS and TEM study of single phase Au Nanoparticle Growth

  Chemistry of Materials

  Given the diverse scientific and technological applications of gold nanoparticles (Au NPs), understanding the impact of macromolecular additives on the distribution of size, shape, and composition is crucial to ensure reproducibility and lower production cost. In situ measurement of the evolution of these distributions challenges current techniques; however, it is critical for in-line manufacturing controls. Using mild Au(I) reduction by tert-butylamine-borane in toluene, the utility and limitations of SAXS, UV/Vis spectroscopy, and TEM are considered by comparing the mean nanoparticle size, size distribution, and relative number density. Individually, these techniques are insufficient to follow these parameters through the initial process of nucleation and growth; either providing insufficient information on the number of particles (UV/Vis), introducing artifacts (TEM), or not providing a unique solution for the shape of the distribution (SAXS). However, when used in conjunction, especially SAXS calibrated with TEM of reaction aliquots, the time evolution of these parameters can be quantified. For the single-phase Au NP synthesis with Au(I) and mild reductants, four distinct experimental regions are revealed that entail two growth mechanisms, which complement previous discussions of single-mode growth kinetics. The kinetics of Au(I):borane reactions are dependent on the Au precursor/reductant ratio and the thiol capping agent length, when reductant concentration is low. The final reaction products exhibit an LSW size distribution.

• Challenges for physical characterization of silver nanoparticles under pristine and environmentally relevant conditions

  Journal of Environmental Monitoring

  The reported size distribution of silver nanoparticles (AgNPs) is strongly affected by the underlying measurement method, agglomeration state, and dispersion conditions. A selection of AgNP materials with vendor-reported diameters ranging from 1 nm to 100 nm, various size distributions, and biocompatible capping agents including citrate, starch and polyvinylpyrrolidone were studied. AgNPs were diluted with either deionized water, moderately hard reconstituted water, or moderately hard reconstituted water containing natural organic matter. Rigorous physico-chemical characterization by consensus methods and protocols where available enables an understanding of how the underlying measurement method impacts the reported size measurements, which in turn provides a more complete understanding of the state (size, size distribution, agglomeration, etc.) of the AgNPs with respect to the dispersion conditions. An approach to developing routine screening is also presented.

• Competitive Adsorption of Thiolated Polyethylene Glycol and Mercaptopropionic Acid on Gold Nanoparticles Measured by Physical Characterization Methods

  Langmuir

  Competitive adsorption kinetics between thiolated polyethylene glycol (SH-PEG) and mercaptopropionic acid (MPA) on gold nanoparticles (Au-NPs) were studied using a prototype physical characterization approach that combines dynamic light scattering (DLS) and electrospray differential mobility analysis (ES-DMA). The change in hydrodynamic particle size (intensity average) due to the formation of SH-PEG coatings on Au-NPs was measured by DLS in both two-component (Au-NP þ MPA or Au-NP @ SH-PEG) and three-component (Au-NP @ MPA @ SH-PEG) systems. ES-DMA was employed to quantify the surface coverage of SH-PEG and establish a correlation between surface coverage and the change in particle size measured by DLS.Achange in the equilibrium binding constant for SH-PEG on Au-NPs at various concentrations of SH-PEG and MPA showed that the presence of MPA reduced the binding affinity of SH-PEG to the Au-NP surface. Kinetic studies showed that SH-PEG was desorbed from the Au-NP surface following a second-order desorption model after subsequently introducing MPA. The desorption rate constant of SH-PEG from the Au-NP surface by MPA displacement was strongly affected by the concentration of MPA and the excess SH-PEG in solution.

• Comparison of Electrical Properties of Viruses Studied by AC Capacitance Scanning Probe Microscopy

  Journal of the American Chemical Society

  Capacitances of five types of viruses, adenovirus type 5 (AV5), herpes simplex virus type 1 (HSV1), simian virus 40 (SV40), vaccinia (MVA), and cowpea mosaic virus (CPMV), were compared by AC capacitance scanning probe microscopy. This technique, using a Pt-coated AFM tip as an electrode to probe capacitance of materials between the tip and a bottom electrode, has been applied to study surface structures of semiconductors and polymers with nanometer spatial resolution; however, biological samples at the nanoscale have not been explored by this technique yet. Because most biological cells are poor conductors, this approach to probe electric properties of cells by capacitance is logical. This scanning probe technique showed that each virus has distinguishable and characteristic capacitance. A series of control experiments were carried out using mutant viruses to validate the origin of the characteristic capacitance responses for different viruses. A mutation on the capsid in HSV1 with green fluorescence proteins increased capacitance from 9 × 10-6 to 1 × 10-5 F/cm2 at the frequency of 104 Hz. Herpes simplex virus type 2 (HSV2) decreased capacitance when its envelope and glycoproteins were chemically extracted. These control experiments indicate that dielectric properties of capsid proteins and envelope glycoproteins significantly influence overall dielectric constants of viruses. Because those capsid proteins and glycoproteins are characteristic of the virus strain, this technique could be applied to detect and identify viruses at the single viron level using their distinct capacitance spectra as fingerprints without labeling.

• Persistence of singly dispersed silver nanoparticles in natural freshwaters, synthetic seawater, and simulated estuarine waters.

  Science of the Total Environment

  This investigation focuses on predicting the persistence of citrate-capped 20nm AgNPs by measuring their colloidal stability in natural freshwaters and synthetic aquatic media. Ultraviolet-visible absorbance spectroscopy, dynamic light scattering, and atomic force microscopy were used to evaluate the colloidal stability of AgNPs in locally-obtained pond water, moderately hard reconstituted water alone or with natural organic matter (NOM), synthetic seawater, and also the individual chemicals most prevalent in seawater. Singly dispersed AgNPs in seawater and waters with greater than 20mmolL(-1) sodium chloride were unstable, with the optical absorbance approaching zero within the first ten hours of mixing. Agglomeration rates as a function of water chemistry and NOM are tested as a hypothesis to explain the rates of disappearance of singly dispersed AgNPs. Other samples, mostly those with lower salinity or NOM, maintained varying degrees of colloidal stability during time studies up to 48h. This indicates likelihood that some AgNPs will be stable long enough in freshwater to successfully enter estuarine or marine systems. These results should enable a more efficient design of nanoEHS risk assessment experiments by predicting the aquatic or soil compartments at greatest potential risk for accumulation of and exposure to citrate capped 20nm AgNPs.

• Newkome-Type Dendron-Stabilized Gold Nanoparticles:Synthesis, Reactivity, and Stability

  Chemistry of Materials

  We report the synthesis and evaluation of four Newkome-type dendrons, G1-COOH, G2-COOH, SH-G1- COOH, and TA-G1-COOH, and their respective gold-dendron conjugates, where GX represents the generation number. G1- and G2-COOH are two-directional symmetric dendrons that have cystamine cores containing a disulfide group. SH-G1- COOH was prepared by treatment of G1-COOH with dithioerythritol to yield a free thiol group to replace the disulfide linkage. TA-G1-COOH has a thioctic acid moiety, which is a five membered ring containing a disulfide group that cleaves to produce two anchoring thiols to bond with the gold surface. All dendrons have peripheral carboxylate groups to afford hydrophilicity and functionality. Gold nanoparticle conjugates were prepared by reaction of each dendron solution with a suspension of gold colloid (nominally 10 nm diameter) and purified by stirred cell ultrafiltration. Chemical structures were confirmed by 1H and 13C nuclear magnetic resonance spectroscopy and matrix assisted laser desorption/ionization time-of-flight mass spectrometry. Particle size and surface plasmon resonance of the conjugates were characterized by dynamic light scattering (DLS) and UV-vis spectroscopy, respectively. X-ray photoelectron spectroscopy (XPS) was utilized to confirm covalent bonding between the thiols on the dendron and the gold surface. XPS also revealed changes in the S/Au intensity ratio as a function of the dendron chemical structure, suggesting steric effects play a role in the reaction and/or conformation of dendrons on the gold surface. The colloidal and chemical stability of the conjugates as a function of temperature, pH, and suspending medium, and with respect to chemical resistance toward KCN, was investigated using DLS and UV-vis absorption.

• Dispersion stabilization of silver nanoparticles in synthetic lung fluid studied under in situ conditions

  Nanotoxicology

  The dispersion stabilization of silver nanoparticles (AgNPs) in synthetic lung fluid was studied to interrogate the effects on colloidal stability due to the principal constituents of the fluid. The colloidal stability of 20 nm citrate-AgNPs dispersed in the presence of each constituent of the synthetic lung fluid (individually, the complete fluid, and without additives) was observed during titration of increasing sodium chloride concentration. A variety of complementary in situ measurement techniques were utilized, including dynamic light scattering, ultraviolet-visible absorption spectroscopy, atomic force microscopy, and small-angle X-ray scattering, which provided a collective set of information that enabled far better understanding of the dispersion behavior in the fluid than any one technique alone. It was observed that AgNPs continued to adsorb bovine serum albumin (BSA) protein from the synthetic lung fluid solution as the sodium chloride concentration increased, until a maximum BSA coating was achieved prior to reaching the physiological sodium chloride concentration of 154 mmol L−1. BSA was determined to be the constituent of the synthetic lung fluid that is required to provide colloidal stability at high salt loadings, though the phospholipid constituent exerts a subtle effect. Additionally, as AgNPs are a distinctly different class of nanoparticles apart from the carbon nanotubes and titanium dioxide nanoparticles initially reported to be dispersible using this fluid, this work also demonstrates the broad applicability of synthetic lung fluid in providing stable dispersions for engineered nanoparticles for use in biological assays.

• Adsorption and Conformation of Serum Albumin Protein on Gold Nanoparticles Investigated Using Dimensional Measurements and in Situ Spectroscopic Methods

  Langmuir

  The adsorption and conformation of bovine serum albumin (BSA) on gold nanoparticles (AuNPs) were interrogated both qualitatively and quantitatively via complementary physicochemical characterization methods. Dynamic light scattering (DLS), asymmetric-flow field flow fractionation (AFFF), fluorescence spectrometry, and attenuated total reflectance- Fourier transform infrared (ATR-FTIR) spectroscopy were combined to characterize BSA-AuNP conjugates under fluid conditions, while conjugates in the aerosol state were characterized by electrospray-differential mobility analysis (ES-DMA). The presence of unbound BSA molecules interferes with DLS analysis of the conjugates, particularly as the AuNP size decreases (i.e., below 30 nm in diameter). Under conditions where the γ value is high, where γ is defined as the ratio of scattering intensity by AuNPs to the scattering intensity by unbound BSA, DLS size results are consistent with results obtained after fractionation by AFFF. Additionally, the AuNP hydrodynamic size exhibits a greater proportional increase due to BSA conjugation at pH values below 2.5 compared with less acidic pH values (3.4-7.3), corresponding with the reversibly denatured (E or F form) conformation of BSA below pH 2.5. Over the pH range from 3.4 to 7.3, the hydrodynamic size of the conjugate is nearly constant, suggesting conformational stability over this range. Because of the difference in the measurement environment, a larger increase of AuNP size is observed following BSA conjugation when measured in the wet state (i.e., by DLS and AFFF) compared to the dry state (by ES-DMA).

• Disinfection action of electrostatic versus steric-stabilized silver nanoparticles on E. coli under different water chemistries

  Colloids and Surfaces B: Biointerfaces

  The capping layer stabilizing silver nanoparticles (AgNPs) affects its aggregation, dissolution, and net disinfection action, especially under conditions of varying water composition, such as, pH, ionic strength and organic matter content. Herein, we correlate the silver ion (Ag+) release and reactive oxygen species (ROS) generation rates for AgNPs of varying functionalization to their net disinfection coefficient on Escherichia coli, under conditions of differing water chemistries. For electrostatically stabilized citrate-capped AgNPs, the rate of ROS generation, as measured using a fluorescent dye, is found to dominate over that of Ag+ release, especially for smaller sized AgNP suspensions (∼10 nm) at low pH (∼6.2). For these AgNPs, the ROS disinfection mechanism is confirmed to dominate net disinfection action, as measured by the live/dead assay, especially at low levels of organic matter. Steric stabilization of AgNPs by protein or starch-capped layers enables disinfection through reducing AgNP aggregation and promoting silver dissolution over ROS generation. We suggest the involvement of protons and dissolved oxygen in causing the independent formation of Ag+ and ROS, regardless of the AgNP capping layer. While protein-capping layers effectively stabilize AgNPs, the generated ROS is likely dissipated by interference with the bulky capping layer, whereas the interference is lower with citrate-capping layers. Steric stabilization of AgNPs enables disinfection within a wide range of water chemistries, whereas effective disinfection can occur under electrostatic stabilization, only at low NaCl (<1 mmol/L) and organic matter (<5 mg/L) levels.

• Tiered guidance for risk-informed environmental health and safety testing of nanotechnologies

  Journal of Nanoparticle Research

• Unexpected Changes in Functionality and Surface Coverage for Au Nanoparticle PEI Conjugates: Implications for Stability and Efficacy in Biological Systems

  Langmuir

• UV-induced photochemical transformations of citrate capped silver nanoparticle suspensions

  Journal of Nanoparticle Research

  Due to the increasing use of silver nanoparticles (AgNPs) in consumer products, it is essential to understand how variables, such as light exposure, may change the physical and chemical characteristics of AgNP suspensions. To this end, the effect of 300 nm ultraviolet (UV) light on (20, 40, 60 and 80) nm citrate-capped AgNP suspensions has been investigated. As a consequence of irradiation, the initial yellow hue of the AgNP suspensions is transformed towards a near colorless solution due to the loss of the surface plasmon resonance (SPR) absorbance. The decrease in SPR absorbance followed a first-order decay process for all particle sizes with a rate constant that increased linearly with the AgNP specific surface area and non-linearly with light intensity. The rate of loss of the SPR absorbance decreased with increasing citrate concentration, suggesting a surface-mediated transformation. Absorbance, atomic force microscopy, and dynamic light scattering results all indicated that AgNP photolysis was accompanied by a diameter decrease and occasional aggregation. Furthermore, in situ transmission electron microscopy imaging using a specialized liquid cell also showed a decrease in the particle size and the formation of a core–shell structure in UV-exposed AgNPs. X-ray photoelectron spectroscopy analysis suggested that this shell consisted of oxidized silver. The SPR in UV-exposed AgNP suspensions could be regenerated by addition of a strong reducing agent (NaBH4), supporting the idea that oxidized silver is present after photolysis. Evidence for UV-enhanced dissolution and the production of silver ions was obtained with the Donnan membrane technique. This study reveals that the physico-chemical properties of aqueous AgNP suspensions will change significantly upon exposure to UV light, with implications for environmental health and safety risk assessments.

• SP 1200-8: Preparation of silver nanoparticle loaded cotton threads to facilitate measurement development for textile applications

  NIST Technical Series Publication

  FOREWORD This NIST Special Publication (SP) is one in a series of NIST SPs that address research needs articulated in the National Nanotechnology Initiative (NNI) Environmental, Health, and Safety Research Strategy published in 2011 [1]. This Strategy identified a Nanomaterial Measurement Infrastructure (NMI) as essential for science-based risk assessment and risk management of nanotechnology-enabled products as pertaining to human health, exposure, and the environment. NIST was identified as the lead federal agency in the NMI core research area of the Strategy. This research area includes development of measurement tools for the detection and characterization of engineered nanomaterials in nanotechnology-enabled products. Textiles containing silver nanoparticles, the focus of this SP, are consumer products of great interest for nanotechnology environmental, health, and safety (nano-EHS) research and for product regulation. The protocol presented in this SP describes a strategy for developing a validation known to contain silver nanoparticles loaded onto the surface of a cotton thread. These validation threads can be used to test the capabilities of various analytical methods to characterize silver nanoparticles in textiles.

• Gold nanorod separation and characterization by asymmetric-flow field flow fractionation with UV–Vis detection

  Analytical Bioanalytical Chemistry

  The application of asymmetric-flow field flow fractionation (A4F) for low aspect ratio gold nanorod (GNR) fractionation and characterization was comprehensively investigated. We report on two novel aspects of this application. The first addresses the analytical challenge involved in the fractionation of positively charged nanoparticles by A4F, due to the interaction that exists between the negatively charged native membrane and the analyte. We show that the mobile phase composition is a critical parameter for controlling fractionation and mitigating the membrane-analyte interaction. A mixture of ammonium nitrate and cetyl trimethyl ammonium bromide at different molar ratios enables separation of GNRs with high recovery. The second aspect is the demonstration of shape-based separation of GNRs in A4F normal mode elution (i.e., Brownian mode). We show that the elution of GNRs is due both to aspect ratio and a steric-entropic contribution for GNRs with the same diameter. This latter effect can be explained by their orientation vector inside the A4F channel. Our experimental results demonstrate the relevance of the theory described by Beckett and Giddings for non-spherical fractionation (Beckett and Giddings, J Colloid and Interface Sci 186(1):53–59, 1997). However, it is shown that this theory has its limit in the case of complex GNR mixtures, and that shape (i.e., aspect ratio) is the principal material parameter controlling elution of GNRs in A4F; the apparent translational diffusion coefficient of GNRs increases with aspect ratio. Finally, the performance of the methodology developed in this work is evaluated by the fractionation and characterization of individual components from a mixture of GNR aspect ratios.

• In-situ UV/Vis, SAXS and TEM study of single phase Au Nanoparticle Growth

  Chemistry of Materials

  Given the diverse scientific and technological applications of gold nanoparticles (Au NPs), understanding the impact of macromolecular additives on the distribution of size, shape, and composition is crucial to ensure reproducibility and lower production cost. In situ measurement of the evolution of these distributions challenges current techniques; however, it is critical for in-line manufacturing controls. Using mild Au(I) reduction by tert-butylamine-borane in toluene, the utility and limitations of SAXS, UV/Vis spectroscopy, and TEM are considered by comparing the mean nanoparticle size, size distribution, and relative number density. Individually, these techniques are insufficient to follow these parameters through the initial process of nucleation and growth; either providing insufficient information on the number of particles (UV/Vis), introducing artifacts (TEM), or not providing a unique solution for the shape of the distribution (SAXS). However, when used in conjunction, especially SAXS calibrated with TEM of reaction aliquots, the time evolution of these parameters can be quantified. For the single-phase Au NP synthesis with Au(I) and mild reductants, four distinct experimental regions are revealed that entail two growth mechanisms, which complement previous discussions of single-mode growth kinetics. The kinetics of Au(I):borane reactions are dependent on the Au precursor/reductant ratio and the thiol capping agent length, when reductant concentration is low. The final reaction products exhibit an LSW size distribution.

• Challenges for physical characterization of silver nanoparticles under pristine and environmentally relevant conditions

  Journal of Environmental Monitoring

  The reported size distribution of silver nanoparticles (AgNPs) is strongly affected by the underlying measurement method, agglomeration state, and dispersion conditions. A selection of AgNP materials with vendor-reported diameters ranging from 1 nm to 100 nm, various size distributions, and biocompatible capping agents including citrate, starch and polyvinylpyrrolidone were studied. AgNPs were diluted with either deionized water, moderately hard reconstituted water, or moderately hard reconstituted water containing natural organic matter. Rigorous physico-chemical characterization by consensus methods and protocols where available enables an understanding of how the underlying measurement method impacts the reported size measurements, which in turn provides a more complete understanding of the state (size, size distribution, agglomeration, etc.) of the AgNPs with respect to the dispersion conditions. An approach to developing routine screening is also presented.

• Competitive Adsorption of Thiolated Polyethylene Glycol and Mercaptopropionic Acid on Gold Nanoparticles Measured by Physical Characterization Methods

  Langmuir

  Competitive adsorption kinetics between thiolated polyethylene glycol (SH-PEG) and mercaptopropionic acid (MPA) on gold nanoparticles (Au-NPs) were studied using a prototype physical characterization approach that combines dynamic light scattering (DLS) and electrospray differential mobility analysis (ES-DMA). The change in hydrodynamic particle size (intensity average) due to the formation of SH-PEG coatings on Au-NPs was measured by DLS in both two-component (Au-NP þ MPA or Au-NP @ SH-PEG) and three-component (Au-NP @ MPA @ SH-PEG) systems. ES-DMA was employed to quantify the surface coverage of SH-PEG and establish a correlation between surface coverage and the change in particle size measured by DLS.Achange in the equilibrium binding constant for SH-PEG on Au-NPs at various concentrations of SH-PEG and MPA showed that the presence of MPA reduced the binding affinity of SH-PEG to the Au-NP surface. Kinetic studies showed that SH-PEG was desorbed from the Au-NP surface following a second-order desorption model after subsequently introducing MPA. The desorption rate constant of SH-PEG from the Au-NP surface by MPA displacement was strongly affected by the concentration of MPA and the excess SH-PEG in solution.

• Comparison of Electrical Properties of Viruses Studied by AC Capacitance Scanning Probe Microscopy

  Journal of the American Chemical Society

  Capacitances of five types of viruses, adenovirus type 5 (AV5), herpes simplex virus type 1 (HSV1), simian virus 40 (SV40), vaccinia (MVA), and cowpea mosaic virus (CPMV), were compared by AC capacitance scanning probe microscopy. This technique, using a Pt-coated AFM tip as an electrode to probe capacitance of materials between the tip and a bottom electrode, has been applied to study surface structures of semiconductors and polymers with nanometer spatial resolution; however, biological samples at the nanoscale have not been explored by this technique yet. Because most biological cells are poor conductors, this approach to probe electric properties of cells by capacitance is logical. This scanning probe technique showed that each virus has distinguishable and characteristic capacitance. A series of control experiments were carried out using mutant viruses to validate the origin of the characteristic capacitance responses for different viruses. A mutation on the capsid in HSV1 with green fluorescence proteins increased capacitance from 9 × 10-6 to 1 × 10-5 F/cm2 at the frequency of 104 Hz. Herpes simplex virus type 2 (HSV2) decreased capacitance when its envelope and glycoproteins were chemically extracted. These control experiments indicate that dielectric properties of capsid proteins and envelope glycoproteins significantly influence overall dielectric constants of viruses. Because those capsid proteins and glycoproteins are characteristic of the virus strain, this technique could be applied to detect and identify viruses at the single viron level using their distinct capacitance spectra as fingerprints without labeling.

• Persistence of singly dispersed silver nanoparticles in natural freshwaters, synthetic seawater, and simulated estuarine waters.

  Science of the Total Environment

  This investigation focuses on predicting the persistence of citrate-capped 20nm AgNPs by measuring their colloidal stability in natural freshwaters and synthetic aquatic media. Ultraviolet-visible absorbance spectroscopy, dynamic light scattering, and atomic force microscopy were used to evaluate the colloidal stability of AgNPs in locally-obtained pond water, moderately hard reconstituted water alone or with natural organic matter (NOM), synthetic seawater, and also the individual chemicals most prevalent in seawater. Singly dispersed AgNPs in seawater and waters with greater than 20mmolL(-1) sodium chloride were unstable, with the optical absorbance approaching zero within the first ten hours of mixing. Agglomeration rates as a function of water chemistry and NOM are tested as a hypothesis to explain the rates of disappearance of singly dispersed AgNPs. Other samples, mostly those with lower salinity or NOM, maintained varying degrees of colloidal stability during time studies up to 48h. This indicates likelihood that some AgNPs will be stable long enough in freshwater to successfully enter estuarine or marine systems. These results should enable a more efficient design of nanoEHS risk assessment experiments by predicting the aquatic or soil compartments at greatest potential risk for accumulation of and exposure to citrate capped 20nm AgNPs.

• Newkome-Type Dendron-Stabilized Gold Nanoparticles:Synthesis, Reactivity, and Stability

  Chemistry of Materials

  We report the synthesis and evaluation of four Newkome-type dendrons, G1-COOH, G2-COOH, SH-G1- COOH, and TA-G1-COOH, and their respective gold-dendron conjugates, where GX represents the generation number. G1- and G2-COOH are two-directional symmetric dendrons that have cystamine cores containing a disulfide group. SH-G1- COOH was prepared by treatment of G1-COOH with dithioerythritol to yield a free thiol group to replace the disulfide linkage. TA-G1-COOH has a thioctic acid moiety, which is a five membered ring containing a disulfide group that cleaves to produce two anchoring thiols to bond with the gold surface. All dendrons have peripheral carboxylate groups to afford hydrophilicity and functionality. Gold nanoparticle conjugates were prepared by reaction of each dendron solution with a suspension of gold colloid (nominally 10 nm diameter) and purified by stirred cell ultrafiltration. Chemical structures were confirmed by 1H and 13C nuclear magnetic resonance spectroscopy and matrix assisted laser desorption/ionization time-of-flight mass spectrometry. Particle size and surface plasmon resonance of the conjugates were characterized by dynamic light scattering (DLS) and UV-vis spectroscopy, respectively. X-ray photoelectron spectroscopy (XPS) was utilized to confirm covalent bonding between the thiols on the dendron and the gold surface. XPS also revealed changes in the S/Au intensity ratio as a function of the dendron chemical structure, suggesting steric effects play a role in the reaction and/or conformation of dendrons on the gold surface. The colloidal and chemical stability of the conjugates as a function of temperature, pH, and suspending medium, and with respect to chemical resistance toward KCN, was investigated using DLS and UV-vis absorption.

• Antibody-Mediated Self-Limiting Self-Assembly for Quantitative Analysis of Nanoparticle Surfaces by Atomic Force Microscopy

  Microscopy and Microanalysis

  Quantification of very low density molecular coatings on large (60 nm) gold nanoparticles (AuNPs) is demonstrated via the use of antibody-mediated self-limiting self-assembly of small and large AuNPs into raspberry-like structures subsequently imaged by atomic force microscopy (AFM). AFM imaging is proposed as an automated, lower-cost, higher-throughput alternative to immunostaining and imaging by transmission electron microscopy. Synthesis of large AuNPs, containing one of three ligand molecules in one of three stoichiometries (1, 2, or 10 ligands per AuNP), and small probe AuNPs with one of three antibody molecules in a one antibody per AuNP ratio, enabled a range of predicted self-limiting self-assembled structures. A model predicting the probability of observing a given small to large AuNP ratio based on a topography measurement such as AFM is described, in which random orientational deposition is assumed and which accounts for the stochastic synthesis method of the library AuNPs with varied ligand ratios. Experimental data were found to agree very well with the predictive models when using an established AFM sample preparation method that avoids drying-induced aggregation.

• Dispersion stabilization of silver nanoparticles in synthetic lung fluid studied under in situ conditions

  Nanotoxicology

  The dispersion stabilization of silver nanoparticles (AgNPs) in synthetic lung fluid was studied to interrogate the effects on colloidal stability due to the principal constituents of the fluid. The colloidal stability of 20 nm citrate-AgNPs dispersed in the presence of each constituent of the synthetic lung fluid (individually, the complete fluid, and without additives) was observed during titration of increasing sodium chloride concentration. A variety of complementary in situ measurement techniques were utilized, including dynamic light scattering, ultraviolet-visible absorption spectroscopy, atomic force microscopy, and small-angle X-ray scattering, which provided a collective set of information that enabled far better understanding of the dispersion behavior in the fluid than any one technique alone. It was observed that AgNPs continued to adsorb bovine serum albumin (BSA) protein from the synthetic lung fluid solution as the sodium chloride concentration increased, until a maximum BSA coating was achieved prior to reaching the physiological sodium chloride concentration of 154 mmol L−1. BSA was determined to be the constituent of the synthetic lung fluid that is required to provide colloidal stability at high salt loadings, though the phospholipid constituent exerts a subtle effect. Additionally, as AgNPs are a distinctly different class of nanoparticles apart from the carbon nanotubes and titanium dioxide nanoparticles initially reported to be dispersible using this fluid, this work also demonstrates the broad applicability of synthetic lung fluid in providing stable dispersions for engineered nanoparticles for use in biological assays.

• Adsorption and Conformation of Serum Albumin Protein on Gold Nanoparticles Investigated Using Dimensional Measurements and in Situ Spectroscopic Methods

  Langmuir

  The adsorption and conformation of bovine serum albumin (BSA) on gold nanoparticles (AuNPs) were interrogated both qualitatively and quantitatively via complementary physicochemical characterization methods. Dynamic light scattering (DLS), asymmetric-flow field flow fractionation (AFFF), fluorescence spectrometry, and attenuated total reflectance- Fourier transform infrared (ATR-FTIR) spectroscopy were combined to characterize BSA-AuNP conjugates under fluid conditions, while conjugates in the aerosol state were characterized by electrospray-differential mobility analysis (ES-DMA). The presence of unbound BSA molecules interferes with DLS analysis of the conjugates, particularly as the AuNP size decreases (i.e., below 30 nm in diameter). Under conditions where the γ value is high, where γ is defined as the ratio of scattering intensity by AuNPs to the scattering intensity by unbound BSA, DLS size results are consistent with results obtained after fractionation by AFFF. Additionally, the AuNP hydrodynamic size exhibits a greater proportional increase due to BSA conjugation at pH values below 2.5 compared with less acidic pH values (3.4-7.3), corresponding with the reversibly denatured (E or F form) conformation of BSA below pH 2.5. Over the pH range from 3.4 to 7.3, the hydrodynamic size of the conjugate is nearly constant, suggesting conformational stability over this range. Because of the difference in the measurement environment, a larger increase of AuNP size is observed following BSA conjugation when measured in the wet state (i.e., by DLS and AFFF) compared to the dry state (by ES-DMA).

• Disinfection action of electrostatic versus steric-stabilized silver nanoparticles on E. coli under different water chemistries

  Colloids and Surfaces B: Biointerfaces

  The capping layer stabilizing silver nanoparticles (AgNPs) affects its aggregation, dissolution, and net disinfection action, especially under conditions of varying water composition, such as, pH, ionic strength and organic matter content. Herein, we correlate the silver ion (Ag+) release and reactive oxygen species (ROS) generation rates for AgNPs of varying functionalization to their net disinfection coefficient on Escherichia coli, under conditions of differing water chemistries. For electrostatically stabilized citrate-capped AgNPs, the rate of ROS generation, as measured using a fluorescent dye, is found to dominate over that of Ag+ release, especially for smaller sized AgNP suspensions (∼10 nm) at low pH (∼6.2). For these AgNPs, the ROS disinfection mechanism is confirmed to dominate net disinfection action, as measured by the live/dead assay, especially at low levels of organic matter. Steric stabilization of AgNPs by protein or starch-capped layers enables disinfection through reducing AgNP aggregation and promoting silver dissolution over ROS generation. We suggest the involvement of protons and dissolved oxygen in causing the independent formation of Ag+ and ROS, regardless of the AgNP capping layer. While protein-capping layers effectively stabilize AgNPs, the generated ROS is likely dissipated by interference with the bulky capping layer, whereas the interference is lower with citrate-capping layers. Steric stabilization of AgNPs enables disinfection within a wide range of water chemistries, whereas effective disinfection can occur under electrostatic stabilization, only at low NaCl (<1 mmol/L) and organic matter (<5 mg/L) levels.

• Tiered guidance for risk-informed environmental health and safety testing of nanotechnologies

  Journal of Nanoparticle Research

• Unexpected Changes in Functionality and Surface Coverage for Au Nanoparticle PEI Conjugates: Implications for Stability and Efficacy in Biological Systems

  Langmuir

• UV-induced photochemical transformations of citrate capped silver nanoparticle suspensions

  Journal of Nanoparticle Research

  Due to the increasing use of silver nanoparticles (AgNPs) in consumer products, it is essential to understand how variables, such as light exposure, may change the physical and chemical characteristics of AgNP suspensions. To this end, the effect of 300 nm ultraviolet (UV) light on (20, 40, 60 and 80) nm citrate-capped AgNP suspensions has been investigated. As a consequence of irradiation, the initial yellow hue of the AgNP suspensions is transformed towards a near colorless solution due to the loss of the surface plasmon resonance (SPR) absorbance. The decrease in SPR absorbance followed a first-order decay process for all particle sizes with a rate constant that increased linearly with the AgNP specific surface area and non-linearly with light intensity. The rate of loss of the SPR absorbance decreased with increasing citrate concentration, suggesting a surface-mediated transformation. Absorbance, atomic force microscopy, and dynamic light scattering results all indicated that AgNP photolysis was accompanied by a diameter decrease and occasional aggregation. Furthermore, in situ transmission electron microscopy imaging using a specialized liquid cell also showed a decrease in the particle size and the formation of a core–shell structure in UV-exposed AgNPs. X-ray photoelectron spectroscopy analysis suggested that this shell consisted of oxidized silver. The SPR in UV-exposed AgNP suspensions could be regenerated by addition of a strong reducing agent (NaBH4), supporting the idea that oxidized silver is present after photolysis. Evidence for UV-enhanced dissolution and the production of silver ions was obtained with the Donnan membrane technique. This study reveals that the physico-chemical properties of aqueous AgNP suspensions will change significantly upon exposure to UV light, with implications for environmental health and safety risk assessments.

• SP 1200-8: Preparation of silver nanoparticle loaded cotton threads to facilitate measurement development for textile applications

  NIST Technical Series Publication

  FOREWORD This NIST Special Publication (SP) is one in a series of NIST SPs that address research needs articulated in the National Nanotechnology Initiative (NNI) Environmental, Health, and Safety Research Strategy published in 2011 [1]. This Strategy identified a Nanomaterial Measurement Infrastructure (NMI) as essential for science-based risk assessment and risk management of nanotechnology-enabled products as pertaining to human health, exposure, and the environment. NIST was identified as the lead federal agency in the NMI core research area of the Strategy. This research area includes development of measurement tools for the detection and characterization of engineered nanomaterials in nanotechnology-enabled products. Textiles containing silver nanoparticles, the focus of this SP, are consumer products of great interest for nanotechnology environmental, health, and safety (nano-EHS) research and for product regulation. The protocol presented in this SP describes a strategy for developing a validation known to contain silver nanoparticles loaded onto the surface of a cotton thread. These validation threads can be used to test the capabilities of various analytical methods to characterize silver nanoparticles in textiles.

• Gold nanorod separation and characterization by asymmetric-flow field flow fractionation with UV–Vis detection

  Analytical Bioanalytical Chemistry

  The application of asymmetric-flow field flow fractionation (A4F) for low aspect ratio gold nanorod (GNR) fractionation and characterization was comprehensively investigated. We report on two novel aspects of this application. The first addresses the analytical challenge involved in the fractionation of positively charged nanoparticles by A4F, due to the interaction that exists between the negatively charged native membrane and the analyte. We show that the mobile phase composition is a critical parameter for controlling fractionation and mitigating the membrane-analyte interaction. A mixture of ammonium nitrate and cetyl trimethyl ammonium bromide at different molar ratios enables separation of GNRs with high recovery. The second aspect is the demonstration of shape-based separation of GNRs in A4F normal mode elution (i.e., Brownian mode). We show that the elution of GNRs is due both to aspect ratio and a steric-entropic contribution for GNRs with the same diameter. This latter effect can be explained by their orientation vector inside the A4F channel. Our experimental results demonstrate the relevance of the theory described by Beckett and Giddings for non-spherical fractionation (Beckett and Giddings, J Colloid and Interface Sci 186(1):53–59, 1997). However, it is shown that this theory has its limit in the case of complex GNR mixtures, and that shape (i.e., aspect ratio) is the principal material parameter controlling elution of GNRs in A4F; the apparent translational diffusion coefficient of GNRs increases with aspect ratio. Finally, the performance of the methodology developed in this work is evaluated by the fractionation and characterization of individual components from a mixture of GNR aspect ratios.

• In-situ UV/Vis, SAXS and TEM study of single phase Au Nanoparticle Growth

  Chemistry of Materials

  Given the diverse scientific and technological applications of gold nanoparticles (Au NPs), understanding the impact of macromolecular additives on the distribution of size, shape, and composition is crucial to ensure reproducibility and lower production cost. In situ measurement of the evolution of these distributions challenges current techniques; however, it is critical for in-line manufacturing controls. Using mild Au(I) reduction by tert-butylamine-borane in toluene, the utility and limitations of SAXS, UV/Vis spectroscopy, and TEM are considered by comparing the mean nanoparticle size, size distribution, and relative number density. Individually, these techniques are insufficient to follow these parameters through the initial process of nucleation and growth; either providing insufficient information on the number of particles (UV/Vis), introducing artifacts (TEM), or not providing a unique solution for the shape of the distribution (SAXS). However, when used in conjunction, especially SAXS calibrated with TEM of reaction aliquots, the time evolution of these parameters can be quantified. For the single-phase Au NP synthesis with Au(I) and mild reductants, four distinct experimental regions are revealed that entail two growth mechanisms, which complement previous discussions of single-mode growth kinetics. The kinetics of Au(I):borane reactions are dependent on the Au precursor/reductant ratio and the thiol capping agent length, when reductant concentration is low. The final reaction products exhibit an LSW size distribution.

• Challenges for physical characterization of silver nanoparticles under pristine and environmentally relevant conditions

  Journal of Environmental Monitoring

  The reported size distribution of silver nanoparticles (AgNPs) is strongly affected by the underlying measurement method, agglomeration state, and dispersion conditions. A selection of AgNP materials with vendor-reported diameters ranging from 1 nm to 100 nm, various size distributions, and biocompatible capping agents including citrate, starch and polyvinylpyrrolidone were studied. AgNPs were diluted with either deionized water, moderately hard reconstituted water, or moderately hard reconstituted water containing natural organic matter. Rigorous physico-chemical characterization by consensus methods and protocols where available enables an understanding of how the underlying measurement method impacts the reported size measurements, which in turn provides a more complete understanding of the state (size, size distribution, agglomeration, etc.) of the AgNPs with respect to the dispersion conditions. An approach to developing routine screening is also presented.

• Competitive Adsorption of Thiolated Polyethylene Glycol and Mercaptopropionic Acid on Gold Nanoparticles Measured by Physical Characterization Methods

  Langmuir

  Competitive adsorption kinetics between thiolated polyethylene glycol (SH-PEG) and mercaptopropionic acid (MPA) on gold nanoparticles (Au-NPs) were studied using a prototype physical characterization approach that combines dynamic light scattering (DLS) and electrospray differential mobility analysis (ES-DMA). The change in hydrodynamic particle size (intensity average) due to the formation of SH-PEG coatings on Au-NPs was measured by DLS in both two-component (Au-NP þ MPA or Au-NP @ SH-PEG) and three-component (Au-NP @ MPA @ SH-PEG) systems. ES-DMA was employed to quantify the surface coverage of SH-PEG and establish a correlation between surface coverage and the change in particle size measured by DLS.Achange in the equilibrium binding constant for SH-PEG on Au-NPs at various concentrations of SH-PEG and MPA showed that the presence of MPA reduced the binding affinity of SH-PEG to the Au-NP surface. Kinetic studies showed that SH-PEG was desorbed from the Au-NP surface following a second-order desorption model after subsequently introducing MPA. The desorption rate constant of SH-PEG from the Au-NP surface by MPA displacement was strongly affected by the concentration of MPA and the excess SH-PEG in solution.

• Comparison of Electrical Properties of Viruses Studied by AC Capacitance Scanning Probe Microscopy

  Journal of the American Chemical Society

  Capacitances of five types of viruses, adenovirus type 5 (AV5), herpes simplex virus type 1 (HSV1), simian virus 40 (SV40), vaccinia (MVA), and cowpea mosaic virus (CPMV), were compared by AC capacitance scanning probe microscopy. This technique, using a Pt-coated AFM tip as an electrode to probe capacitance of materials between the tip and a bottom electrode, has been applied to study surface structures of semiconductors and polymers with nanometer spatial resolution; however, biological samples at the nanoscale have not been explored by this technique yet. Because most biological cells are poor conductors, this approach to probe electric properties of cells by capacitance is logical. This scanning probe technique showed that each virus has distinguishable and characteristic capacitance. A series of control experiments were carried out using mutant viruses to validate the origin of the characteristic capacitance responses for different viruses. A mutation on the capsid in HSV1 with green fluorescence proteins increased capacitance from 9 × 10-6 to 1 × 10-5 F/cm2 at the frequency of 104 Hz. Herpes simplex virus type 2 (HSV2) decreased capacitance when its envelope and glycoproteins were chemically extracted. These control experiments indicate that dielectric properties of capsid proteins and envelope glycoproteins significantly influence overall dielectric constants of viruses. Because those capsid proteins and glycoproteins are characteristic of the virus strain, this technique could be applied to detect and identify viruses at the single viron level using their distinct capacitance spectra as fingerprints without labeling.

• Persistence of singly dispersed silver nanoparticles in natural freshwaters, synthetic seawater, and simulated estuarine waters.

  Science of the Total Environment

  This investigation focuses on predicting the persistence of citrate-capped 20nm AgNPs by measuring their colloidal stability in natural freshwaters and synthetic aquatic media. Ultraviolet-visible absorbance spectroscopy, dynamic light scattering, and atomic force microscopy were used to evaluate the colloidal stability of AgNPs in locally-obtained pond water, moderately hard reconstituted water alone or with natural organic matter (NOM), synthetic seawater, and also the individual chemicals most prevalent in seawater. Singly dispersed AgNPs in seawater and waters with greater than 20mmolL(-1) sodium chloride were unstable, with the optical absorbance approaching zero within the first ten hours of mixing. Agglomeration rates as a function of water chemistry and NOM are tested as a hypothesis to explain the rates of disappearance of singly dispersed AgNPs. Other samples, mostly those with lower salinity or NOM, maintained varying degrees of colloidal stability during time studies up to 48h. This indicates likelihood that some AgNPs will be stable long enough in freshwater to successfully enter estuarine or marine systems. These results should enable a more efficient design of nanoEHS risk assessment experiments by predicting the aquatic or soil compartments at greatest potential risk for accumulation of and exposure to citrate capped 20nm AgNPs.

• Newkome-Type Dendron-Stabilized Gold Nanoparticles:Synthesis, Reactivity, and Stability

  Chemistry of Materials

  We report the synthesis and evaluation of four Newkome-type dendrons, G1-COOH, G2-COOH, SH-G1- COOH, and TA-G1-COOH, and their respective gold-dendron conjugates, where GX represents the generation number. G1- and G2-COOH are two-directional symmetric dendrons that have cystamine cores containing a disulfide group. SH-G1- COOH was prepared by treatment of G1-COOH with dithioerythritol to yield a free thiol group to replace the disulfide linkage. TA-G1-COOH has a thioctic acid moiety, which is a five membered ring containing a disulfide group that cleaves to produce two anchoring thiols to bond with the gold surface. All dendrons have peripheral carboxylate groups to afford hydrophilicity and functionality. Gold nanoparticle conjugates were prepared by reaction of each dendron solution with a suspension of gold colloid (nominally 10 nm diameter) and purified by stirred cell ultrafiltration. Chemical structures were confirmed by 1H and 13C nuclear magnetic resonance spectroscopy and matrix assisted laser desorption/ionization time-of-flight mass spectrometry. Particle size and surface plasmon resonance of the conjugates were characterized by dynamic light scattering (DLS) and UV-vis spectroscopy, respectively. X-ray photoelectron spectroscopy (XPS) was utilized to confirm covalent bonding between the thiols on the dendron and the gold surface. XPS also revealed changes in the S/Au intensity ratio as a function of the dendron chemical structure, suggesting steric effects play a role in the reaction and/or conformation of dendrons on the gold surface. The colloidal and chemical stability of the conjugates as a function of temperature, pH, and suspending medium, and with respect to chemical resistance toward KCN, was investigated using DLS and UV-vis absorption.

• Antibody-Mediated Self-Limiting Self-Assembly for Quantitative Analysis of Nanoparticle Surfaces by Atomic Force Microscopy

  Microscopy and Microanalysis

  Quantification of very low density molecular coatings on large (60 nm) gold nanoparticles (AuNPs) is demonstrated via the use of antibody-mediated self-limiting self-assembly of small and large AuNPs into raspberry-like structures subsequently imaged by atomic force microscopy (AFM). AFM imaging is proposed as an automated, lower-cost, higher-throughput alternative to immunostaining and imaging by transmission electron microscopy. Synthesis of large AuNPs, containing one of three ligand molecules in one of three stoichiometries (1, 2, or 10 ligands per AuNP), and small probe AuNPs with one of three antibody molecules in a one antibody per AuNP ratio, enabled a range of predicted self-limiting self-assembled structures. A model predicting the probability of observing a given small to large AuNP ratio based on a topography measurement such as AFM is described, in which random orientational deposition is assumed and which accounts for the stochastic synthesis method of the library AuNPs with varied ligand ratios. Experimental data were found to agree very well with the predictive models when using an established AFM sample preparation method that avoids drying-induced aggregation.

• Humic Acid-Induced Silver Nanoparticle Formation Under Environmentally Relevant Conditions

  Environmental Science & Technology

  The formation of silver nanoparticles (AgNPs) via reduction of silver ions (Ag+) in the presence of humic acids (HAs) under various environmentally relevant conditions is described. HAs tested originated from the Suwannee River (SUW), and included samples of three sedimentary HAs (SHAs), and five soils obtained across the state of Florida. The time required to form AgNPs varied depending upon the type and concentration of HA, as well as temperature. SUW and all three SHAs reduced Ag+ at 22 °C. However, none of the soil HAs formed absorbance-detectable AgNPs at room temperature when allowed to react for a period of 25 days, at which time experiments were halted. The appearance of the characteristic surface plasmon resonance (SPR) of AgNPs was observed by ultraviolet−visible spectroscopy in as few as 2−4 days at 22 °C for SHAs and SUW. An elevated temperature of 90 °C resulted in the accelerated appearance of the SPR within 90 min for SUW and all SHAs. The formation of AgNPs at 90 °C was usually complete within 3 h. Transmission electron microscopy and atomic force microscopy images showed that the AgNPs formed were typically spherical and had a broad size distribution. Dynamic light scattering also revealed polydisperse particle size distributions. HAs appeared to colloidally stabilize AgNPs based on lack of any significant change in the spectral characteristics over a period of two months. The results suggest the potential for direct formation of AgNPs under environmental conditions from Ag+ sources, implying that not all AgNPs observed in natural waters today may be of anthropogenic origin.

• Dispersion stabilization of silver nanoparticles in synthetic lung fluid studied under in situ conditions

  Nanotoxicology

  The dispersion stabilization of silver nanoparticles (AgNPs) in synthetic lung fluid was studied to interrogate the effects on colloidal stability due to the principal constituents of the fluid. The colloidal stability of 20 nm citrate-AgNPs dispersed in the presence of each constituent of the synthetic lung fluid (individually, the complete fluid, and without additives) was observed during titration of increasing sodium chloride concentration. A variety of complementary in situ measurement techniques were utilized, including dynamic light scattering, ultraviolet-visible absorption spectroscopy, atomic force microscopy, and small-angle X-ray scattering, which provided a collective set of information that enabled far better understanding of the dispersion behavior in the fluid than any one technique alone. It was observed that AgNPs continued to adsorb bovine serum albumin (BSA) protein from the synthetic lung fluid solution as the sodium chloride concentration increased, until a maximum BSA coating was achieved prior to reaching the physiological sodium chloride concentration of 154 mmol L−1. BSA was determined to be the constituent of the synthetic lung fluid that is required to provide colloidal stability at high salt loadings, though the phospholipid constituent exerts a subtle effect. Additionally, as AgNPs are a distinctly different class of nanoparticles apart from the carbon nanotubes and titanium dioxide nanoparticles initially reported to be dispersible using this fluid, this work also demonstrates the broad applicability of synthetic lung fluid in providing stable dispersions for engineered nanoparticles for use in biological assays.

• Adsorption and Conformation of Serum Albumin Protein on Gold Nanoparticles Investigated Using Dimensional Measurements and in Situ Spectroscopic Methods

  Langmuir

  The adsorption and conformation of bovine serum albumin (BSA) on gold nanoparticles (AuNPs) were interrogated both qualitatively and quantitatively via complementary physicochemical characterization methods. Dynamic light scattering (DLS), asymmetric-flow field flow fractionation (AFFF), fluorescence spectrometry, and attenuated total reflectance- Fourier transform infrared (ATR-FTIR) spectroscopy were combined to characterize BSA-AuNP conjugates under fluid conditions, while conjugates in the aerosol state were characterized by electrospray-differential mobility analysis (ES-DMA). The presence of unbound BSA molecules interferes with DLS analysis of the conjugates, particularly as the AuNP size decreases (i.e., below 30 nm in diameter). Under conditions where the γ value is high, where γ is defined as the ratio of scattering intensity by AuNPs to the scattering intensity by unbound BSA, DLS size results are consistent with results obtained after fractionation by AFFF. Additionally, the AuNP hydrodynamic size exhibits a greater proportional increase due to BSA conjugation at pH values below 2.5 compared with less acidic pH values (3.4-7.3), corresponding with the reversibly denatured (E or F form) conformation of BSA below pH 2.5. Over the pH range from 3.4 to 7.3, the hydrodynamic size of the conjugate is nearly constant, suggesting conformational stability over this range. Because of the difference in the measurement environment, a larger increase of AuNP size is observed following BSA conjugation when measured in the wet state (i.e., by DLS and AFFF) compared to the dry state (by ES-DMA).

• Disinfection action of electrostatic versus steric-stabilized silver nanoparticles on E. coli under different water chemistries

  Colloids and Surfaces B: Biointerfaces

  The capping layer stabilizing silver nanoparticles (AgNPs) affects its aggregation, dissolution, and net disinfection action, especially under conditions of varying water composition, such as, pH, ionic strength and organic matter content. Herein, we correlate the silver ion (Ag+) release and reactive oxygen species (ROS) generation rates for AgNPs of varying functionalization to their net disinfection coefficient on Escherichia coli, under conditions of differing water chemistries. For electrostatically stabilized citrate-capped AgNPs, the rate of ROS generation, as measured using a fluorescent dye, is found to dominate over that of Ag+ release, especially for smaller sized AgNP suspensions (∼10 nm) at low pH (∼6.2). For these AgNPs, the ROS disinfection mechanism is confirmed to dominate net disinfection action, as measured by the live/dead assay, especially at low levels of organic matter. Steric stabilization of AgNPs by protein or starch-capped layers enables disinfection through reducing AgNP aggregation and promoting silver dissolution over ROS generation. We suggest the involvement of protons and dissolved oxygen in causing the independent formation of Ag+ and ROS, regardless of the AgNP capping layer. While protein-capping layers effectively stabilize AgNPs, the generated ROS is likely dissipated by interference with the bulky capping layer, whereas the interference is lower with citrate-capping layers. Steric stabilization of AgNPs enables disinfection within a wide range of water chemistries, whereas effective disinfection can occur under electrostatic stabilization, only at low NaCl (<1 mmol/L) and organic matter (<5 mg/L) levels.

• Tiered guidance for risk-informed environmental health and safety testing of nanotechnologies

  Journal of Nanoparticle Research

• Unexpected Changes in Functionality and Surface Coverage for Au Nanoparticle PEI Conjugates: Implications for Stability and Efficacy in Biological Systems

  Langmuir

• UV-induced photochemical transformations of citrate capped silver nanoparticle suspensions

  Journal of Nanoparticle Research

  Due to the increasing use of silver nanoparticles (AgNPs) in consumer products, it is essential to understand how variables, such as light exposure, may change the physical and chemical characteristics of AgNP suspensions. To this end, the effect of 300 nm ultraviolet (UV) light on (20, 40, 60 and 80) nm citrate-capped AgNP suspensions has been investigated. As a consequence of irradiation, the initial yellow hue of the AgNP suspensions is transformed towards a near colorless solution due to the loss of the surface plasmon resonance (SPR) absorbance. The decrease in SPR absorbance followed a first-order decay process for all particle sizes with a rate constant that increased linearly with the AgNP specific surface area and non-linearly with light intensity. The rate of loss of the SPR absorbance decreased with increasing citrate concentration, suggesting a surface-mediated transformation. Absorbance, atomic force microscopy, and dynamic light scattering results all indicated that AgNP photolysis was accompanied by a diameter decrease and occasional aggregation. Furthermore, in situ transmission electron microscopy imaging using a specialized liquid cell also showed a decrease in the particle size and the formation of a core–shell structure in UV-exposed AgNPs. X-ray photoelectron spectroscopy analysis suggested that this shell consisted of oxidized silver. The SPR in UV-exposed AgNP suspensions could be regenerated by addition of a strong reducing agent (NaBH4), supporting the idea that oxidized silver is present after photolysis. Evidence for UV-enhanced dissolution and the production of silver ions was obtained with the Donnan membrane technique. This study reveals that the physico-chemical properties of aqueous AgNP suspensions will change significantly upon exposure to UV light, with implications for environmental health and safety risk assessments.

• SP 1200-8: Preparation of silver nanoparticle loaded cotton threads to facilitate measurement development for textile applications

  NIST Technical Series Publication

  FOREWORD This NIST Special Publication (SP) is one in a series of NIST SPs that address research needs articulated in the National Nanotechnology Initiative (NNI) Environmental, Health, and Safety Research Strategy published in 2011 [1]. This Strategy identified a Nanomaterial Measurement Infrastructure (NMI) as essential for science-based risk assessment and risk management of nanotechnology-enabled products as pertaining to human health, exposure, and the environment. NIST was identified as the lead federal agency in the NMI core research area of the Strategy. This research area includes development of measurement tools for the detection and characterization of engineered nanomaterials in nanotechnology-enabled products. Textiles containing silver nanoparticles, the focus of this SP, are consumer products of great interest for nanotechnology environmental, health, and safety (nano-EHS) research and for product regulation. The protocol presented in this SP describes a strategy for developing a validation known to contain silver nanoparticles loaded onto the surface of a cotton thread. These validation threads can be used to test the capabilities of various analytical methods to characterize silver nanoparticles in textiles.

• Gold nanorod separation and characterization by asymmetric-flow field flow fractionation with UV–Vis detection

  Analytical Bioanalytical Chemistry

  The application of asymmetric-flow field flow fractionation (A4F) for low aspect ratio gold nanorod (GNR) fractionation and characterization was comprehensively investigated. We report on two novel aspects of this application. The first addresses the analytical challenge involved in the fractionation of positively charged nanoparticles by A4F, due to the interaction that exists between the negatively charged native membrane and the analyte. We show that the mobile phase composition is a critical parameter for controlling fractionation and mitigating the membrane-analyte interaction. A mixture of ammonium nitrate and cetyl trimethyl ammonium bromide at different molar ratios enables separation of GNRs with high recovery. The second aspect is the demonstration of shape-based separation of GNRs in A4F normal mode elution (i.e., Brownian mode). We show that the elution of GNRs is due both to aspect ratio and a steric-entropic contribution for GNRs with the same diameter. This latter effect can be explained by their orientation vector inside the A4F channel. Our experimental results demonstrate the relevance of the theory described by Beckett and Giddings for non-spherical fractionation (Beckett and Giddings, J Colloid and Interface Sci 186(1):53–59, 1997). However, it is shown that this theory has its limit in the case of complex GNR mixtures, and that shape (i.e., aspect ratio) is the principal material parameter controlling elution of GNRs in A4F; the apparent translational diffusion coefficient of GNRs increases with aspect ratio. Finally, the performance of the methodology developed in this work is evaluated by the fractionation and characterization of individual components from a mixture of GNR aspect ratios.

• In-situ UV/Vis, SAXS and TEM study of single phase Au Nanoparticle Growth

  Chemistry of Materials

  Given the diverse scientific and technological applications of gold nanoparticles (Au NPs), understanding the impact of macromolecular additives on the distribution of size, shape, and composition is crucial to ensure reproducibility and lower production cost. In situ measurement of the evolution of these distributions challenges current techniques; however, it is critical for in-line manufacturing controls. Using mild Au(I) reduction by tert-butylamine-borane in toluene, the utility and limitations of SAXS, UV/Vis spectroscopy, and TEM are considered by comparing the mean nanoparticle size, size distribution, and relative number density. Individually, these techniques are insufficient to follow these parameters through the initial process of nucleation and growth; either providing insufficient information on the number of particles (UV/Vis), introducing artifacts (TEM), or not providing a unique solution for the shape of the distribution (SAXS). However, when used in conjunction, especially SAXS calibrated with TEM of reaction aliquots, the time evolution of these parameters can be quantified. For the single-phase Au NP synthesis with Au(I) and mild reductants, four distinct experimental regions are revealed that entail two growth mechanisms, which complement previous discussions of single-mode growth kinetics. The kinetics of Au(I):borane reactions are dependent on the Au precursor/reductant ratio and the thiol capping agent length, when reductant concentration is low. The final reaction products exhibit an LSW size distribution.

• Challenges for physical characterization of silver nanoparticles under pristine and environmentally relevant conditions

  Journal of Environmental Monitoring

  The reported size distribution of silver nanoparticles (AgNPs) is strongly affected by the underlying measurement method, agglomeration state, and dispersion conditions. A selection of AgNP materials with vendor-reported diameters ranging from 1 nm to 100 nm, various size distributions, and biocompatible capping agents including citrate, starch and polyvinylpyrrolidone were studied. AgNPs were diluted with either deionized water, moderately hard reconstituted water, or moderately hard reconstituted water containing natural organic matter. Rigorous physico-chemical characterization by consensus methods and protocols where available enables an understanding of how the underlying measurement method impacts the reported size measurements, which in turn provides a more complete understanding of the state (size, size distribution, agglomeration, etc.) of the AgNPs with respect to the dispersion conditions. An approach to developing routine screening is also presented.

• Competitive Adsorption of Thiolated Polyethylene Glycol and Mercaptopropionic Acid on Gold Nanoparticles Measured by Physical Characterization Methods

  Langmuir

  Competitive adsorption kinetics between thiolated polyethylene glycol (SH-PEG) and mercaptopropionic acid (MPA) on gold nanoparticles (Au-NPs) were studied using a prototype physical characterization approach that combines dynamic light scattering (DLS) and electrospray differential mobility analysis (ES-DMA). The change in hydrodynamic particle size (intensity average) due to the formation of SH-PEG coatings on Au-NPs was measured by DLS in both two-component (Au-NP þ MPA or Au-NP @ SH-PEG) and three-component (Au-NP @ MPA @ SH-PEG) systems. ES-DMA was employed to quantify the surface coverage of SH-PEG and establish a correlation between surface coverage and the change in particle size measured by DLS.Achange in the equilibrium binding constant for SH-PEG on Au-NPs at various concentrations of SH-PEG and MPA showed that the presence of MPA reduced the binding affinity of SH-PEG to the Au-NP surface. Kinetic studies showed that SH-PEG was desorbed from the Au-NP surface following a second-order desorption model after subsequently introducing MPA. The desorption rate constant of SH-PEG from the Au-NP surface by MPA displacement was strongly affected by the concentration of MPA and the excess SH-PEG in solution.

• Comparison of Electrical Properties of Viruses Studied by AC Capacitance Scanning Probe Microscopy

  Journal of the American Chemical Society

  Capacitances of five types of viruses, adenovirus type 5 (AV5), herpes simplex virus type 1 (HSV1), simian virus 40 (SV40), vaccinia (MVA), and cowpea mosaic virus (CPMV), were compared by AC capacitance scanning probe microscopy. This technique, using a Pt-coated AFM tip as an electrode to probe capacitance of materials between the tip and a bottom electrode, has been applied to study surface structures of semiconductors and polymers with nanometer spatial resolution; however, biological samples at the nanoscale have not been explored by this technique yet. Because most biological cells are poor conductors, this approach to probe electric properties of cells by capacitance is logical. This scanning probe technique showed that each virus has distinguishable and characteristic capacitance. A series of control experiments were carried out using mutant viruses to validate the origin of the characteristic capacitance responses for different viruses. A mutation on the capsid in HSV1 with green fluorescence proteins increased capacitance from 9 × 10-6 to 1 × 10-5 F/cm2 at the frequency of 104 Hz. Herpes simplex virus type 2 (HSV2) decreased capacitance when its envelope and glycoproteins were chemically extracted. These control experiments indicate that dielectric properties of capsid proteins and envelope glycoproteins significantly influence overall dielectric constants of viruses. Because those capsid proteins and glycoproteins are characteristic of the virus strain, this technique could be applied to detect and identify viruses at the single viron level using their distinct capacitance spectra as fingerprints without labeling.

• Persistence of singly dispersed silver nanoparticles in natural freshwaters, synthetic seawater, and simulated estuarine waters.

  Science of the Total Environment

  This investigation focuses on predicting the persistence of citrate-capped 20nm AgNPs by measuring their colloidal stability in natural freshwaters and synthetic aquatic media. Ultraviolet-visible absorbance spectroscopy, dynamic light scattering, and atomic force microscopy were used to evaluate the colloidal stability of AgNPs in locally-obtained pond water, moderately hard reconstituted water alone or with natural organic matter (NOM), synthetic seawater, and also the individual chemicals most prevalent in seawater. Singly dispersed AgNPs in seawater and waters with greater than 20mmolL(-1) sodium chloride were unstable, with the optical absorbance approaching zero within the first ten hours of mixing. Agglomeration rates as a function of water chemistry and NOM are tested as a hypothesis to explain the rates of disappearance of singly dispersed AgNPs. Other samples, mostly those with lower salinity or NOM, maintained varying degrees of colloidal stability during time studies up to 48h. This indicates likelihood that some AgNPs will be stable long enough in freshwater to successfully enter estuarine or marine systems. These results should enable a more efficient design of nanoEHS risk assessment experiments by predicting the aquatic or soil compartments at greatest potential risk for accumulation of and exposure to citrate capped 20nm AgNPs.

• Newkome-Type Dendron-Stabilized Gold Nanoparticles:Synthesis, Reactivity, and Stability

  Chemistry of Materials

  We report the synthesis and evaluation of four Newkome-type dendrons, G1-COOH, G2-COOH, SH-G1- COOH, and TA-G1-COOH, and their respective gold-dendron conjugates, where GX represents the generation number. G1- and G2-COOH are two-directional symmetric dendrons that have cystamine cores containing a disulfide group. SH-G1- COOH was prepared by treatment of G1-COOH with dithioerythritol to yield a free thiol group to replace the disulfide linkage. TA-G1-COOH has a thioctic acid moiety, which is a five membered ring containing a disulfide group that cleaves to produce two anchoring thiols to bond with the gold surface. All dendrons have peripheral carboxylate groups to afford hydrophilicity and functionality. Gold nanoparticle conjugates were prepared by reaction of each dendron solution with a suspension of gold colloid (nominally 10 nm diameter) and purified by stirred cell ultrafiltration. Chemical structures were confirmed by 1H and 13C nuclear magnetic resonance spectroscopy and matrix assisted laser desorption/ionization time-of-flight mass spectrometry. Particle size and surface plasmon resonance of the conjugates were characterized by dynamic light scattering (DLS) and UV-vis spectroscopy, respectively. X-ray photoelectron spectroscopy (XPS) was utilized to confirm covalent bonding between the thiols on the dendron and the gold surface. XPS also revealed changes in the S/Au intensity ratio as a function of the dendron chemical structure, suggesting steric effects play a role in the reaction and/or conformation of dendrons on the gold surface. The colloidal and chemical stability of the conjugates as a function of temperature, pH, and suspending medium, and with respect to chemical resistance toward KCN, was investigated using DLS and UV-vis absorption.

• Antibody-Mediated Self-Limiting Self-Assembly for Quantitative Analysis of Nanoparticle Surfaces by Atomic Force Microscopy

  Microscopy and Microanalysis

  Quantification of very low density molecular coatings on large (60 nm) gold nanoparticles (AuNPs) is demonstrated via the use of antibody-mediated self-limiting self-assembly of small and large AuNPs into raspberry-like structures subsequently imaged by atomic force microscopy (AFM). AFM imaging is proposed as an automated, lower-cost, higher-throughput alternative to immunostaining and imaging by transmission electron microscopy. Synthesis of large AuNPs, containing one of three ligand molecules in one of three stoichiometries (1, 2, or 10 ligands per AuNP), and small probe AuNPs with one of three antibody molecules in a one antibody per AuNP ratio, enabled a range of predicted self-limiting self-assembled structures. A model predicting the probability of observing a given small to large AuNP ratio based on a topography measurement such as AFM is described, in which random orientational deposition is assumed and which accounts for the stochastic synthesis method of the library AuNPs with varied ligand ratios. Experimental data were found to agree very well with the predictive models when using an established AFM sample preparation method that avoids drying-induced aggregation.

• Humic Acid-Induced Silver Nanoparticle Formation Under Environmentally Relevant Conditions

  Environmental Science & Technology

  The formation of silver nanoparticles (AgNPs) via reduction of silver ions (Ag+) in the presence of humic acids (HAs) under various environmentally relevant conditions is described. HAs tested originated from the Suwannee River (SUW), and included samples of three sedimentary HAs (SHAs), and five soils obtained across the state of Florida. The time required to form AgNPs varied depending upon the type and concentration of HA, as well as temperature. SUW and all three SHAs reduced Ag+ at 22 °C. However, none of the soil HAs formed absorbance-detectable AgNPs at room temperature when allowed to react for a period of 25 days, at which time experiments were halted. The appearance of the characteristic surface plasmon resonance (SPR) of AgNPs was observed by ultraviolet−visible spectroscopy in as few as 2−4 days at 22 °C for SHAs and SUW. An elevated temperature of 90 °C resulted in the accelerated appearance of the SPR within 90 min for SUW and all SHAs. The formation of AgNPs at 90 °C was usually complete within 3 h. Transmission electron microscopy and atomic force microscopy images showed that the AgNPs formed were typically spherical and had a broad size distribution. Dynamic light scattering also revealed polydisperse particle size distributions. HAs appeared to colloidally stabilize AgNPs based on lack of any significant change in the spectral characteristics over a period of two months. The results suggest the potential for direct formation of AgNPs under environmental conditions from Ag+ sources, implying that not all AgNPs observed in natural waters today may be of anthropogenic origin.

• A framework for identifying performance targets for sustainable nanomaterials

  Sustainable Materials and Technologies

  An issue in the application of nano-enabled products is howcanwe evaluate sustainable solutions to current system problems based on performance criteria? This work describes the application of an Input–Process–Output (IPO) model as a framework for a life-cycle analysis approach to identify performance metrics and criteria for evaluating the application of nanomaterials to improve the sustainability of a system. A case study is presented describing a scenario whereby a nano-enabled biocidal paint is considered for a remediation effort to reduce growth of dark molds and bacteria on refrigerated warehouses. The framework is applied to support identification of the energy-consuming steps (such as increased refrigeration energy burden, cleaning and repainting), selection of performance metrics for evaluating consumption, and determination of thresholds to measure sustainability outcomes.

• Dispersion stabilization of silver nanoparticles in synthetic lung fluid studied under in situ conditions

  Nanotoxicology

  The dispersion stabilization of silver nanoparticles (AgNPs) in synthetic lung fluid was studied to interrogate the effects on colloidal stability due to the principal constituents of the fluid. The colloidal stability of 20 nm citrate-AgNPs dispersed in the presence of each constituent of the synthetic lung fluid (individually, the complete fluid, and without additives) was observed during titration of increasing sodium chloride concentration. A variety of complementary in situ measurement techniques were utilized, including dynamic light scattering, ultraviolet-visible absorption spectroscopy, atomic force microscopy, and small-angle X-ray scattering, which provided a collective set of information that enabled far better understanding of the dispersion behavior in the fluid than any one technique alone. It was observed that AgNPs continued to adsorb bovine serum albumin (BSA) protein from the synthetic lung fluid solution as the sodium chloride concentration increased, until a maximum BSA coating was achieved prior to reaching the physiological sodium chloride concentration of 154 mmol L−1. BSA was determined to be the constituent of the synthetic lung fluid that is required to provide colloidal stability at high salt loadings, though the phospholipid constituent exerts a subtle effect. Additionally, as AgNPs are a distinctly different class of nanoparticles apart from the carbon nanotubes and titanium dioxide nanoparticles initially reported to be dispersible using this fluid, this work also demonstrates the broad applicability of synthetic lung fluid in providing stable dispersions for engineered nanoparticles for use in biological assays.

• Adsorption and Conformation of Serum Albumin Protein on Gold Nanoparticles Investigated Using Dimensional Measurements and in Situ Spectroscopic Methods

  Langmuir

  The adsorption and conformation of bovine serum albumin (BSA) on gold nanoparticles (AuNPs) were interrogated both qualitatively and quantitatively via complementary physicochemical characterization methods. Dynamic light scattering (DLS), asymmetric-flow field flow fractionation (AFFF), fluorescence spectrometry, and attenuated total reflectance- Fourier transform infrared (ATR-FTIR) spectroscopy were combined to characterize BSA-AuNP conjugates under fluid conditions, while conjugates in the aerosol state were characterized by electrospray-differential mobility analysis (ES-DMA). The presence of unbound BSA molecules interferes with DLS analysis of the conjugates, particularly as the AuNP size decreases (i.e., below 30 nm in diameter). Under conditions where the γ value is high, where γ is defined as the ratio of scattering intensity by AuNPs to the scattering intensity by unbound BSA, DLS size results are consistent with results obtained after fractionation by AFFF. Additionally, the AuNP hydrodynamic size exhibits a greater proportional increase due to BSA conjugation at pH values below 2.5 compared with less acidic pH values (3.4-7.3), corresponding with the reversibly denatured (E or F form) conformation of BSA below pH 2.5. Over the pH range from 3.4 to 7.3, the hydrodynamic size of the conjugate is nearly constant, suggesting conformational stability over this range. Because of the difference in the measurement environment, a larger increase of AuNP size is observed following BSA conjugation when measured in the wet state (i.e., by DLS and AFFF) compared to the dry state (by ES-DMA).

• Disinfection action of electrostatic versus steric-stabilized silver nanoparticles on E. coli under different water chemistries

  Colloids and Surfaces B: Biointerfaces

  The capping layer stabilizing silver nanoparticles (AgNPs) affects its aggregation, dissolution, and net disinfection action, especially under conditions of varying water composition, such as, pH, ionic strength and organic matter content. Herein, we correlate the silver ion (Ag+) release and reactive oxygen species (ROS) generation rates for AgNPs of varying functionalization to their net disinfection coefficient on Escherichia coli, under conditions of differing water chemistries. For electrostatically stabilized citrate-capped AgNPs, the rate of ROS generation, as measured using a fluorescent dye, is found to dominate over that of Ag+ release, especially for smaller sized AgNP suspensions (∼10 nm) at low pH (∼6.2). For these AgNPs, the ROS disinfection mechanism is confirmed to dominate net disinfection action, as measured by the live/dead assay, especially at low levels of organic matter. Steric stabilization of AgNPs by protein or starch-capped layers enables disinfection through reducing AgNP aggregation and promoting silver dissolution over ROS generation. We suggest the involvement of protons and dissolved oxygen in causing the independent formation of Ag+ and ROS, regardless of the AgNP capping layer. While protein-capping layers effectively stabilize AgNPs, the generated ROS is likely dissipated by interference with the bulky capping layer, whereas the interference is lower with citrate-capping layers. Steric stabilization of AgNPs enables disinfection within a wide range of water chemistries, whereas effective disinfection can occur under electrostatic stabilization, only at low NaCl (<1 mmol/L) and organic matter (<5 mg/L) levels.

• Tiered guidance for risk-informed environmental health and safety testing of nanotechnologies

  Journal of Nanoparticle Research

• Unexpected Changes in Functionality and Surface Coverage for Au Nanoparticle PEI Conjugates: Implications for Stability and Efficacy in Biological Systems

  Langmuir

• UV-induced photochemical transformations of citrate capped silver nanoparticle suspensions

  Journal of Nanoparticle Research

  Due to the increasing use of silver nanoparticles (AgNPs) in consumer products, it is essential to understand how variables, such as light exposure, may change the physical and chemical characteristics of AgNP suspensions. To this end, the effect of 300 nm ultraviolet (UV) light on (20, 40, 60 and 80) nm citrate-capped AgNP suspensions has been investigated. As a consequence of irradiation, the initial yellow hue of the AgNP suspensions is transformed towards a near colorless solution due to the loss of the surface plasmon resonance (SPR) absorbance. The decrease in SPR absorbance followed a first-order decay process for all particle sizes with a rate constant that increased linearly with the AgNP specific surface area and non-linearly with light intensity. The rate of loss of the SPR absorbance decreased with increasing citrate concentration, suggesting a surface-mediated transformation. Absorbance, atomic force microscopy, and dynamic light scattering results all indicated that AgNP photolysis was accompanied by a diameter decrease and occasional aggregation. Furthermore, in situ transmission electron microscopy imaging using a specialized liquid cell also showed a decrease in the particle size and the formation of a core–shell structure in UV-exposed AgNPs. X-ray photoelectron spectroscopy analysis suggested that this shell consisted of oxidized silver. The SPR in UV-exposed AgNP suspensions could be regenerated by addition of a strong reducing agent (NaBH4), supporting the idea that oxidized silver is present after photolysis. Evidence for UV-enhanced dissolution and the production of silver ions was obtained with the Donnan membrane technique. This study reveals that the physico-chemical properties of aqueous AgNP suspensions will change significantly upon exposure to UV light, with implications for environmental health and safety risk assessments.

• SP 1200-8: Preparation of silver nanoparticle loaded cotton threads to facilitate measurement development for textile applications

  NIST Technical Series Publication

  FOREWORD This NIST Special Publication (SP) is one in a series of NIST SPs that address research needs articulated in the National Nanotechnology Initiative (NNI) Environmental, Health, and Safety Research Strategy published in 2011 [1]. This Strategy identified a Nanomaterial Measurement Infrastructure (NMI) as essential for science-based risk assessment and risk management of nanotechnology-enabled products as pertaining to human health, exposure, and the environment. NIST was identified as the lead federal agency in the NMI core research area of the Strategy. This research area includes development of measurement tools for the detection and characterization of engineered nanomaterials in nanotechnology-enabled products. Textiles containing silver nanoparticles, the focus of this SP, are consumer products of great interest for nanotechnology environmental, health, and safety (nano-EHS) research and for product regulation. The protocol presented in this SP describes a strategy for developing a validation known to contain silver nanoparticles loaded onto the surface of a cotton thread. These validation threads can be used to test the capabilities of various analytical methods to characterize silver nanoparticles in textiles.

• Gold nanorod separation and characterization by asymmetric-flow field flow fractionation with UV–Vis detection

  Analytical Bioanalytical Chemistry

  The application of asymmetric-flow field flow fractionation (A4F) for low aspect ratio gold nanorod (GNR) fractionation and characterization was comprehensively investigated. We report on two novel aspects of this application. The first addresses the analytical challenge involved in the fractionation of positively charged nanoparticles by A4F, due to the interaction that exists between the negatively charged native membrane and the analyte. We show that the mobile phase composition is a critical parameter for controlling fractionation and mitigating the membrane-analyte interaction. A mixture of ammonium nitrate and cetyl trimethyl ammonium bromide at different molar ratios enables separation of GNRs with high recovery. The second aspect is the demonstration of shape-based separation of GNRs in A4F normal mode elution (i.e., Brownian mode). We show that the elution of GNRs is due both to aspect ratio and a steric-entropic contribution for GNRs with the same diameter. This latter effect can be explained by their orientation vector inside the A4F channel. Our experimental results demonstrate the relevance of the theory described by Beckett and Giddings for non-spherical fractionation (Beckett and Giddings, J Colloid and Interface Sci 186(1):53–59, 1997). However, it is shown that this theory has its limit in the case of complex GNR mixtures, and that shape (i.e., aspect ratio) is the principal material parameter controlling elution of GNRs in A4F; the apparent translational diffusion coefficient of GNRs increases with aspect ratio. Finally, the performance of the methodology developed in this work is evaluated by the fractionation and characterization of individual components from a mixture of GNR aspect ratios.

• In-situ UV/Vis, SAXS and TEM study of single phase Au Nanoparticle Growth

  Chemistry of Materials

  Given the diverse scientific and technological applications of gold nanoparticles (Au NPs), understanding the impact of macromolecular additives on the distribution of size, shape, and composition is crucial to ensure reproducibility and lower production cost. In situ measurement of the evolution of these distributions challenges current techniques; however, it is critical for in-line manufacturing controls. Using mild Au(I) reduction by tert-butylamine-borane in toluene, the utility and limitations of SAXS, UV/Vis spectroscopy, and TEM are considered by comparing the mean nanoparticle size, size distribution, and relative number density. Individually, these techniques are insufficient to follow these parameters through the initial process of nucleation and growth; either providing insufficient information on the number of particles (UV/Vis), introducing artifacts (TEM), or not providing a unique solution for the shape of the distribution (SAXS). However, when used in conjunction, especially SAXS calibrated with TEM of reaction aliquots, the time evolution of these parameters can be quantified. For the single-phase Au NP synthesis with Au(I) and mild reductants, four distinct experimental regions are revealed that entail two growth mechanisms, which complement previous discussions of single-mode growth kinetics. The kinetics of Au(I):borane reactions are dependent on the Au precursor/reductant ratio and the thiol capping agent length, when reductant concentration is low. The final reaction products exhibit an LSW size distribution.

• Challenges for physical characterization of silver nanoparticles under pristine and environmentally relevant conditions

  Journal of Environmental Monitoring

  The reported size distribution of silver nanoparticles (AgNPs) is strongly affected by the underlying measurement method, agglomeration state, and dispersion conditions. A selection of AgNP materials with vendor-reported diameters ranging from 1 nm to 100 nm, various size distributions, and biocompatible capping agents including citrate, starch and polyvinylpyrrolidone were studied. AgNPs were diluted with either deionized water, moderately hard reconstituted water, or moderately hard reconstituted water containing natural organic matter. Rigorous physico-chemical characterization by consensus methods and protocols where available enables an understanding of how the underlying measurement method impacts the reported size measurements, which in turn provides a more complete understanding of the state (size, size distribution, agglomeration, etc.) of the AgNPs with respect to the dispersion conditions. An approach to developing routine screening is also presented.

• Competitive Adsorption of Thiolated Polyethylene Glycol and Mercaptopropionic Acid on Gold Nanoparticles Measured by Physical Characterization Methods

  Langmuir

  Competitive adsorption kinetics between thiolated polyethylene glycol (SH-PEG) and mercaptopropionic acid (MPA) on gold nanoparticles (Au-NPs) were studied using a prototype physical characterization approach that combines dynamic light scattering (DLS) and electrospray differential mobility analysis (ES-DMA). The change in hydrodynamic particle size (intensity average) due to the formation of SH-PEG coatings on Au-NPs was measured by DLS in both two-component (Au-NP þ MPA or Au-NP @ SH-PEG) and three-component (Au-NP @ MPA @ SH-PEG) systems. ES-DMA was employed to quantify the surface coverage of SH-PEG and establish a correlation between surface coverage and the change in particle size measured by DLS.Achange in the equilibrium binding constant for SH-PEG on Au-NPs at various concentrations of SH-PEG and MPA showed that the presence of MPA reduced the binding affinity of SH-PEG to the Au-NP surface. Kinetic studies showed that SH-PEG was desorbed from the Au-NP surface following a second-order desorption model after subsequently introducing MPA. The desorption rate constant of SH-PEG from the Au-NP surface by MPA displacement was strongly affected by the concentration of MPA and the excess SH-PEG in solution.

• Comparison of Electrical Properties of Viruses Studied by AC Capacitance Scanning Probe Microscopy

  Journal of the American Chemical Society

  Capacitances of five types of viruses, adenovirus type 5 (AV5), herpes simplex virus type 1 (HSV1), simian virus 40 (SV40), vaccinia (MVA), and cowpea mosaic virus (CPMV), were compared by AC capacitance scanning probe microscopy. This technique, using a Pt-coated AFM tip as an electrode to probe capacitance of materials between the tip and a bottom electrode, has been applied to study surface structures of semiconductors and polymers with nanometer spatial resolution; however, biological samples at the nanoscale have not been explored by this technique yet. Because most biological cells are poor conductors, this approach to probe electric properties of cells by capacitance is logical. This scanning probe technique showed that each virus has distinguishable and characteristic capacitance. A series of control experiments were carried out using mutant viruses to validate the origin of the characteristic capacitance responses for different viruses. A mutation on the capsid in HSV1 with green fluorescence proteins increased capacitance from 9 × 10-6 to 1 × 10-5 F/cm2 at the frequency of 104 Hz. Herpes simplex virus type 2 (HSV2) decreased capacitance when its envelope and glycoproteins were chemically extracted. These control experiments indicate that dielectric properties of capsid proteins and envelope glycoproteins significantly influence overall dielectric constants of viruses. Because those capsid proteins and glycoproteins are characteristic of the virus strain, this technique could be applied to detect and identify viruses at the single viron level using their distinct capacitance spectra as fingerprints without labeling.

• Persistence of singly dispersed silver nanoparticles in natural freshwaters, synthetic seawater, and simulated estuarine waters.

  Science of the Total Environment

  This investigation focuses on predicting the persistence of citrate-capped 20nm AgNPs by measuring their colloidal stability in natural freshwaters and synthetic aquatic media. Ultraviolet-visible absorbance spectroscopy, dynamic light scattering, and atomic force microscopy were used to evaluate the colloidal stability of AgNPs in locally-obtained pond water, moderately hard reconstituted water alone or with natural organic matter (NOM), synthetic seawater, and also the individual chemicals most prevalent in seawater. Singly dispersed AgNPs in seawater and waters with greater than 20mmolL(-1) sodium chloride were unstable, with the optical absorbance approaching zero within the first ten hours of mixing. Agglomeration rates as a function of water chemistry and NOM are tested as a hypothesis to explain the rates of disappearance of singly dispersed AgNPs. Other samples, mostly those with lower salinity or NOM, maintained varying degrees of colloidal stability during time studies up to 48h. This indicates likelihood that some AgNPs will be stable long enough in freshwater to successfully enter estuarine or marine systems. These results should enable a more efficient design of nanoEHS risk assessment experiments by predicting the aquatic or soil compartments at greatest potential risk for accumulation of and exposure to citrate capped 20nm AgNPs.

• Newkome-Type Dendron-Stabilized Gold Nanoparticles:Synthesis, Reactivity, and Stability

  Chemistry of Materials

  We report the synthesis and evaluation of four Newkome-type dendrons, G1-COOH, G2-COOH, SH-G1- COOH, and TA-G1-COOH, and their respective gold-dendron conjugates, where GX represents the generation number. G1- and G2-COOH are two-directional symmetric dendrons that have cystamine cores containing a disulfide group. SH-G1- COOH was prepared by treatment of G1-COOH with dithioerythritol to yield a free thiol group to replace the disulfide linkage. TA-G1-COOH has a thioctic acid moiety, which is a five membered ring containing a disulfide group that cleaves to produce two anchoring thiols to bond with the gold surface. All dendrons have peripheral carboxylate groups to afford hydrophilicity and functionality. Gold nanoparticle conjugates were prepared by reaction of each dendron solution with a suspension of gold colloid (nominally 10 nm diameter) and purified by stirred cell ultrafiltration. Chemical structures were confirmed by 1H and 13C nuclear magnetic resonance spectroscopy and matrix assisted laser desorption/ionization time-of-flight mass spectrometry. Particle size and surface plasmon resonance of the conjugates were characterized by dynamic light scattering (DLS) and UV-vis spectroscopy, respectively. X-ray photoelectron spectroscopy (XPS) was utilized to confirm covalent bonding between the thiols on the dendron and the gold surface. XPS also revealed changes in the S/Au intensity ratio as a function of the dendron chemical structure, suggesting steric effects play a role in the reaction and/or conformation of dendrons on the gold surface. The colloidal and chemical stability of the conjugates as a function of temperature, pH, and suspending medium, and with respect to chemical resistance toward KCN, was investigated using DLS and UV-vis absorption.

• Antibody-Mediated Self-Limiting Self-Assembly for Quantitative Analysis of Nanoparticle Surfaces by Atomic Force Microscopy

  Microscopy and Microanalysis

  Quantification of very low density molecular coatings on large (60 nm) gold nanoparticles (AuNPs) is demonstrated via the use of antibody-mediated self-limiting self-assembly of small and large AuNPs into raspberry-like structures subsequently imaged by atomic force microscopy (AFM). AFM imaging is proposed as an automated, lower-cost, higher-throughput alternative to immunostaining and imaging by transmission electron microscopy. Synthesis of large AuNPs, containing one of three ligand molecules in one of three stoichiometries (1, 2, or 10 ligands per AuNP), and small probe AuNPs with one of three antibody molecules in a one antibody per AuNP ratio, enabled a range of predicted self-limiting self-assembled structures. A model predicting the probability of observing a given small to large AuNP ratio based on a topography measurement such as AFM is described, in which random orientational deposition is assumed and which accounts for the stochastic synthesis method of the library AuNPs with varied ligand ratios. Experimental data were found to agree very well with the predictive models when using an established AFM sample preparation method that avoids drying-induced aggregation.

• Humic Acid-Induced Silver Nanoparticle Formation Under Environmentally Relevant Conditions

  Environmental Science & Technology

  The formation of silver nanoparticles (AgNPs) via reduction of silver ions (Ag+) in the presence of humic acids (HAs) under various environmentally relevant conditions is described. HAs tested originated from the Suwannee River (SUW), and included samples of three sedimentary HAs (SHAs), and five soils obtained across the state of Florida. The time required to form AgNPs varied depending upon the type and concentration of HA, as well as temperature. SUW and all three SHAs reduced Ag+ at 22 °C. However, none of the soil HAs formed absorbance-detectable AgNPs at room temperature when allowed to react for a period of 25 days, at which time experiments were halted. The appearance of the characteristic surface plasmon resonance (SPR) of AgNPs was observed by ultraviolet−visible spectroscopy in as few as 2−4 days at 22 °C for SHAs and SUW. An elevated temperature of 90 °C resulted in the accelerated appearance of the SPR within 90 min for SUW and all SHAs. The formation of AgNPs at 90 °C was usually complete within 3 h. Transmission electron microscopy and atomic force microscopy images showed that the AgNPs formed were typically spherical and had a broad size distribution. Dynamic light scattering also revealed polydisperse particle size distributions. HAs appeared to colloidally stabilize AgNPs based on lack of any significant change in the spectral characteristics over a period of two months. The results suggest the potential for direct formation of AgNPs under environmental conditions from Ag+ sources, implying that not all AgNPs observed in natural waters today may be of anthropogenic origin.

• A framework for identifying performance targets for sustainable nanomaterials

  Sustainable Materials and Technologies

  An issue in the application of nano-enabled products is howcanwe evaluate sustainable solutions to current system problems based on performance criteria? This work describes the application of an Input–Process–Output (IPO) model as a framework for a life-cycle analysis approach to identify performance metrics and criteria for evaluating the application of nanomaterials to improve the sustainability of a system. A case study is presented describing a scenario whereby a nano-enabled biocidal paint is considered for a remediation effort to reduce growth of dark molds and bacteria on refrigerated warehouses. The framework is applied to support identification of the energy-consuming steps (such as increased refrigeration energy burden, cleaning and repainting), selection of performance metrics for evaluating consumption, and determination of thresholds to measure sustainability outcomes.

• Colloidal stability of silver nanoparticles in biologically relevant conditions

  Journal of Nanoparticle Research

  Understanding the colloidal stability of nanoparticles (NPs) plays a key role in phenomenological interpretation of toxicological experiments, particularly if single NPs or their aggregates or agglomerates determine the dominant experimental result. This report examines a variety of instrumental techniques for surveying the colloidal stability of aqueous suspensions of silver nanoparticles (AgNPs), including atomic force microscopy, dynamic light scattering, and colorimetry. It was found that colorimetry can adequately determine the concentration of single AgNPs that remained in solution if morphological information about agglomerates is not required. The colloidal stability of AgNPs with various surface capping agents and in various solvents ranging from cell culture media to different electrolytes of several concentrations, and in different pH conditions was determined. It was found that biocompatible bulky capping agents, such as bovine serum albumin or starch, that provided steric colloidal stabilization, as opposed to purely electrostatic stabilization such as with citrate AgNPs, provided better retention of single AgNPs in solution over a variety of conditions for up to 64 h of observation.

• Dispersion stabilization of silver nanoparticles in synthetic lung fluid studied under in situ conditions

  Nanotoxicology

  The dispersion stabilization of silver nanoparticles (AgNPs) in synthetic lung fluid was studied to interrogate the effects on colloidal stability due to the principal constituents of the fluid. The colloidal stability of 20 nm citrate-AgNPs dispersed in the presence of each constituent of the synthetic lung fluid (individually, the complete fluid, and without additives) was observed during titration of increasing sodium chloride concentration. A variety of complementary in situ measurement techniques were utilized, including dynamic light scattering, ultraviolet-visible absorption spectroscopy, atomic force microscopy, and small-angle X-ray scattering, which provided a collective set of information that enabled far better understanding of the dispersion behavior in the fluid than any one technique alone. It was observed that AgNPs continued to adsorb bovine serum albumin (BSA) protein from the synthetic lung fluid solution as the sodium chloride concentration increased, until a maximum BSA coating was achieved prior to reaching the physiological sodium chloride concentration of 154 mmol L−1. BSA was determined to be the constituent of the synthetic lung fluid that is required to provide colloidal stability at high salt loadings, though the phospholipid constituent exerts a subtle effect. Additionally, as AgNPs are a distinctly different class of nanoparticles apart from the carbon nanotubes and titanium dioxide nanoparticles initially reported to be dispersible using this fluid, this work also demonstrates the broad applicability of synthetic lung fluid in providing stable dispersions for engineered nanoparticles for use in biological assays.

• Adsorption and Conformation of Serum Albumin Protein on Gold Nanoparticles Investigated Using Dimensional Measurements and in Situ Spectroscopic Methods

  Langmuir

  The adsorption and conformation of bovine serum albumin (BSA) on gold nanoparticles (AuNPs) were interrogated both qualitatively and quantitatively via complementary physicochemical characterization methods. Dynamic light scattering (DLS), asymmetric-flow field flow fractionation (AFFF), fluorescence spectrometry, and attenuated total reflectance- Fourier transform infrared (ATR-FTIR) spectroscopy were combined to characterize BSA-AuNP conjugates under fluid conditions, while conjugates in the aerosol state were characterized by electrospray-differential mobility analysis (ES-DMA). The presence of unbound BSA molecules interferes with DLS analysis of the conjugates, particularly as the AuNP size decreases (i.e., below 30 nm in diameter). Under conditions where the γ value is high, where γ is defined as the ratio of scattering intensity by AuNPs to the scattering intensity by unbound BSA, DLS size results are consistent with results obtained after fractionation by AFFF. Additionally, the AuNP hydrodynamic size exhibits a greater proportional increase due to BSA conjugation at pH values below 2.5 compared with less acidic pH values (3.4-7.3), corresponding with the reversibly denatured (E or F form) conformation of BSA below pH 2.5. Over the pH range from 3.4 to 7.3, the hydrodynamic size of the conjugate is nearly constant, suggesting conformational stability over this range. Because of the difference in the measurement environment, a larger increase of AuNP size is observed following BSA conjugation when measured in the wet state (i.e., by DLS and AFFF) compared to the dry state (by ES-DMA).

• Disinfection action of electrostatic versus steric-stabilized silver nanoparticles on E. coli under different water chemistries

  Colloids and Surfaces B: Biointerfaces

  The capping layer stabilizing silver nanoparticles (AgNPs) affects its aggregation, dissolution, and net disinfection action, especially under conditions of varying water composition, such as, pH, ionic strength and organic matter content. Herein, we correlate the silver ion (Ag+) release and reactive oxygen species (ROS) generation rates for AgNPs of varying functionalization to their net disinfection coefficient on Escherichia coli, under conditions of differing water chemistries. For electrostatically stabilized citrate-capped AgNPs, the rate of ROS generation, as measured using a fluorescent dye, is found to dominate over that of Ag+ release, especially for smaller sized AgNP suspensions (∼10 nm) at low pH (∼6.2). For these AgNPs, the ROS disinfection mechanism is confirmed to dominate net disinfection action, as measured by the live/dead assay, especially at low levels of organic matter. Steric stabilization of AgNPs by protein or starch-capped layers enables disinfection through reducing AgNP aggregation and promoting silver dissolution over ROS generation. We suggest the involvement of protons and dissolved oxygen in causing the independent formation of Ag+ and ROS, regardless of the AgNP capping layer. While protein-capping layers effectively stabilize AgNPs, the generated ROS is likely dissipated by interference with the bulky capping layer, whereas the interference is lower with citrate-capping layers. Steric stabilization of AgNPs enables disinfection within a wide range of water chemistries, whereas effective disinfection can occur under electrostatic stabilization, only at low NaCl (<1 mmol/L) and organic matter (<5 mg/L) levels.

• Tiered guidance for risk-informed environmental health and safety testing of nanotechnologies

  Journal of Nanoparticle Research

• Unexpected Changes in Functionality and Surface Coverage for Au Nanoparticle PEI Conjugates: Implications for Stability and Efficacy in Biological Systems

  Langmuir

• UV-induced photochemical transformations of citrate capped silver nanoparticle suspensions

  Journal of Nanoparticle Research

  Due to the increasing use of silver nanoparticles (AgNPs) in consumer products, it is essential to understand how variables, such as light exposure, may change the physical and chemical characteristics of AgNP suspensions. To this end, the effect of 300 nm ultraviolet (UV) light on (20, 40, 60 and 80) nm citrate-capped AgNP suspensions has been investigated. As a consequence of irradiation, the initial yellow hue of the AgNP suspensions is transformed towards a near colorless solution due to the loss of the surface plasmon resonance (SPR) absorbance. The decrease in SPR absorbance followed a first-order decay process for all particle sizes with a rate constant that increased linearly with the AgNP specific surface area and non-linearly with light intensity. The rate of loss of the SPR absorbance decreased with increasing citrate concentration, suggesting a surface-mediated transformation. Absorbance, atomic force microscopy, and dynamic light scattering results all indicated that AgNP photolysis was accompanied by a diameter decrease and occasional aggregation. Furthermore, in situ transmission electron microscopy imaging using a specialized liquid cell also showed a decrease in the particle size and the formation of a core–shell structure in UV-exposed AgNPs. X-ray photoelectron spectroscopy analysis suggested that this shell consisted of oxidized silver. The SPR in UV-exposed AgNP suspensions could be regenerated by addition of a strong reducing agent (NaBH4), supporting the idea that oxidized silver is present after photolysis. Evidence for UV-enhanced dissolution and the production of silver ions was obtained with the Donnan membrane technique. This study reveals that the physico-chemical properties of aqueous AgNP suspensions will change significantly upon exposure to UV light, with implications for environmental health and safety risk assessments.

• SP 1200-8: Preparation of silver nanoparticle loaded cotton threads to facilitate measurement development for textile applications

  NIST Technical Series Publication

  FOREWORD This NIST Special Publication (SP) is one in a series of NIST SPs that address research needs articulated in the National Nanotechnology Initiative (NNI) Environmental, Health, and Safety Research Strategy published in 2011 [1]. This Strategy identified a Nanomaterial Measurement Infrastructure (NMI) as essential for science-based risk assessment and risk management of nanotechnology-enabled products as pertaining to human health, exposure, and the environment. NIST was identified as the lead federal agency in the NMI core research area of the Strategy. This research area includes development of measurement tools for the detection and characterization of engineered nanomaterials in nanotechnology-enabled products. Textiles containing silver nanoparticles, the focus of this SP, are consumer products of great interest for nanotechnology environmental, health, and safety (nano-EHS) research and for product regulation. The protocol presented in this SP describes a strategy for developing a validation known to contain silver nanoparticles loaded onto the surface of a cotton thread. These validation threads can be used to test the capabilities of various analytical methods to characterize silver nanoparticles in textiles.

• Gold nanorod separation and characterization by asymmetric-flow field flow fractionation with UV–Vis detection

  Analytical Bioanalytical Chemistry

  The application of asymmetric-flow field flow fractionation (A4F) for low aspect ratio gold nanorod (GNR) fractionation and characterization was comprehensively investigated. We report on two novel aspects of this application. The first addresses the analytical challenge involved in the fractionation of positively charged nanoparticles by A4F, due to the interaction that exists between the negatively charged native membrane and the analyte. We show that the mobile phase composition is a critical parameter for controlling fractionation and mitigating the membrane-analyte interaction. A mixture of ammonium nitrate and cetyl trimethyl ammonium bromide at different molar ratios enables separation of GNRs with high recovery. The second aspect is the demonstration of shape-based separation of GNRs in A4F normal mode elution (i.e., Brownian mode). We show that the elution of GNRs is due both to aspect ratio and a steric-entropic contribution for GNRs with the same diameter. This latter effect can be explained by their orientation vector inside the A4F channel. Our experimental results demonstrate the relevance of the theory described by Beckett and Giddings for non-spherical fractionation (Beckett and Giddings, J Colloid and Interface Sci 186(1):53–59, 1997). However, it is shown that this theory has its limit in the case of complex GNR mixtures, and that shape (i.e., aspect ratio) is the principal material parameter controlling elution of GNRs in A4F; the apparent translational diffusion coefficient of GNRs increases with aspect ratio. Finally, the performance of the methodology developed in this work is evaluated by the fractionation and characterization of individual components from a mixture of GNR aspect ratios.

• In-situ UV/Vis, SAXS and TEM study of single phase Au Nanoparticle Growth

  Chemistry of Materials

  Given the diverse scientific and technological applications of gold nanoparticles (Au NPs), understanding the impact of macromolecular additives on the distribution of size, shape, and composition is crucial to ensure reproducibility and lower production cost. In situ measurement of the evolution of these distributions challenges current techniques; however, it is critical for in-line manufacturing controls. Using mild Au(I) reduction by tert-butylamine-borane in toluene, the utility and limitations of SAXS, UV/Vis spectroscopy, and TEM are considered by comparing the mean nanoparticle size, size distribution, and relative number density. Individually, these techniques are insufficient to follow these parameters through the initial process of nucleation and growth; either providing insufficient information on the number of particles (UV/Vis), introducing artifacts (TEM), or not providing a unique solution for the shape of the distribution (SAXS). However, when used in conjunction, especially SAXS calibrated with TEM of reaction aliquots, the time evolution of these parameters can be quantified. For the single-phase Au NP synthesis with Au(I) and mild reductants, four distinct experimental regions are revealed that entail two growth mechanisms, which complement previous discussions of single-mode growth kinetics. The kinetics of Au(I):borane reactions are dependent on the Au precursor/reductant ratio and the thiol capping agent length, when reductant concentration is low. The final reaction products exhibit an LSW size distribution.

• Challenges for physical characterization of silver nanoparticles under pristine and environmentally relevant conditions

  Journal of Environmental Monitoring

  The reported size distribution of silver nanoparticles (AgNPs) is strongly affected by the underlying measurement method, agglomeration state, and dispersion conditions. A selection of AgNP materials with vendor-reported diameters ranging from 1 nm to 100 nm, various size distributions, and biocompatible capping agents including citrate, starch and polyvinylpyrrolidone were studied. AgNPs were diluted with either deionized water, moderately hard reconstituted water, or moderately hard reconstituted water containing natural organic matter. Rigorous physico-chemical characterization by consensus methods and protocols where available enables an understanding of how the underlying measurement method impacts the reported size measurements, which in turn provides a more complete understanding of the state (size, size distribution, agglomeration, etc.) of the AgNPs with respect to the dispersion conditions. An approach to developing routine screening is also presented.

• Competitive Adsorption of Thiolated Polyethylene Glycol and Mercaptopropionic Acid on Gold Nanoparticles Measured by Physical Characterization Methods

  Langmuir

  Competitive adsorption kinetics between thiolated polyethylene glycol (SH-PEG) and mercaptopropionic acid (MPA) on gold nanoparticles (Au-NPs) were studied using a prototype physical characterization approach that combines dynamic light scattering (DLS) and electrospray differential mobility analysis (ES-DMA). The change in hydrodynamic particle size (intensity average) due to the formation of SH-PEG coatings on Au-NPs was measured by DLS in both two-component (Au-NP þ MPA or Au-NP @ SH-PEG) and three-component (Au-NP @ MPA @ SH-PEG) systems. ES-DMA was employed to quantify the surface coverage of SH-PEG and establish a correlation between surface coverage and the change in particle size measured by DLS.Achange in the equilibrium binding constant for SH-PEG on Au-NPs at various concentrations of SH-PEG and MPA showed that the presence of MPA reduced the binding affinity of SH-PEG to the Au-NP surface. Kinetic studies showed that SH-PEG was desorbed from the Au-NP surface following a second-order desorption model after subsequently introducing MPA. The desorption rate constant of SH-PEG from the Au-NP surface by MPA displacement was strongly affected by the concentration of MPA and the excess SH-PEG in solution.

• Comparison of Electrical Properties of Viruses Studied by AC Capacitance Scanning Probe Microscopy

  Journal of the American Chemical Society

  Capacitances of five types of viruses, adenovirus type 5 (AV5), herpes simplex virus type 1 (HSV1), simian virus 40 (SV40), vaccinia (MVA), and cowpea mosaic virus (CPMV), were compared by AC capacitance scanning probe microscopy. This technique, using a Pt-coated AFM tip as an electrode to probe capacitance of materials between the tip and a bottom electrode, has been applied to study surface structures of semiconductors and polymers with nanometer spatial resolution; however, biological samples at the nanoscale have not been explored by this technique yet. Because most biological cells are poor conductors, this approach to probe electric properties of cells by capacitance is logical. This scanning probe technique showed that each virus has distinguishable and characteristic capacitance. A series of control experiments were carried out using mutant viruses to validate the origin of the characteristic capacitance responses for different viruses. A mutation on the capsid in HSV1 with green fluorescence proteins increased capacitance from 9 × 10-6 to 1 × 10-5 F/cm2 at the frequency of 104 Hz. Herpes simplex virus type 2 (HSV2) decreased capacitance when its envelope and glycoproteins were chemically extracted. These control experiments indicate that dielectric properties of capsid proteins and envelope glycoproteins significantly influence overall dielectric constants of viruses. Because those capsid proteins and glycoproteins are characteristic of the virus strain, this technique could be applied to detect and identify viruses at the single viron level using their distinct capacitance spectra as fingerprints without labeling.

• Persistence of singly dispersed silver nanoparticles in natural freshwaters, synthetic seawater, and simulated estuarine waters.

  Science of the Total Environment

  This investigation focuses on predicting the persistence of citrate-capped 20nm AgNPs by measuring their colloidal stability in natural freshwaters and synthetic aquatic media. Ultraviolet-visible absorbance spectroscopy, dynamic light scattering, and atomic force microscopy were used to evaluate the colloidal stability of AgNPs in locally-obtained pond water, moderately hard reconstituted water alone or with natural organic matter (NOM), synthetic seawater, and also the individual chemicals most prevalent in seawater. Singly dispersed AgNPs in seawater and waters with greater than 20mmolL(-1) sodium chloride were unstable, with the optical absorbance approaching zero within the first ten hours of mixing. Agglomeration rates as a function of water chemistry and NOM are tested as a hypothesis to explain the rates of disappearance of singly dispersed AgNPs. Other samples, mostly those with lower salinity or NOM, maintained varying degrees of colloidal stability during time studies up to 48h. This indicates likelihood that some AgNPs will be stable long enough in freshwater to successfully enter estuarine or marine systems. These results should enable a more efficient design of nanoEHS risk assessment experiments by predicting the aquatic or soil compartments at greatest potential risk for accumulation of and exposure to citrate capped 20nm AgNPs.

• Newkome-Type Dendron-Stabilized Gold Nanoparticles:Synthesis, Reactivity, and Stability

  Chemistry of Materials

  We report the synthesis and evaluation of four Newkome-type dendrons, G1-COOH, G2-COOH, SH-G1- COOH, and TA-G1-COOH, and their respective gold-dendron conjugates, where GX represents the generation number. G1- and G2-COOH are two-directional symmetric dendrons that have cystamine cores containing a disulfide group. SH-G1- COOH was prepared by treatment of G1-COOH with dithioerythritol to yield a free thiol group to replace the disulfide linkage. TA-G1-COOH has a thioctic acid moiety, which is a five membered ring containing a disulfide group that cleaves to produce two anchoring thiols to bond with the gold surface. All dendrons have peripheral carboxylate groups to afford hydrophilicity and functionality. Gold nanoparticle conjugates were prepared by reaction of each dendron solution with a suspension of gold colloid (nominally 10 nm diameter) and purified by stirred cell ultrafiltration. Chemical structures were confirmed by 1H and 13C nuclear magnetic resonance spectroscopy and matrix assisted laser desorption/ionization time-of-flight mass spectrometry. Particle size and surface plasmon resonance of the conjugates were characterized by dynamic light scattering (DLS) and UV-vis spectroscopy, respectively. X-ray photoelectron spectroscopy (XPS) was utilized to confirm covalent bonding between the thiols on the dendron and the gold surface. XPS also revealed changes in the S/Au intensity ratio as a function of the dendron chemical structure, suggesting steric effects play a role in the reaction and/or conformation of dendrons on the gold surface. The colloidal and chemical stability of the conjugates as a function of temperature, pH, and suspending medium, and with respect to chemical resistance toward KCN, was investigated using DLS and UV-vis absorption.

• Antibody-Mediated Self-Limiting Self-Assembly for Quantitative Analysis of Nanoparticle Surfaces by Atomic Force Microscopy

  Microscopy and Microanalysis

  Quantification of very low density molecular coatings on large (60 nm) gold nanoparticles (AuNPs) is demonstrated via the use of antibody-mediated self-limiting self-assembly of small and large AuNPs into raspberry-like structures subsequently imaged by atomic force microscopy (AFM). AFM imaging is proposed as an automated, lower-cost, higher-throughput alternative to immunostaining and imaging by transmission electron microscopy. Synthesis of large AuNPs, containing one of three ligand molecules in one of three stoichiometries (1, 2, or 10 ligands per AuNP), and small probe AuNPs with one of three antibody molecules in a one antibody per AuNP ratio, enabled a range of predicted self-limiting self-assembled structures. A model predicting the probability of observing a given small to large AuNP ratio based on a topography measurement such as AFM is described, in which random orientational deposition is assumed and which accounts for the stochastic synthesis method of the library AuNPs with varied ligand ratios. Experimental data were found to agree very well with the predictive models when using an established AFM sample preparation method that avoids drying-induced aggregation.

• Humic Acid-Induced Silver Nanoparticle Formation Under Environmentally Relevant Conditions

  Environmental Science & Technology

  The formation of silver nanoparticles (AgNPs) via reduction of silver ions (Ag+) in the presence of humic acids (HAs) under various environmentally relevant conditions is described. HAs tested originated from the Suwannee River (SUW), and included samples of three sedimentary HAs (SHAs), and five soils obtained across the state of Florida. The time required to form AgNPs varied depending upon the type and concentration of HA, as well as temperature. SUW and all three SHAs reduced Ag+ at 22 °C. However, none of the soil HAs formed absorbance-detectable AgNPs at room temperature when allowed to react for a period of 25 days, at which time experiments were halted. The appearance of the characteristic surface plasmon resonance (SPR) of AgNPs was observed by ultraviolet−visible spectroscopy in as few as 2−4 days at 22 °C for SHAs and SUW. An elevated temperature of 90 °C resulted in the accelerated appearance of the SPR within 90 min for SUW and all SHAs. The formation of AgNPs at 90 °C was usually complete within 3 h. Transmission electron microscopy and atomic force microscopy images showed that the AgNPs formed were typically spherical and had a broad size distribution. Dynamic light scattering also revealed polydisperse particle size distributions. HAs appeared to colloidally stabilize AgNPs based on lack of any significant change in the spectral characteristics over a period of two months. The results suggest the potential for direct formation of AgNPs under environmental conditions from Ag+ sources, implying that not all AgNPs observed in natural waters today may be of anthropogenic origin.

• A framework for identifying performance targets for sustainable nanomaterials

  Sustainable Materials and Technologies

  An issue in the application of nano-enabled products is howcanwe evaluate sustainable solutions to current system problems based on performance criteria? This work describes the application of an Input–Process–Output (IPO) model as a framework for a life-cycle analysis approach to identify performance metrics and criteria for evaluating the application of nanomaterials to improve the sustainability of a system. A case study is presented describing a scenario whereby a nano-enabled biocidal paint is considered for a remediation effort to reduce growth of dark molds and bacteria on refrigerated warehouses. The framework is applied to support identification of the energy-consuming steps (such as increased refrigeration energy burden, cleaning and repainting), selection of performance metrics for evaluating consumption, and determination of thresholds to measure sustainability outcomes.

• Colloidal stability of silver nanoparticles in biologically relevant conditions

  Journal of Nanoparticle Research

  Understanding the colloidal stability of nanoparticles (NPs) plays a key role in phenomenological interpretation of toxicological experiments, particularly if single NPs or their aggregates or agglomerates determine the dominant experimental result. This report examines a variety of instrumental techniques for surveying the colloidal stability of aqueous suspensions of silver nanoparticles (AgNPs), including atomic force microscopy, dynamic light scattering, and colorimetry. It was found that colorimetry can adequately determine the concentration of single AgNPs that remained in solution if morphological information about agglomerates is not required. The colloidal stability of AgNPs with various surface capping agents and in various solvents ranging from cell culture media to different electrolytes of several concentrations, and in different pH conditions was determined. It was found that biocompatible bulky capping agents, such as bovine serum albumin or starch, that provided steric colloidal stabilization, as opposed to purely electrostatic stabilization such as with citrate AgNPs, provided better retention of single AgNPs in solution over a variety of conditions for up to 64 h of observation.

• Highly Stable Positively Charged Dendron-Encapsulated Gold Nanoparticles

  Langmuir

• Dispersion stabilization of silver nanoparticles in synthetic lung fluid studied under in situ conditions

  Nanotoxicology

  The dispersion stabilization of silver nanoparticles (AgNPs) in synthetic lung fluid was studied to interrogate the effects on colloidal stability due to the principal constituents of the fluid. The colloidal stability of 20 nm citrate-AgNPs dispersed in the presence of each constituent of the synthetic lung fluid (individually, the complete fluid, and without additives) was observed during titration of increasing sodium chloride concentration. A variety of complementary in situ measurement techniques were utilized, including dynamic light scattering, ultraviolet-visible absorption spectroscopy, atomic force microscopy, and small-angle X-ray scattering, which provided a collective set of information that enabled far better understanding of the dispersion behavior in the fluid than any one technique alone. It was observed that AgNPs continued to adsorb bovine serum albumin (BSA) protein from the synthetic lung fluid solution as the sodium chloride concentration increased, until a maximum BSA coating was achieved prior to reaching the physiological sodium chloride concentration of 154 mmol L−1. BSA was determined to be the constituent of the synthetic lung fluid that is required to provide colloidal stability at high salt loadings, though the phospholipid constituent exerts a subtle effect. Additionally, as AgNPs are a distinctly different class of nanoparticles apart from the carbon nanotubes and titanium dioxide nanoparticles initially reported to be dispersible using this fluid, this work also demonstrates the broad applicability of synthetic lung fluid in providing stable dispersions for engineered nanoparticles for use in biological assays.

• Adsorption and Conformation of Serum Albumin Protein on Gold Nanoparticles Investigated Using Dimensional Measurements and in Situ Spectroscopic Methods

  Langmuir

  The adsorption and conformation of bovine serum albumin (BSA) on gold nanoparticles (AuNPs) were interrogated both qualitatively and quantitatively via complementary physicochemical characterization methods. Dynamic light scattering (DLS), asymmetric-flow field flow fractionation (AFFF), fluorescence spectrometry, and attenuated total reflectance- Fourier transform infrared (ATR-FTIR) spectroscopy were combined to characterize BSA-AuNP conjugates under fluid conditions, while conjugates in the aerosol state were characterized by electrospray-differential mobility analysis (ES-DMA). The presence of unbound BSA molecules interferes with DLS analysis of the conjugates, particularly as the AuNP size decreases (i.e., below 30 nm in diameter). Under conditions where the γ value is high, where γ is defined as the ratio of scattering intensity by AuNPs to the scattering intensity by unbound BSA, DLS size results are consistent with results obtained after fractionation by AFFF. Additionally, the AuNP hydrodynamic size exhibits a greater proportional increase due to BSA conjugation at pH values below 2.5 compared with less acidic pH values (3.4-7.3), corresponding with the reversibly denatured (E or F form) conformation of BSA below pH 2.5. Over the pH range from 3.4 to 7.3, the hydrodynamic size of the conjugate is nearly constant, suggesting conformational stability over this range. Because of the difference in the measurement environment, a larger increase of AuNP size is observed following BSA conjugation when measured in the wet state (i.e., by DLS and AFFF) compared to the dry state (by ES-DMA).

• Disinfection action of electrostatic versus steric-stabilized silver nanoparticles on E. coli under different water chemistries

  Colloids and Surfaces B: Biointerfaces

  The capping layer stabilizing silver nanoparticles (AgNPs) affects its aggregation, dissolution, and net disinfection action, especially under conditions of varying water composition, such as, pH, ionic strength and organic matter content. Herein, we correlate the silver ion (Ag+) release and reactive oxygen species (ROS) generation rates for AgNPs of varying functionalization to their net disinfection coefficient on Escherichia coli, under conditions of differing water chemistries. For electrostatically stabilized citrate-capped AgNPs, the rate of ROS generation, as measured using a fluorescent dye, is found to dominate over that of Ag+ release, especially for smaller sized AgNP suspensions (∼10 nm) at low pH (∼6.2). For these AgNPs, the ROS disinfection mechanism is confirmed to dominate net disinfection action, as measured by the live/dead assay, especially at low levels of organic matter. Steric stabilization of AgNPs by protein or starch-capped layers enables disinfection through reducing AgNP aggregation and promoting silver dissolution over ROS generation. We suggest the involvement of protons and dissolved oxygen in causing the independent formation of Ag+ and ROS, regardless of the AgNP capping layer. While protein-capping layers effectively stabilize AgNPs, the generated ROS is likely dissipated by interference with the bulky capping layer, whereas the interference is lower with citrate-capping layers. Steric stabilization of AgNPs enables disinfection within a wide range of water chemistries, whereas effective disinfection can occur under electrostatic stabilization, only at low NaCl (<1 mmol/L) and organic matter (<5 mg/L) levels.

• Tiered guidance for risk-informed environmental health and safety testing of nanotechnologies

  Journal of Nanoparticle Research

• Unexpected Changes in Functionality and Surface Coverage for Au Nanoparticle PEI Conjugates: Implications for Stability and Efficacy in Biological Systems

  Langmuir

• UV-induced photochemical transformations of citrate capped silver nanoparticle suspensions

  Journal of Nanoparticle Research

  Due to the increasing use of silver nanoparticles (AgNPs) in consumer products, it is essential to understand how variables, such as light exposure, may change the physical and chemical characteristics of AgNP suspensions. To this end, the effect of 300 nm ultraviolet (UV) light on (20, 40, 60 and 80) nm citrate-capped AgNP suspensions has been investigated. As a consequence of irradiation, the initial yellow hue of the AgNP suspensions is transformed towards a near colorless solution due to the loss of the surface plasmon resonance (SPR) absorbance. The decrease in SPR absorbance followed a first-order decay process for all particle sizes with a rate constant that increased linearly with the AgNP specific surface area and non-linearly with light intensity. The rate of loss of the SPR absorbance decreased with increasing citrate concentration, suggesting a surface-mediated transformation. Absorbance, atomic force microscopy, and dynamic light scattering results all indicated that AgNP photolysis was accompanied by a diameter decrease and occasional aggregation. Furthermore, in situ transmission electron microscopy imaging using a specialized liquid cell also showed a decrease in the particle size and the formation of a core–shell structure in UV-exposed AgNPs. X-ray photoelectron spectroscopy analysis suggested that this shell consisted of oxidized silver. The SPR in UV-exposed AgNP suspensions could be regenerated by addition of a strong reducing agent (NaBH4), supporting the idea that oxidized silver is present after photolysis. Evidence for UV-enhanced dissolution and the production of silver ions was obtained with the Donnan membrane technique. This study reveals that the physico-chemical properties of aqueous AgNP suspensions will change significantly upon exposure to UV light, with implications for environmental health and safety risk assessments.

• SP 1200-8: Preparation of silver nanoparticle loaded cotton threads to facilitate measurement development for textile applications

  NIST Technical Series Publication

  FOREWORD This NIST Special Publication (SP) is one in a series of NIST SPs that address research needs articulated in the National Nanotechnology Initiative (NNI) Environmental, Health, and Safety Research Strategy published in 2011 [1]. This Strategy identified a Nanomaterial Measurement Infrastructure (NMI) as essential for science-based risk assessment and risk management of nanotechnology-enabled products as pertaining to human health, exposure, and the environment. NIST was identified as the lead federal agency in the NMI core research area of the Strategy. This research area includes development of measurement tools for the detection and characterization of engineered nanomaterials in nanotechnology-enabled products. Textiles containing silver nanoparticles, the focus of this SP, are consumer products of great interest for nanotechnology environmental, health, and safety (nano-EHS) research and for product regulation. The protocol presented in this SP describes a strategy for developing a validation known to contain silver nanoparticles loaded onto the surface of a cotton thread. These validation threads can be used to test the capabilities of various analytical methods to characterize silver nanoparticles in textiles.

• Gold nanorod separation and characterization by asymmetric-flow field flow fractionation with UV–Vis detection

  Analytical Bioanalytical Chemistry

  The application of asymmetric-flow field flow fractionation (A4F) for low aspect ratio gold nanorod (GNR) fractionation and characterization was comprehensively investigated. We report on two novel aspects of this application. The first addresses the analytical challenge involved in the fractionation of positively charged nanoparticles by A4F, due to the interaction that exists between the negatively charged native membrane and the analyte. We show that the mobile phase composition is a critical parameter for controlling fractionation and mitigating the membrane-analyte interaction. A mixture of ammonium nitrate and cetyl trimethyl ammonium bromide at different molar ratios enables separation of GNRs with high recovery. The second aspect is the demonstration of shape-based separation of GNRs in A4F normal mode elution (i.e., Brownian mode). We show that the elution of GNRs is due both to aspect ratio and a steric-entropic contribution for GNRs with the same diameter. This latter effect can be explained by their orientation vector inside the A4F channel. Our experimental results demonstrate the relevance of the theory described by Beckett and Giddings for non-spherical fractionation (Beckett and Giddings, J Colloid and Interface Sci 186(1):53–59, 1997). However, it is shown that this theory has its limit in the case of complex GNR mixtures, and that shape (i.e., aspect ratio) is the principal material parameter controlling elution of GNRs in A4F; the apparent translational diffusion coefficient of GNRs increases with aspect ratio. Finally, the performance of the methodology developed in this work is evaluated by the fractionation and characterization of individual components from a mixture of GNR aspect ratios.

• In-situ UV/Vis, SAXS and TEM study of single phase Au Nanoparticle Growth

  Chemistry of Materials

  Given the diverse scientific and technological applications of gold nanoparticles (Au NPs), understanding the impact of macromolecular additives on the distribution of size, shape, and composition is crucial to ensure reproducibility and lower production cost. In situ measurement of the evolution of these distributions challenges current techniques; however, it is critical for in-line manufacturing controls. Using mild Au(I) reduction by tert-butylamine-borane in toluene, the utility and limitations of SAXS, UV/Vis spectroscopy, and TEM are considered by comparing the mean nanoparticle size, size distribution, and relative number density. Individually, these techniques are insufficient to follow these parameters through the initial process of nucleation and growth; either providing insufficient information on the number of particles (UV/Vis), introducing artifacts (TEM), or not providing a unique solution for the shape of the distribution (SAXS). However, when used in conjunction, especially SAXS calibrated with TEM of reaction aliquots, the time evolution of these parameters can be quantified. For the single-phase Au NP synthesis with Au(I) and mild reductants, four distinct experimental regions are revealed that entail two growth mechanisms, which complement previous discussions of single-mode growth kinetics. The kinetics of Au(I):borane reactions are dependent on the Au precursor/reductant ratio and the thiol capping agent length, when reductant concentration is low. The final reaction products exhibit an LSW size distribution.

• Challenges for physical characterization of silver nanoparticles under pristine and environmentally relevant conditions

  Journal of Environmental Monitoring

  The reported size distribution of silver nanoparticles (AgNPs) is strongly affected by the underlying measurement method, agglomeration state, and dispersion conditions. A selection of AgNP materials with vendor-reported diameters ranging from 1 nm to 100 nm, various size distributions, and biocompatible capping agents including citrate, starch and polyvinylpyrrolidone were studied. AgNPs were diluted with either deionized water, moderately hard reconstituted water, or moderately hard reconstituted water containing natural organic matter. Rigorous physico-chemical characterization by consensus methods and protocols where available enables an understanding of how the underlying measurement method impacts the reported size measurements, which in turn provides a more complete understanding of the state (size, size distribution, agglomeration, etc.) of the AgNPs with respect to the dispersion conditions. An approach to developing routine screening is also presented.

• Competitive Adsorption of Thiolated Polyethylene Glycol and Mercaptopropionic Acid on Gold Nanoparticles Measured by Physical Characterization Methods

  Langmuir

  Competitive adsorption kinetics between thiolated polyethylene glycol (SH-PEG) and mercaptopropionic acid (MPA) on gold nanoparticles (Au-NPs) were studied using a prototype physical characterization approach that combines dynamic light scattering (DLS) and electrospray differential mobility analysis (ES-DMA). The change in hydrodynamic particle size (intensity average) due to the formation of SH-PEG coatings on Au-NPs was measured by DLS in both two-component (Au-NP þ MPA or Au-NP @ SH-PEG) and three-component (Au-NP @ MPA @ SH-PEG) systems. ES-DMA was employed to quantify the surface coverage of SH-PEG and establish a correlation between surface coverage and the change in particle size measured by DLS.Achange in the equilibrium binding constant for SH-PEG on Au-NPs at various concentrations of SH-PEG and MPA showed that the presence of MPA reduced the binding affinity of SH-PEG to the Au-NP surface. Kinetic studies showed that SH-PEG was desorbed from the Au-NP surface following a second-order desorption model after subsequently introducing MPA. The desorption rate constant of SH-PEG from the Au-NP surface by MPA displacement was strongly affected by the concentration of MPA and the excess SH-PEG in solution.

• Comparison of Electrical Properties of Viruses Studied by AC Capacitance Scanning Probe Microscopy

  Journal of the American Chemical Society

  Capacitances of five types of viruses, adenovirus type 5 (AV5), herpes simplex virus type 1 (HSV1), simian virus 40 (SV40), vaccinia (MVA), and cowpea mosaic virus (CPMV), were compared by AC capacitance scanning probe microscopy. This technique, using a Pt-coated AFM tip as an electrode to probe capacitance of materials between the tip and a bottom electrode, has been applied to study surface structures of semiconductors and polymers with nanometer spatial resolution; however, biological samples at the nanoscale have not been explored by this technique yet. Because most biological cells are poor conductors, this approach to probe electric properties of cells by capacitance is logical. This scanning probe technique showed that each virus has distinguishable and characteristic capacitance. A series of control experiments were carried out using mutant viruses to validate the origin of the characteristic capacitance responses for different viruses. A mutation on the capsid in HSV1 with green fluorescence proteins increased capacitance from 9 × 10-6 to 1 × 10-5 F/cm2 at the frequency of 104 Hz. Herpes simplex virus type 2 (HSV2) decreased capacitance when its envelope and glycoproteins were chemically extracted. These control experiments indicate that dielectric properties of capsid proteins and envelope glycoproteins significantly influence overall dielectric constants of viruses. Because those capsid proteins and glycoproteins are characteristic of the virus strain, this technique could be applied to detect and identify viruses at the single viron level using their distinct capacitance spectra as fingerprints without labeling.

• Persistence of singly dispersed silver nanoparticles in natural freshwaters, synthetic seawater, and simulated estuarine waters.

  Science of the Total Environment

  This investigation focuses on predicting the persistence of citrate-capped 20nm AgNPs by measuring their colloidal stability in natural freshwaters and synthetic aquatic media. Ultraviolet-visible absorbance spectroscopy, dynamic light scattering, and atomic force microscopy were used to evaluate the colloidal stability of AgNPs in locally-obtained pond water, moderately hard reconstituted water alone or with natural organic matter (NOM), synthetic seawater, and also the individual chemicals most prevalent in seawater. Singly dispersed AgNPs in seawater and waters with greater than 20mmolL(-1) sodium chloride were unstable, with the optical absorbance approaching zero within the first ten hours of mixing. Agglomeration rates as a function of water chemistry and NOM are tested as a hypothesis to explain the rates of disappearance of singly dispersed AgNPs. Other samples, mostly those with lower salinity or NOM, maintained varying degrees of colloidal stability during time studies up to 48h. This indicates likelihood that some AgNPs will be stable long enough in freshwater to successfully enter estuarine or marine systems. These results should enable a more efficient design of nanoEHS risk assessment experiments by predicting the aquatic or soil compartments at greatest potential risk for accumulation of and exposure to citrate capped 20nm AgNPs.

• Newkome-Type Dendron-Stabilized Gold Nanoparticles:Synthesis, Reactivity, and Stability

  Chemistry of Materials

  We report the synthesis and evaluation of four Newkome-type dendrons, G1-COOH, G2-COOH, SH-G1- COOH, and TA-G1-COOH, and their respective gold-dendron conjugates, where GX represents the generation number. G1- and G2-COOH are two-directional symmetric dendrons that have cystamine cores containing a disulfide group. SH-G1- COOH was prepared by treatment of G1-COOH with dithioerythritol to yield a free thiol group to replace the disulfide linkage. TA-G1-COOH has a thioctic acid moiety, which is a five membered ring containing a disulfide group that cleaves to produce two anchoring thiols to bond with the gold surface. All dendrons have peripheral carboxylate groups to afford hydrophilicity and functionality. Gold nanoparticle conjugates were prepared by reaction of each dendron solution with a suspension of gold colloid (nominally 10 nm diameter) and purified by stirred cell ultrafiltration. Chemical structures were confirmed by 1H and 13C nuclear magnetic resonance spectroscopy and matrix assisted laser desorption/ionization time-of-flight mass spectrometry. Particle size and surface plasmon resonance of the conjugates were characterized by dynamic light scattering (DLS) and UV-vis spectroscopy, respectively. X-ray photoelectron spectroscopy (XPS) was utilized to confirm covalent bonding between the thiols on the dendron and the gold surface. XPS also revealed changes in the S/Au intensity ratio as a function of the dendron chemical structure, suggesting steric effects play a role in the reaction and/or conformation of dendrons on the gold surface. The colloidal and chemical stability of the conjugates as a function of temperature, pH, and suspending medium, and with respect to chemical resistance toward KCN, was investigated using DLS and UV-vis absorption.

• Antibody-Mediated Self-Limiting Self-Assembly for Quantitative Analysis of Nanoparticle Surfaces by Atomic Force Microscopy

  Microscopy and Microanalysis

  Quantification of very low density molecular coatings on large (60 nm) gold nanoparticles (AuNPs) is demonstrated via the use of antibody-mediated self-limiting self-assembly of small and large AuNPs into raspberry-like structures subsequently imaged by atomic force microscopy (AFM). AFM imaging is proposed as an automated, lower-cost, higher-throughput alternative to immunostaining and imaging by transmission electron microscopy. Synthesis of large AuNPs, containing one of three ligand molecules in one of three stoichiometries (1, 2, or 10 ligands per AuNP), and small probe AuNPs with one of three antibody molecules in a one antibody per AuNP ratio, enabled a range of predicted self-limiting self-assembled structures. A model predicting the probability of observing a given small to large AuNP ratio based on a topography measurement such as AFM is described, in which random orientational deposition is assumed and which accounts for the stochastic synthesis method of the library AuNPs with varied ligand ratios. Experimental data were found to agree very well with the predictive models when using an established AFM sample preparation method that avoids drying-induced aggregation.

• Humic Acid-Induced Silver Nanoparticle Formation Under Environmentally Relevant Conditions

  Environmental Science & Technology

  The formation of silver nanoparticles (AgNPs) via reduction of silver ions (Ag+) in the presence of humic acids (HAs) under various environmentally relevant conditions is described. HAs tested originated from the Suwannee River (SUW), and included samples of three sedimentary HAs (SHAs), and five soils obtained across the state of Florida. The time required to form AgNPs varied depending upon the type and concentration of HA, as well as temperature. SUW and all three SHAs reduced Ag+ at 22 °C. However, none of the soil HAs formed absorbance-detectable AgNPs at room temperature when allowed to react for a period of 25 days, at which time experiments were halted. The appearance of the characteristic surface plasmon resonance (SPR) of AgNPs was observed by ultraviolet−visible spectroscopy in as few as 2−4 days at 22 °C for SHAs and SUW. An elevated temperature of 90 °C resulted in the accelerated appearance of the SPR within 90 min for SUW and all SHAs. The formation of AgNPs at 90 °C was usually complete within 3 h. Transmission electron microscopy and atomic force microscopy images showed that the AgNPs formed were typically spherical and had a broad size distribution. Dynamic light scattering also revealed polydisperse particle size distributions. HAs appeared to colloidally stabilize AgNPs based on lack of any significant change in the spectral characteristics over a period of two months. The results suggest the potential for direct formation of AgNPs under environmental conditions from Ag+ sources, implying that not all AgNPs observed in natural waters today may be of anthropogenic origin.

• A framework for identifying performance targets for sustainable nanomaterials

  Sustainable Materials and Technologies

  An issue in the application of nano-enabled products is howcanwe evaluate sustainable solutions to current system problems based on performance criteria? This work describes the application of an Input–Process–Output (IPO) model as a framework for a life-cycle analysis approach to identify performance metrics and criteria for evaluating the application of nanomaterials to improve the sustainability of a system. A case study is presented describing a scenario whereby a nano-enabled biocidal paint is considered for a remediation effort to reduce growth of dark molds and bacteria on refrigerated warehouses. The framework is applied to support identification of the energy-consuming steps (such as increased refrigeration energy burden, cleaning and repainting), selection of performance metrics for evaluating consumption, and determination of thresholds to measure sustainability outcomes.

• Colloidal stability of silver nanoparticles in biologically relevant conditions

  Journal of Nanoparticle Research

  Understanding the colloidal stability of nanoparticles (NPs) plays a key role in phenomenological interpretation of toxicological experiments, particularly if single NPs or their aggregates or agglomerates determine the dominant experimental result. This report examines a variety of instrumental techniques for surveying the colloidal stability of aqueous suspensions of silver nanoparticles (AgNPs), including atomic force microscopy, dynamic light scattering, and colorimetry. It was found that colorimetry can adequately determine the concentration of single AgNPs that remained in solution if morphological information about agglomerates is not required. The colloidal stability of AgNPs with various surface capping agents and in various solvents ranging from cell culture media to different electrolytes of several concentrations, and in different pH conditions was determined. It was found that biocompatible bulky capping agents, such as bovine serum albumin or starch, that provided steric colloidal stabilization, as opposed to purely electrostatic stabilization such as with citrate AgNPs, provided better retention of single AgNPs in solution over a variety of conditions for up to 64 h of observation.

• Highly Stable Positively Charged Dendron-Encapsulated Gold Nanoparticles

  Langmuir

• Tumor necrosis factor interaction with gold nanoparticles

  Nanoscale

  We report on a systematic investigation of molecular conjugation of tumor necrosis factor-α (TNF) protein onto gold nanoparticles (AuNPs) and the subsequent binding behavior to its antibody (anti-TNF). We employ a combination of physical and spectroscopic characterization methods, including electrospray-differential mobility analysis, dynamic light scattering, polyacrylamide gel electrophoresis, attenuated total reflectance-Fourier transform infrared spectroscopy, fluorescence assay, and enzyme-linked immunosorbent assay. The native TNF used in this study exists in the active homotrimer configuration prior to conjugation. After binding to AuNPs, the maximum surface density of TNF is (0.09 ± 0.02) nm−2 with a binding constant of 3 × 106 (mol L−1)−1. Dodecyl sulfate ions induce desorption of monomeric TNF from the AuNP surface, indicating a relatively weak intermolecular binding within the AuNP-bound TNF trimers. Anti-TNF binds to both TNF-conjugated and citrate-stabilized AuNPs, showing that non-specific binding is significant. Based on the number of anti-TNF molecules adsorbed, a substantially higher binding affinity was observed for the TNF-conjugated surface. The inclusion of thiolated polyethylene glycol (SH–PEG) on the AuNPs inhibits the binding of anti-TNF, and the amount of inhibition is related to the number ratio of surface bound SH–PEG to TNF and the way in which the ligands are introduced. This study highlights the challenges in quantitatively characterizing complex hybrid nanoscale conjugates, and provides insight on TNF–AuNP formation and activity.