Mobility of Capped Silver Nanoparticles under Environmentally

Publication Date (Web): December 1, 2011. Copyright © 2011 American .... Environmental Science & Technology 2013 47 (2), 757-764 ... Parvaneh Ghaderi...
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Mobility of Capped Silver Nanoparticles under Environmentally Relevant Conditions Beng Joo Reginald Thio,† Milka O. Montes,† Mahmoud A. Mahmoud,‡ Dong-Woog Lee,§ Dongxu Zhou,† and Arturo A. Keller†,* †

Bren School of Environmental Science & Management, University of California at Santa Barbara, California 93106, United States School of Chemistry & Biochemistry, Georgia Institute of Technology, Georgia 30332, United States § Chemical Engineering, University of California at Santa Barbara, California 93106, United States ‡

S Supporting Information *

ABSTRACT: The mobility and deposition of capped silver (Ag) nanoparticles (NPs) on silica surfaces were characterized over a wide range of pH and ionic strength (IS) conditions, including seawater and freshwater. Two common organic capping agents (citrate and PVP) were evaluated. Both the capped Ag NPs and the silica surfaces were negatively charged under these environmentally relevant conditions, resulting in net repulsive electrostatics under most conditions. The steric repulsion introduced by the capping agents significantly reduced aggregation and deposition. In addition, the presence of natural organic matter in solution further decreased the deposition of either Ag NP on silica. Ag NPs were found to be highly mobile under these environmentally relevant conditions, with little or no deposition.

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ions on their surfaces.12 The presence of surface charges on the nanoparticles in the natural water environment affects their aggregation, dissolution, deposition, and attachment behavior. These processes have been studied for several engineered metal and metal oxide NPs, ranging from Au to ZnO, in simple aquatic systems with monovalent electrolytes, divalent electrolytes and natural organic matter (NOM). In particular, recent studies with Ag nanoparticles have addressed their aggregation in synthetic matrices with variations in pH, ionic strength, and NOM.13−15 The objective of this study was to quantify key fate and transport processes, such as aggregation, sedimentation, attachment and dissolution, needed to predict the behavior of capped Ag NPs in environmentally relevant conditions in aqueous systems. This information can then be used to make an assessment of the likely exposure to Ag NPs and their dissolution products. In addition to novel approaches for determining deposition and attachment rates, we study the complex behavior of these NPs in natural waters, and interpret the results using simplified synthetic aqueous matrices to support our interpretations.

INTRODUCTION Metallic silver nanoparticles have attracted considerable industrial and research interest in the fields of catalysis,1 optics,2 textiles,3,4 and electronics5 due to their unique sizedependent optical, electronic and antibacterial properties.6 Silver nanoparticles (Ag NPs) are a known antimicrobial agent6 and their toxicity is primarily due to cell metabolic inactivation resulting from the interaction of Ag+ ions with biomolecules, particularly sulfhydryl groups of metabolic enzymes.7 However, little is known on the transport of such nanomaterials once they are emitted to the environment. Previous studies3,4 have found that Ag NPs are readily released from Ag-impregnated textiles and fabrics during washing cycles to wastewaters. To better predict the fate and transport of Ag nanoparticles in aquatic systems, it is important to understand their interactions (aggregation, deposition and attachment) with important geochemical surfaces such as silica over a broad range of environmentally relevant physicochemical conditions. Natural and synthetic ligands or polymers are widely used as protecting agents to prevent aggregation in the syntheses and preparation of metal nanoparticles.8−10 Two very commonly used capping agents are citrate ions8,9 and poly (vinyl pyrrolidone) (PVP)10,11 due to their high chemical stability, very low toxicity and excellent solubility in polar solvents such as water. Metal NPs suspended in aqueous media acquire a charge through oxidation of the metal surface and via adsorption of © 2011 American Chemical Society

Special Issue: Transformations of Nanoparticles in the Environment Received: Revised: Accepted: Published: 6985

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Ag NP Suspensions. Ag-PVP and Ag-cit NP suspensions were prepared by pipetting 10 μL of stock suspension into different salt solutions (ionic strength, IS = 1 and 100 mM) with two different electrolytes (NaCl and CaCl2) at various pH for a total volume of 10 mL. pH adjustment to target conditions (pH 4, 6, 7, 8, and 10) was done by careful titration using 100 mM NaOH or HCl. The suspension was sonicated for 30 min prior to TLC and QCM-D experiments. Each TLC and QCM-D experiment was repeated using at least three different Ag NP suspensions prepared on different days. The same concentration (1 mg/L Ag NPs) was used in all measurements and experiments for comparison, except as noted. Ag Dissolution Rates. Ag NPs at 10 mg/L in either seawater or borate-buffered deionized water (pH 8) were put on a roller at 60 rpm At 1, 5, and 15 days, 5 mL were pipetted into Amicon centrifugal filter units (10 kD, Millipore, MA) and centrifuged at 8000 rpm for 40 min. One mL of 70% HNO3 was added in the filtrate to dissolve any small particles formed after filtration. [Ag] of filtrates were analyzed by ICP-AES (iCAP 6300, Thermo, MA). Quartz Crystal Microbalance with Dissipation. A QCM-D D300 system (Q-Sense AB, Gothenburg, Sweden) was used to examine the deposition of Ag NPs on silica surfaces. QCM-D experiments were performed by simultaneously monitoring the changes in frequency (Δf) and energy dissipation (ΔD) of a 5 MHz AT-cut quartz sensor crystal with silica coated surface (QSX-303). This technique can measure with very high precision the deposition of particles on the crystal surface, by measuring Δf and ΔD.21 Before use, the crystal, chamber and tubing were cleaned by rinsing with at least 10 mL DI water and 5 mL of ethanol. The crystal was further cleaned in a UV-ozone chamber for 30 min before being mounted onto the chamber. Deposition experiments were performed in batch mode at 25 °C. Background electrolyte (1 mL) at pH and IS of interest was injected into the QCM-D chamber for a minimum of 1 h or until a stable baseline was obtained (drift of average normalized frequency less than 0.2 Hz within 30 min). Subsequently, 0.5 mL Ag NP suspension (prepared 30 min before QCM-D run) was injected and Δf and ΔD were recorded. Shifts in the normalized third overtone frequency and dissipation (Δf 3 and ΔD3) were monitored since this overtone usually has the best signal-to-noise ratio.22 Since the working volume of the D300 chamber is 80 μL,23 0.5 mL of suspension is sufficient to displace the background electrolyte completely. Thin Layer Chromatography. Ten mL of Ag-PVP or Agcit suspension was placed in a 50 mL beaker. Flexible silicaTLC plates (Selecto Inc., Suwannee, GA) were first cut into 3 × 5.5 cm rectangles and placed in the beakers at a 30° angle to the vertical. The plates were removed from the beakers once the solvent front had reached 1 cm from the top of the plate and allowed to dry. The concentrations of the Ag on the silica in both the air exposed (Top, T) and submerged (Bottom, B) regions were analyzed by ICP-AES, and their ratios were expressed as T/B. Three TLC plates were used for each pH and IS condition and the T/B ratio is an average of the three runs. Chemical Analysis. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES, iCAP 6300, Thermo) was used to measure Ag concentrations on the TLC plates. The submerged and the top 2 cm of the Ag NP-exposed silica were carefully scraped off the plastic backing with a safety razor and placed separately into two 15 mL disposable centrifuge tubes.

MATERIALS AND METHODS

Materials. Reagent-grade AgNO3, NaS, NaCl, CaCl2, NaOH, HCl, H2SO4, acetone and ethylene glycol were purchased from Sigma-Aldrich and used without further purification. Poly (vinyl pyrrolidone) (PVP, Mw = 40 000 g/ mol) was used as purchased from Fisher Scientific. PVP-coated Ag NPs (Ag-PVP) were synthesized following Mahmoud and El-Sayed16 (see Supporting Information for more details). Citrate coated Ag nanospheres (Ag-cit, ca. 40 nm) suspended in DI water (1 mg/mL) were purchased from nanoComposix, Inc. (San Diego, CA) and used as received. Deionized (DI) water (Milli-Q, Millipore) with a resistivity of 18.2 MΩ·cm was used for the preparation of the electrolyte solutions. Santa Barbara (CA) seawater was obtained from the Marine Science Institute (UCSB) laboratory water system, and 0.2 μm membrane-filtered. To evaluate the key parameters (IS and dissolved organic content) of natural waters that influence Ag NP mobility, artificial seawater, and freshwater were prepared. Artificial seawater was prepared using the Marine Biological Laboratory method17 while artificial freshwater was prepared by dissolving similar amounts of the main electrolytes (NaCl and CaCl2) and organic carbon content using Suwannee River humic acid (SRHA) (Standard II, International Humic Substances Society). Approximately 50% by mass of SRHA is composed of organic carbon.18 The characterization of the seawater and freshwater samples were done previously19 and the relevant parameters of the natural and artificial waters are presented in SI Table S1. Briefly, seawater is high in ionic strength, mostly due to high NaCl, and low in organic matter, while the freshwater used had low ionic strength and high organic matter. Total organic carbon (TOC) was measured using a Shimadzu TOC-V instrument (Shimadzu Scientific Instruments). The pH was measured using an Oakton pH meter (Ion 510 series, Fisher Scientific). Inductively coupled plasma atomic emission spectroscopy (ICP-AES, Thermo iCAP 6300 ICP) was used to measure the concentration of five metal ions. Chloride ion concentration was determined by the argentometric method, following Standard Methods (19th Ed., Method 4500-Cl-B). Phosphate, sulfate, nitrite, and nitrate ions were measured via colorimetry (HACH portable DR/890, HACH Company, Loveland, CO). HCO3− was determined by titration via a phenolphthalein/total alkalinity test (model WAT-MP-DR, Lamotte Chemical Products, MD). Ag NP Characterization. Primary sizes of Ag NPs were estimated using scanning electron microscopy (SEM) (XL40 Sirion FEG Digital SEM w/EDS, FEI). Hydrodynamic diameter was determined via Dynamic Light Scattering (DLS, Malvern Zetasizer Nano ZS-90). The particles were sonicated in an ultrasound bath (Branson 2510, Danbury, CT) for 30 min prior to DLS measurement. Laser Doppler velocimetry (Malvern Zetasizer Nano ZS-90) was used to characterize the electrophoretic mobility (EPM). Measured EPMs were converted to ζ-potential using the Smoluchowski equation. 200 nm SiO2 particles (Brinker Nanostructures Research Group, University of New Mexico) were used as a surrogate to estimate the ζ-potentials of the silica surfaces of the quartz crystal microbalance with deposition (QCM-D) sensor crystal and thin-layer chromatography (TLC) plate, following Jiang et al.20 The hydrodynamic diameter and sedimentation rates were measured with the Malvern Zetasizer Nano ZS-90. 6986

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slight decrease with increasing pH in these solutions. There was a slight increase in hydrodynamic diameter with increasing IS. The diameter of the Ag-PVP was smaller in the presence of Ca2+ than in Na+. In contrast, the Ag-cit diameters generally decreased significantly at higher pH, and increased slightly with increasing salt concentration at constant pH (Figure 1b). At low IS, the cation type did not make a difference for Ag-Cit, but at high IS the hydrodynamic diameters were larger for Ca2+ than for Na+. Cumberland and Lead found that for their 15 nm Ag-cit NPs, increasing Ca2+ led to complete instability of the suspension, but could be stabilized if NOM was present.13 El Badawy et al. also found similar aggregation behavior for a range of Ag NPs under different pH and background electrolytes.15 The stability of Ag-PVP compared to Ag-cit with respect to aggregation can be explained by the difference in the chemistry of their surface coatings. The PVP used in this study is a large bulky neutral polymer (molecular mass ∼40 000 g/mol) and it prevents the aggregation of the Ag-PVP by steric hindrance. The small difference in observed hydrodynamic diameter compared to dry diameter reflects the bulkiness of PVP, but essentially Ag-PVPs remain as primary particles. On the other hand, Ag-cit rely more on the negative electrostatic charge of the citrate ion for colloidal stability. Increasing pH makes the Ag surfaces more negative and results in increasing repulsion, whereas lower pH protonates the citrate ion and results in decreased repulsion,15 increasing hydrodynamic diameter. Citrate anion has a molecular mass of 189 g/mol, so there is considerably less steric hindrance and some bare contact points may result in small aggregates of Ag-cit NPs. This phenomenon is in general agreement with the extended DLVO model of colloidal interactions.24−26 The hydrodynamic diameter of the two Ag NPs in natural waters (Figure 1c) are noticeably greater, ranging from 220 to 311 nm in seawater and 140 to 160 nm in freshwater, for either capping agent. The PVP cap is a slightly better stabilizer, although in the very high IS of seawater aggregation of several primary particles is likely occurring. In freshwater, aggregation is lower, perhaps involving only a few nanoparticles. The behavior is similar for natural and artificial waters, indicating that the observed aggregation is mostly a function of IS and [NOM], and not other ions or natural constituents. Both types of Ag NPs were negatively charged over the range of IS and pH conditions studied (Figure 2), except at high [Ca2+] and pH 10. The silica surface was negatively charged under all conditions (Figure 2c). All NPs become less negatively charged with an increase in either NaCl or CaCl2 due to the compression of the electrostatic double layer on the NP surface by the electrolyte ions. Figure 2 also shows that both the Ag NPs and silica have more negative ζ-potentials in NaCl than CaCl2 under similar IS conditions. Due to its divalent nature the screening of Ag NP and silica surface charges by Ca2+ is more effective than Na+, and reverses the negative charge at pH 10 and 100 mM CaCl2. 20,27 Ag-PVP NPs are less negatively charged than Ag-cit NPs under similar solution conditions. As described in the previous section, this difference can be attributed to the difference in their surface coatings. PVP is a neutral bulky polymer that shields the Ag metal from electrolyte electrostatic charges, whereas the citrate anion is much smaller and more susceptible than PVP to electrolyte cation surface adsorption and neutralization. There is very little influence of pH over the range studied except at pH 10, regardless of IS and the nature of the cations, indicating

DI water (10.2 mL) with 1.8 mL of aqua regia were then added to each of the tubes. All Elemental standard solution (1000 μg/ mL Ag in 2% HNO3, High-Purity Standards Inc., Charleston, SC) was employed to prepare diluted standard solutions for ICP-AES optimization.

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RESULTS AND DISCUSSION Characterization. SI Figure S1a and b show the dry pristine primary sizes of the Ag NPs used in our study obtained using SEM. These Ag NPs were dispersed in DI water and were not exposed to any electrolytes or NOM to minimize imaging of artifacts arising from the aggregation of nanoparticles. AgPVP have a cube-like structure and an average diameter of about 50 nm, whereas the bulk of Ag-cit were observed to be spherical and have a primary particle diameter of approximately 40 nm. However, their hydrodynamic diameters (based on the intensity data) in the electrolyte solutions at various pHs are 60−120 nm (Figure 1a and b). In the stock suspensions, the

Figure 1. Hydrodynamic diameters over a range of pH and IS for (a) Ag-PVP, (b) Ag-citrate NP suspensions, and in (c) natural and artificial waters.

hydrodynamic diameters are 42.9 nm for Ag-Cit and 53.44 nm for Ag-PVP. Ag-PVP increased up to twice of their average dry diameter,whereas Ag-cit increased from 2 to 3 times their primary size. The Ag-PVP diameters were almost constant under varying pH and IS conditions (Figure 1a), with only a 6987

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ΔD3 under these conditions, indicating little or no deposition after 60 min. Figure 3 shows the summary of Δf 3 for the

Figure 3. Frequency shifts Δf3 under varying pH and IS conditions for (a) Ag-PVP, (b) Ag-cit NPs, and (c) in natural and artificial waters.

deposition of Ag NPs onto silica in the various aqueous media. For both types of Ag NPs there was negligible deposition under most conditions except at low pH (4−5) and higher IS, particularly in the presence of Ca2+, where the electrostatic repulsion is lower due to the neutralization of the charge on the Ag NPs and silica. There was no observed deposition in natural or artificial seawater and freshwater, at this time frame (60 min). The negative ζ-potentials of both types of Ag NPs and silica, as well as the steric repulsive interaction of the PVP, citrate and NOM in natural waters, result in minimal deposition. Even at 100 mM CaCl2 and pH 10, where the ζpotential of the Ag NP was +17 mV and for silica −7.25 mV, the Δf 3 was 0, indicating that the PVP coating provides sufficient steric hindrance for deposition. Figure 4 shows the sedimentation of Ag-cit and Ag-PVP in fresh and sea waters. While both Ag NPs were stable in freshwater for 6 h, they settled out rapidly in seawater. The sedimentation rate of Ag-PVP in seawater was slightly lower than that of Ag-cit, confirming that the steric hindrance provided by adsorbed PVP is more effective in preventing Ag NPs from attaching to each other than for citrate. The high sedimentation rates in seawater reflect the higher initial NP

Figure 2. ζ-potentials based on electrophoretic mobility measurements as a function of pH and IS for (a) Ag-PVP, (b) Ag-citrate and (c) silica; and in (d) natural and artificial waters.

that the ζ-potential is mostly a function of the capping agent. Ag-PVP are nearly neutral in seawater (Figure 2d), whereas Agcit are considerably more negative. Both Ag NPs are more negatively charged in freshwater than in the seawater. The higher content of NOM in freshwater serves to make the ζpotential more negative, and this helps to explain the lower diameter in freshwater than seawater, in addition to the effect of IS. Deposition and Mobility of Ag NPs. SI Figure S2 shows a representative normalized frequency shift (Δf) and dissipation (ΔD) at the third overtone for Ag-cit NPs interacting with silica in natural freshwater. There is virtually no change in Δf 3 and 6988

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Figure 4. Sedimentation of Ag-cit and Ag-PVP NPs in seawater and freshwater. Initial [Ag-cit] = [Ag-PVP] = 100 mg/L.

concentration (100 mg/L) in these sedimentation experiments compared to the DLS measurements (1 mg/L). Sedimentation is sensitive to NP concentration,20 since the characteristic Brownian aggregation time is inversely proportional to NP concentration.28 Therefore, Ag NPs in [Ag NP] = 1 mg/L suspension will settle much slower (on the order of hundreds of hours) than [Ag NP] = 100 mg/L suspension in seawater. The TLC experiments capture the combined effect of mobility, deposition and attachment. An NP with high aggregation would exhibit high sedimentation rates. If attachment is strong, then one would expect to detect high deposition. Low aggregation would result in high mobility, potentially leading to attachment higher up in the TLC plate as the particles are able to migrate with the capillary film. However, if attachment is weak, there will be little difference in the amount of NP present in either the top (T) or bottom (B) of the TLC plate. A T/B ratio of 1 indicates that the NPs are mobile on the silica surface as the liquid suspension of NPs wets the surface via capillary action. On the other extreme end, a T/B ratio of 0 would indicate there is no transport of the NPs along the substrate surface as the water spreads upward on the silica surface. Figure 5 shows the T/B ratios of the two types of Ag NPs on silica, with values ranging from 0.7 to 1. Thus, under all conditions high mobility is observed, due to the low sedimentation rates and low attachment efficiencies. The highest Ag NP mobility was observed at 1 mM NaCl for all pH, when the charge on the Ag NPs is very negative. For other conditions, increasing IS or the presence of Ca2+, there was increased mobility as pH increased. This is explained in part by the lower particle sizes at higher pH and the low deposition and attachment due to the capping on the Ag NPs. High mobility was also observed in natural and artificial waters. There was no significant difference in mobility between Ag-cit and Ag-PVP, despite the large difference in size of the capping agents. Analysis of the mass of Ag NPs deposited on the TLC plates indicates that although there was high mobility, some deposition and attachment does occur (Figure 6). Deposition is low for Ag-PVP (Figure 6a) under all conditions, and slightly trends downward as pH increases. In general, higher deposition is observed at higher IS. For Ag-cit, deposition generally increases with increasing pH, particularly in 100 mM CaCl2. The deposition of either Ag-cit or Ag-PVP is not influenced by pH significantly when only Na+ is present, even at high IS. The

Figure 5. Mobility on silica under varying pH and IS conditions, measured as the T/B ratio, for (a) Ag-PVP, (b) Ag-citrate, and (c) in natural and artificial waters. Estimated error for measurements in (a) and (b) is up to ±15%; error bars not shown for clarity.

deposition of either Ag NP in natural waters is low, due to the additional steric hindrance provided by the presence of NOM. The dissolution of Ag NPs is fairly slow. Both Ag-cit and AgPVP dissolved progressively throughout a 30-day period in natural seawater, with roughly 6% Ag detected as free ion for Ag-cit sample at day 30 and 8% for Ag-PVP, with [Ag]initial = 10 mg/L (Figure 7). In contrast, less than 1% of either Ag NP dissolved in freshwater after 30 days. Dissolution of Ag-cit was also monitored in deionized (DI) water buffered at pH 8 (borate buffer) with and without 35 mM NaCl. With 35 mM NaCl, less than 1% of Ag-cit dissolved within 15 days, whereas with no NaCl addition, no dissolution of Ag-cit was detected (Ag ion concentration was below the detection limit of the ICP). There was minimal dissolution in freshwater after 15 days, as Ag+ reaches equilibrium in the absence of Cl−. This agrees with prior results29 that the presence of Cl− enhances Ag NP dissolution.

4. ENVIRONMENTAL IMPLICATIONS The surface mobility and deposition of capped Ag NPs on silica over a wide range of environmentally relevant solution pH, IS and presence of NOM were investigated. Capped Ag NPs were found to be highly mobile, with little or no deposition on the silica surface under the conditions studied, including natural waters. The overall deposition and transport behavior of Ag NPs on silica was shown to be only weakly dependent on the 6989

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *Phone: 01-805-453-1822; fax: 01-805-456-3807; e-mail: [email protected].

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation and the Environmental Protection Agency under Cooperative Agreement Number DBI-0830117. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation or the Environmental Protection Agency. This work has not been subjected to EPA review and no official endorsement should be inferred. We thank Prof. Jacob Israelachvili for access to the QCM-D, Ingmar Prokop and Gabriel Rubio for QCM-D measurements, Annabelle Lee for Ag-PVP synthesis, John Conway and Gabriel Rubio for collecting ζ-potential data, and acknowledge Zhaoxia Ji for assistance with purchase of Ag-cit NPs.

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Figure 6. Deposition of (a) Ag-PVP and (b) Ag-cit NPs on silica surfaces in different aqueous matrices.

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Figure 7. Dissolution of Ag-cit and Ag-PVP NPs in seawater, freshwater and DI water adjusted to pH 8 and [NaCl] = 35 mM. [Ag]initial =10 mg/L.

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