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Article pubs.acs.org/est Effects of Humic and Fulvic Acids on Silver Nanoparticle Stability, Dissolution, and Toxicity Ian L. Gunsolus, Maral P. S. Mousavi, Kadir Hussein, Philippe Bühlmann,* and Christy L. Haynes* Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States S Supporting Information * ABSTRACT: The colloidal stability of silver nanoparticles (AgNPs) in natural aquatic environments influences their transport and environmental persistence, while their dissolution to Ag+ influences their toxicity to organisms. Here, we characterize the colloidal stability, dissolution behavior, and toxicity of two industrially relevant classes of AgNPs (i.e., AgNPs stabilized by citrate or polyvinylpyrrolidone) after exposure to natural organic matter (NOM, i.e., Suwannee River Humic and Fulvic Acid Standards and Pony Lake Fulvic Acid Reference). We show that NOM interaction with the nanoparticle surface depends on (i) the NOM’s chemical composition, where sulfur- and nitrogen-rich NOM more significantly increases colloidal stability, and (ii) the affinity of the capping agent for the AgNP surface, where nanoparticles with loosely bound capping agents are more effectively stabilized by NOM. Adsorption of NOM is shown to have little effect on AgNP dissolution under most experimental conditions, the exception being when the NOM is rich in sulfur and nitrogen. Similarly, the toxicity of AgNPs to a bacterial model (Shewanella oneidensis MR-1) decreases most significantly in the presence of sulfur- and nitrogen-rich NOM. Our data suggest that the rate of AgNP aggregation and dissolution in aquatic environments containing NOM will depend on the chemical composition of the NOM, and that the toxicity of AgNPs to aquatic microorganisms is controlled primarily by the extent of nanoparticle dissolution. ■ mode of AgNP toxicity to microorganisms.9−12 Among the potential transformations of AgNPs entering natural aquatic environments, the least understood are those affected by NOM. A survey of the literature reveals variable effects of NOM on AgNP stability and dissolution, which appears to be caused by the high heterogeneity of NOM and the many AgNP models (in terms of size and surface chemistry) that have been employed. Several studies have demonstrated that addition of purified, naturally extracted NOM at low parts-per-million concentrations decreases homoaggregation rates (i.e., increases colloidal stability) of AgNPs; this applies to both AgNPs electrostatically stabilized with a citrate capping agent and sterically stabilized with polyvinylpyrrolidone (PVP) as the capping agent.13−15 Similarly, increased stability of citratecapped AgNPs in unpurified NOM suspensions was observed.16 A few notable exceptions to this trend were reported. For example, fulvic acids isolated from a reference site in a Norwegian lake, despite having elemental composition very similar to Suwannee River fulvic acid models that were shown INTRODUCTION Silver nanoparticles (AgNPs) are the most commonly used engineered nanomaterial in consumer products, serving primarily as antimicrobial agents (e.g., in fabrics and ointments).1 Common product uses can result in leaching of AgNPs into water (e.g., through laundering or skin cleansing), which is expected to be the major route for AgNPs to enter the wastewater supply.2,3 Although a recent study demonstrated high removal efficiency of AgNPs in municipal wastewater treatment plants,4 AgNPs are also expected to enter natural environments through direct discharge from manufacturing and disposal of consumer and medical products that may circumvent wastewater treatment.3,5,6 Given the potential for AgNP entry into environments and their known toxicity to microorganisms,7 significant efforts are being made to identify the material and environmental parameters that control AgNP behavior and environmental impact. AgNPs that enter natural aquatic environments encounter variable temperature, pH, light illumination, ionic strength, dissolved molecular oxygen concentration, and natural organic matter (NOM) concentration and composition. Each of these parameters has the potential to influence nanoparticle colloidal stability. These factors can also influence AgNP dissolution to give Ag+ (a process that depends on proton and molecular oxygen concentration),8 which is suggested to be the primary © 2015 American Chemical Society Received: Revised: Accepted: Published: 8078 March 24, 2015 June 2, 2015 June 5, 2015 June 5, 2015 DOI: 10.1021/acs.est.5b01496 Environ. Sci. Technol. 2015, 49, 8078−8086 Article Environmental Science & Technology fundamental and generalizable conclusions about AgNP−NOM interactions. Using in situ characterization, we avoid sample preparation errors that may have contributed to conflicting interpretations of prior results. to stabilize AgNPs, had no effect on AgNP stability at equivalent or higher NOM concentrations.17 Additionally, decreased colloidal stability of PVP-capped AgNPs following addition of cysteine (a simple model for protein-rich NOM) was observed in at least two studies.18,19 Our current understanding of NOM’s impact on AgNP colloidal stability is complicated by results obtained using a wide range of nanomaterial-stabilizing agents and NOM types, and the general notion that NOM, despite its high chemical heterogeneity, can be considered as a class of molecules to have common patterns of interaction with AgNPs. Here, we identify the characteristics of NOM that most significantly impact the colloidal stability of AgNPs by employing in a single study a series of NOM types with variable chemical composition and nanoparticle capping agents. The kinetics of AgNP dissolution, the equilibrium concentration of released Ag+, and complexation reactions of released Ag+ have been studied under variable solution conditions and with variable AgNP types. Discrepancies exist in the literature regarding the effect of NOM (either macromolecular or small molecule NOM models) on the extent of AgNP dissolution. Liu et al. observed decreased AgNP dissolution in the presence of thiol-containing species (e.g., cysteine and glutathione), which they attributed to a reduction in surface sites prone to oxidation.20 In contrast, Gondikas et al. demonstrated increased dissolution of citrate- and PVP-capped AgNPs in the presence of cysteine.18 The latter authors attributed the discrepancy between the two studies to differences in sample preparation (specifically, the possibility for analyte retention and loss when using centrifugal filter units).18 Further studies employing other NOM models observed either significantly increased21,22 or decreased8,23 dissolution of AgNPs with increasing NOM concentration. We note that the majority of dissolution studies employ measurements of total Ag concentration, without discriminating between free Ag+ and Ag+−NOM complexes, though related work of ours demonstrated that Ag+ binding to NOM can in some cases mitigate Ag+ toxicity to bacteria.24 In light of the important role of dissolved Ag+ (and Ag+−NOM complexes) to AgNP toxicity,9,24 and to address existing discrepancies in the literature, we used fluorous-phase Ag+ ionselective electrodes (ISEs) for in situ detection of AgNP dissolution by monitoring the Ag+ concentration. Fluorousphase Ag+ ISEs were previously shown to be powerful tools for dynamic monitoring of AgNP dissolution in complex media.25 Several studies observed reduced toxicity of AgNPs toward a number of organismal models in the presence of NOM, but the mechanism of this effect remains unclear. Studies using bacterial models such as Pseudomonas f luorescens26 and Escherichia coli23 suggested the primary mechanism to be complexation of Ag+ with NOM, reducing its bioavailability or bactericidal activity. Other studies using Pseudomonas f luorescens27 and the nematode Caenorhabditis elegans28 suggested that NOM adsorption to AgNP surfaces (possibly decreasing total Ag+ release or modulating nanoparticle adsorption to or internalization by organisms) is the primary mechanism of toxicity mitigation. Through parallel measurements of Ag+ concentration and AgNP toxicity to a bacterium (Shewanella oneidensis MR-1), this study provides a more direct means to evaluate the mechanism of NOM mitigation of AgNP toxicity than was previously possible. It seeks to provide new insight on the molecular interaction of NOM with commercially relevant AgNPs stabilized with citrate or PVP. By employing a series of NOM models to represent major NOM classes, we arrive at ■ EXPERIMENTAL SECTION Citrate-capped AgNPs were prepared using a reported method.16 PVP-capped AgNPs were prepared by incubating citrate-capped AgNPs with excess PVP-10 (average molecular weight 10 000 g/mol, Sigma-Aldrich), followed by purification. Ligand exchange was confirmed by zeta potential measurements, while particle size was determined using transmission electron microscopy. For details see the Supporting Information (SI). Stock solutions of 10 g/L NOM (Suwannee River Humic Acid Standard II, Suwannee River Fulvic Acid Standard II or Pony Lake Fulvic Acid Reference, International Humic Substances Society, St. Paul, MN) were prepared in deionized water and mixed with aliquots of purified and concentrated AgNP suspensions to achieve a 600 mg/L NOM concentration. Nanoparticles were incubated with NOM in the dark without mixing for 18 h, followed by redispersion in 0.1 M ionic strength potassium phosphate buffer (pH 7.5, 5 mg Ag/L, 10 mg/L of NOM; acrylic cuvettes). The colloidal stability of the resulting 3.0-mL AgNP samples was monitored over 2 days using UV−visible extinction spectroscopy and dark-field microscopy, and over 8 days using dynamic light scattering (DLS). Dissolution was monitored over 5 h in an identical buffer using fluorousphase Ag+ ISEs, prepared as described elsewhere24 (see also the SI and Figure S4). Toxicity of AgNPs to Shewanella oneidensis MR-1 was evaluated using the LIVE/DEAD Cell Viability Assay (Invitrogen). For consistency, the cells were suspended in the buffer described above. For details see the SI. ■ RESULTS AND DISCUSSION Impact of NOM and Nanoparticle Capping Agents on AgNP Colloidal Stability. AgNPs were prepared with either a citrate or PVP-10 capping agent to represent two major classes of industrially relevant AgNPs. TEM micrographs (SI Figure S1) reveal no appreciable change in AgNP morphology after exchanging citrate for PVP-10, demonstrating that nanoparticle surface functionalization can be varied independently from morphology. The average particle diameters of citrate- and PVP-capped AgNPs were calculated to be 12.1 ± 2.4 and 15.5 ± 4.1 nm, respectively, based on TEM analysis of 500 nanoparticles, indicating a minor, though statistically significant (p < 0.001), difference in nanoparticle size. Replacement of citrate by PVP-10 was probed by measuring the nanoparticle zeta potential. The latter decreased significantly with exchange to PVP-10, from −32.8 ± 2.2 to −13.6 ± 3.6 mV. These values are consistent with at least partial replacement of negatively charged citrate with neutral PVP-10 (SI Figure S1), and are in agreement with literature values for PVP-capped AgNPs prepared directly.18,29,30 Initially, AgNPs were exposed to 10 mg/L NOM, chosen to fall within the concentration range of natural freshwaters (1−60 mg/L).31 Three types of NOM were used: Suwannee River fulvic acid (SRFA) and Suwannee River humic acid (SRHA) have similar elemental compositions32 (SI Table S1) and represent NOM fractions derived primarily from decomposition of vegetation.33 Pony Lake fulvic acid (PLFA) represents 8079 DOI: 10.1021/acs.est.5b01496 Environ. Sci. Technol. 2015, 49, 8078−8086 Article Environmental Science & Technology Figure 1. UV−visible extinction spectra show that NOM improves the colloidal stability of citrate-capped AgNPs more significantly than PVPcapped AgNPs, and that the degree of colloidal stabilization conferred by NOM for both particle types was SRFA < SRHA ≪ PLFA. Spectra of citrate-capped (top) and PVP-capped (bottom) AgNPs in pH 7.5 phosphate buffer were collected after incubation of AgNPs with NOM type specified. Shown are spectra measured at 1−6 (red), 11, 20, 24, 30, and 46 h (violet) after particle redispersion; arrows indicate directions of peak intensity changes. The peak near 390 nm corresponds to extinction by the primary (12-nm-diameter) particle population and peaks at longer wavelengths correspond to aggregates. Agregate settling decreases peak intensities as it removes particles from the probed sample volume. The feature observed around 650 nm is an instrumental artifact. Results were duplicated in independent experiments. NOM rich in sites with high affinity for metallic silver and Ag+ (due to high sulfur and nitrogen content,34 a subset of which has a high affinity for silver and Ag+;24 SI Table S1). It is derived exclusively from microbial matter decomposition.33 None of these had a detectable effect on AgNP colloidal stability when present at a concentration of 10 mg/L, as determined by UV−visible extinction spectroscopy (SI Figure S2). This technique was used to demonstrate changes in AgNP aggregation by monitoring the intensity and position of sizedependent extinction peaks due to the localized surface plasmon resonance effect. In subsequent experiments, AgNPs were exposed to a larger concentration of NOM (600 mg/L) prior to colloidal stability assessment to promote NOM interaction with the AgNP surface. This simulates, on an accelerated time-scale, particle acquisition of adsorbed NOM, which is expected to take place over longer time periods in natural aquatic environments containing lower NOM concentrations. Following redispersion in a high ionic strength (0.1 M) buffer, the bulk NOM concentration during colloidal stability assessment was 10 mg/L. The ionic strength was chosen to ensure that the interaction of NOM with the AgNP surface was not purely electrostatic while remaining representative of natural aquatic systems.35 Nanoparticle colloidal stability was monitored over 2 days again using UV−visible extinction spectroscopy (Figure 1). The plasmon resonance of spherical metal nanoparticles causes light extinction features that are sensitive to interparticle interactions.36 Such interactions shift the particle plasmon extinction peak to higher wavelengths.36 In our study, a primary extinction peak attributable to the plasmon resonance frequency of nonagglomerated 12-nm-diameter AgNPs was observed at 391 and 394 nm for citrate- and PVP-capped AgNPs, respectively. The formation of variable-sized AgNP aggregates resulted in the appearance of a broader peak at longer wavelengths; larger aggregates produced broader and more red-shifted peaks. Similar observations of red-shifted UV−visible extinction spectra in response to AgNP aggregation were reported previously.37 Nanoparticle aggregation was further characterized using dynamic light scattering (DLS) to track hydrodynamic particle diameter over time following dispersion in buffer (Figure 2). Extinction spectroscopy and DLS demonstrate that incubation with NOM stabilized citrateand PVP-capped AgNPs against homoaggregation in a high ionic strength buffer (0.1 M) relative to their pristine (noNOM) counterparts. The aggregation behavior of AgNPs is known to depend on surface characteristics, where surface coatings that promote steric repulsion are typically more effective at maintaining AgNP colloidal stability in high ionic strength environments than their counterparts promoting only electrostatic repulsion.38,39 However, surface-coating-dependent behavior of AgNPs in the presence of NOM has not been studied thoroughly. It was observed that citrate- and PVPcapped AgNPs aggregated similarly with increasing ionic strength after addition of cysteine, which was used as a low molecular weight model for NOM.18 However, higher molecular weight NOM, such as that used in our study, is expected to induce different effects, given its greater potential to increase steric repulsion between particles. In this study, the 8080 DOI: 10.1021/acs.est.5b01496 Environ. Sci. Technol. 2015, 49, 8078−8086 Article Environmental Science & Technology nm nanoparticles. Appearance of a broader, secondary extinction peak at red-shifted wavelengths was also slower, suggesting slower aggregate formation. In most cases, the average hydrodynamic particle diameter increased following dispersion in buffer, but increases were dramatically slower than for pristine nanoparticles. This result suggests that ligandstabilized AgNPs that encounter high concentrations of NOM have significantly higher colloidal stability than their pristine counterparts. Whereas nanoparticle transport in natural aquatic environments depends not only on homoaggregation as evaluated here, but also on heteroaggregation and nanoparticle adsorption onto collector surfaces, our result suggests that AgNPs stabilized by NOM may be transported through aquatic environments more efficiently than their pristine counterparts, since homoaggregation and nanoparticle settling is reduced. Second, the extent to which NOM increases AgNP colloidal stability depends on the affinity of the original organic capping agent for the nanoparticle surface. A fraction of PVP-capped AgNPs previously incubated with SRFA or SRHA aggregated immediately after dispersion in NOM-free buffer, resulting in a broad extinction peak between 500 and 750 nm (Figure 1, bottom). Equivalently prepared citrate-capped AgNPs aggregated more slowly and formed smaller aggregates, as indicated by slower growth of a narrower secondary extinction peak between 500 and 600 nm (Figure 1, top) and DLS measurement of hydrodynamic diameters (Figure 2). These results indicate that PVP-capped AgNPs are less effectively stabilized by NOM than citrate-capped AgNPs, which may be caused by the higher affinity of PVP than citrate for the AgNP surface. Citrate is generally thought to be weakly bound to the AgNP surface,37 and citrate-capping of AgNPs is widely used in industry to provide stable precursors for other functionalization schemes due to the labile nature of this agent. At the high NOM concentrations employed in the current study, NOM may displace citrate from the nanoparticle surface. In contrast, PVP coordinates with the AgNP surface through van der Waals interactions and direct bonding interactions with the Ag dband.40,41 Computational studies showed that the latter occurs through bonding orbitals of the 2-pyrrolidone subunit, localized on the oxygen (∼60%) and nitrogen (∼25%).40,41 This is consistent with spectroscopic studies of PVP interaction with Ag, which suggested that direct bonding interactions occur through either only oxygen or a combination of oxygen and nitrogen.42−44 Because of direct bonding interactions with Ag, PVP is harder to displace than citrate, which may result in a greater barrier to NOM interaction with the nanoparticle surface. Alternatively, NOM might adsorb to either citrate or PVP on the AgNP surface, rather than displacing them. Under this assumption, our results suggest that more NOM binds to adsorbed citrate than to PVP since exposure to NOM induces a more significant increase in the colloidal stability of citrate- than PVP-capped AgNPs. However, at the pH of this system (pH 7.5), greater electrostatic repulsion exists between the negatively charged acidic residues of NOM and citrate (which carries three negative charges) than PVP (which is neutral). On the basis of our results, we conclude that AgNPs stabilized with easily displaceable organic capping agents will be more effectively stabilized by NOM. Third, the extent to which NOM increases AgNP colloidal stability, regardless of the org ...
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Article Summary



The article is on the various effects of both fulvic and humic acids on Silver
Nanoparticle stability, dissolution, and toxicity. The article was published on 5th June 2015 at
the University of Minnesota in the United States (Kaegi,nVoegelin, Sinnet, Zuleeg,
Hagendorfer, Burkhardt, Siegrist, 2011). The silver nanoparticle is the often used materials in
the manufacture of several consumer products like ointments and fabrics. Use of common
product may lead to leaching of silver nanoparticle into water.
To begin with, fulvic and humic acids in low concentrations leads to a decrease in the
rates of homoaggregation of the ...

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