Article pubs.acs.org/est
Natural Organic Matter Concentration and Hydrochemistry Influence Aggregation Kinetics of Functionalized Engineered Nanoparticles Junfeng Liu,† Samuel Legros,† Frank von der Kammer,*,† and Thilo Hofmann*,† †
Department of Environmental Geosciences, University of Vienna, Althanstrasse 14 A-1090 Vienna S Supporting Information *
ABSTRACT: Understanding the colloidal stability of functionalized engineered nanoparticles (FENPs) in aquatic environments is of paramount importance in order to assess the risk related to FENPs. In this study, gold nanoparticles (GNPs) of 68 and 43 nm diameter, coated with citrate and 11mercaptoundecanoic acid (MUA) respectively, were used as models of FENPs. Time-resolved dynamic light scattering was employed to investigate the aggregation kinetics of two types of GNPs. The results show that without Suwannee river natural organic matter (SRNOM), MUA coating resulted in greater stability than citrate coating for GNPs. Cations have a destabilizing effect on both GNPs following the order Ca2+ ≈ Mg2+ ≫ Na+; different anions (Cl− and SO42−) showed no difference in effects. In the fast aggregation regime, adding SRNOM enhanced the stability of MUA-coated GNPs in both Ca2+ and Mg2+ solutions. However citrate-coated GNPs were only stabilized in Mg2+ solution but enhanced aggregation occurred in high Ca2+ concentration due to interparticle bridging. For the investigated GNPs and in the presence of SRNOM, Ca2+ does not always act as a strong coagulant. This indicates that for the new materials emerging from the application of nanotechnology the well-described aggregation mechanisms of colloids in the environment require a detailed re-examination.
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INTRODUCTION Engineered nanoparticles (ENPs) are functionalized through the application of a variety of surface coatings. This functionalization prevents ENPs from aggregating or dissolving,1,2 and changes their physicochemical properties to suit specific applications. Such functionalized engineered nanoparticles (FENPs) have been widely used in biomedical, optical, and cosmetic applications, as well as for catalysis.3−6 Gold nanoparticles (GNPs) have, for example, been functionalized with transferrin for drug delivery,7 and nanoscale zerovalent iron has been coated with polyelectrolytes to enhance its stability and migration in the subsurface for groundwater remediation.8 The production and use of FENPs inevitably results in their eventual release into the hydrosphere.9−12 This release introduces a risk to the environment since toxic effects have been reported in recent studies, e.g., for mammal cells.13 Hoshino et al.14 have found that the cytotoxicity of nanoscale quantum dots is more related to their surface coatings than to the core material. The relationship between toxicity and nanoparticle size has also been demonstrated in several studies.15−17 Investigations are therefore required into the potential impacts of ENPs once they are released into a natural aquatic environment. The environmental impact of engineered nanomaterials is dependent on, beside others, both their size and their colloidal stability.18,19 The colloidal stability of FENPs in aquatic © 2013 American Chemical Society
environments has been investigated by observing their aggregation in aqueous suspensions with different hydrochemical compositions.20−22 The principal environmental parameters investigated to date have been pH, ionic strength, and natural organic matter (NOM).23−25 NOM is recognized as an important factor for the colloidal stability of engineered nanoparticles, since it is ubiquitous in aquatic systems.22 NOM may, for example, enhance colloidal stability in aquatic systems due to its adsorption onto nanoparticle surfaces.26,27 Such NOM-coated nanoparticles are also more likely to be destabilized by divalent cation bridging.22,28,29 Natural waters usually contain NOM concentrations that range from ∼1 to ∼50 mg/L dissolved organic carbon (DOC).30 However, research on the stability of FENPs has generally only considered a single fixed concentration of NOM, and may therefore not have been sufficiently representative of conditions in natural aquatic systems. There has to date been no systematic investigation into the influence of different NOM concentrations on FENP stability. The surface coatings of FENPs also influence their colloidal stability in a number of ways. First, the functionalization may increase the colloidal stability of FENPs relative to uncoated Received: Revised: Accepted: Published: 4113
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and quantitatively assessed, we decided to not distinguish between agglomerates and aggregates. Following the tradition in colloid chemistry we term attached particles aggregates independent of the magnitude of forces between the particles.
particles of the same core material, under similar environmental conditions.31 Second, the character of the surface coating molecules and their surface adsorption density may also affect the stability of the FENPs. Citrate- and 11-mercaptoundecanoic acid (MUA)-coated GNPs, for example, show different pH related aggregation when adjusting pH from 12 to 2. Liu et al.32 attributed this to the different pKa values of the functional groups of the surface coating molecules. In addition, Huynh et al.24 found that citrate-coated silver nanoparticles (AgNPs) showed a lower colloidal stability than AgNPs coated with polyvinylpyrrolidone (PVP), in NaCl and CaCl2 solutions. The nature of the coating will thus have a greater effect on the behavior of the FENPs within a natural environment than their core material.31 A number of studies have compared the effects of different cations on the aggregation kinetics of FENPs. For instance, Ca2+ has been shown to be more efficient at screening the surface charge of citrate- or PVP-coated GNPs than Na+.24 Stankus et al.22 found that Ca2+ also had a greater tendency than K+ to induce aggregation of GNPs with different surface coatings. With respect to interactions with NOM, divalent cations have been found to show bridging effects, while monovalent cations do not.33 Nevertheless, none of these studies considered the role that the type of anion plays for the stabilization. The objective of this study was to investigate the impact of electrolyte compositions (CaCl2, MgCl2, CaSO4, and Na2SO4) on the colloidal stability of FENPs in association with different concentrations of NOM. These electrolytes represent variations in the cation valence (Ca2+ or Na+), the anion valence (Cl− or SO42−), and the cation nature (Ca2+ or Mg2+). To understand how different functionalization of the particle surface does influence the behavior of ENPs, GNPs were chosen for the following reasons as proxy and an ideal template: GNPs are resistant against dissolution, easy to be detected, and comparably easy to functionalize due to their strong binding of sulfur groups. Therefore, even though GNPs are not expected to be released to the environment in large quantities, we chose GNPs with two different types of surface coating (citrate and 11-mercaptoundecanoic acid), as models for FENPs. Attachment efficiencies (α) were derived from series of time-resolved dynamic light scattering (DLS) measurements to elucidate the interactions between particle surface coatings and NOM, at different electrolyte concentrations (Supporting Information Figure S1). First, the aggregation behavior of the two functionalized GNPs was studied as a function of ionic strength of the investigated electrolytes. Second, the influence of NOM on the aggregation of the two functionalized GNPs was investigated by addition of NOM to the gold suspension in the diffusion limited (fast) aggregation regime (α = 1). The diffusion limited conditions were induced by adding the investigated electrolytes at and above their respective critical coagulation concentrations (CCCs). The change of particle attachment efficiency (α) in fast aggregation (α = 1) was examined as function of the NOM concentration. It was demonstrated that variations in the type of surface coating on FENPs, the NOM concentration, the electrolyte composition, and the electrolyte concentration all induce different interactions and behavior in FENPs. This information is of importance for exposure modeling and risk assessment for FENPs in aquatic environments. Since the differentiation of strong or weak bonds between attached particles can not be practically measured or routinely
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MATERIALS AND METHODS Functionalized Gold Nanoparticles. Two types of functionalized GNPs with different surface coatings have been investigated (Table 1). The stock of citrate-coated GNP was Table 1. Characteristics of Citrate-Coated GNP and 11Mercaptoundecanoic Acid (MUA)-Coated GNP Stock suspensions GNPs
pH
particle diameter (nm)
concentration (mg/L)
zeta potential (mV)
citratecoated GNP MUAcoated GNP
7.16
67.90 ± 0.39
46.63 ± 0.0035
−53.93 ± 1.31
6.56
43.26 ± 2.86
38.70 ± 0.0017
−49.40 ± 3.56
purchased from British Biocell International. The stock of GNP coated with MUA was synthesized by the Chemistry Department of the University of Alberta in Edmonton, Canada. Details of the synthesis method can be found elsewhere.32 The detailed methods for characterization of the applied functionalized GNPs are provided in the Supporting Information. Solution Chemistry. Ultrapure water with a resistivity of 18 MΩ/cm was used in all experiments. A Millipore Advantage A10 system (Millipore, Billerica, ME) equipped with a Bio-Pak ultra filter (5000 g/mol molecular weight cutoff) was used to produce the ultrapure water with a minimum background of particles. Stock solutions of 0.1 M CaCl2, 0.1 M MgCl2, 0.01 M CaSO4, and 1 M Na2SO4 were prepared by dissolving analytical-grade CaCl2, MgCl2, CaSO4, and Na2SO4 in Millipore water. These were then used to adjust electrolyte concentrations. Suwannee river natural organic matter (SRNOM, International Humic Substances Society) was used, being a well-studied model of NOM. A 1 g/L SRNOM stock solution was prepared and then diluted to 0.1 g/L and 0.01 g/L for use in sample preparation. The DOC of the SRNOM stock solution (0.4 ± 0.015 g/L) was determined using a Shimadzu TOC-V CPH TOC analyzer (Duisburg, Germany), following filtration with a 0.45 μm Whatman cellulose acetate membrane. Determination of Aggregation Kinetics and Attachment Efficiencies. The initial increase in the DLS-determined hydrodynamic radius rh(t) with respect to time (t) is proportional to the primary particle concentration and the aggregation rate constant k11 (eq 1), which can be identified as the formation rate of doublets (1 to 1). Only in the very early stage of aggregation, the formation of doublet particles from single particles prevails. At later stages, when larger aggregates have formed, the particle−cluster and cluster−cluster aggregation dominates the reaction. Hence the aggregation rate constant is determined by measuring the increase in hydrodynamic radius rh0 at t = 0 until the time at which it reaches 1.38 rh0.34 For extremely slow aggregation rates at low electrolyte concentrations, the aggregation rate constant k11 was determined before hydrodynamic radius reaching 1.38 rh. In a few cases of extremely fast aggregation, the points chosen for the determination of k11 went beyond 1.38 rh0, but still showed the same linear relationship. In this study, all of the 4114
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Figure 1. Attachment efficiencies of citrate-coated GNPs as a function of background electrolyte concentration: (a) CaCl2, (b) CaSO4, (c) MgCl2, and (d) Na2SO4. The error bars represent the standard deviation from triplicate measurements. The dashed line provides a visual guide to distinguish the two aggregation regimes.
⎡ drh ⎤ ⎢⎣ dt ⎥ k11 t→0 ⎦ α= = k fast ⎡ drh ⎤fast ⎢⎣ dt ⎥ t→0 ⎦
time-resolved DLS measurements were conducted at 20 °C. The pH of the samples after measurements ranged between 6.5 and 7.0. In this pH range, our previous study shows that the solution pH does not influence the stability of the two functionalized GNPs.32 ⎛ drh ⎞ ⎜ ⎟ ∝ k11C ⎝ dt ⎠ t → 0
( )
( )
(2)
Time-Resolved Dynamic Light Scattering Measurements. All time-resolved particle size determinations to obtain aggregation rates were carried out on a Malvern ZetaSizer NanoZS system (Malvern, UK) equipped with a 4 mW He−Ne laser (633 nm). Experiments were performed in standard 1 × 1 cm disposable cuvettes. The hydrodynamic diameter of the particles was calculated from the determined particle diffusion coefficient (D), applying the Stokes−Einstein equation. Determination of the z-average particle size was based on the cumulant method for fitting the autocorrelation function.35 Each aggregation experiment consisted of 30 individual measurements, with every measurement consisting of 5 replicates, each with a 6 s collection time. All experiments were performed in triplicate, with the optimum measurement position and attenuator position being determined from preliminary tests and then fixed, in order to save the time in which the instrument requires to set the parameters automatically. Sample Preparation. The electrolytes chosen for this study were CaCl2, CaSO4, MgCl2, and Na2SO4. They were used to investigate the influence of cation nature (CaCl2 or MgCl2), cation valence (CaCl2 or Na2SO4, also including results from our previous work32 on NaCl), and the valence of counteranions (CaCl2 or CaSO4) on the aggregation kinetics of FENPs. The aggregation kinetics of the GNPs were investigated in both the absence and the presence of SRNOM. The
(1)
In this study the attachment efficiency (α) was used to describe the stability of the dispersions, which is the inverse of the stability ratio W and was calculated from eq 2. The attachment efficiency (α) represents the number of particle− particle collisions that succeed in forming a doublet, as a proportion of the total number of particle−particle collisions. It is calculated by normalizing the actual aggregation rate constant k11 to the fastest possible aggregation rate constant kfast (obtained in the diffusion limited aggregation regime, where k11 = kfast and α = 1). The concentration of the electrolyte at which attachment efficiency (α) becomes unity is defined as the critical coagulation concentration (CCC) of that electrolyte for the particular suspension being investigated. At electrolyte concentrations above the CCC, the attachment efficiency (α) shows no dependency on electrolyte concentration and the system is fully destabilized. The aggregation rate constant is totally dependent on the particle number concentration, the diffusion of the particles, and the resulting frequency of particle collisions. This is known as a diffusion-limited regime.35 4115
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Figure 2. Attachment efficiencies of MUA-coated GNPs as a function of background electrolyte concentration: (a) CaCl2, (b) CaSO4, (c) MgCl2, and (d) Na2SO4. The error bars represent the standard deviation from triplicate measurements. The dashed line provides a visual guide to distinguish the two aggregation regimes.
or Mg2+) promote aggregation much more strongly than monovalent cations (Na+). In addition, the CCCs for NaCl reported in our previous study32 are approximately twice as high as the CCCs for Na2SO4. This provides strong support for the results from the electrolytes with divalent cations, which indicate that the valence of the anion (Cl− or SO42−) has a negligible effect on the aggregation kinetics in the absence of NOM. The substantially smaller CCCs of citrate- and MUA-coated GNPs in divalent cation solutions (Ca2+ and Mg2+) than for monovalent cation solutions (Na+) are consistent with the Schultz−Hardy rule36,37 which predicts a proportionality fraction of Z−6 (with Z being the valence of the cation). In our case, the proportionality of the CCC between divalent cation solutions (Ca2+ and Mg2+) and monovalent cation solutions (Na+) is Z−5.3 for citrate-coated GNPs and Z−5.7 for MUA-coated GNPs (Table S1). In contrast, Buttner et al.38 found that, for cerium nanoparticles, this proportionality between CaCl2 and NaCl decreased to Z−2.3, which is much lower than the Schultz−Hardy rule that would predict (Z−6). Such observations can be attributed to the specific adsorption of Ca2+ on the surface of the metal oxide nanoparticles which may reduce aggregation in the regime of positive zeta potentials where Cl− would be the monovalent anion.39 However, the greater efficiency of Ca2+ at screening the surface charge that we observed has also been reported in studies on multiwalled carbon nanotubes and boron nanoparticles.29,40 Comparison of the Aggregation Kinetics of CitrateCoated and MUA-Coated GNPs. The CCC of MUA-coated GNPs for CaCl2, CaSO4, and MgCl2 (3 mM) is higher than that of citrate-coated GNPs (2 mM). The CCC of MUA-coated GNPs in the presence of Na2SO4 (equivalent to 160 mM Na+)
aggregation experiments were carried out according to the procedure described in a previous publication.32 To investigate the influence of SRNOM concentrations on aggregation, GNPs were added to solutions containing different SRNOM concentrations at (1) electrolyte concentrations equal to the electrolyte’s CCC, and (2) electrolyte concentrations exceeding the electrolyte’s CCC. The sample preparation procedure was the same as that used for samples in the absence of SRNOM,32 except that the required amount of SRNOM (from 1 g/L stock) was also introduced into the cuvette containing the electrolyte solution. Each measurement was performed in triplicate.
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RESULTS AND DISCUSSION
Aggregation Kinetics of GNPs with Different Types of Cation and Anion. The aggregation behavior of the functionalized GNPs was characterized using the attachment efficiency (α), which is a well-established parameter in colloidal chemistry (eq 2). The CCC corresponds to the minimum electrolyte concentration needed to induce fast aggregation, at which an attachment efficiency (α) of 1 is reached. The CCCs determined for citrate-coated GNPs with respect to CaCl2, CaSO4, and MgCl2 were all 2 mM (Figure 1a−c). The CCCs determined for MUA-coated GNPs with the same electrolytes were all ∼50% higher, at 3 mM (Figure 2a−c). In our experiments neither the nature of the cation (Ca2+ or Mg2+) nor the valence of the anion (Cl− or SO42−) could be seen to exert any identifiable influence on the aggregation kinetics. Using Na2SO4 as the electrolyte, the CCC was determined to be 40 mM for citrate-coated GNPs and 80 mM for MUA-coated GNPs (Figures 1d and 2d). This was to be expected since it is well established that divalent cations (Ca2+ 4116
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Figure 3. Influence of SRNOM on the fast aggregations of (a, b) citrate- and (c, d) MUA-coated GNPs induced by divalent electrolytes. The attachment efficiencies were calculated by normalizing the aggregation rates to the fast aggregation rate at the CCC. The error bars represent the standard deviation from triplicate measurements. The dashed line provides a visual guide to distinguish the two aggregation regimes.
coated GNPs showed a greater tendency to aggregate in the presence of SRNOM than its absence. An aggregation processing with attachment efficiencies greater than 1 could be explained by a bridging effect,28,42 which arises from the affinity of Ca2+ for the RCOO− groups of SRNOM. This behavior is consistent with the results obtained by Huynh and Chen24 on AgNPs. They observed that the addition of humic acid (HA) decreased the attachment efficiencies of citratecoated AgNPs at CaCl2 concentrations below 9 mM. However, the attachment efficiencies of citrate-coated AgNPs increased at CaCl2 concentrations above 9 mM due to an interparticle bridging through the complexation of Ca2+ with HA. In addition to the bridging effect, we also noticed that the increase in attachment efficiencies was greater in CaCl2 solution than in CaSO4 solution. The presence of SO42− in the suspension reduced the bridging effect. This suggests that, in the presence of SRNOM, the anion of the background electrolyte also exerted an influence on the aggregation kinetics. The SO42− thus appears to have been more effective than Cl− at countering the destabilizing effect of Ca2+. In the presence of 8 mM MgCl2, the addition of SRNOM resulted in only a slight reduction in the attachment efficiencies of citrate-coated GNPs (Figure 3b). No evidence of a bridging effect was observed. This may be related to the greater electronegativity of Mg2+ (1.31), compared to Ca2+ (1.00), inducing a larger hydrated radius for Mg2+. As a result, Mg2+ is less likely to complex with SRNOM than Ca2+.43 Thus, the addition of SRNOM is not sufficient to increase the aggregation rate of citrate-coated GNPs in MgCl2 solution.22,44 The nature of the cation in the electrolyte is therefore an important factor for affecting the aggregation of FENPs in the presence of SRNOM, if the NOM interacts with the FENPs. Similar effects
is even more significantly higher than that of citrate-coated GNPs (equivalent to 80 mM Na+). This higher stability of MUA-coated GNPs can be attributed to the steric stabilization contributed by the long chain of MUA molecule, which is greater than that contributed by the citrate coating.32 Influence of SRNOM on Citrate-Coated GNPs in Divalent Cation Solutions. For studying the influence of SRNOM we opted for an approach which started at the CCC of the respective particles in the selected electrolyte and then we added SRNOM to investigate its influence on aggregation kinetics. The attachment efficiency (α) of citrate-coated GNPs in the presence of 2 mM of CaCl2, CaSO4, or MgCl2 is unity. Increasing the SRNOM concentration from 0 to 100 mg/L decreased the attachment efficiency (α) for citrate-coated GNPs to 0 (Figure 3a). This stabilizing effect is likely to be related to the adsorption of SRNOM onto the surface of citrate-coated GNPs. Indeed, SRNOM contains negatively charged functional groups such as RCOO− groups but even more importantly RS− groups.41 These functional groups may have facilitated SRNOM in replacing the citrate on the GNPs because of the physically weak adsorption of citrate onto the gold surface.23,32 The adsorption of SRNOM may have resulted in electrosteric repulsion and increased the stability of citrate-coated GNPs. The effect of SRNOM on the aggregation induced by an electrolyte concentration above the CCC was also considered. When the concentration of CaCl2 or CaSO4 was elevated to 8 mM, the attachment efficiency (α) of citrate-coated GNPs was still 1 in the diffusion limited regime. However, an increase in SRNOM concentration resulted in attachment efficiencies greater than 1, which continued to increase with SRNOM concentration (Figure 3b). Under these conditions the citrate4117
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have also been observed by Chen and Elimelech28 on fullerene nanoparticles (C60). Adding HA (1 mg/L TOC) increased the attachment efficiency (α) of C60 at CaCl2 concentrations above 0.01 M, but decreased its attachment efficiency (α) at MgCl2 concentrations above 0.01 M. A lower affinity of Mg2+ to R COO− groups of NOM has also been reported in a previous study on boron nanoparticles.40 Influence of SRNOM on MUA-Coated GNPs in Divalent Cation Solutions. The addition of 10 mg/L of SRNOM dramatically reduced the attachment efficiencies of MUA-coated GNPs in 3 mM of CaCl2 or CaSO4 solution (Figure 3c). This reduction can be interpreted by electrosteric repulsion through the adsorption of SRNOM onto the surface of MUA-coated GNPs, or an overcoating of the MUA layer. In contrast to citrate, which is electrostatically adsorbed onto gold, MUA is covalently bonded to the gold surface via its RS− groups.41,45 Replacement of MUA is therefore unlikely to occur. However, Ca2+ may have facilitated the adsorption of SRNOM onto the surface of MUA-coated GNPs. This adsorption could be caused either by bridging between the MUA and RCOO− groups of SRNOM, or by a reduction in the net surface charge of both the MUA-coated GNPs and the SRNOM. Chen and Elimelech28 found that Ca2+ can complex with dissolved HA by interaction with the RCOO− group on HA. In our case, both MUA-coated GNPs and SRNOM carry RCOO− groups and Ca2+ is thus very likely to associate with SRNOM and MUA-coated GNPs. To understand Ca2+ association behavior, we need to distinguish between the screening of surface charge by Ca2+ and the elimination of surface charge, introduction of charge heterogeneity, and reversal of surface charge polarity, by specific interactions between the electrolyte and RCOO− groups on MUA-coated GNPs. Nevertheless, the reduced attachment efficiencies suggest that the binary association of Ca2+ may result in the adsorption of SRNOM onto the surface of MUA-coated GNPs and hence reduce their aggregation. Ca2+ has also been found to facilitate the adsorption of NOM onto the surface of UVirradiated fullerene nanoparticles (7DUV-nC60).46 This adsorption increased the stability of 7DUV-nC60 in CaCl2 due to the specific interaction of Ca2+ with RCOO− groups on 7DUV-nC60 and HA. The negative charges on 7DUV-nC60 surfaces and HA were probably neutralized by Ca2+ complexation, resulting in bridging between 7DUV-nC60 and HA.46 A similar effect has also been reported by Feng et al.47 on clay mineral surfaces. In contrast to the observations made with CaCl2 and CaSO4, the attachment efficiencies in 3 mM MgCl2 were reduced to a far lesser extent by adsorption of SRNOM, even at SRNOM concentrations of up to 100 mg/L. This may be due to the lower affinity of Mg2+ for the RCOO− group surface, resulting in fewer SRNOM molecules adsorbed onto the surface of MUA-coated GNPs.22 For the fast aggregation induced by 8 mM CaCl2, CaSO4, or MgCl2, adding SRNOM decreased the attachment efficiencies of MUA-coated GNPs to a lesser extent than adding SRNOM to 3 mM CaCl2, CaSO4, or MgCl2 (Figure 3d). In contrast to citrate-coated GNPs, there was no increased attachment with MUA-coated GNPs following the addition of SRNOM. These results show that a bridging effect is less likely to occur with MUA-coated GNPs than with citrate-coated GNPs in the presence of SRNOM. Furthermore, with increasing SRNOM concentrations the attachment efficiencies of MUA-coated GNPs were reduced to a greater extent in CaSO4 than in CaCl2. Thus, the presence of SO42− in the Ca2+ dominant suspension
does increase the stability of MUA-coated GNPs slightly more than Cl−. The SO42− countered the destabilizing effect of Ca2+ in the presence of SRNOM to a greater extent than the Cl−, as has already been observed for citrate-coated GNPs. Influence of SRNOM on Citrate-Coated GNPs in Monovalent Cation Solutions. The addition of SRNOM into the suspension increased the attachment efficiencies of citrate-coated GNPs in 0.04 M Na2SO4 (corresponding to the CCC) as well as in 0.1 M Na2SO4 (Figure S2a, b). The increase of the attachment efficiency (α) must be related to the anion SO42−, since in a previous study32 adding SRNOM (100 mg/L) to citrate-coated GNPs in 0.2 M NaCl stabilized the suspension. Influence of SRNOM on MUA-Coated GNPs in Monovalent Cation Solutions. No significant change in attachment efficiency (α) was observed when different SRNOM concentrations were introduced into the suspension of MUA-coated GNPs in the presence of 0.08 and 0.2 M Na2SO4 (Figure S2c, d). These results indicate that SRNOM does not adsorb onto the surface of MUA-coated GNPs under such conditions, and therefore has no effect on the stability of MUA-coated GNPs. This observed behavior in the presence of Na2SO4 differs from that in the presence of CaCl2 and CaSO4. We attribute this difference to the valence of the dominant cation in the suspension. Unlike the bridging effect of Ca2+ observed in our study, Na+ does not permit a binary association between SRNOM and MUA-coated GNPs to stabilize the suspension, as has also been reported for 7DUV-nC60 and NOM by Li et al.46
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ENVIRONMENTAL IMPLICATIONS In principle, homoaggregation between particles of the same kind may be of less environmental importance compared to heteroaggregation processes with environmentally relevant natural particles of nano- or microsize. However, in the current situation of lacking analytical tools to investigate near-natural heteroaggregation processes, we still gain valuable mechanistic knowledge from experiments looking at homoaggregation under the influence of NOM and multiple charged ions. The water chemistry may exert a strong influence on the colloidal stability of FENPs released into an aquatic environment. Understanding the mechanisms that affect the state of aggregation is therefore essential for predicting the transport and eventual fate of FENPs. In this study the attachment efficiencies of GNPs coated by citrate and by MUA were measured under different conditions that included a suite of different electrolytes and SRNOM concentrations. In the absence of SRNOM, the valence of the cations (Ca2+, Mg2+, Na+) was found to be the dominant parameter controlling the aggregation kinetics of the investigated GNPs. Divalent cations were more efficient at screening the particle surface charge. They induced fast aggregation at very low concentrations (e.g., ∼2 mM Ca2+/Mg2+ for citrate-coated GNPs and ∼3 mM Ca2+/Mg2+ for MUA-coated GNPs), while monovalent cations induced aggregation at higher concentrations (e.g., ∼80 mM Na+ for citrate-coated GNPs and 160 mM Na+ for MUA-coated GNPs). The anions (Cl− or SO42−) showed no impact on the stability of FENPs, nor were any differences observed between different divalent cations. The type of the coating on the FENPs influenced the particle stability: citrate-coated GNPs were less stable than MUAcoated GNPs under the same conditions. River systems and freshwater streams often contain low organic matter concen4118
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trations, a Ca2+ concentration of less than ∼1 mM, less than ∼0.4 mM Mg2+, and less than ∼5 mM Na+.39,44,48 Citrate- or MUA-coated GNPs should therefore be relatively stable under the typical hydrochemical conditions of freshwater aquatic environments. The citrate- or MUA-coated GNPs should remain well-dispersed once released into such systems. However, the water in lakes and rivers derived from industrial areas can reach Ca2+ concentrations up to ∼2.1 mM,44 which would result in the aggregation of both of the functionalized GNPs investigated. Our results suggest a more complex situation for functionalized GNPs release into aquatic systems with high DOC concentrations, such as the peat bogwater (at Tanner Moor, Austria), which has 37.2 mg/L DOC.39 Functionalized GNPs will show a high level of stability under these conditions. Wastewater inflow might contain up to ∼100 mg/L DOC,49 with an elevated ionic strength and very complex electrolyte compositions. In this situation the nature and valence of the cation may have a strong effect on the colloidal stability of the functionalized GNPs. The valence of the anions (Cl− or SO42−) may also influence the functionalized GNPs stability at elevated NOM concentrations. SO42− associated with NOM may further destabilized citrate-coated GNPs, especially in Na+ dominated systems, but has no effect on MUA-coated GNPs. We deliberately selected the SRNOM as a representative for NOM in aquatic environments. However, the SRNOM represents only a certain type of surface water NOM and has not been produced as a surrogate for true natural NOM. Additionally, the influence of the fractionation of NOM (e. g., humic and fulvic acids, carbohydrates, etc.) should be further considered in future work focused on mechanistic understanding and the role of different fractions of NOM on particle aggregation. For the systems influenced by nanoparticle surface coating, NOM, and divalent cations, we observed a behavior that is contrary to the generalizations made in environmental colloid chemistry. In the presence of NOM and certain particles with specific coatings, Ca2+ does not always act as a strong coagulant. The well-described aggregation mechanisms of colloids in the environment therefore need to be re-examined, adapted, and extended to take into account the new (and challenging) materials emerging from the application of nanotechnology. In general, MUA-coated GNPs showed a higher colloidal stability than citrate-coated GNPs under all of the electrolyte backgrounds and SRNOM concentrations investigated. The aggregation kinetics are controlled by the surface coatings of FENPs and the interactions between the surface coating groups and NOM. This information is of crucial importance for understanding the aggregation, transport, and the eventual fate of FENPs, and provides a basis for extending the classical exposure models in risk assessment. We would like to point out that the experiments were conducted with nanoparticles having particle sizes considerably above 10 nm in diameter. Particles smaller than 10 nm in diameter will approach a regime where particle core size, chain length of coating molecules, and electrostatic double layer thickness approach similar dimensions. DLVO theory, which is the predominant conceptual model we use to interpret our results, will no longer be valid in this particle size region.
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ASSOCIATED CONTENT
S Supporting Information *
Flowchart of the overall study layout, the methods for determination of size distribution and mass concentration of the applied GNPs in stock, table of the CCCs of citrate- and MUA-coated GNPs for the investigated electrolytes, and figure of the influence of SRNOM on the fast aggregations of citrateand MUA-coated GNPs induced by Na2SO4. This information is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +43-1-4277-53380; fax: +43-1-4277-9533; e-mail: frank.
[email protected] (F.v.d.K.); Tel.: +43-1-427753320; fax: +43-1-4277-9533; e-mail: thilo.hofmann@univie. ac.at (T.H.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank European Chemical Industry Council (Cefic) for their financial support (Project: Detection, Fate and Uptake of Engineered Nanoparticles in Aquatic Systems). We thank Jonathan Veinot, Chemistry Department of University of Alberta in Edmonton, AB, Canada, for providing the gold stock suspension. We are also grateful to the China Scholarship Council and the University of Vienna for supporting scholarships provided to J.L.
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