Article pubs.acs.org/est
Determining the Concentration Dependent Transformations of Ag Nanoparticles in Complex Media: Using SP-ICP-MS and Au@Ag Core−Shell Nanoparticles as Tracers Ruth C. Merrifield,† Chady Stephan,‡ and Jamie Lead*,† †
CENR, University of South Carolina, Columbia, South Carolina United States PerkinElmer, 501 Rowntree Dairy Road Unit 6, Woodbridge, Ontario L4L8H1, Canada
‡
S Supporting Information *
ABSTRACT: The fate, behavior, and impact of engineered nanoparticles (NPs) in toxicological and environmental media are driven by complex processes which are difficult to quantify. A key limitation is the ability to perform measurements at low and environmentally relevant concentrations, since concentration may be a key factor determining fate and effects. Here, we use single particle inductively coupled mass spectroscopy (SP-ICP-MS) to measure directly NP diameter and particle number concentration of suspensions containing gold−silver core−shell (Au@Ag) NPs in EPA moderately hard water (MHW) and MHW containing 2.5 mg L−1 Suwannee River fulvic acid. The Au core of the Au@Ag NPs acts as an internal standard, and aids in the analysis of the complex Ag transformations. The high sensitivity of SP-ICP-MS, along with the Au@Ag NPs, enabled us to track the NP transformations in the range 0.01 and 50 μg L−1, without further sample preparation. On the basis of the analysis of both Au and Ag parameters (size, size distribution, and particle number), concentration was shown to be a key factor in NP behavior. At higher concentration, NPs were in an aggregation-dominated regime, while at the lower and environmentally representative concentrations, dissolution of Ag was dominant and aggregation was negligible. In addition, further formation of ionic silver as Ag NPs in the form of AgS or AgCl was shown to occur. Between 1 and 10 μg L−1, both aggregation and dissolution were important. The results suggest that, under realistic conditions, the role of NP homoaggregation may be minimal. In addition, the complexity of exposure and dose in dose−response relationships is highlighted.
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INTRODUCTION Manufactured nanoparticles (NPs) are widely used in both industrial processes and consumer products1 and, therefore, have well-controlled properties and characteristics for their specific applications.2−4 Through their use, NPs will inevitably make their way into the environment and are known to be potentially toxic.5 However, it can be expected that that NP concentrations in the environment will be much lower than those produced and used in industry 6 and modeled concentrations are available7 and currently in the 0.05 were considered not significantly different.
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EXPERIMENTAL SECTION NPs. NP suspensions were purchased from Nanocomposix (San Diego, CA, USA) and the National Institute of Standards Technology (NIST, Gaithersburg MD, USA). Core−shell (Au@Ag) 60 nm (core size 30 nm, shell thickness 15 nm) NPs from Nanocomposix were used after dilution between 1 × 104 and 4.4 × 107 particles mL−1 for experiments. The solution concentrations correspond to approximately 0.01 to 46 μg L−1. Further dilution in UHP water of the highest concentrations to 1 × 105 particles mL−1 was performed to allow analysis. The 30 nm Au NIST NPs were used to calculate the transport efficiency20 at a concentration of 1 × 105 particles mL−1. Single-Particle Inductively Coupled Plasma Mass Spectroscopy (SP-ICP-MS). All SP-ICP-MS21−25 data was acquired with a NexION 350D operating in single particle mode with the Syngistix Nano Application Module. The sample introduction system was standard using a Meinhard glass concentric nebulizer, glass cyclonic spray chamber, and 2 mm ID quartz injector. The sample uptake rate was 0.31 mL min−1. Data acquisition conditions were performed at an RF power of 1600 W, 100 μs dwell time, 0 μs settling time and 60−300 s acquisition times. The transport efficiency was found to be media dependent and between 9 and 11% in MHW water and MHW water with FA, respectively, because of the varying viscosities of these media. The measured analytes were Ag 107 and Au 197. All samples were run twice: once while monitoring Ag and once for Au. Each analysis provided the following information: particle size, particle concentration, most frequent size, mean size, and dissolved (ionic) concentration. A 3 min rinse cycle was preformed after each sample to ensure that the sample introduction system was cleaned between each injection. This rinse cycle was important to ensure that all salts, organic matter and metals were removed from the system as a buildup of these could contribute to crosscontamination, clog the nebulizer, or change the transport efficiency. The rinse cycle consisted of 2 min of 1% aqua regia and 1 min of high purity water (this was increased to a 10 min cycle for the 1 × 104 particles mL−1 samples: 5 min 1% aqua regia and 5 min water, due to the increased sampling time and thus the potential for increased contamination). The size and concentration of freshly made Au@Ag solutions and Au 30 nm NIST particles were measured before and after each time point and periodically between exposure samples, as a QA/QC check. In these analyses, the SP-ICP-MS measured one element at a time. While analyzing a Au@Ag core−shell NP by SP-ICP-MS, only the Ag or Au atoms will be detected, even though the entire particle has been fully ionized in the plasma. This results in two sizes of NP, one for the core and one for the shell. The software estimated particle size for the core material will be accurate. However, the geometry of the core−shell NP26 was
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RESULTS AND DISCUSSION By monitoring the particle size and number concentration for both the Ag shell and the Au core over time, the degree of NP transformation (including aggregation, dissolution, and dissolution/reprecipitation) was quantified. Explicitly, we interpreted (1) an increase in both Au and Ag particle size and a corresponding decrease in particle number as an aggregationdominated regime, (2) a decrease in Ag particle size, and stable Au particle size as dissolution-dominated, and (3) a decrease in Ag size, increase in Ag number concentration while the Au size remained stable as dissolution-dominated, followed by transformations, such as reprecipitation of Ag NPs and AgCl particles. It is important to note that geometrical calculations30 (Table S1) show that if a 3 nm shell is removed from a 60 nm B
DOI: 10.1021/acs.est.6b05178 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology
environmentally relevant.32,33 Table 1 lists all the particle concentrations used, their equivalent Ag and Au mass concentrations and the calculated ratio of fulvic acid molecules to NP numbers and surface area for each concentration. We do not assume that all FA is bound to the NP surface and indeed this is unlikely due to the large ratio between available NP surface and number of molecules of organic matter. Figure 1A shows the change in NP diameter over a simulated ecotoxicology exposure period of 48 h for three concentrations of 4.4 × 107, 1 × 105, and 1 × 104 particles mL−1 in MHW (for space reasons concentrations of 2.2 × 107, 2.5 × 105, and 5 × 104 can be found in Figure S3 and Table S3). The measured particle number concentrations for the same samples are shown in Figure 2A. At the highest concentrations, there is a significant increase in both the Ag and Au particle diameter (Figure 1A) and corresponding decrease in particle number concentration (Figure 2A). The particle number drops in a nearly identical manner for both the Ag and Au particles. As these are core−shell particles, this stoichiometric change at high concentration suggests that the particles are primarily undergoing aggregation. This is further supported by the analysis of the size distribution data at 0 and 48 h, Figure 3 (Table S4). Using the geometry of the original particles and the estimated number of atoms in the core and shell, that change appears equivalent to the formation aggregates of doublets and triplets of NPs. The size distribution in MHW at 0 h is similar to that of the NPs in water (Figure S5 and Table S5) showing that the original suspension contains a small amount of aggregated particles already (
