Analysis of Silver Nanoparticles in Antimicrobial Products Using

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Analysis of Silver Nanoparticles in Antimicrobial Products Using Surface-Enhanced Raman Spectroscopy (SERS) Huiyuan Guo,† Zhiyun Zhang,‡ Baoshan Xing,*,† Arnab Mukherjee,§ Craig Musante,§ Jason C. White,§ and Lili He*,‡ †

Stockbridge School of Agriculture, University of Massachusetts, Amherst, Massachusetts 01003, United States Department of Food Science, University of Massachusetts, Amherst, Massachusetts 01003, United States § Department of Analytical Chemistry, The Connecticut Agricultural Experiment Station, New Haven, Connecticut 06511, United States ‡

S Supporting Information *

ABSTRACT: Silver nanoparticles (AgNPs) are the most commonly used nanoparticles in consumer products. Concerns over human exposure to and risk from these particles have resulted in increased interest in novel strategies to detect AgNPs. This study investigated the feasibility of surface-enhanced Raman spectroscopy (SERS) as a method for the detection and quantification of AgNPs in antimicrobial products. By using ferbam (ferric dimethyl-dithiocarbamate) as an indicator molecule that binds strongly onto the nanoparticles, AgNPs detection and discrimination were achieved based on the signature SERS response of AgNPs-ferbam complexes. SERS response with ferbam was distinct for silver ions, silver chloride, silver bulk particles, and AgNPs. Two types of AgNPs with different coatings, citrate and polyvinylpirrolidone (PVP), both showed strong interactions with ferbam and induced strong SERS signals. SERS was effectively applicable for detecting Ag particles ranging from 20 to 200 nm, with the highest signal intensity in the 60−100 nm range. A linear relationship (R2 = 0.9804) between Raman intensity and citrate-AgNPs concentrations (60 nm; 0−20 mg/L) indicates the potential for particle quantification. We also evaluated SERS detection of AgNPs in four commercially available antimicrobial products. Combined with ICP-MS and TEM data, the results indicated that the SERS response is primarily dependent on size, but also affected by AgNPs concentration. The findings demonstrate that SERS is a promising analytical platform for studying environmentally relevant levels of AgNPs in consumer products and related matrices.

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INTRODUCTION The application of silver nanoparticles (AgNPs) in various sectors and consumer products continues to increase at a rapid pace. According to a well-known and publicly available inventory from the Woodrow Wilson Center for International Scholars’ Project on Emerging Nanotechnologies (www. nanotechproject.org), ∼24% of commercially available 1628 nanomaterial-based products (October 2013) contain AgNPs. These applications can be grouped into three categories: food production/packaging, biomedicine and consumer goods. AgNPs uses in the food industry are largely as coatings to prevent bacterial growth. Specifically, applications include coatings on food preparation equipment, refrigerator storage containers and packaging materials.1 Because of the large surface area and high antimicrobial effectiveness, AgNPs have also found many uses in medical products, such as antibacterial surface coatings of stents, breathing tubes, heart valves, catheters, surgical masks, and wound dressings, in spite of their higher cost than ionic silver.2,3 Nanotechnology-based consumer products are third important application of AgNPs, © XXXX American Chemical Society

with widespread usage in personal care and cosmetics, textiles, electronics, household products/home improvement, and filtration/purification/sanitization devices.1 The widespread use is due not only to the higher antimicrobial activity but also to the enhanced thermal and electrical conductivity relative to non-nanoscale forms of the element. However, the large scale application of AgNPs increases the likelihood of human exposure and highlights the importance of thoroughly understanding nanoparticle fate and effects in biological systems. For example, previous studies have shown that AgNPs can enter human body through ingestion, inhalation and dermal contact, causing largely unknown risk and harm.4−6 To accurately assess the environmental and human health risks associated with AgNPs exposure, reliable and robust analytical techniques are necessary to characterize and quantify Received: January 21, 2015 Revised: March 12, 2015 Accepted: March 16, 2015

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DOI: 10.1021/acs.est.5b00370 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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or biological analytes. However, the current investigation is seeking to validate the use of SERS to detect AgNPs in complex matrices. The central hypothesis is that added Raman indicator molecules which bind to the surface of AgNPs will enable the detection of the NPs-indicator complexes. In this study, ferbam (ferric dimethyl-dithiocarbamate) (Figure 1) was chosen as a Raman indicator since it has previously been shown to have strong interactions with AgNPs through thiol groups, producing a significant SERS fingerprint spectrum after complexation with the metal nanoparticle.32,33 We first examined the potential of SERS to discriminate AgNPs from other silver species, including AgCl, Ag ions, and Ag bulk particles. The effectiveness and sensitivity of SERS for detecting AgNPs of different surface coatings, sizes or concentrations was then evaluated. Lastly, the developed approach was used to detect AgNPs in four commercially available antimicrobial products. SERS results were compared against the data from both TEM and ICP-MS.

AgNPs in these products. However, few techniques for identifying and quantifying nanoparticles such as Ag are currently available. In published studies focused on AgNPscontaining consumer products such as textiles and plush toys,7−9 scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS) were used for particle identification and characterization. However, SEM and TEM can only provide size information in highly localized scanning areas and EDS is necessary for elemental identification. In addition, sample preparation and detection processes are labor intensive, somewhat destructive, and may alter the sample in unknown ways. In terms of elemental quantification, graphite furnace atomic absorption spectrometry (GFAA), inductively coupled plasma optical emission spectrometer (ICP-OES) or inductively coupled plasma mass spectrometry (ICP-MS) can be used to determine Ag content. Although highly sensitive, spectrometric methods cannot differentiate AgNPs from silver ions or bulk particles and are less than ideal for providing spatial distribution information within the sample. Laser ablation ICP-MS can be used to gain spatial information on elemental distribution within the sample but still only provides total element content with no information on particle size or transition state.10 Single particle ICP-MS (sp-ICP-MS) can distinguish particle size but the resolution can be low; in addition, this technique requires assumptions about particle morphology and highly complex data analysis.11 Field flow fractionation ICP-MS (FFF-ICP-MS) has superb resolution but is expensive and labor intensive.11 Therefore, the development of a simple, rapid and accurate method for the detection and measurement of AgNPs in complex matrices is greatly needed. Surface-enhanced Raman spectroscopy (SERS) is an emerging technique that combines Raman spectroscopy with nanotechnology. The analytical platform retains the inherent advantages of Raman spectroscopy, including small sample size, minimal sample preparation, rapid spectrum collection and characteristic fingerprint for specific analytes.12 However, SERS also overcomes two major drawbacks of Raman spectroscopy: low sensitivity and fluorescence interference.13 Nanoscale metals, such as silver and gold are common SERS substrates, and can enhance the sensitivity of Raman spectroscopy by as much as 1014 to 1015 fold.14 The enhancement mechanism lies in the electromagnetic field enhancement attributed to localized surface plasmon resonance (LSPR),15,16 as well as chemical enhancement due to charge transfer between the analyte and the substrate.17,18 With these enhancements, SERS is sensitive enough for detecting ultratrace analytes as low as pico- and femto-molar levels, and perhaps even single molecules.19 Slight alterations in molecular orientation and structure can also be resolved by SERS.13,16 In addition, low background autofluorescence can be achieved in SERS by selecting less energetic excitation or by detecting the analytes close to the SERS-active metal surface with a quenching effect.13 These characteristics make SERS a simple, rapid, nondestructive, reliable, and sensitive technique that is finding increasing use in chemistry,20,21 molecular biology,22−24 medicine,25,26 food analysis,27−29 and environmental contaminant detection.30,31 Given that AgNPs possess LSPR and are one of the most commonly used substrates for SERS,29 the method appears to hold great potential as a technique for the detection of AgNPs in consumer products. SERS-based investigations have been focusing on the use of Ag and/or Au nanosubstrates to detect metal-sorbed chemical

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MATERIALS AND METHODS Materials. Acetone (≥99.5%), AgNO3 (99.8%), AgCl (99+ %), 0.5−1 μm Ag powder (99.9%), ferbam, sodium citrate, and Au slides (for SERS) were all purchased from Fisher Scientific (Pittsburgh, PA). 2−3.5 μm Ag powder (≥99.9%) was purchased from Sigma-Aldrich (St. Louis, MO). Citrate-capped AgNPs colloids with a nominal diameter of 20 nm, 60 nm, 100 or 200 nm and a mass concentration of 20 mg/L were purchased from NanoComposix (San Diego, CA); a 60 nm PVP-coated AgNPs solution of similar concentration was also acquired from NanoComposix. The characterization data provided by the vendor is shown in Supporting Information Table S1. Four antimicrobial products claiming to contain AgNPs and intended for human application were separately purchased from http://www.amazon.com and http://www. taobao.com. Additional information about these products is listed below. (a) A throat spray whose label lists colloidal silver (30 mg/L) and deionized water as the only ingredients. (b) A nasal spray which is claimed to contain colloidal silver (10 mg/L) and deionized water. (c) An antifungal/antibacterial disinfecting spray which lists 99.99% pure colloidal silver and ultrapure water as the ingredients. The colloidal silver concentration is listed at 30 mg/L, while average particle size is described as less than 1 nm. (d) An antibacterial hydrogel. “Nanosilver” hydrogel is the only ingredient listed but no size or surface chemistry information can be found. We use “SEJ” to represent this product. Characterization of AgNPs in the Test Products. Transmission electron microscopy (TEM, JEOL JEM2000FX) was used to characterize the AgNPs in the antimicrobial products. To reduce the matrix interference when visualizing the AgNPs by TEM, all four products were diluted with ultrapure water. After dilution, the samples were sonicated by a probe sonicator (Sonic Dismembrator, Model 100) at 100 W and 30 kHz for 5 min to ensure AgNPs dispersion in solution. The TEM samples were prepared by placing aliquots of 10 μL of AgNPs suspension onto copper grids coated by an unbroken carbon film. The samples were placed in a laminar flow hood to air-dry for approximately 3 h. The dry samples were then examined by TEM. ImageJ software B

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Environmental Science & Technology (NIH, Bethesda, MD, http://www.rsb.info.nih.gov/ij) was used to analyze the size distribution of AgNPs in each product based on acquired TEM images. The total silver content in each product was determined by inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7500ce, Santa Clara, CA). The products were acid digested based on the method of Reed et al.44 First, 5 mL throat spray, nasal spray, antifungal spray or 531.0 mg SEJ was digested with 5 mL HNO3 (Fisher Scientific, ACS Reagent) and 10 mL ultrapure water on a hot plate at 200 °C. When the mixture volume was reduced to ∼5 mL, the solution was then diluted with 2% HNO3 to 50 mL. In order to separate silver particles smaller than 200 nm from larger particles in select samples, we passed the solutions of antimicrobial products through 0.2 μm (25 mm diameter) hydrophilic PTFE filters (Millipore Omnipore, Billerica, MA). Before filtration, the three sprays were diluted 1:10 with ultrapure water; 149.6 mg of SEJ was diluted with 100 mL ultrapure water. Subsequently, a 3-KDa-cutoff centrifuge filtering unit (Millipore Amicon Ultra) was used to isolate nanoparticulate silver from ionic silver. The size-fractionized samples were digested using the same method as those without filtration. Conjugation of Ferbam onto AgNPs. AgNPs stock solutions were mixed with 10 mg/L ferbam in a volume ratios of 3:17. The mixture was incubated on a Nutating mixer (Fisher Scientific) at room temperature operating at 24 rpm for 45 min; this was to ensure complete ferbam complexation with nanoparticles through Ag-thiol bond formation. The solution was then centrifuged at 10 000 rpm for 5 min to concentrate the AgNPs-ferbam complexes to the bottom of the vessel. The sediment was then deposited onto a gold slide for drying in a fume hood. Ferbam alone was used as a blank control while mixture of ferbam and AgNO3, AgCl, or Ag bulk particles were considered negative controls. All controls were treated the same as AgNPs stock solutions that were amended with ferbam. In addition, the four commercially available products were amended with ferbam using a method similar to the one described above for the AgNPs stock solutions. Detection of AgNPs by SERS Using Ferbam as an Indicator. The dry samples were immediately analyzed by a DXR Raman Spectro-microscope (Thermo Scientific, Madison, WI). The experimental settings involved a 780 nm laser with an output power of 5 mW, a 10 × confocal microscope objective with 3 μm spot diameter and 5 cm−1 spectral resolution, with a 50 μm slit width for 2 s integration time.45 Long-wavelength excitation (780 nm) was chosen to minimize the photodegradation of ferbam or photochemical transformation of AgNPs resulting from intense irradiation. OMNIC software (version 9.1) was used to manage the detection progress. Eight discrete locations were randomly chosen in each sample and analyzed within a spectrum range of 400−1700 cm−1. Data Analysis. TQ Analyst software (version 8.0, Thermo Scientific) was used to analyze SERS spectral data. The spectra from eight locations in each sample were averaged to get a final spectrum, which was then compared across all samples/ treatments.

Figure 1. Ability of SERS to detect and discriminate AgNPs with ferbam (10 mg/L) as an indicator. The chemical structure of ferbam is shown in the top-right corner.

Figure 2. (A) SERS spectra of PVP- and citrate-AgNPs (60 nm) amended with ferbam. (B) Comparisons of enhancement effects from Ag particles of distinct sizes shown in overlaid spectra. [Ferbam] = 10 mg/L, all the Ag particles were compared at the same concentration (20 mg/L).

silver bulk particles at 20 mg/L. As shown in Figure 1, ferbam alone yielded a negligible Raman signal at this concentration. In fact, only AgNPs were able to enhance the Raman signal of ferbam; this effect was due both to the electromagnetic enhancement induced by LSPR from the nanoparticles and to the chemical enhancement produced by the charge transfer between AgNPs and ferbam through the Ag-thiol groups. The largest peak was located at 1379 cm−1, which can be attributed to the ν(C−N) stretching that was coupled to the symmetric CH3 deformation of ferbam.34,35 The additional peak assign-

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RESULTS AND DISCUSSION Effectiveness of SERS-Based Method to Detect and Identify AgNPs. A 10 mg/L ferbam solution was separately amended with citrate-AgNPs, silver ions, silver chloride, or C

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Figure 4. SERS spectra for four AgNPs-containing antimicrobial products. Negative controls were prepared using the solvent of ferbam (water/acetone, 1:1, V/V).

easily adsorb onto the surface of AgNPs through strong Ag−S interactions regardless of coating type. In AgNPs synthesis studies, 27% stabilizing agents were sodium citrate, the most commonly used one, followed by PVP (18%).38 These results demonstrate the likelihood of this method being widely used to detect synthetic AgNPs. The potential for SERS to detect different size ranges of Ag particles was also evaluated (Figure 2B, Supporting Information Figure S1). Six representative size ranges of Ag particles (20 nm, 60 nm, 100 nm, 200 nm, 0.5−1 μm and 2−3.5 μm) were used to assess the detection range of SERS. For 20, 60, and 100 nm AgNPs, strong and largely consistent SERS responses representing the various vibrational modes of AgNPs-ferbam complexes were observed. Interestingly, the Raman signal intensity increased from 20 to 100 nm, but began to decrease at 200 nm relative to the smaller size particles (Figure 2B, Supporting Information Figure S1). The underlying mechanism of this phenomenon lies in the size-dependent excitation of dipolar plasmons, which play a key role in electromagnetic enhancement and are the major type of LSPR for nanoparticles. Notably, when larger particles are irradiated by optical fields, higher order plasmon multipoles that are nonradiative and inefficient in SERS enhancement results, are induced increasingly with size, ultimately yielding the decreased SERS response,39 as evident in Figure 2B and Supporting Information Figure S1. Little or no signal response was noticeable at the two large size fractions: 0.5−1 μm and 2−3.5 μm. The size-dependent nature of SERS signal enhancement has been reported previously. Moskovits observed that the optimum size of coinage metals (Ag, Au, and Cu) for SERS enhancement ranged from ∼10 to 100 nm.39 Stamplecoskie and Scaiano investigated AgNPs with varying sizes (20.6−69.5 nm) using SERS and found the optimum range to be ∼50−60 nm,40 consistent with our observation. A similar study investigating the SERS activity of Au nanoparticles of different sizes (30−90 nm) demonstrated that SERS intensity increased as particle size increased, indicating that gold nanoparticles displayed particle size dependent response as well.41 Nie and Emory42 used high-resolution atomic force microscope (AFM) to generate images of enhancement-efficient Ag particles, and demonstrated that most particles were within a narrow size range of 110 to 120 nm. This is in general agreement with our work, where 100 nm was shown to be in the optimum size range.

Figure 3. Concentration-dependent SERS spectra of AgNPs (citrate, 60 nm) with ferbam (10 mg/L) as an indicator. (A) Common scale. (B) Full scale. (C) The linear relationship between Raman intensity and AgNPs concentration. The error bars represent the standard errors of five parallel SERS measurements.

ments are listed in Supporting Information Table S2.36 A similar SERS pattern of ferbam was also reported by other studies.32,33,37 Notably, the other three silver species provided no enhancement to the Raman scattering of ferbam, indicating the signal enhancement is indeed nanoparticle-specific and may be used to distinguish AgNPs from other silver species. Detection Capability of SERS for AgNPs of Different Coatings or Sizes. To examine the feasibility of SERS for detecting AgNPs with different coatings, we chose two common nanoparticles for evaluation: citrate-capped and PVP-capped AgNPs. As shown in Figure 2A, SERS had excellent response for both types of nanoparticles. In fact, not only did both particle types yield the characteristic signals listed in Supporting Information Table S2, but the SERS enhancements were quite dramatic. The data indicate that ferbam can D

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Environmental Science & Technology Table 1. TEM, ICP-MS and SERS Data for Four Antimicrobial Products product throat spray nasal spray antifungal spray SEJ a

average size (nm, TEM)a

advertised concentration of AgNPs (mg/L)

± ± ± ±

30 10 30 −b

28.9 33.2 15.4 14.0

0.8 1.0 0.5 0.4

total silver concentration (mg/L)a 29.9 10.2 30.2 340.7

± ± ± ±

0.2 0.1 0.2 3.1c

concentration of Ag (3 kDa-200 nm, mg/L)a

SERS intensity

± ± ± ±

6335 1253 78 141

21.5 6.5 10.0 75.3

0.6 0.1 1.2 2.1c

Mean ± standard Error. bThe total Ag concentration of SEJ was not provided by the vendor. cThe unit is mg/kg.

Figure 5. Characterization of AgNPs in the antimicrobial products. Representative TEM images demonstrate the spherical silver particles detected in four samples are less than 100 nm. (a) Throat spray, (b) nasal spray, (c) antifungal spray, (d) SEJ.

AgNPs Quantitation by SERS. To estimate the sensitivity of this method and determine its suitability for particle quantitation, we investigated the profile of SERS response to 60 nm citrate-capped AgNPs over a concentration range of 0− 20 mg/L (before ferbam addition). As evident in Figure 3, Raman signal intensity was positively correlated with AgNPs concentrations in the tested range (equivalent to 0−3 mg/L of AgNPs in the SERS mixture). This is understandable because higher initial concentrations yield more Ag particles in the system, which are then available to surface associate with additional ferbam molecules. However, SERS can still

sensitively respond to low concentrations of AgNPs and enable differentiation from background levels/response. In Figure 3A, the common-scale spectrum of 0.5 mg/L AgNPs could be easily differentiated from 0 mg/L, whereas in full-scale spectra (Figure 3B), 0.1 mg/L AgNPs had distinct spectrum from the blank control. To further examine the method’s quantitative ability, a linear calibration curve (R2 = 0.9804, Figure 3C) was created based on the Raman intensity of the strong feature band at 1379 cm−1 as a function of AgNPs concentration. The linear relationship between SERS intensity and AgNPs concentration clearly demonstrates the potential of this method E

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Figure 6. Size distribution of AgNPs in four antimicrobial products based on TEM data. For each products, 100 particles were randomly selected for size measurement.

signature AgNPs-ferbam peaks were still clearly observable. In Supporting Information Figure S2, the SERS intensity sequence based on the intensity of the feature signal at 1379 cm−1 is throat spray > nasal spray > antifungal spray. However, the sequence of advertised silver concentration is throat spray = antifungal spray > nasal spray (Table 1). This may be because not all silver is present as nanoscale particles and those that are in the nanoscale may have different size distributions. As noted above, these factors will alter SERS signal intensity and would need to be confirmed by TEM. Validation of the SERS Detection Using TEM and ICPMS. To validate the SERS data, TEM and ICP-MS were used to analyze the sizes and concentrations of AgNPs within the four commercial products (Table 1, Figure 5, and Figure 6). No Au was detected by ICP-MS. The total silver concentrations as determined by ICP-MS in throat spray, nasal spray and antifungal spray were 29.9 ± 0.2 mg/L, 10.2 ± 0.1 mg/L and 30.2 ± 0.2 mg/L, respectively, which were amazingly similar to the label-stated concentrations of 30 mg/L, 10 mg/L and 30 mg/L. The total silver concentration in SEJ was 340.7 ± 3.1 mg/kg; no label-stated concentration was reported. The size partition experiment revealed that silver particles between 3 kDa and 200 nm in throat spray, nasal spray, antifungal spray and SEJ accounted for 71.9%, 63.7%, 33.1%, and 22.1% of the total silver, respectively. For SERS detection, three aqueous products (throat spray, nasal spray and antifungal spray) were used directly while the gel-like SEJ was diluted by adding 1 mL ultrapure water into 265.6 mg product. Therefore, the order of Ag concentrations (3 kDa-200 nm) in the product samples for

to quantify AgNPs in simple media. However, in complex environmental media where the size, shape, composition and aggregation status of the AgNPs are unknown, accurate quantitative measurement will be more difficult. The use of a combination of SERS with TEM to assess Ag particle physical properties will strengthen quantitative estimates. Another method weakness that must be addressed in quantitative analysis is SERS signal variation.31 The variation is mainly from the difficulty in controlling AgNPs aggregation and distribution as the particles are dried on the slide. In this study, we attempted to mitigate the variation by high replicate numbers. Furthermore, evenly distributed nanoparticles may be achieved by solution-based SERS technique, which may significantly decrease the variation and should be explored in the future.43 Detection of AgNPs in Real Antimicrobial Products. In order to validate the applicability of this method for detecting AgNPs in actual consumer product matrices, similar analyses as above were performed on four different antimicrobial products. After interacting with 10 mg/L ferbam, all the antimicrobial products produced signals characteristic of AgNPs-ferbam complexes (e.g., 1379 and 1139 cm−1) (Figure 4), indicating the presence of the metal nanoparticle in these products. In contrast, the negative controls with water/acetone (solvent of ferbam) showed broad, indistinct or low-intensity spectra. After incubation with ferbam (10 mg/L), the spectra of throat spray and SEJ had the similar pattern as AgNPs-ferbam standard solutions, indicating little matrix influence on the detection of nanoparticles. For the nasal spray and antifungal spray, the SERS spectra were more affected by the matrices, but the F

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SERS detection was throat spray (21.5 ± 0.6 mg/L) > SEJ (20.0 ± 0.6 mg/kg) > antifungal spray (10.0 ± 1.2 mg/L) > nasal spray (6.5 ± 0.1 mg/L), which is inconsistent with the sequence of SERS signal intensity reported above. Clearly there may be additional factors influencing SERS performance. Further, we analyzed the size distributions of silver particles using TEM (Table 1, Figure 5 and Figure 6). The average sizes of AgNPs in throat spray and nasal spray were 28.9 ± 0.8 nm and 33.2 ± 1.0 nm, respectively, while smaller AgNPs were present in antifungal spray (15.4 ± 0.5 nm) and SEJ (14.0 ± 0.4 nm). As we discussed above in the section on particle size effects, at equivalent concentrations of AgNPs, SERS response will vary with particle size. Signal intensity escalates with increasing particle size until 100 nm, with an optimum often in the 60−100 nm range. Thus, it is reasonable to speculate that throat spray and nasal spray would produce stronger SERS signals (closer to the optimum) than antifungal spray and SEJ at the equivalent concentrations. Interestingly, the nasal spray had a lower concentration of SERS-active Ag particles than both SEJ and antifungal spray. The reason why it induced a higher intensity of SERS signals is likely due to the particle size effects. However, for instances where the particle sizes are similar, the concentration of Ag (3 kDa-200 nm) will dominate the nature of the SERS response. That is why the throat spray with higher concentration generated stronger response than the nasal spray and similar phenomenon can be found with the SEJ and antifungal spray. Overall, the SERS spectra accurately reflected the size and concentration information on Ag particles in antimicrobial products and the findings were consistent with the ICP-MS and TEM data.

AUTHOR INFORMATION

Corresponding Authors

*(B.X.) Phone: +1 413 545 5212; fax: +1 413 577 0242; e-mail: [email protected]. *(L.H.) Phone: +1 413 545 5847; fax: +1 413 545 1262; email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge USDA NIFA Hatch Program (MAS00475) and USDA-NIFA (2011-67006-30181 and 2015-67017-23070) for supporting this work.

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ENVIRONMENTAL IMPLICATION The SERS-based method developed in this study not only shows high potential for differentiating AgNPs from other silver species, but also has applicability for detecting AgNPs of different coatings, sizes, and concentrations. The underlying mechanism for the detection of AgNPs is mainly based on the distinct optical properties of AgNPs, namely the LSPR and induced local electromagnetic field enhancement. The strong interactions between AgNPs and ferbam through Ag-thiol binding make ferbam a reliable indicator for the presence of the metal NPs. The direct environmental significance of the established SERS method is that once optimized, it can be used as a fast, simple and sensitive technique to detect AgNPs with minimal sample preparation. In this work, we successfully measured AgNPs in four antimicrobial products using SERS without removing matrices and the findings were successfully validated by ICP-MS and TEM. Overall, the method developed in this study opens a new analytical window for reliably determining AgNPs in consumer products and potentially other complex matrices. Furthermore, the SERS method provides a platform for investigating the interactions of AgNPs with coexisting organic compounds, such as pesticides and sulfhydryl-containing natural ligands that will likely alter the fate, behavior and toxicity of AgNPs in the environment and biota.

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ASSOCIATED CONTENT

S Supporting Information *

Characterization information on AgNPs stock solutions and additional results. This material is available free of charge via the Internet at http://pubs.acs.org. G

DOI: 10.1021/acs.est.5b00370 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.est.5b00370 Environ. Sci. Technol. XXXX, XXX, XXX−XXX