A Raman-Based Imaging Method for ... - ACS Publications

C. Stahler, John Ryan, Steven R. Higgins, and Ioana E. Sizemore*. 4. Department of Chemistry, Wright State University, 3640 Colonel Glenn Hwy., Dayton...
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Article Cite This: Environ. Sci. Technol. 2018, 52, 2854−2862

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A Raman-Based Imaging Method for Characterizing the Molecular Adsorption and Spatial Distribution of Silver Nanoparticles on Hydrated Mineral Surfaces Seth W. Brittle, Daniel P. Foose, Kevin A. O’Neil, Janice M. Sikon, Jasmine K. Johnson, Adam C. Stahler, John Ryan, Steven R. Higgins, and Ioana E. Sizemore* Department of Chemistry, Wright State University, 3640 Colonel Glenn Highway, Dayton, Ohio 45435, United States S Supporting Information *

ABSTRACT: Although minerals are known to affect the environmental fate and transformation of heavy-metal ions, little is known about their interaction with the heavily exploited silver nanoparticles (AgNPs). Proposed here is a combination of hitherto under-utilized micro-Raman-based mapping and chemometric methods for imaging the distribution of AgNPs on various mineral surfaces and their molecular interaction mechanisms. The feasibility of the Raman-based imaging method was tested on two macro- and microsized mineral models, muscovite [KAl2(AlSi3O10)(OH)2] and corundum (α-Al2O3), under key environmental conditions (ionic strength and pH). Both AgNPs− and AgNPs+ were found to covalently attach to corundum (pHpzc = 9.1) through the formation of Ag−O−Al− bonds and thereby to potentially experience reduced environmental mobility. Because label-free Raman imaging showed no molecular interactions between AgNPs− and muscovite (pHpzc = 7.5), a label-enhanced Raman imaging approach was developed for mapping the scarce spatial distribution of AgNPs− on such mineral surfaces. Raman maps comprising of n = 625−961 spectra for each sample/control were rapidly analyzed in Vespucci, a free open-source software, and the results were confirmed via ICPOES, AFM, and SEM-EDX. The proposed Raman-based imaging requires minimum to no sample preparation; is sensitive, noninvasive, cost-effective; and might be extended to other environmentally relevant systems.

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INTRODUCTION

diverse soil components as well as the lack of cost- and timeefficient methodologies for examining these aspects.5−12 Current research suggests that released AgNPs are most likely to be immobilized in soils due to their adsorption to natural organic matter (NOM) and minerals.6−8 Most of these studies focused on the adsorption of AgNPs to NOM because of their high chemical affinity for the sulfhydryl-rich functional groups present in NOM.7−9 Because NOM constitutes only ∼5% of soil,10 it is important to also examine the interaction of AgNPs with the main soil component, minerals (∼45%). Although there is extensive research available on the interactions between metal ions and soil minerals, only a few studies were reported on nanometals.11−13 The research that does exist on AgNPs is generally focused on simulations of ideal interaction scenarios or employs advanced methodologies (e.g., X-ray absorption spectroscopy and electron microscopy) that usually require extensive sample preparation and expensive resources.8,14−25 Proposed here is a combination of hitherto

The expansion of nanotechnologies in consumer products has raised increasing concern about their potential impact on human and environmental health. Special emphasis has been placed on silver nanoparticles (AgNPs); 54% of the total number of consumer products containing nanomaterials primarily harness the antimicrobial properties of AgNPs. About 1230 tons of the total silver produced worldwide is allocated to the fabrication of AgNPs.1 The increased use of AgNPs may result in a high Ag content for the biosolids produced by wastewater treatment plants (WWTP). For example, a 2010 study reported that the level of AgNPs released from antibacterial athletic socks was significant2 but removable by WWTPs.2 The resulting biosolids may then be sold as fertilizer, thus providing a pathway for these engineered nanomaterials to be introduced into soils through irrigation and rainfall. Numerous studies have already examined the health effects of AgNPs on both aquatic and terrestrial organisms, including humans, and primarily attributed their toxicity to the release of Ag+ ions.3−5 However, the impact AgNPs have on environmental health remains under investigation in view of their possible transformations during the interaction with © 2018 American Chemical Society

Received: Revised: Accepted: Published: 2854

October 3, 2017 December 22, 2017 January 31, 2018 January 31, 2018 DOI: 10.1021/acs.est.7b04884 Environ. Sci. Technol. 2018, 52, 2854−2862

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Environmental Science & Technology

[KAl2(AlSi3O10)(OH)2] were selected because they contain several of the most abundant elements in the earth’s crust (O, Al, and Si).10 In addition, both minerals are rock-forming and encountered in streams and sands.10 While the Creighton synthesis is one of the most widely used bottom-up fabrication approaches of colloidal AgNPs of negative charge due to its simplicity, time, and cost efficiency,14,29 the Sun method is generally employed for the synthesis of AgNPs of positive charge.36,37 A concentration of 1 mg L−1 of AgNPs was used in order to ensure submonolayer coverage at the mineral surface and to surpass the maximum contaminant level (MCL) set by the U.S. Environmental Agency (EPA) for Ag+ in drinking water from both natural and anthropogenic sources (0.1 mg L−1).38 The interaction mechanism of corundum and muscovite in macro form was investigated at the pH of the colloid (8.2 for Creighton and 6.9 for Sun) because both minerals have a pH at point of zero charge (pHpzc of 9.1 for corundum39 and 7.5 for muscovite40) close to this pH value. For illustrative purposes, the interaction mechanism between corundum in micro form and AgNPs was also investigated in the pH range from 6 to 11, typical of the pH of soils (3.5−9).10,41 Aggregation and settling of AgNPs in the corundum mixtures were noticed at smaller (11) pH values. In addition, several other wellestablished techniques, like inductively coupled plasma optical emission spectroscopy (ICP-OES), atomic force microscopy (AFM), and scanning electron microscopy coupled with energy dispersive X-ray (SEM-EDX), were employed to confirm and complement the Raman results.

under-utilized micro-Raman-based mapping and chemometric methods15−17 for the simple and cost-effective imaging of the spatial distribution of AgNPs to various mineral surfaces and their molecular interaction mechanisms. It was suggested that AgNPs released into the environment through wastewater, fertilizers, or landfill runoff might exist in multiple forms.8 AgNPs may persist in their nanoscale form, oxidize to aqueous Ag+, aggregate, or precipitate into salts (Ag2S, AgCl, Ag2SO4, AgOH, Ag2CO3, and Ag2O).3,8,18 Thus, a methodology capable of observing AgNP distribution on mineral surfaces and their molecular interaction mechanisms is essential to the determination of their fate and transport. Raman and surface-enhanced Raman spectroscopy (SERS) offer a unique approach because of their molecular fingerprinting and multiplex detection capabilities, nondestructive and aqueous compatible nature, and little to no requirement for sample preparation.19−30 Furthermore, modern Raman systems can collect multiple point spectra in x,y-raster patterns (Raman maps) to micro- or nanomap molecular interactions across large surfaces. The herein proposed Raman-based imaging approach is novel in that it offers the possibility of performing either label-free or label-enhanced SERS measurements. In general, minerals (e.g., quartz, calcite, and corundum) have been decorated with noble-metal nanoparticles (e.g., silver and gold) for the development of new SERS platforms, and in this context, they have been examined through the acquisition of Raman control spectra.31−35 However, no approaches were yet proposed for the rigorous mapping of the molecular adsorption and spatial distribution behavior of AgNPs onto environmentally relevant mineral surfaces. Here it is demonstrated that the proposed Raman-based imaging methodology can achieve these goals for different types of minerals. If no direct molecular interaction occurs between minerals and AgNPs, a Ramanactive label of large scattering cross-section [e.g., rhodamine 6G (R6G) dye] and high affinity toward AgNPs may be utilized to make AgNPs “Raman-visible” and to indirectly image their spatial distribution onto mineral surfaces with increased sensitivity. Our group has already reported single-molecule SERS detection events of R6G (1 fM) adsorbed onto Creighton AgNPs under the excitation of visible laser lines and with less than 20 s acquisition times.23 When a target species is located in the immediate vicinity of an individual AgNP or at the nanosized interstitial site of aggregated AgNPs, the SERS effect occurs and further boosts the sensitivity of the Raman-based detection method to unprecedented levels.19−22 The SERS enhancement is largely due to the increase in the magnitude of both the incident and the scattered electromagnetic fields resulting from the excitation of localized surface plasmon resonances (LSPR) of AgNPs.21,23−26 Thus, the labelfree and label-enhanced Raman mapping of mineral surfaces exposed to AgNPs results in the collection of a large number of SERS spectra characteristic of the direct or indirect interaction of the two systems, respectively. An additional element of novelty in this study is the coupling of Raman mapping with chemometric methods for the facile hyperspectral analysis of large volumes of Raman data in a reproducible manner. This was achieved using a free, open-source, stand-alone analysis software package, Vespucci.27 The feasibility of the Raman-mapping chemometric method was demonstrated on two representative minerals in micro and macro form and unfunctionalized (Creighton) AgNPs− and cetyltrimethylammonium bromide (CTAB)-functionalized (Sun method) AgNPs+. Corundum (α-Al2O3) and muscovite

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MATERIALS AND METHODS Single-crystal muscovite and corundum were purchased from Ward’s Earth Science and Marketech International, respectively. Macro-muscovite (i.e., single-crystal) was prepared by freshly cleaving 10 × 10 mm squares. Macro-corundum (i.e., singlecrystal) consisted of cylindrical, single α-phase crystals cut and polished on the (1120̅ ) plane, 25 mm wide and 5 mm thick. Micro-corundum was purchased as fused, 1-μm-sized particles (99% fused α-Al2O3) from Alfa Aesar. High-quality (HQ) water (18.2 MΩ cm) was produced in a LabConco system and utilized throughout the course of all experiments unless otherwise specified. All other materials were purchased from Fisher Scientific and used without further modification unless specified. Synthesis and Characterization of AgNPs. Negatively charged, colloidal AgNPs were fabricated using a slightly modified Creighton method in water.19,29,30 Specifically, a 2:1 mM ratio of sodium borohydride (300 mL of 2 × 10−3 M of NaBH4) to silver nitrate (50 mL of 1 × 10−3 M of AgNO3) solutions was used to minimize the amount of excess reagents and byproducts. Positively charged, colloidal AgNPs were synthesized using a slightly modified Sun method in water through the reduction of AgNO3 (300 mL of 5 × 10−3 M) with CTAB (20 mL of 2 × 10−2 M) and NaBH4 [0.2 mL of 1% (w/ v)].36,50 The physicochemical properties of AgNPs were then characterized by Raman spectroscopy, ICP-OES, AFM, SEMEDX, and UV−vis absorption spectroscopy. Sample Preparation. The flat macro-corundum surfaces were sequentially washed with acetone (HPLC grade), methanol (HPLC grade), and nitric acid (70% OPTIMA grade) in a sonic bath for 10 min each. Next, they were annealed in air at 1250 °C for 12 h to provide a “clean” terraced surface. Both macro-corundum and macro-muscovite samples were then submerged in the colloid (100 mL of 1 mg L−1 of 2855

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Environmental Science & Technology AgNPs−) mixed with 100 μL of 5 M of sodium nitrate (NaNO3), as an ionic strength adjuster (ISA), in order to simulate the ion strength of freshwater environmental samples. Macro-muscovite samples were removed after 30 min and dried with a stream of nitrogen gas. Macro-muscovite received an additional exposure to rhodamine 6G (R6G), a standard SERS probe, in order to make adsorbed-AgNPs− Raman “active”. Briefly, 10 μL of 10−4 M of R6G solution was dispensed onto the mineral covering a surface area of ∼1 cm2. Next, the surface was washed three times with water after 30 min incubation. Control groups with ISA included mineral exposure to water, CTAB (for AgNPs+ only), AgNPs (positively or negatively charged), or 10−3 M of R6G (for macro-muscovite only). Approximately 1.2 g of micro-corundum was incubated with 100 mL of 1 mg L−1 of AgNPs−, 100 μL of 5 M of NaNO3, and pH adjusters (5−100 μL of 0.1 M of HNO3 or 0.1 M of NaOH). This amount of micro-corundum yields submonolayer coverage when assuming total AgNP− adsorption and a specific surface area (SSA) of 6.5 ± 0.2 m2 g−1. Similar experiments were carried out with CTAB-functionalized AgNPs+ for illustrative purposes. The pH was measured with a SevenGo Duo pro model pH meter that was calibrated daily with four pH buffers (4.0, 7.0, 10.0, and 12.0). After stirring for 30 min, the samples were centrifuged for 2 min at 5000 G in an AccuSpin Micro 17/17R model centrifuge. The centrifuge supernatants containing free, unbound AgNPs− were saved for ICP-OES analysis, while the centrifuge pellets consisting of corundum particles with bound AgNPs− were collected for Raman imaging. Control groups with ISA included mineral exposed to AgNPs− and hydrated mineral only at each experimental pH value (6, 7, 8, 9, 10, and 11). Specific Surface Area (SSA) Analysis. BET42 surface area analysis was performed on the corundum powder using a Micromeritics Tristar II 3020 surface area and porosity analyzer. A five-point N2 BET isotherm with relative pressures in the range P/Po = 0−0.25 was used for the analysis, where Po is the saturation pressure. Prior to the analysis of the sample material, a standard reference material specified as alumina powder was measured to validate the method employed. The standard alumina was determined to have a specific surface area of 0.28 ± 0.02 m2 g−1, which compared well with the specified surface area of 0.29 ± 0.02 m2 g−1. The Alfa-Aesar corundum was found to have a specific surface area of 6.5 ± 0.2 m2 g−1, which is within the range of 6−8 m2 g−1 specified by the supplier. Raman Spectroscopy Analysis. Raman data were collected using a LabRam HR800 Raman system coupled to an Olympus BX41 confocal microscope (100× objective) and a motorized stage. Samples were irradiated with HeNe (632.8 nm) and Nd:YAG (532.134 nm) lasers with powers at the sample of 15 and 19 mW, respectively. The 532 nm excitation line is close to the R6G resonance at 530 nm (resonant conditions). The backscattered photons were measured using a thermoelectrically cooled Andor CCD camera with a resolution of 1024 × 256 pixels. The following parameters were selected: 300 μm confocal hole, 600 grooves mm−1 holographic grating, 3 s (muscovite) and 1 s (macro-corundum) acquisition times, and two or three averaging cycles. Under these conditions, the spectral resolution, i.e., the distance between points on the spectral abscissa, was ∼1.18 cm−1. For each macro-muscovite sample, n = 900 spectra were measured in a 150 × 150 μm grid, at 5 μm increments. For each macro-corundum sample, n = 961 spectra were collected in a 31 × 31 μm grid, at 1 μm

increments. The Raman system proved successful in the collection of data with nanospatial resolution but at higher time costs. All spectra were collected in the 100−1700 cm−1 spectral range. Samples from the micro-corundum experiment were smeared onto clean glass slides and 11 × 11 μm areas were mapped in 1 μm increments for each sample to obtain a representative molecular picture of the interaction between AgNPs− and corundum particles. Because increased dispersity was anticipated with these nonflat samples, statistical confidence was strengthened by measuring n = 3 maps from n = 3 individually prepared samples, totaling nine maps for each pH (i.e., n = 1089 spectra for each pH). For illustrative purposes, point Raman spectra were also collected from random areas on n = 3 independently prepared microcorundum samples exposed to AgNPs+ in the presence of ISA, at different pH values (6−11). Raman Data Analysis. Raman maps were processed in Vespucci.27 All spectra were smoothed with a median filter (window size 7). Muscovite spectra were baseline-corrected using a rolling-ball approach43 in order to remove the fluorescent background and then normalized to the 1-norm. Univariate analysis to determine peak area and intensity against a local linear baseline was performed on the following regions of interest: the Ag−O stretch (235 cm−1) arising from interaction between AgNPs and the corundum surface in corundum samples and several R6G bands19,23 in the muscovite samples. Macro-corundum spectra were min/max-normalized by subtracting the minimum value of the spectrum from each value of the spectrum and then subsequently dividing by the maximum value of the spectrum, so that the smallest value of each spectrum was 0 and the largest value of each spectrum was 1. The micro-corundum data were further analyzed using the Kruskal−Wallis test, with the Dunn’s test performed posthoc to determine the significance of differences in the Raman signal between pH groups. AFM Analysis. Topographic images were created with an Agilent AFM operated in intermittent contact mode (i.e., ac or tapping mode), in air. Cantilevers were obtained from Nanoworld (NCHR, Pointprobe, noncontact mode) and were fabricated from single-crystal Si and coated with Al, with a nominal resonance frequency of 320 kHz and a nominal force constant of 42 N/m. Scanning speeds were typically set to 1−2 Hz and 2 × 2 μm images of 256 × 256 pixels were recorded to adequately display features between 1 and 100 nm (i.e., AgNPs−). AgNP− adsorption was then interpreted by differentiating topographic profiles inbetween controls and samples. ICP-OES Analysis. Quantitative characterization of the total Ag content reacted with the microsized corundum was determined by difference using a Varian 710 ICP-OES system. Briefly, original colloids and supernatant samples containing free, unbound AgNPs− were chemically digested and diluted in trace-metal-grade nitric acid (HNO3) following the US EPA method 200.11.44,45 An 11-point external calibration curve (0, 5, 10, 15, 20, 25, 50, 75, 100, 125, and 150 μg L−1) was then constructed, and the Ag concentrations were determined by interpolation from the calibration curve. SEM-EDX Analysis. Prior to the data collection with an FEI Quanta FEG 250 SEM system, all samples were sputter-coated with 2 nm of Ir to increase their stability during exposure to relatively high beam voltages. Each sample was then loaded onto an aluminum stub using Cu tape and exposed to a 6 kV electron beam at a working distance of 10 mm. An EDAX EDS 2856

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Environmental Science & Technology detector coupled to the microscope was utilized in the collection of spot-size EDX spectra. At least 20 000 counts were obtained from the dominant element present within each spectrum. A holographic peak deconvolution was then employed to qualify spectra by ruling out noise and validating the surface signal.

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RESULTS AND DISCUSSION Synthesis and Characterization of AgNPs. Three batches of AgNPs were synthesized for each experiment and were characterized in accordance with the US EPA’s recommendations.46,47 Both AgNPs− and AgNPs+ exhibited a surface plasmon resonance peak at ∼400 nm, in good agreement with the literature.50 ICP-OES revealed an average Ag colloidal concentration of 15.4 ± 0.8 and 300.6 ± 0.6 mg L−1 for AgNPs− and AgNPs+, respectively. Raman spectroscopy confirmed (a) the absence of organic impurities or silver oxide peaks in the colloids and (b) the presence of CTAB at the AgNP+ surface [Figure S1, Supporting Information (SI)]. Electron microscopy data showed that AgNPs− are spherical and have an average diameter of 14.1 ± 13.4 nm and a moderate breadth size distribution in the 1−100 nm range.23 The specific AgNP− surface area was then estimated to be 20.9−814.8 m2 g−1 on the basis of this size range and assumed sphericalness.48 AgNPs+ were also found to be spherical but had a narrower size distribution (1−40 nm) than AgNPs−.50 ζPotential measurements demonstrated that both AgNPs are charged and stable at the experimental pH of this study (ζpotential of −41.47 mV at pH 8.2 for AgNPs− and +34 mV at pH 7 for AgNPs+).14,48 These colloidal AgNPs contain less than 10% Ag+ ions.49,50 Macro-Muscovite. The label-free Raman spectra of macromuscovite exposed to AgNPs− indicated no direct covalent interaction between the two (Figure S2, SI). Thus, labelenhanced Raman spectra were acquired on these samples using R6G as a SERS label of large scattering cross-section in order to indirectly image the AgNP− distribution on the macromuscovite surfaces. It is known that R6G cations have a high affinity toward the negatively charged AgNP surface, where they experience large Raman signal enhancements associated with the SERS effect.19 As a result, R6G fluorescence is quenched and detailed SERS spectra may be collected from R6G molecules located in the immediate vicinity of individual AgNPs− or AgNP− aggregates interacting with mineral surfaces. Raman controls (n = 900 spectra) consisting of hydrated macro-muscovite surfaces (no AgNPs− or R6G) exhibited solely the characteristic vibrational modes of muscovite (e.g., 170, 170, 195, 215, 267, 382, 413, 638, 707, 757, 915, 959, and 1120 cm−1), which were identified in good agreement with the literature51−54 (Table S1, SI). Macro-muscovite controls with just AgNPs− yielded no detectable additional peaks and appeared vibrationally similar to the muscovite control (Figure S2, SI). The macro-muscovite control sample exposed to 10−3 M R6G (no AgNPs−) displayed a fluorescent background with no characteristic Raman R6G peaks. In order to mitigate R6G fluorescence in the label-enhanced SERS spectra, R6G concentration was lowered to 10−4 M (R6G footprint sufficient for AgNP− coverage). The R6G−AgNP−−mineral samples revealed significant differences with respect to controls: strong Raman spectral signatures of R6G and muscovite were simultaneously detected [Figures 1 and S2 (SI)]. The loadings of all principal components (PCs) that accounted for >1% of variance exhibited both R6G and muscovite Raman signatures.

Figure 1. (A) Raman images of four R6G−AgNP−−macro-muscovite samples (25 × 25 μm each) constructed from the first PC and (B) the loading for the first PC corresponding to R6G and muscovite signals. The sample was prepared by immersing muscovite into 1 mg L−1 of AgNPs− for 30 min. Colors are mapped to the score for the first PC (accounting for 46% of variance), with purple regions representing spectra with negative scores (R6G absorbed onto AgNPs−) and brown regions representing spectra with positive scores (muscovite only). A larger score in the first PC is associated with a stronger muscovite Raman signature relative to the R6G signature, as seen in the plot of the first PC loading.

The presence of strong R6G peaks (e.g., 611, 776, 1310, 1361, 1508, and 1652 cm−1)19,23,55,56 indicates that enhancement had occurred in the areas where R6G-labeled AgNPs− have interacted with muscovite. Because no other Raman bands (e.g., Ag−O stretching mode) were detected in the n = 3600 analyzed spectra, the interaction between AgNPs− and muscovite was deemed to be of a noncovalent nature. Macro-Corundum. The interaction mechanism and spatial distribution of AgNPs− on the mineral surface were investigated via label-free Raman imaging [Figures 2 and S3 (SI)]. The corundum controls exhibited the typical corundum peaks from the literature (e.g., 379, 417, 429, 451, 576, 644, and 750 cm−1, Figure S3 and Table S2, SI),57−59 while corundum exposed to AgNPs− revealed additional molecular vibrations in the 225− 255 cm−1 spectral range, matching the literature for Ag−O stretching modes.20,23,60 Therefore, a covalent attachment of AgNPs− to corundum was inferred. An illustrative Raman image is given in Figure 2, where the gray-black areas correspond to the presence of corundum only (quantified through the integrated Raman area). The spectrum containing the most intense Ag−O peak from n = 961 analyzed spectra is represented by the white pixel in the chemical image. The high 2857

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Figure 2. (A) Raman image of a AgNP−−macro-corundum window (52 × 52 μm) prepared by immersing corundum into 1 mg L−1 AgNPs− for 30 min. The white pixel (4 μm2) corresponds to the covalent attachment of AgNP(s)− to the mineral surface, while the gray-black pixels are indicative of corundum alone. (B) The corresponding Raman spectrum illustrates the appearance of a Ag− O stretching mode at 244 cm−1 and confirms the presence of corundum. The spectrum with the minimum Euclidean distance to the mean of all spectra in the image is shown.

sensitivity of the Raman-based imaging methodology was demonstrated through the detection of scarce molecular interaction events of AgNPs− on the macro-mineral surface. Further AFM and SEM-EDX analyses were then carried out to complement the Raman results and to further interrogate the interaction mechanism. Topographic AFM profiles of corundum windows revealed a step and terrace structure on the (112̅0) plane surface (Figure 3). Control hydrated mineral surfaces originally appeared devoid of any nanoscale features but exhibited protrusions and holes characteristic to macro-minerals (Figure 3A). The addition of ISA (NaNO3) to the control mineral surfaces resulted in the formation of a few nanoscale features (n = ∼ 2.5/μm2, 1−30 nm, Figure 3B). This is probably due to the precipitation of the electrolyte from the thin water film that was not completely removed with the stream of nitrogen gas during drying. The EDX elemental analysis (Figure S3C, SI) confirmed the chemical nature of the ISA-derived nanostructures (Na peak at 1.05 keV); they appeared as white features in the collected SEM images (Figure S3A, SI). The corundum windows were then submerged in 1 mg L−1 colloidal AgNPs− for 30 min (with 0.005 M NaNO3 at pH 8) and dried again with nitrogen gas. The resultant AFM images of the exposed windows showed AgNPs− of a larger size distribution (n > 10/ μm2, 1−100 nm) on the mineral surfaces without any real indication of avoidance of or preference for adsorption at the

Figure 3. AFM topographic images (2 × 2 μm) of macro-corundum windows. (A) Mineral control: a water-washed and annealed mineral window displaying step-and-terrace structure with minimal nanosized features. (B) Mineral control with ISA (no AgNPs−): the same window submerged in 0.005 M NaNO3 for 30 min leading to the formation of a few nanosized features (white spots) due to the NaNO3 precipitation during the evaporation of the solution film on the surface. (C) Mineral sample with AgNPs−: the same window submerged in 1 mg L−1 AgNPs− generating additional, larger nanosized features (white spots).

step edges (white features, Figure 3C). SEM-EDX measurements (Figure S4, SI) further established the presence of AgNPs− (Ag peak at 2.99 keV) on the mineral surface (O and Al peaks at 0.53 and 1.5 keV, respectively). 2858

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Figure 4. Raman data and their statistical analysis for AgNP−−micro-corundum at all experimental pH values (6−11). Average Raman spectra of (A) corundum control with ISA and (B) corundum sample with ISA exposed to 1 mg L−1 AgNPs− for 30 min at each pH. (C) The total number of Raman spectra of the AgNP−−micro-corundum samples exhibiting Ag−O stretching modes at each pH value. (D) Mean of the integrated area of the Ag−O stretching mode at each examined pH value with 95% confidence intervals. The values obtained at each pH represent an average of n = 1089 spectra from n = 3 separate trials.

To further analyze the interaction mechanism, the total number of Raman spectra exhibiting the Ag−O stretching mode was plotted as a function of pH (Figure 4C). Although the largest number of AgNP−−corundum interactions was detected at pH ≥9 (n = 902−1015 spectra) when compared to all other examined pH values (n = 796−862), the difference was not statistically significant (p > 0.05). Thus, the adsorption process of AgNPs− was not deemed to be pH-dependent under the employed experimental conditions, i.e., 1.2 g of microcorundum particles (SSA ∼ 6.5 ± 0.2 m2 g−1) exposed to 100 mL of 1 mg L−1 AgNPs− for 30 min at each pH. The integrated area under each Ag−O peak was also estimated to further confirm this pH-independent behavior (Figure 4D). Again, the differences among all pH values were not statistically significant (p > 0.05), despite pH 8 and 9 yielding the largest average integrated area. Similar Ag−O interactions were detected between AgNPs+ and micro-corundum at each pH (Figure S1, SI), but no statistically significant trend could again be established with the change in pH. It should be noted that possible trends in the integrated area of the corresponding Ag− O stretching peak (235−250 cm−1) might have been skewed by the presence of a small CTAB vibrational mode at ∼238 cm−1 (Figure S1, SI). Future work that establishes the relative contributions of CTAB to the Ag−O band profile, such as a pH dependence study on CTAB alone or the use of different AgNPs+ colloidal systems, would lead to a better quantitative description of the Ag−O interactions.

The macro-corundum experiment was conducted at pH 8, but a wide range of pH values are possible in natural aquatic systems. Because mineral surface charge and particle oxidation depend on pH, the next set of experiments were conducted on micro-corundum in the absence and presence of AgNPs− and AgNPs+, in the pH range of 6−11. Furthermore, multiple crystalline faces exposed on mineral grains are more common in natural systems. Thus, the incorporation of a micromineral model can offer additional insight into the mineral−AgNP interaction behavior, where additional crystallographic planes other than the (1120̅ ) surface are exposed. Micro-Corundum Particles. As expected, no Ag−O peaks were observed in the micro-corundum Raman controls similarly to macro-corundum (Figure 4A), when no AgNPs were present. Upon exposure to AgNPs−, a Ag−O stretching band appeared at ∼230 cm−1 at each pH (Figure 4B), confirming the covalent interaction between AgNPs− and corundum (for both micro- and macro-corundum). However, for n > 0.73, more Ag−O interactions per μm2 were detected in the micro- than macro-corundum samples, probably due to the different number of lattice planes exposed at the surface; AgNP− concentration and mineral surface area were factored into these calculations. While macro-corundum has only a single lattice plane exposed at the surface [an ordered lattice in the aplane (112̅0)61], micro-corundum has many different lattice planes that could potentially favor the Ag−O interactions. 2859

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Environmental Science & Technology Because the Raman experiments conducted on both macroand micro-corundum revealed the appearance of Ag−O stretching modes, the surface complexation of AgNPs− to the terminus corundum oxygen atoms (Ag−O−Al−) was considered to play a key role in their interaction. Furthermore, the highest chemisorption level was detected around a pH value of 9, where corundum has approximately zero net surface charge and other interaction mechanisms may be at play. This is illustrated in the proposed mechanistic scheme (Figure 5). In

Figure 6. Average percent adsorption of AgNPs− to micro-corundum as a function of pH. The Ag content was determined by ICP-OES after 30 min incubation of 1.2 g of corundum particles (SSA of 6.5 ± 0.2 m2 g−1) with 100 mL of 1 mg L−1 AgNPs− followed by the separation of free AgNPs− from corundum-bound AgNPs− through centrifugation. Error bars represent the standard deviation of n = 9 independently prepared samples.

occurring at each experimental pH. For example, at pH >9.1, corundum particles have an overall negative charge and are thereby expected to experience electrostatic repulsions to the negatively charged AgNPs−. Nevertheless, over 80% of the total amount of available AgNPs− was found to be adsorbed on the mineral surface at these pH values (ICP-OES data). Thus, the chemisorption of AgNPs− to corundum through the formation of Ag−O covalent bonds (Raman data) may offset to a certain degree the contributions of the electrostatic repulsions to the overall interaction scheme. Future work that establishes the relative amounts of AgNP− sorbed via electrostatic and chemisorption interactions, such as via a study of the effects of ionic strength on sorption, would lead to a better quantitative description of the speciation of sorbed AgNPs−. On the other hand, at pH 9.1 and 0.05). Again, this is likely due to the multiple interaction mechanisms possibly 2860

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Environmental Science & Technology

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characteristics (e.g., size, concentration, surface charge, and surface coating) could also be explored. In addition, both cationic and anionic Raman labels could be employed to further investigate the interactions with mineral surfaces of permanent negative or positive charge. The knowledge gained from such Raman-based imaging projects could raise awareness with respect to the safe manufacture, processing, and disposal of engineered nanomaterials into the environment.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.7b04884. Raman spectra of micro-corundum exposed to CTABcapped AgNPs+ with ISA and the corresponding controls at different pH values, representative Raman spectra of muscovite controls, a Raman image of a AgNP−−macrocorundum control window, assignments of the main Raman vibrational modes observed for the muscovite and corundum controls, backscattered SEM images, and EDX spectra of micro-corondum exposed to AgNPs− with ISA and the corresponding controls (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected]. ORCID

Steven R. Higgins: 0000-0003-0609-9139 Ioana E. Sizemore: 0000-0002-9494-5843 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Chemistry Department at WSU and the U.S. National Science Foundation Award #1438340 are gratefully acknowledged for the financial support of this project. Garrett VanNess and Joseph Solch are thanked for their assistance in the maintenance and operation of the ICP-OES system at WSU. Dr. Dhriti Nepal is acknowledged for providing the access to the SEM-EDX system at Wright Patterson Air Force Base, Ohio. The authors are grateful to Cody Fourman and Garrett VanNess for conducting BET surface area analysis on the mineral samples. The authors also acknowledge the Wright State University Research Challenge program for financial support of the surface area analysis instrumentation.

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REFERENCES

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