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in situ methods for monitoring silver nanoparticle sulfidation in simulated waters John M. Pettibone, and Jingyu Liu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03023 • Publication Date (Web): 16 Sep 2016 Downloaded from http://pubs.acs.org on September 20, 2016

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Environmental Science & Technology

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in situ methods for monitoring silver nanoparticle sulfidation in simulated waters

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John M. Pettibone* and Jingyu Liu

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Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD,

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20899

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*Corresponding author: John Pettibone; 100 Bureau Dr., Gaithersburg, MD 20899 Phone: +1(301) 975-5656; email: [email protected]

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Abstract To probe the transformation pathways of metallic nanomaterials, measurement tools capable of detecting

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and characterizing the broad distribution of products with limited perturbation are required. Here, we

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demonstrate the detection of transformation products resulting from 40 kDa PVP-coated silver

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nanoparticles (AgNPs) reacted in aerated, sulfide-containing water and EPA moderately hard

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reconstituted water standard. Using single particle inductively coupled plasma mass spectrometry, silver

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mass preservation in primary AgNP populations during sulfidation was observed under all reaction

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conditions examined. Disparate sensitivities of Ag+ and AgNPs to different media were observed, limiting

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confidence in the measured dissolved fraction. Examination with hyphenated asymmetric flow field-flow

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fractionation (A4F) methods supported similar mass preservation. Using flow-cell FTIR measurements,

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we provide direct evidence for the preservation of PVP-coatings in the presence of Na2S and fulvic acid,

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which we attributed to the observed, unprecedented Ag preservation. Using A4F and X-ray scattering, sub

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10 nm AgNP populations, which have gone nearly unstudied in environmental systems, were detected

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and characterized in all the pristine and transformed product distributions examined. Furthermore, by

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distinguishing Ag+ from individual AgNPs, quantification of each population becomes tractable, which is

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a critical measurement need for toxicity testing and predicting NP fate in engineered and natural systems.

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Introduction

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The design and implementation of nanomaterials with superior properties in commercial and industrial

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applications that also have minimal impact to human and ecological health requires knowledge of their

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fate, including both their transformations and transport in the possible environmental compartments

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exposed during their lifecycle. Experimental and computational methods that provide necessary inputs for

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developing predictive models are required for more accurately predicting the fate of current and emerging

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nanomaterial based products.1-3 Of particular importance are measurement methods that can detect and

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distinguish the entire mass distribution of transforming nanomaterial populations in complex media, such

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as natural or engineered environments. They would provide tools to better elucidate the contributions of

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individual variables (e.g., composition, surface coating, media constituents, etc.) on the reaction network

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and relative rates associated with the overall fate. The ability to detect and quantify the entire mass

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distribution as it transforms in different media is also critical for toxicity testing, which requires

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information on both the toxin and dose, because the intrinsic properties of nanomaterials are known to

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change with composition, size and physical state.4

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Silver nanoparticles (AgNPs) represent one of the highest priority nanomaterials for environmental risk

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assessment due to their potential adverse impacts on the ecosystem and increasing incorporation into

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consumer products and biomedical applications5. However, detecting and quantifying the broad mass

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distribution that results from AgNP transformations in complex media remains a major measurement

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challenge.6,7 During these transformations, the Ag mass distribution can undergo dynamic changes in

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composition, size and physical states, where additional complexity resulting from the formation of

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progeny products in the presence of natural organic matter (NOM),8-10 polyvinyl-pyrrolidone (PVP),11 and

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other natural (photo)reduction processes12,13 have also been reported. Thus, much of previous work has

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examined final transformation products of corrosion (e. g. oxidation and sulfidation), which is the

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prevalent processing route for AgNPs in the environmental systems, as function of AgNP coating,

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corrosive agents, NP size, and environmental factors (pH, temperature, dissolved oxygen, biota, etc.).11,14-

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salient physicochemical properties associated with specific transformation pathways.

However, further work is required to understand multi-component complexity and better identify the


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Because AgNP sulfidation is a dynamic process, it is an ideal case study to develop the measurement

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methods capable of tracking the entire mass distribution during transformations. By evaluating evolution

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of distinct species, transformation pathways with relative rates can be elucidated; thus, the abilities to

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observe distinct processing provide insight into the principal factors contributing to their fate, which can

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be used as a comparison for re-evaluating batch results from previous studies. The overall sulfidation

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reaction for AgNPs in natural waters (pH > 7) is:

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4Ag0 + O2 + 2HS- 2Ag2S + 2OH-

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where the oxidation of the Ag0 initiates the formation of Ag2S.14 In general, non-covalently bound ligand

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caps have not been reported to limit the extent of sulfidation (similar to oxidative processes), which also

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can include NOM.14,26,27,28 However, AgNP sulfidation has generally been reported to form aggregated,

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bridged, and coalescence particles over a broad range of sulfide to Ag molar ratios.14,26 An example of

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preserved colloidal stability has been reported for tween-capped AgNPs that were further stabilized by

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fulvic acid during sulfidation.23 Some evidence in the literature suggests NOM may provide stabilization

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for the corrosion products, similar to pristine and progeny AgNPs,9,14,29-33 where different NOM sources

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were also reported to have distinct capabilities for stabilizing the AgNPs.34 The authors posited the

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stabilities were associated with distinct surface interactions,34 but no direct evidence for examining the

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NOM-AgNP surface interaction was provided. Furthermore, undetermined interactions of humic acid

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with citrate-coated AgNPs during sulfidation were also reported to direct the formation of hollow sphere

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products during a sulfidation processes.28 This result suggests that transformations (reactions and rates)

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are affected by the surface properties and the mechanism for chemical and colloidal stability should be

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further investigated with spectroscopic methods that can directly interrogate the interaction to better

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design engineered coatings in the presence of NOM and ultimately more accurately predict the NPs fate.

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Notably, the majority of the studies examining structure were conducted under high vacuum conditions

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and require sometimes significant sample processing. Known origins of possible measurement artifacts

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during sample preparation (e.g., drop casting solutions for microscopy, washing, filtering, centrifuging or

(eq. 1)



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other mechanical sample perturbations) support the development of measurement methods that can

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minimize or eliminate these uncertainties to examine the entire mass distribution during sulfidation or

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other prevalent processing routes. The small Ag-based NP populations (sub 10 nm diameter) have not

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been extensively investigated in large part due to the dearth of measurement methods that can probe these

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species with limited sample perturbation.35 However, they are known to possess distinct optical and

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catalytic properties from the bulk,36,37 and recent study proposed a different toxic mechanism from ions or

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larger AgNPs.38

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In the current study, we develop and apply a validated method to examine the entire product distribution

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resulting from the sulfidation of PVP-protected AgNPs. The measurement method development for

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monitoring the metallic mass distribution for both pristine and processed materials was achieved using

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research grade reference materials and appropriate calibrants39 and examined over a broad AgNP

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concentration range, which afforded the opportunity to evaluate both transformation mechanisms of the

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reference materials and processing of unknown samples at environmentally relevant concentrations.

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Surfaces of pristine and processed AgNPs were directly probed spectroscopically to investigate the

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relationship engineered coatings and Suwannee River fulvic acid (FA) have on observed physical

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behavior of the AgNPs during corrosion and in increasing complex media. Overall, the work provides a

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methodology and data set to compare to other experimental results with increasing heterogeneity in

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sample type (size, surface coverage, etc.) and increasing complexity in system type to better predict NP-

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environmental surface interactions contributing to their fate and risk.

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Materials and Methods

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Chemicals The 40 kDa PVP-coated AgNPs used were research grade NIST RM 8017 (nominal 75 nm diameter).40

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Sodium sulfide nonahydrate (≥ 99.99%, ACS reagent) stored at 4 °C, sodium citrate, PVP (average

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molecular weight of 40 kDa), sodium hydroxide, and nitric acid were purchased from Sigma-Aldrich. The

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Suwannee River fulvic acid standard I (FA) was obtained from International Humic Substances Society

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and dissolved in DI water. In single particle inductively coupled plasma mass spectrometry (SP-ICP-MS) 4 ACS Paragon Plus Environment

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analysis, NIST RM 8013 Gold Nanoparticles (nominal 60 nm diameter) were used as size standard for

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calculating the transport efficiency; and NIST SRM 3121 Au standard solution and SRM 3151 Ag

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standard solution were used for preparing soluble calibration standards. The moderately hard

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reconstituted water (MHRW) solution was prepared according to an EPA protocol.41

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Instrumentation UV-Vis spectra were collected on a Perkin Elmer (Waltham, MA) Lambda 750 spectrophotometer.

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Fractionation was conducted with an Eclipse 3+ asymmetric flow field-flow fractionation (A4F) system

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(Wyatt Technology, Santa Barbara, CA) and coupled to a detection chain comprising a diode array

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detector (DAD, 1200 series, Agilent Technologies) with a spectral range from (190 to 950) nm and a

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sampling rate of 20 Hz and an online multi-angle light scattering (MALS) detector (DAWN HELEOS,

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Wyatt Technology) equipped with quasi-elastic light scattering (DynaPro, Wyatt Technology). We

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followed our previous protocol42 to find optimal parameters: 250 µm spacer that formed a tapered 26.5

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cm length channel; polyethersulfone (PES) membrane with a 10 kDa cut-off; channel flow (Vp) = 0.5 mL

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min-1 and cross flow (Vc) = 0.3 mL min-1. The diffusion coefficient, D, of the analytes can be determined

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based on retention time, tR:

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‫= ܴݐ‬

߱2 ܸܿ ln ቆ1 + ቇ 6‫ܦ‬ ܸ‫݌‬ Eq. 2

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where ω is the effective channel thickness and was determined using online light scattering. Developing

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methods for tR-based D provide a tool for evaluating unknown samples, which is a critical need for

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environmental systems. Details on the development of optimal parameters, calibration of tR, and

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concentration ranges appropriate for the hyphenated methods to determine average size (D) are found in

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the Supporting Information (SI). Stokes-Einstein equation was used to calculate average hydrodynamic

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diameter, DH, from D. The reported DH values from online DLS measurements are z-average values with

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uncertainties representing the standard deviation (1σ) of the entire measured population with sufficient

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signal intensity. Recovery measurements were conducted using hyphenated A4F-ICP-MS after focusing



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for select samples in the presence of FA, because DAD measurements convoluted the total absorbance.

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All reported samples had recoveries of total silver > 90%, which were determined by the percentage of

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the integrated

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focusing. All samples were run in triplicate.

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SP-ICP-MS measurements were conducted on a ThermoFisher X series II quadrupole ICP-MS (Waltham,

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MA) equipped with a microflow concentric nebulizer (PFA-ST, Elemental Scientific, Omaha, NE) and a

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glass impact bead spray chamber cooled to 2 oC. Instrument was tuned daily for maximum

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sensitivity and minimum oxide level (156CeO /

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107

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Both small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) data were obtained

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from the ultra-small-angle X-ray scattering (USAXS) facility45 at sector 9-ID of the Advanced photon

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Source (APS) at Argonne National Laboratory, Argonne, IL using an X-ray energy of 17.5 keV. More

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details on the SAXS and WAXS measurements are found in SI. Attenuated total reflectance Fourier

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transform infrared spectroscopy (ATR-FTIR) was conducted on Nicolet Spectra 750 using a 45° ZnSe

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horizontal ATR crystal. Samples were drop casted onto the crystal and allowed to dry overnight before

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processing, including the Ag2S NP films. PVP was introduced prior to FA to simulate the pristine AgNP

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properties. All spectra represent 250 scans collected at 2 cm-1 resolution.

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AgNP sulfidation experiments The complete details for experimental design, including sample preparation, processing, and handling can

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be found in the SI. Briefly, we made all stock solutions by diluting the reconstituted and purified research

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grade RM 8017.46 All samples were prepared in 20 mL glass vials that were protected from light using

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aluminum foil and were adjusted to an initial pH ≥ 7.1 ± 0.2. Aliquots from the batch reaction solutions

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were directly examined with UV-Vis, hyphenated A4F, SP-ICP-MS and SAXS after any necessary

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dilutions without further modification unless otherwise annotated, which limited any sample perturbation.

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The completion of the sulfidation reaction was determined based on unchanging optical signatures

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Ag intensities, IAg, of the sample with applied Vc to IAg with no applied Vc after sample

140

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In

Ce < 2%), which is also the condition for optimized

Ag signal. Detailed information for calibration, operation and data processing43,44 are found in the SI.


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(change in relative absorbance < 2%). Statistical comparison of pristine NP distribution to the

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transformed product was done using a two-tailed Student’s t-test with a threshold p-value of 0.05. In

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addition, Ag2S NPs were synthesized following a previously reported method with modification by

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mixing AgNO3 with Na2S under agitation.47 All experiments were run in at least duplicate.

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Results To examine the role of FA during the sulfidation of AgNPs at pH 7.3 ± 0.2, we first examined the

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evolution of the research grade RM8017 AgNPs using UV-vis spectroscopy, which detects global

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changes to AgNP optical properties in aqueous environments. Using different molar ratios of Na2S and

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AgNPs, notated as [S]/[Ag] herein, that reportedly to result in partial to complete conversion of Ag0 to

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Ag2S,26 the relative changes to the optical signatures of the AgNPs at (6 and 24) h could be quantified. In

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Figure 1, the normalized absorbances, (A/A0), in the presence and absence of FA at the two specified time

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points are presented as a function of [S]/[Ag], where 25 mg L-1 AgNP suspension data are offset by

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[S]/[Ag]=0.05. For 25 mg L-1 AgNPs in the absence of FA, the intensity of the plasmon band centered

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near 458 nm decreases and continuously blue shifts with increasing [S]/[Ag] (Fig. S1). The (A/A0) at (6

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and 24) h are less than 2% different based on replicate samples. For 25 mg L-1 AgNP samples containing

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FA, a larger but statistically insignificant change based on 1σ of duplicate measurements was observed,

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which converged with the samples without FA at [S]/[Ag]=1. For 25 mg L-1 AgNP suspensions, (A/A0)

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values for FA containing samples were always equal to or greater than the (A/A0) for suspensions without

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FA.



[Ag] mg L-1 [FA]/[Ag] 25 0 25 10 0.5 0 0.5 10 Open symbols: 6 h Solid symbols: 24 h

Normalized Absorbance (A/A0)

1

0.8

0.6

0.4

0.2

0

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0.2

0.4

0.6

0.8

1

[S]/[Ag] (mol/mol)

171 172 173 174 175

Figure 1. Representative normalized absorbances for pH adjusted (7.3 ± 0.2) 0.5 mg L-1 and 25 mg L-1 suspensions of AgNPs in the presence and absence of FA at 6 h (open symbols) and 24 h (solid symbols) after Na2S addition. FA was added at 0.1 mass fraction of Ag to FA. The UV-Vis measurements are able to monitor and quantify relative changes to the optical spectra of the AgNPs. 25 mg L-1 AgNP suspension data are offset by [S]/[Ag]=0.05 for clarity.

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Suspensions containing 0.5 mg L-1 AgNPs in the presence and absence of FA at [S]/[Ag] = 0 resulted in

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changes to (A/A0) that were < 5% at 6 h from t = 0 h based on peak height (Fig. 1 and Fig. S2). For

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samples containing [S]/[Ag] = 0.3, decrease in (A/A0) from 6 h to 24 h was observed in the absence and

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presence of FA. At the highest molar sulfide ratio, [S]/[Ag] = 1.0, (A/A0) didn’t continually change from

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(6 to 24) h. The relative change in absorbance at 24 h for the 0.5 mg L-1 samples containing [S]/[Ag] = 0.3

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and 1.0 were similar, which was not observed in samples containing 25 mg L-1 AgNPs. Furthermore,

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evaluation of the raw spectra suggests the extent of conversion from Ag0 to Ag2S is greater in the

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presence of FA at 24 h based on the extinction intensity at longer wavelengths.

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Wide-angle X-ray scattering (WAXS) measurements also indicated the conversion of Ag0 to Ag2S

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occurred and representative diffraction data is presented (Fig. S3), which provided confirmation that the

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reaction conditions (e.g., [S]/[Ag]) resulted in the formation of Ag2S.26 However, because UV-vis cannot

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determine the size distribution and properties of distinct populations in heterogeneous samples, additional

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efforts are necessary to identify transformation pathways and associated products.


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To examine the evolution of the primary AgNP distribution with increasing [S]/[Ag], we first employed

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SP-ICP-MS to track changes in Ag mass. Purification processes did not change the mass distribution and

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average particle size (Fig. S4A), thus, all samples presented subsequently were the purified sample. In

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Figure 2, SP-ICP-MS results are presented for sulfidation experiments conducted in water with initial pH

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7.3 ± 0.2 and [S]/[Ag] = x, 0 ≤ x ≤ 1. With increasing [S]/[Ag], the mean particle mass and the total

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particle number remains unchanged at t = 6 h (Table 1 and Fig. S4B). At t = 24 h, no significant changes

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to the particle mass distribution were observed (Fig. 2A). A similar set of experiments over the same

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[S]/[Ag] range was conducted with a 0.1 mass fraction of Ag to FA (Fig. 2B and Fig. S4B). The presence

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of FA did not change the distribution of observed masses significantly and the total particle number

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remained similar, which is summarized in Table 1.

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Table 1. Equivalent NP size, detected NP number and mass recovery measurements for AgNP sulfidation in water at 6 h and 24 h from SP-ICP-MS1 Sample

[S] / [Ag] (mol / mol)

Equivalent diameter2 (nm) FA = 0 FA = 10× Ag

Detected NP3 (min-1) FA = 0 FA = 10× Ag

Mass recovery4 (%) FA = 0 FA = 10× Ag

Purified

0

71 ± 10

111 ± 12

96

Sulfidation t=6h

0 0.1 0.3 0.5 1

71 ± 13 72 ± 10 73 ± 14 73 ± 12 72 ± 12

71 ± 10 72 ± 12 72 ± 14 72 ± 13 72 ± 14

97 ± 10 103 ± 15 106 ± 7 113 ± 24 94 ± 6

97 ± 12 108 ± 11 103 ± 10 115 ± 15 104 ± 8

Sulfidation t = 24 h

201 202 203 204 205 206 207 208 209

87 87 91 95 84

82 88 85 92 87

0 72 ± 12 71 ± 11 109 ± 2 101 ± 19 88 81 0.1 72 ± 12 72 ± 12 112 ± 8 106 ± 7 91 85 0.3 73 ± 12 72 ± 13 125 ± 10 123 ± 18 100 100 0.5 73 ± 11 72 ± 13 119 ± 11 119 ± 19 99 98 1 73 ± 12 72 ± 13 106 ± 16 125 ± 13 88 100 1 Particle size and number of processed samples were compared with that of purified AgNPs. Under all conditions, the difference is not statistically significant (P>0.05). 2 The equivalent diameter represents the 10% trimmed mean and 1σ of particle size distribution that calculated from the measured mass of silver in individual particle assuming chemical composition and density of 100% Ag and spherical geometry. 3 Particle number detected by SP-ICP-MS in a 180 s measurement was counted every 60 s. Data represent the average and 1σ of detected NP number in three consecutive 60 s. 4 Mass recovery was determined by dividing the total measured Ag concentration by Ag concentration in the sample.



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Figure 2. SP-ICP-MS measurements of AgNP sulfidation process at 25 mg L-1 and increasing [S]/[Ag] = x, (0 ≤ x ≤ 1) at 24 h and suspension pH ≈ 7.3 ± 0.2. A) Intensity distribution of AgNP sulfidation in the absence of FA; and B) intensity histograms of AgNP sulfidation in the presence of FA with a 0.1 mass fraction of Ag to FA. The mean particle silver mass and number remain similar during the sulfidation process for the range of [S]/[Ag] studied.

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Analysis of sulfidated AgNP products resulted in mass recovery ≥ 84 % (Table 1). In both the absence

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and presence of FA, no signs of aggregation were observed for any conditions examined. Together with

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the relatively unchanging mass distribution (Fig. 2), these data sets demonstrate the preservation of mass

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and colloidal stabilization of the primary AgNPs during sulfidation under examined experimental

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conditions.

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Dissolved fraction measured by SP-ICP-MS showed continuous decrease after addition of sulfide (Table

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S1), which is consistent with previously reported findings.26 However, accurate quantification of the

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dissolved Ag fraction represents a significant challenge to SP-ICP-MS due to possible interferences

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associated with sulfide and/or FA. Additionally, in theory the dissolved fraction contains both soluble Ag

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species (Ag+, dissolved complexes) and any NPs (pristine or transformed AgNPs, Ag2S) smaller than the

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size detection limit (15 nm for AgNPs in current study), which makes differentiation of specific

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populations in this size regime intractable with SP-ICP-MS. When similar experiments were conducted in

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MHRW, the equivalent diameters of the NPs remained unchanged for all samples. However, particle

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number and mass recovery in the absence of FA is significantly lower than sulfidation in water (Fig. S5

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and Table S2), and the soluble fractions were near the limit of detection (LOD) of the instrument. Control

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experiments using SP-ICP-MS to quantify dissolved Ag+ (AgNO3) under similar water conditions 10 ACS Paragon Plus Environment

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demonstrate the sensitivity of Ag ions was severely depressed in MHRW and in the presence of sulfide

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and FA (Fig. S6A and S6B). The MHRW contains species that can interact with Ag+ such as Cl-, HxSx-2,

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HxCO3x-2, etc. and decrease solubility. In contrast, PVP-AgNPs produce similar intensities irrespective of

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water chemistry (Fig. S6C). The different responses of silver ions (and small species produced by

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interaction of Ag+ with water components) and NPs suggest dissolved silver is more susceptible to the

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changes in solution matrices. FA addition enhanced primary NP mass recovery in the MHRW solutions.

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Despite the observed difficulties to quantify the dissolved fraction, the SP-ICP-MS data do suggest the

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PVP-stabilized AgNPs behave like a closed system (mass preservation) in sulfide-rich corrosive

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environments in the absence and presence of FA near pH 7.

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To monitor the interaction of FA and PVP on pristine and transformed Ag, we used ATR-FTIR

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spectroscopy (Figure 3). In the presence of FA, AgNPs displayed features for both the engineered PVP

242

coating and FA (Fig. 3A). When fully sulfidated PVP-coated AgNPs (i.e., Ag2S NPs) were exposed to

243

excess amounts of FA to PVP (mass fractions > 0.99), no FA signatures were detected; furthermore, the

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spectra demonstrate the persistence of the PVP coating based on the differential of relative peak

245

intensities prior to and after FA addition (Fig. 3B).



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A

Normalized absorbance (A\A0)

5x FA + PVP AgNP

PVP AgNP

SRFA I

B

FA + PVP Ag2S

PVP Ag2S

2000

246

1800

1600

1400

1200

Wavenumber (cm -1)

1000

247 248 249 250 251

Figure 3. A) ATR-FTIR spectra for dried FA (pH 7), PVP AgNPs, and purified PVP AgNPs in the presence of excess FA on the ZnSe crystal. FA attachment to the unsulfidated AgNP surface is clearly observed. B) ATR-FTIR spectra of Ag2S NP films first exposed to PVP and washed with water (black trace) and subsequently exposed to FA solutions adjusted to pH 7.1 (blue trace). The addition of FA mass fractions to PVP > 0.99 did not result in observed changes to the PVP-Ag2S spectrum, indicating FA adsorption was below the LOD for the instrument.

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To develop methods that can fully examine the pristine NPs and nearly the complete product distribution

253

resulting from transformations, the detection and characterization of the sample population below the SP-

254

ICP-MS detection limit (

272

90% (blue trace). In addition, the calculated DH values from tR were consistent with the online DLS

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measurements for [S]/[Ag] = 0.3, (96 nm ± 7 nm). Besides the primary NP population, a small population

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of with calculated DH values < 10 nm (tR ≤ 10 min) were observed in the fractogram. The addition of FA

275

did not change tR for either population (Fig. S8B). Further experiments on samples containing the FA

276

additive were conducted at 0.1 mg L-1 AgNP concentrations with hyphenated A4F-ICP-MS, which

277

resulted in nearly identical tR values observed for the 2.5 mg L-1 suspensions (Figure 4B). The data at both

278

AgNP concentrations were also consistent with mass preservation exhibited in SP-ICP-MS results (Fig. 2,

279

Table 1), providing further confidence that the addition of FA as an additive to homogenize analyte-

280

membrane interactions does not affect the accuracy of the tR-based diffusion calculation (Eq. 2).

281

The capabilities of A4F to determine D (size) also extend to the sub 15 nm population range that is

282

currently inaccessible with SP-ICP-MS, which provides a more complete assessment of the initial and

283

transforming product distributions. At both concentrations examined, small nanoparticle populations that

284

would likely not be distinguished by SP-ICP-MS could be identified in the fractograms (Fig. 4B and Fig.



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S8B). The observed populations persisted after the addition of sulfide, [S]/[Ag] = 0.3 (Fig. 4, sky blue and

286

grey traces).

287

To examine the small population and confirm that the observed signals are in fact solid, small NPs (Fig.

288

4B, blue trace), 0.3 molar equivalents of Ag+ (AgNO3) were added to the AgNP sample (Fig. 4B, red

289

trace). The hyphenated A4F-ICPMS data did not exhibit a distinguishable change in the integrated ratios

290

of the peaks representing the small population defined as (0 ≤ tR ≤ 5) min, Region 1 (Fig. 4B inset), and

291

the primary NP population defined as (18 ≤ tR ≤ 31) min, Region 2. The ratio of integrated areas of

292

Region 1 and 2 for AgNP and AgNP + Ag+ samples were 0.014 ± 0.004 and 0.013 ± 0.004, respectively,

293

for repeat experiments. The unchanging ratio demonstrates the dissolved ions are efficiently removed

294

during the focusing step in the channel and ions can be differentiated from small nanoparticles.

A

296 297 298 299 300 301 302 303 304

Intensity (CPS) 0

1

2 3 Time (min)

4

5

107Ag

0

295

107Ag

Intensity (CPS)

Noramlized LS @ 90° (a.u.)

B

4

8

12

16

20

24

28

32

0

4

8

Time (min)

12

16

20

Time (min)

24

28

32

Figure 4. A) Fractograms for 75 nm AgNPs 24 h after [S]/[Ag] = 0.3 addition (red trace) and the same sulfidated product after sequential addition of FA (blue trace) using the LS signal at 90°, demonstrating the redispersion of sulfidated AgNPs in the channel by FA. Although the optimal conditions for the samples in the absence and presence of FA are different, the relative unchanging peak position demonstrates that D (size) remains similar for all cases, which is consistent between all measurement methods employed. B) Hyphenated A4F-ICPMS fractograms for samples that include 0.1 mg L-1 Ag and 0.83 mass fraction of FA to Ag. AgNPs (blue trace), AgNPs and 0.3 molar equivalents of AgNO3 (red trace), AgNPs in the presence of [S]/[Ag] = 0.3 at t = 10 min (sky blue trace), and AgNPs in the presence of [S]/[Ag] = 0.3 at t = 1 h (grey trace) are shown. The primary NP distribution remained unchanged based on tR and no additional signal intensity was observed in samples spiked with Ag ions based on


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305 306 307

integration of the peak intensities in Region 1 (inset), and Region 2 that represents the primary NP population (18 ≤ tR ≤ 31) min. These results indicate the method can differentiate dissolved ions from NPs. The small NP populations initially present were preserved after sulfide addition.

308

Small angle X-Ray scattering (SAXS) results, described in detail in the SI, provide further evidence for

309

the persistence, transformation, and dissolution of these small species and provide the highest resolution

310

for size in the sub 10 nm regime.49 The Ag concentrations necessary for SAXS are > 1000 mg L-1,

311

allowing examination of the mechanistic factors at high AgNP concentrations that are likely not

312

environmentally relevant but could be useful in comparison studies or evaluating suitability of individual

313

measurements. In the SAXS data (Figure S3), small species (diameter < 10 nm) were observed prior to

314

and more than 24 h after the addition of Na2S for experiments in MHRW (and DI water), which was

315

consistent with hyphenated A4F data (Fig. 4 and S8).

316

Discussion

317

Few established methods can follow, characterize and quantify chemical and physical transformations of

318

metallic NPs in increasingly complex media. This is especially true for systems that can undergo

319

significant changes to NP core composition, NP surface properties, agglomeration state, and significant

320

changes to the size distribution. Here a methodology capable of achieving the aforementioned goals for

321

the sulfidation of PVP-AgNPs is presented, which was validated through the self-consistency of the data.

322

We believe the unprecedented ability of the hyphenated A4F method to determine tR-based diffusion for

323

both the pristine and transformed AgNPs is the first reported method to not require a priori information to

324

determine mass distribution of both analytes when the engineered coating is preserved.

325

As has been previously demonstrated in numerous works, UV-Vis measurements provide insight into the

326

transformation of the PVP-coated AgNPs, where the roles of FA and rate of sulfidation can be evaluated

327

based on the total changes to the system (i.e., sensitive probe to changes in optical properties, but

328

different processing events cannot be distinguished). Thus, two general conclusions that could be derived

329

from the UV-Vis data were i) FA containing samples always resulted in equivalent or higher values for

330

A/A0 at both Ag concentrations studied (0.5 mg L-1 and 25 mg L-1), suggesting previously unreported 15 ACS Paragon Plus Environment


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contributions to AgNP sulfidation rates under the current conditions, and ii) changes in relative global

332

rates of processing of the competing reaction pathways were most clearly observed in 0.5 mg L-1 AgNP

333

suspensions containing [S]/[Ag] = 0.3. The first finding indicates more Ag0 in the NP was preserved at 24

334

h in the presence of FA, which was in contrast to previous studies that reported no contribution to the rate

335

of observed change during sulfidation. The latter point indicates that the relative rates of competing

336

reactions (e.g., sulfidation and oxidation) are sufficiently similar to affect the subsequent product

337

distribution after the addition of Na2S. Thus, it is reasonable to surmise the continued decrease observed

338

between (6 and 24) h, which was not observed in suspensions with higher [S]/[Ag], would result in

339

different product distributions. However, the correlation between the optical signature, which are affected

340

similarly by aggregation or change in refractive index due to the formation of Ag2S shells,50 and product

341

distribution cannot be determined by UV-Vis alone. Also, because the system continues to change,

342

preparation of samples to examine the products with electron microscopy (EM) methods would introduce

343

further uncertainty to the known challenges for determining size distributions. Therefore, the need for

344

higher resolution measurements to understand contributions from engineered coatings, media

345

components, and FA and their specific impact on the observed results was clearly identified.

346

SP-ICP-MS measurements demonstrated a constant mass in the primary NP distribution, which has not

347

been reported previously for sulfidation of PVP-AgNP systems. The preservation of the primary NPs was

348

also corroborated by the SAXS and USAXS data (Fig. S3) and hyphenated A4F results showing a slight

349

increase of tR consistent with the density change from Ag0 to Ag2S (Fig. 4A and S8). The nearly constant

350

relative intensity of the primary AgNPs irrespective of solution conditions (Fig. S6) provides confidence

351

in the use of SP-ICP-MS to track their mass change in complex media. Furthermore, changes were

352

observed in the quantified dissolved fraction, which is generally thought to include soluble ions and

353

smaller species below the LOD of the ICP-MS. A common interpretation of the decreasing dissolved

354

fraction would be removal of this population (e.g., ripening or coalescence of smaller particles generating

355

larger species). However, the resulting change to the equivalent Ag size was rather subtle and no


Page 17 of 24


356

statistically significant difference in the primary NP distribution or appearance of other species was

357

observed. Combining these observations with our reported disparate sensitivities of ions and NPs (Fig.

358

S6) demonstrates the care that should be taken when reporting the mass distribution with SP-ICP-MS and

359

possible pitfalls with the dissolved fraction component.

360

The implementation of FA as additive in the current study achieved both i) diffusion-based retention

361

behavior that is unaffected by the additive (FA) over a broad concentration range and ii) did not change

362

the quantified amounts of Ag that were retained in the channel (Fig. 4), which overcomes the persistent

363

measurement challenges associated with calibrating ω based on heterogeneity in core and surface

364

composition.39,42 Based on SAXS, online DLS and SP-ICP-MS, FA optimizes the membrane-analyte

365

interactions in the channel, but colloidal stability of the sulfidation products remained statistically

366

unchanged in all measurements and at all concentration ranges in the presence and absence of FA. The

367

FA addition does allow ω to be calibrated for the sulfidated species because retention behavior in the

368

channel remained unchanged. Thus, accurate determination of D (and size through Stokes-Einstein

369

equation)39 using tR can be achieved, which is critical step for samples of unknown origin and surface

370

properties. Together, these capabilities allowed the development of the first reported A4F method capable

371

of monitoring and quantifying the change in the mass distribution for both the pristine and transformed

372

products resulting from AgNP sulfidation over the entire nano-size regime. The hyphenated A4F-ICP-

373

MS system can examine sample concentrations well below the LOD of online LS and optical detectors --

374

< 100 µg L-1 Ag range.

375

The first finding allows evaluation of the evolving mass distribution during sulfidation of the research

376

grade PVP-coated AgNP at a broad range of NP concentrations to examine the relative contributions from

377

competing reactions (e.g., increased oxidation rate at lower concentrations), which has not been

378

previously well-characterized due to the paucity of methods capable of distinguishing transformation

379

products from sample preparation artifacts. Consistent with SP-ICP-MS data, the preservation of the

380

primary AgNPs were observed with hyphenated A4F, and an increase in tR (size) associated with the 17 ACS Paragon Plus Environment


Page 18 of 24

381

conversion of Ag0 to Ag2S was observed that is expected by the observed mass preservation and ~30%

382

decrease in bulk Ag density. The second finding demonstrates the ability to distinguish and quantify small

383

NPs from the dissolved fraction (Fig. 4), where the latter has gone nearly unstudied in environmental

384

systems. The observed unchanging mass distribution after AgNO3 addition that increases after [S]/[Ag] =

385

0.3 addition demonstrates a possible tool for estimating the true dissolved fraction (Fig. 4). The presence

386

of the small NP populations prior to sulfide addition and their preservation after transforming was also

387

demonstrated with SAXS measurements, which further corroborates the veracity of the hyphenated A4F

388

results. Overall, the ability to quantify the nascent population and monitor the evolution allows the

389

competing processes that were evident in the UV-Vis data (Fig. 1), but unable to be distinguished, to be

390

probed and identified during AgNP transformations.

391

The development and implementation of in situ measurement methods capable of examining the

392

sulfidation of PVP-AgNPs provides a platform for better understanding the contribution different system

393

factors (coating, NOM, [S]/[Ag], etc.) have on the transformation product distribution, especially with

394

distinct size populations. The PVP persistence resulted in nearly complete mass preservation in the

395

primary NPs and seemingly affect the rates of sulfidation. Complete conversion of Ag0 to Ag2S was

396

expected at the highest [S]/[Ag] after 24 h, but the current results support the composition of the AgNP

397

core contained Ag0 based on WAXS data and optical signatures. Furthermore, the optical signatures of

398

AgNPs in the presence of FA suggest distinct rates of processing (Fig. S1), which were also accompanied

399

by changing affinities of the FA to the sulfidated surface (Fig. 3). By coupling the mass distribution

400

measurements with in situ X-ray-based methods and surface sensitive techniques (e.g., ATR-FTIR, XPS,

401

etc.), the reaction rate of Ag0 to Ag2S for individual populations can now be directly measured for range

402

of system and sample parameters to directly determine their weighted contributions on overall fate in

403

environmental or engineered systems. The mechanisms and associated rates of sulfidation will be

404

presented in subsequent work using the current methodologies, which will compare the use of proxy

405

measurements and examine uncertainty (overestimation) in rate determination.


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Environmental Implications

407

The development and validation of a methodology for probing and characterizing the evolution of AgNPs

408

in sulfide-containing media allows evaluation of a high priority material class for the environmental,

409

health and safety community. Direct evidence for the persistence of the PVP coating during chemical

410

transformation of the core and in the presence of excess FA using ATR-FTIR was provided. We surmised

411

that the PVP coating contributed to both the observed mass and colloidal stability in the system. The

412

persistence of the engineered coating could also affect downstream processing in engineered or natural

413

systems.51,52 The capabilities of the A4F methods to distinguish and quantify NPs from ions (Fig. 4)

414

provides the opportunity to better identify and quantify dissolution from other mass loss channels.

415

Because both Ag0 and Ag2S species in the sub 10 nm size regime are known to possess distinct properties

416

from their ionic and larger NP counterparts,36,37 detection and distinction from the mass distribution may

417

be necessary to better understand environmental response endpoints, which have not been reported to date

418

but can begin to be evaluated with the current methods.

419

Besides the surface coatings, other sample specific factors may also be contributing to the distinct

420

preservation of the primary NP populations observed in the current work. The use of research grade

421

AgNP reference materials with narrow size distributions is one general distinction from previously

422

reported sulfidation experiments. The NP size distribution (heterogeneity) and the coverage of the

423

engineered coating, which may change with NP size and molar mass, could also be a factor dictating

424

primary particle integrity. Although our goal was not to specifically identify the origins of the diverging

425

results, the current results and application of the methods should provide a data set to better understand

426

the contributing factors that contribute to metallic NP fate in complex systems, which should lead to

427

improved risk assessment for these materials.

428

Implementing the reported methodology specifically affords a tool box to monitor and follow changes in

429

distinct populations of analytes with limited sample perturbation, which allows individual transformation

430

pathways of NPs in complex environment to be identified and quantified with more accurate rate



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431

coefficients, thus facilitating better prediction of the fate, impact and risks of engineered nanomaterials

432

under more realistic scenarios. The ability to identify distinct populations and dissolved ions is also

433

critical for toxicity testing, which should allow the risk of distinct species to be more accurately evaluated

434

by quantifying the dose of transformed products present during exposure. The method expands on the

435

information that can be gleaned from combining UV-Vis, batch LS and electron microscopy

436

measurements, and can be applied to examine unknown samples at environmentally relevant

437

concentrations.

438

ASSOCIATED CONTENT

439

Supporting Information

440

Detailed data acquisition and processing of SP-ICP-MS, detailed sample preparation and additional

441

results are provided in Supporting Information. This material is available free of charge via the Internet at

442

http://pubs.acs.org.

443

Acknowledgements

444

The authors thank Andrew Allen and Fan Zhang of NIST, Gaithersburg, MD, for SAXS and WAXS

445

measurements. This research used resources of the Advanced Photon Source, a U.S. Department of

446

Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne

447

National Laboratory under Contract No. DE-AC02-06CH11357.

448 449 450 451 452 453 454 455 456 457 458 459 460

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