A Raman-Based Imaging Method for Characterizing the Molecular

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A Raman-Based Imaging Method for Characterizing the Molecular Adsorption and Spatial Distribution of Silver Nanoparticles to Hydrated Mineral Surfaces Seth W Brittle, Daniel P. Foose, Kevin A O'Neil, Janice M Sikon, Jasmine K Johnson, Adam C Stahler, John David Ryan, Steven R Higgins, and Ioana Emilia Sizemore Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04884 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on February 1, 2018

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Environmental Science & Technology

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A Raman-Based Imaging Method for Characterizing the Molecular Adsorption and Spatial Distribution of Silver Nanoparticles to Hydrated Mineral Surfaces

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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*

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Department of Chemistry, Wright State University, 3640 Colonel Glenn Hwy., Dayton, OH, 45435

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*Corresponding author: [email protected]

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ABSTRACT

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Although minerals are known to affect the environmental fate and transformation of heavy metal

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ions, little is known about their interaction with the heavily-exploited silver nanoparticles (AgNPs).

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Proposed here is a combination of hitherto under-utilized micro-Raman-based mapping and

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chemometric methods for imaging the distribution of AgNPs to various mineral surfaces and

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their molecular interaction mechanisms. The feasibility of the Raman-based imaging method was

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tested on two macro- and micro-sized mineral models, muscovite (KAl2(AlSi3O10)(OH)2) and

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corundum (α-Al2O3), under key environmental conditions (ionic strength and pH). Both AgNPs-

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and AgNPs+ were found to covalently attach to corundum (pHpzc = 9.1) through the formation of

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Ag-O-Al- bonds and thereby to potentially experience reduced environmental mobility. Because

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label-free Raman imaging showed no molecular interactions between AgNPs- and muscovite

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(pHpzc = 7.5), a label-enhanced Raman imaging approach was developed for mapping the scarce

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spatial distribution of AgNPs- onto such mineral surfaces. Raman maps comprising of n = 625-

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961 spectra for each sample/control were rapidly analyzed in Vespucci, a free open-source

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software, and the results were confirmed via ICP-OES, AFM, and SEM-EDX. The proposed

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Raman-based imaging requires minimum to no sample preparation, is sensitive, non-invasive,

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cost-effective, and might be extended to other environmentally relevant systems.

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ACS Paragon Plus Environment


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Keywords

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Raman imaging, chemometrics, silver nanoparticles, corundum, muscovite, ICP-OES, AFM, SEM-EDX

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INTRODUCTION

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The expansion of nanotechnologies in consumer products has raised increasing concern

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about their potential impact on human and environmental health. Special emphasis has been

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placed on silver nanoparticles (AgNPs); 54% of the total number of consumer products

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containing nanomaterials primarily harness the antimicrobial properties of AgNPs. About 1,230

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tons of the total silver produced worldwide is allocated to the fabrication of AgNPs.1 The

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increased use of AgNPs may result in a high Ag content for the biosolids produced by

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wastewater treatment plants (WWTP). For example, a 2010 study reported that AgNPs released

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from antibacterial athletic socks was significant2 but removable by WWTPs2. The resulting

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biosolids may then be sold as fertilizer, thus providing a pathway for these engineered

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nanomaterials to be introduced into soils through irrigation and rainfall. Numerous studies have

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already examined the health effects of AgNPs on both aquatic and terrestrial organisms including

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humans, and primarily attributed their toxicity to the release of Ag+ ions.3-5 However, the impact

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AgNPs have on environmental health remains under investigation in view of their possible

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transformations during the interaction with diverse soil components as well as the lack of cost-

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and time-efficient methodologies for examining these aspects.5-12

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Current research suggests that released AgNPs are most likely to be immobilized in soils

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due to the adsorption to natural organic matter (NOM) and minerals.6–8 Most of these studies

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focused on the adsorption of AgNPs to NOM because of their high chemical affinity to the

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sulfhydryl-rich functional groups present in NOM.7–9 Because NOM constitutes only ~ 5% of

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soil,10 it is important to also examine the interaction of AgNPs with the main soil component,


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minerals (~ 45%). Although there is extensive research available on the interactions between

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metal ions and soil minerals, only a few studies were reported on nanometals.11–13 The research

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that does exist on AgNPs is generally focused on simulations of ideal interaction scenarios or

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employs advanced methodologies (e.g., X-ray absorption spectroscopy and electron microscopy)

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that usually requires extensive sample preparation and expensive resources.8,14-25 Proposed here

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is a combination of hitherto under-utilized micro-Raman-based mapping and chemometric

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methods15–17 for the simple and cost-effective imaging of the spatial distribution of AgNPs to

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various mineral surfaces and their molecular interaction mechanisms.

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It was suggested that AgNPs released into the environment through wastewater,

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fertilizers or landfill runoff might exist in multiple forms.8 AgNPs may persist in their nanoscale

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form, oxidize to aqueous Ag+, aggregate or precipitate into salts (Ag2S, AgCl, Ag2SO4, AgOH,

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Ag2CO3, and Ag2O).3,8,18 Thus, a methodology capable of observing AgNP distribution on

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mineral surfaces and their molecular interaction mechanisms is essential to the determination of

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their fate and transport. Raman and surface-enhanced Raman spectroscopy (SERS) offer a

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unique approach because of their molecular fingerprinting and multiplex detection capabilities,

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non-destructive and aqueous compatible nature, and little to no requirement for sample

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preparation.19–30 Furthermore, modern Raman systems can collect multiple point spectra in x,y-

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raster patterns (Raman maps) to micro- or nano-map molecular interactions across large surfaces.

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The herein proposed Raman-based imaging approach is novel in that it offers the possibility of

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performing either label-free or label-enhanced SERS measurements. In general, minerals (e.g.,

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quartz, calcite, and corundum) have been decorated with noble metal nanoparticles (e.g., silver

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and gold) for the development of new SERS platforms and in this context, they have been

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examined through the acquisition of Raman control spectra.31–35 However, no approaches were



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yet proposed for the rigorous mapping of the molecular adsorption and spatial distribution

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behavior of AgNPs onto environmentally relevant mineral surfaces. Here it is demonstrated that

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the proposed Raman-based imaging methodology can achieve these goals for different types of

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minerals. If no direct molecular interaction occurs between minerals and AgNPs, a Raman active

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label of large scattering cross-section (e.g., rhodamine 6G (R6G) dye) and high affinity toward

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AgNPs may be utilized to make AgNPs “Raman-visible” and to indirectly image their spatial

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distribution onto mineral surfaces with increased sensitivity. Our group has already reported

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single-molecule SERS detection events of R6G (1 fM) adsorbed onto Creighton AgNPs under

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the excitation of visible laser lines and with less than 20 s acquisition times.23 When a target

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species is located in the immediate vicinity of an individual AgNP or at the nano-sized interstitial

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site of aggregated AgNPs, the SERS effect occurs and further boosts the sensitivity of the

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Raman-based detection method to unprecedented levels.19–22 The SERS enhancement is largely

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due to the increase in the magnitude of both the incident and the scattered electromagnetic fields

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resulting from the excitation of localized surface plasmon resonances (LSPR) of AgNPs.21,23–26

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Thus, the label-free and label-enhanced Raman-mapping of mineral surfaces exposed to AgNPs

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results in the collection of a large number of SERS spectra characteristic to the direct or indirect

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interaction of the two systems, respectively. An additional element of novelty in this study is the

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coupling of Raman mapping with chemometric methods for the facile hyperspectral analysis of

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large volumes of Raman data in a reproducible manner. This was achieved using a free, open-

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source, stand-alone analysis software package, Vespucci.27

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The feasibility of the Raman-mapping chemometric method was demonstrated on two

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representative minerals in micro and macro form and unfunctionalized (Creighton) AgNPs- and

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cetyltrimethylammonium bromide (CTAB)-functionalized (Sun method) AgNPs+. Corundum (α-


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Al2O3) and muscovite (KAl2(AlSi3O10)(OH)2) were selected because they contain several of the

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most abundant elements in the earth’s crust (O, Al, and Si).10 In addition, both minerals are rock-

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forming and encountered in streams and sands.10 While the Creighton synthesis is one of the

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most widely-used bottom-up fabrication approaches of colloidal AgNPs of negative charge due

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to its simplicity, time, and cost efficiency,14,29 the Sun method is generally employed for the

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synthesis of AgNPs of positive charge.36, 37 A concentration of 1 mg L-1 of AgNPs was used in

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order to ensure sub-monolayer coverage at the mineral surface and to surpass the maximum

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contaminant level (MCL) set by the U.S. Environmental Agency (EPA) for Ag+ in drinking

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water from both natural and anthropogenic sources (0.1 mg L-1).38 The interaction mechanism of

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corundum and muscovite in macro form was investigated at the pH of the colloid (8.2 for

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Creighton and 6.9 for Sun) because both minerals have a pH at point of zero charge (pHpzc of 9.1

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for corundum39 and 7.5 for muscovite40) close to this pH value. For illustrative purposes, the

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interaction mechanism between corundum in micro form and AgNPs was also investigated in the

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pH range from 6 to 11, typical to the pH of soils (3.5–9)10,41. Aggregation and settling of AgNPs

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in the corundum mixtures were noticed at smaller (< 6) or larger (> 11) pH values. In addition,

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several other well-established techniques, inductively coupled plasma optical emission

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spectroscopy (ICP-OES), atomic force microscopy (AFM), and scanning electron microscopy

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coupled with energy dispersive X-Ray (SEM-EDX) were employed to confirm and complement

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the Raman results.

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MATERIALS AND METHODS

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Single-crystal muscovite and corundum were purchased from Ward’s Earth Science and

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Marketech International, respectively. Macro-muscovite (i.e., single-crystal) was prepared by

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freshly cleaving 10 mm × 10 mm squares. Macro-corundum (i.e., single-crystal) consisted of



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cylindrical, single α-phase crystals cut and polished on the ( 1120 ) plane, 25 mm wide and 5 mm

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thick. Micro-corundum was purchased as fused, 1-µm-sized particles (99% fused α-Al2O3)

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from Alfa Aesar. High-quality (HQ) water (18.2 MΩ ·cm) was produced in a LabConco system

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and utilized throughout the course of all experiments unless otherwise specified. All other

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materials were purchased from Fisher Scientific and used without further modification unless

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specified.

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Synthesis and Characterization of AgNPs: Negatively charged, colloidal AgNPs were

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fabricated using a slightly modified Creighton method in water.19,29,30 Specifically, a 2:1 mM

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ratio of sodium borohydride (300 mL of 2 x 10-3 M of NaBH4) to silver nitrate (50 mL of 1 x 10-3

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M of AgNO3) solutions was used to minimize the amount of excess reagents and byproducts.

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Positively charged, colloidal AgNPs were synthesized using a slightly modified Sun method in

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water through the reduction of AgNO3 (300 mL of 5 x 10-3 M) with CTAB (20 mL of 2 x 10-2

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M) and NaBH4 (0.2 mL of 1% (w/v)).36,50 The physicochemical properties of AgNPs were then

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characterized by Raman spectroscopy, ICP-OES, AFM, SEM-EDX, and UV-Vis absorption

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spectroscopy.

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Sample Preparation: The flat macro-corundum surfaces were sequentially washed with

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acetone (HPLC grade), methanol (HPLC grade), and nitric acid (70% OPTIMA grade) in a sonic

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bath for 10 min each. Next, they were annealed in air at 1250°C for 12 h to provide a “clean”

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terraced surface. Both macro-corundum and macro-muscovite samples were then submerged in

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the colloid (100 mL of 1 mg L-1 of AgNPs-) mixed with 100 µL of 5 M of sodium nitrate

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(NaNO3), as an ionic strength adjuster in order to simulate the ion strength of freshwater

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environmental samples. Macro-muscovite samples were removed after 30 min and dried with a

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stream of nitrogen gas. Macro-muscovite received an additional exposure to rhodamine 6G


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(R6G), a standard SERS probe, in order to make adsorbed-AgNPs- Raman "active". Briefly, 10

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µL of 10-4 M of R6G solution was dispensed onto the mineral covering a surface area of ~1 cm2.

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Next, the surface was washed three times with water after 30 min incubation. Control groups

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with ISA included mineral exposure to water, CTAB (for AgNPs+ only), AgNPs (positively or

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negatively charged), or 10-3 M of R6G (for macro-muscovite only).

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Approximately 1.2 g micro-corundum was incubated with 100 mL of 1 mg L-1 of AgNPs-

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, 100 µL of 5 M of NaNO3, and pH adjusters (5-100 µL of 0.1 M of HNO3 or 0.1 M of sodium

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hydroxide (NaOH)). This amount of micro-corundum yields submonolayer coverage when

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assuming total AgNP- adsorption and a specific surface area (SSA) of 6.5 ± 0.2 m2 g-1. Similar

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experiments were carried out with CTAB-functionalized AgNPs+ for illustrative purposes. The

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pH was measured with a SevenGo Duo pro model pH meter that was calibrated daily with four

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pH buffers (4.0, 7.0 10.0, and 12.0). After stirring for 30 min, the samples were centrifuged for 2

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min at 5000 G in an AccuSpin Micro 17/17R model centrifuge. The centrifuge supernatants

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containing free, unbound AgNPs- were saved for ICP-OES analysis, while the centrifuge pellets

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consisting of corundum particles with bound AgNPs- were collected for Raman imaging. Control

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groups with ISA included mineral exposed to AgNPs- and hydrated mineral only at each

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experimental pH value (6, 7, 8, 9, 10, and 11).

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Specific Surface Area (SSA) Analysis: BET42 surface area analysis was performed on

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the corundum powder using a Micromeritics Tristar II 3020 surface area and porosity analyzer.

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A 5-point N2 BET isotherm with relative pressures in the range P/Po = 0–0.25 was used for the

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analysis, where Po is the saturation pressure. Prior to the analysis of the sample material, a

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standard reference material specified as alumina powder was measured to validate the method

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employed. The standard alumina was determined to have a specific surface area of 0.28 ± 0.02



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m2 g-1, which compared well with the specified surface area of 0.29 ± 0.02 m2 g-1. The Alfa-

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Aesar corundum was found to have a specific surface area of 6.5 ± 0.2 m2 g-1, which is within the

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range of 6-8 m2 g-1 specified by the supplier.

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Raman Spectroscopy Analysis: Raman data was collected using a LabRam HR800

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Raman system coupled to an Olympus BX41 confocal microscope (100x objective) and a

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motorized stage. Samples were irradiated with HeNe (632.8 nm) and Nd:YAG (532.134 nm)

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lasers with powers at the sample of 15 and 19 mW, respectively. The 532 nm excitation line is

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close to the R6G resonance at 530 nm (resonant conditions). The backscattered photons were

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measured using a thermoelectrically-cooled Andor CCD camera with a resolution of 1024×256

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pixels. The following parameters were selected: 300 µm confocal hole, 600 grooves mm-1

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holographic grating, 3 s (muscovite) and 1 s (macro-corundum) acquisition times, and 2 or 3

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averaging cycles. Under these conditions, the spectral resolution, i.e., the distance between points

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on the spectral abscissa, was ~1.18 cm-1. For each macro-muscovite sample, n = 900 spectra

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were measured in a 150×150 µm grid, at 5 µm increments. For each macro-corundum sample, n

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= 961 spectra were collected in a 31×31 µm grid, at 1 µm increments. The Raman system proved

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successful in the collection of data with nano-spatial resolution but at higher time costs. All

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spectra were collected in the 100–1700 cm-1 spectral range. Samples from the micro-corundum

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experiment were smeared onto clean glass slides and 11×11 µm areas were mapped in 1 µm

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increments for each sample to obtain a representative molecular picture of the interaction

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between AgNPs- and corundum particles. Because increased dispersity was anticipated with

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these non-flat samples, statistical confidence was strengthened by measuring n = 3 maps from n

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= 3 individually prepared samples, totaling 9 maps for each pH (i.e., n = 1089 spectra for each

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pH). For illustrative purposes, point Raman spectra were also collected from random areas on n


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= 3 independently prepared micro-corundum samples exposed to AgNPs+ in the presence of ISA,

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at different pH values (6-11).

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Raman Data Analysis: Raman maps were processed in Vespucci27. All spectra were

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smoothed with a median filter (window size 7). Muscovite spectra were baseline corrected using

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a rolling-ball approach43 in order to remove the fluorescent background and then normalized to

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the 1-norm. Univariate analysis to determine peak area and intensity against a local linear

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baseline was performed on the following regions of interest: the Ag-O stretch (235 cm-1) arising

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from interaction between AgNPs and the corundum surface in corundum samples and several

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R6G bands19,23 in the muscovite samples. Macro-corundum spectra were min/max normalized by

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subtracting the minimum value of the spectrum from each value of the spectrum, then

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subsequently dividing by the maximum value of the spectrum, so that the smallest value of each

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spectrum was 0, and the largest value of each spectrum was 1. The micro-corundum data were

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further analyzed using the Kruskal-Wallis test, with Dunn's test performed post-hoc to determine

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the significance of differences in Raman signal between pH groups.

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AFM analysis: Topographic images were created with an Agilent AFM operated in

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intermittent contact mode (i.e., AC or Tapping Mode), in air. Cantilevers were obtained from

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Nanoworld (NCHR, Pointprobe, non-contact mode) and were fabricated from single-crystal Si

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and coated with Al, with a nominal resonance frequency of 320 kHz and a nominal force

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constant of 42 N/m. Scanning speeds were typically set to 1–2 Hz and 2×2 µm images of

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256×256 pixels were recorded to adequately display features between 1–100 nm (i.e., AgNPs-).

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AgNP- adsorption was then interpreted by differentiating topographic profiles inbetween controls

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and samples.



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ICP-OES Analysis: Quantitative characterization of the total Ag content reacted with the

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micro-sized corundum was determined by difference using a Varian 710 ICP-OES system.

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Briefly, original colloids and supernatant samples containing free, unbound AgNPs- were

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chemically digested and diluted in trace metal grade nitric acid (HNO3) following the U.S. EPA

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method 200.11.44,45 An eleven-point external calibration curve (0, 5, 10, 15, 20, 25, 50, 75, 100,

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125, and 150 µg L-1) was then constructed, and the Ag concentrations were determined by

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interpolation from the calibration curve.

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SEM-EDX Analysis: Prior to the data collection with an FEI Quanta FEG 250 SEM

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system, all samples were sputter coated with 2-nm of Ir to increase their stability during exposure

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to relatively high beam voltages. Each sample was then loaded onto an aluminum stub using Cu

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tape and exposed to a 6-kV electron beam at a working distance of 10-mm. An EDAX EDS

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detector coupled to the microscope was utilized in the collection of spot size EDX spectra. At

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least 20,000 counts were obtained from the dominant element present within each spectrum. A

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holographic peak deconvolution was then employed to qualify spectra by ruling out noise and

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validating surface signal.

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RESULTS AND DISCUSSION Synthesis and Characterization of AgNPs: n = 3 batches of AgNPs were synthesized

227 228

for

each

experiment

and

were

characterized

in

229

recommendations.46,47 Both AgNPs- and AgNPs+ exhibited a surface plasmon resonance peak at

230

~400 nm in good agreement with the literature.50 ICP-OES revealed an average Ag colloidal

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concentration of 15.4 ± 0.8 and 300.6 ±0.6 mg L-1 for AgNPs- and AgNPs+, respectively. Raman

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spectroscopy confirmed a) the absence of organic impurities or silver oxide peaks in the colloids

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and b) the presence of CTAB at the AgNP+ surface (Figure S1). Electron microscopy data


accordance

with

U.S.

EPA’s

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showed that AgNPs- are spherical, have an average diameter of 14.1 ± 13.4 nm and a moderate

235

breadth size distribution in the 1-100 nm range.23 The specific AgNP- surface area was then

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estimated to be 20.9 - 814.8 m2 g-1 based on this size range and assumed sphericalness.48 AgNPs+

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were also found to be spherical but had a narrower size distribution (1-40 nm) than AgNPs-.50

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Zeta-potential measurements demonstrated that both AgNPs are charged and stable at the

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experimental pH of this study (ζ-potential of -41.47 mV at pH = 8.2 for AgNPs- and +34 mV at

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pH = 7 for AgNPs+).14,48 These colloidal AgNPs contain less than 10% Ag+ ions.49,50

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Macro-Muscovite: The label-free Raman spectra of macro-muscovite exposed to

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AgNPs- indicated no direct covalent interaction between the two (Figure S2). Thus, label-

243

enhanced Raman spectra were acquired on these samples using R6G as a SERS label of large

244

scattering cross-section in order to indirectly image the AgNP- distribution on the macro-

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muscovite surfaces. It is known that R6G cations have a high affinity towards the negatively-

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charged AgNP surface, where they experience large Raman signal enhancements associated with

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the SERS effect19. As a result, R6G fluorescence is quenched and detailed SERS spectra may be

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collected from R6G molecules located in the immediate vicinity of individual AgNPs- or AgNP--

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aggregates interacting with mineral surfaces.

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Raman controls (n = 900 spectra) consisting of hydrated macro-muscovite surfaces (no

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AgNPs- or R6G) exhibited solely the characteristic vibrational modes of muscovite (e.g., 170,

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170, 195, 215, 267, 382, 413, 638, 707, 757, 915, 959, and 1120 cm-1), which were identified in

253

good agreement with the literature51-54 (Table S1). Macro-muscovite controls with just AgNPs-

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yielded no detectable additional peaks and appeared vibrationally similar to muscovite control

255

(Figure S2). The macro-muscovite control sample exposed to 10-3 M of R6G (no AgNPs-)

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displayed a fluorescent background with no characteristic Raman R6G peaks. In order to



257

mitigate R6G fluorescence in the label-enhanced SERS spectra, R6G concentration was lowered

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to 10-4 M (R6G footprint sufficient for AgNP- coverage). The R6G-AgNP--mineral samples

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revealed significant differences with respect to controls: strong Raman spectral signatures of

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R6G and muscovite were simultaneously detected (Figures 1, S2). The loadings of all PCs that

261

accounted for > 1% of variance exhibited both R6G and muscovite Raman signatures. The

262

presence of strong R6G peaks (e.g., 611, 776, 1310, 1361, 1508, and 1652 cm-1)19,23,55,56 indicate

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enhancement had occurred in the areas where R6G-labeled AgNPs- have interacted with

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muscovite. Because no other Raman bands (e.g., Ag-O stretching mode) were detected in the n =

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3600 analyzed spectra, the interaction between AgNPs- and muscovite was deemed to be of a

266

non-covalent nature.

267 268

Figure 1. A) Raman images of four R6G-AgNP--macro-muscovite samples (25 x 25 µm each)

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constructed from the first PC and B) the loading for the first PC corresponding to R6G and


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muscovite signals. The sample was prepared by immersing muscovite into 1 mg L-1 of AgNPs-

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for 30 min. Colors are mapped to the score for the first PC (accounting for 46% of variance),

272

with purple regions representing spectra with negative scores (R6G absorbed onto AgNPs-) and

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brown regions representing spectra with positive scores (muscovite only). A larger score in the

274

first PC is associated with a stronger muscovite Raman signature relative to the R6G signature,

275

as seen in the plot of the first PC loading.

276 277

Macro-Corundum—The interaction mechanism and spatial distribution of AgNPs- on the

278

mineral surface were investigated via label-free Raman imaging (Figures 2 and S3). The

279

corundum controls exhibited the typical corundum peaks from literature (e.g., 379, 417, 429,

280

451, 576, 644, and 750 cm-1, Figure S3, Table S2)57-59, while corundum exposed to AgNPs-

281

revealed additional molecular vibrations in the 225-255 cm-1 spectral range, matching the

282

literature for Ag-O stretching modes.20,23,60 Therefore, a covalent attachment of AgNPs- to

283

corundum was inferred. An illustrative Raman image is given in Figure 2, where the gray-black

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areas correspond to the presence of corundum only (quantified through the integrated Raman

285

area). The spectrum containing the most intense Ag-O peak from n = 961 analyzed spectra is

286

represented by the white pixel in the chemical image. The high sensitivity of the Raman-based

287

imaging methodology was demonstrated through the detection of scarce molecular interaction

288

events of AgNPs- on the macro-mineral surface. Further AFM and SEM-EDX analyses were

289

then carried out to complement the Raman results and to further interrogate the interaction

290

mechanism.



291 292

Figure 2. A). Raman image of a AgNP--macro-corundum window (52 µm x 52 µm) prepared by

293

immersing corundum into 1 mg L-1 of AgNPs- for 30 min. The white pixel (4 µm2) corresponds

294

to the covalent attachment of AgNP(s)- to the mineral surface, while the gray-black pixels are

295

indicative of corundum alone. B) The corresponding Raman spectra illustrate the appearance of a

296

Ag-O stretching mode at 244 cm-1 and confirm the presence of corundum. The spectrum with the

297

minimum Euclidean distance to the mean of all spectra in the image is shown.

298

299

Topographic AFM profiles of corundum windows revealed a step and terrace structure on

300

the ( 1120 ) plane surface (Figure 3). Control hydrated mineral surfaces originally appeared

301

devoid of any nanoscale features but exhibited protrusions and holes characteristic to macro-

302

minerals (Figure 3A). The addition of ISA (NaNO3) to the control mineral surfaces resulted into

303

the formation of a few nanoscale features (n = ~2.5 per 1 µm2, 1-30 nm, Figure 3B). This is

304

probably due to the precipitation of the electrolyte from the thin water film that was not


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completely removed with the stream of nitrogen gas during drying. The EDX elemental analysis

306

(Figure S3C) confirmed the chemical nature of the ISA-derived nanostructures (Na peak at 1.05

307

keV); they appeared as white features in the collected SEM images (Figure S3A). The corundum

308

windows were then submerged in 1 mg L-1 of colloidal AgNPs- for 30 min (with 0.005 M of

309

NaNO3 at pH 8) and dried again with nitrogen gas. The resultant AFM images of the exposed

310

windows showed AgNPs- of a larger size distribution (n > 10 per 1 µm2, 1-100 nm) on the

311

mineral surfaces without any real indication of avoidance of or preference to adsorption at the

312

step edges (white features, Figure 3C). SEM-EDX measurements (figure S4) further established

313

the presence of AgNPs- (Ag peak at 2.99 keV) to the mineral surface (O and Al peaks at 0.53

314

keV and 1.5 keV, respectively).

315

316

317

318

319

320

321

322

323



324

Figure 3. 2 µm x 2 µm AFM topographic images of macro-corundum windows. A) Mineral

325

control: a water-washed and annealed mineral window displaying step-and-terrace structure with

326

minimal nano-sized features. B) Mineral control with ISA (no AgNPs-): the same window

327

submerged in 0.005 M of NaNO3 for 30 min leading to the formation of few nano-sized features

328

(white spots) due to the NaNO3 precipitation during the evaporation of the solution film on the

329

surface. C) Mineral sample with AgNPs-: the same window submerged in 1 mg L-1 of AgNPs-

330

generating additional, larger nano-sized features (white spots).

331

332

The macro-corundum experiment was conducted at pH = 8, but a wide range of pH

333

values are possible in natural aquatic systems. Because mineral surface charge and particle


Page 16 of 33

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334

oxidation depend on pH, the next set of experiments were conducted on micro-corundum in the

335

absence and presence of AgNPs- and AgNPs+, in the 6-11 pH range. Furthermore, multiple

336

crystalline faces exposed on mineral grains are more common in natural systems. Thus, the

337

incorporation of a micro-mineral model can offer additional insight into the mineral-AgNP

338

interaction behavior where additional crystallographic planes other than the ( 1120 ) surface are

339

exposed.

340

Micro-corundum particles: As expected, no Ag-O peaks were observed in the micro-

341

corundum Raman controls similarly to macro-corundum (Figure 4A), when no AgNPs were

342

present. Upon exposure to AgNPs-, a Ag-O stretching band appeared at ~ 230 cm-1 at each pH

343

(Figure 4B), confirming the covalent interaction between AgNPs- and corundum (for both

344

micro- and macro-corundum). However, n > 0.73 more Ag-O interactions per µm2 were

345

detected in the micro- than macro-corundum samples probably due to the different number of

346

lattice planes exposed at the surface; AgNP- concentration and mineral surface area were

347

factored into these calculations. While macro-corundum has only a single lattice plane exposed

348

at the surface (an ordered lattice in the a-plane (112ത0)61), micro-corundum has many different

349

lattice planes that could potentially favor the Ag-O interactions.

350

To further analyze the interaction mechanism, the total number of Raman spectra

351

exhibiting the Ag-O stretching mode was plotted as a function of pH (Figure 4C). Although the

352

largest number of AgNP--corundum interactions was detected at pH ≥ 9 (n = 902-1015 spectra)

353

when compared to all other examined pH values (n = 796-862), the difference was not

354

statistically significant (p > 0.05). Thus, the adsorption process of AgNPs- was deemed not pH

355

dependent under the employed experimental conditions, i.e., 1.2 g of micro-corundum particles

356

(SSA ~ 6.5 ± 0.2 m2 g-1) exposed to 100 mL of 1 mg L-1 of negatively charged AgNPs- for 30



357

min at each pH. The integrated area under each Ag-O peak was also estimated to further

358

confirm this pH independent behavior (Figure 4D). Again, the differences amongst all pH

359

values were not statistically significant (p > 0.05), despite pH 8 and 9 yielding the largest

360

average integrated area. Similar Ag-O interactions were detected between AgNPs+ and micro-

361

corundum at each pH (Figure S1), but no statistically significant trend could again be

362

established with the change in pH. It should be noted that possible trends in the integrated area

363

of the corresponding Ag-O stretching peak (235-250 cm-1) might have been skewed by the

364

presence of a small CTAB vibrational mode at ~238 cm-1 (Figure S1). Future work that

365

establishes the relative contributions of CTAB to the Ag-O band profile , such as a pH

366

dependence study on CTAB alone or the use of different AgNPs+ colloidal systems, would lead

367

to a better quantitative description of the Ag-O interactions.


Page 18 of 33

Page 19 of 33


368 369

Figure 4. Raman data and their statistical analysis for AgNP--micro-corundum at all

370

experimental pH values (6 – 11). Average Raman spectra of A) corundum control with ISA and

371

B) corundum sample with ISA exposed to 1 mg L-1 of AgNPs- for 30 min at each pH. C) The

372

total number of Raman spectra of the AgNP--micro-corundum samples exhibiting Ag-O

373

stretching modes at each pH value. D) Mean of the integrated area of the Ag-O stretching mode

374

at each examined pH value with 95% confidence intervals. The values obtained at each pH

375

represent an average of n = 1089 spectra from n = 3 separate trials.

376 377

Because the Raman experiments conducted on both macro- and micro-corundum

378

revealed the appearance of Ag-O stretching modes, the surface complexation of AgNPs- to the

379

terminus corundum oxygen atoms (Ag-O-Al-) was considered to play a key role in their



380

interaction. Furthermore, the highest chemisorption level was detected around a pH value of 9,

381

where corundum has approximately zero net surface charge and other interaction mechanisms

382

may be at play. This is illustrated in the proposed mechanistic scheme (Figure 5). In addition to

383

the favored Ag-O covalent interactions at pH ~ 9, AgNPs- may also experience electrostatic

384

repulsions and attractions to the negative and positive moieties, respectively, of the mineral

385

surface. The formation of hydrogen bonding may also occur at all pH values due to the

386

hydration of corundum. In fact, literature shows that the hydration of corundum is accompanied

387

by the formation of multiple hydroxyl terminations onto its surface,59 which can facilitate this

388

interaction. Some hydroxyl groups will undergo deprotonation above the pHpzc (pH = 9.1) of

389

corundum to create an overall negative charge, whereas positively-charged surface sites will be

390

in the majority due to surface site protonation below the pHpzc. Thus, electrostatic repulsions

391

and attractions are expected at pH values > 9.1 and < 9.1, respectively.

392 393

Figure 5. Schematic of the proposed interaction mechanisms between AgNPs- and micro-

394

corundum at the investigated pH values (6-11). Red atoms = Al, blue = O, gray = H, and green =

395

Ag. Drawing is for illustrative purposes and is not to scale.


Page 20 of 33

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396 397

To support the Raman analysis, the total Ag content on the mineral surface was measured

398

by ICP-OES. It was estimated by the difference between the Ag content of the initial AgNP-

399

colloid and that of the supernatant containing free AgNPs-, which were isolated from AgNPs-

400

bound to micro-corundum particles through centrifugation. This was interpreted as an average

401

percent of AgNPs- adsorbed onto micro-corundum. Quantitative measurements of the total

402

amount of Ag was also made for all controls at each pH; no Ag was detected in the absence of

403

AgNPs-. In good agreement with the Raman results, no significant dependency on pH was

404

observed for AgNP- adsorption to micro-corundum (Figure 6, p > 0.05). Again, this is likely due

405

to the multiple interaction mechanisms possibly occurring at each experimental pH. For example,

406

at pH > 9.1, corundum particles have an overall negative charge and are thereby expected to

407

experience electrostatic repulsions to the negatively charged AgNPs-. Nevertheless, over 80% of

408

the total amount of available AgNPs- was found to be adsorbed on the mineral surface at these

409

pH values (ICP-OES data). Thus, the chemisorption of AgNPs- to corundum through the

410

formation of Ag-O covalent bonds (Raman data) may offset to a certain degree the contributions

411

of the electrostatic repulsions to the overall interaction scheme. Future work that establishes the

412

relative amounts of AgNP- sorbed via electrostatic and chemisorption interactions, such as via a

413

study of the effects of ionic strength on sorption, would lead to a better quantitative description

414

of the speciation of sorbed AgNPs-. On the other hand, at pH < 9.1, corundum particles have an

415

overall positive charge and are electrostatically attracted to the negatively charged AgNPs-.

416

These electrostatic contributions should add up to the chemisorption ones leading to statistically

417

higher levels of AgNP- adsorption to corundum, which was not observed here (figure 6). Thus,



418

additional interaction mechanisms may be at play here and should be the subject of future

419

investigations.

420 421

Figure 6. Average percent adsorption of AgNPs- to micro-corundum as a function of pH. The

422

Ag content was by determined by ICP-OES after 30 min incubation of 1.2 g of corundum

423

particles (SSA of 6.5 ± 0.2 m2 g-1) with 100 mL of 1 mg L-1 of AgNPs- followed by the

424

separation of free AgNPs- from corundum-bound AgNPs- through centrifugation. Error bars

425

represent the standard deviation of n = 9 independently prepared samples.

426 427

Overall, this study addresses a fundamental methodology gap by demonstrating the

428

capability of the Raman-based imaging and chemometrics approaches in characterizing the

429

molecular adsorption behavior and distribution of AgNPs to hydrated mineral surfaces with

430

nano- or micro-spatial resolution (available in modern Raman systems). This Raman method

431

requires minimum to no sample preparation, is sensitive and non-invasive. Furthermore, label-

432

enhanced Raman imaging could substitute for label-free Raman imaging to indirectly map the

433

distribution of AgNPs onto mineral surfaces, when no direct molecular interaction occurs

434

between the two. The proposed Raman method could be extended to a large variety of

435

environmentally relevant minerals (e.g., quartz, kaolinite, orthoclase, montmorillonite, hematite,

436

and calcite) and nanomaterials (e.g., Au, Cu, C, Zn, CdSe, TiO2, and so on). For example,


Page 22 of 33

Page 23 of 33


437

Raman spectroscopy was successfully utilized in the identification of nano-TiO2 phases, crystal

438

size, quantum confinement, annealing effects, and so on. Furthermore, most of the mentioned

439

nanomaterials were found to enhance the Raman signal of analytes physisorbed or chemisorbed

440

on their surface through the SERS effect. The effects of key solution variables (e.g., pH and ionic

441

strength) and nanomaterial characteristics (e.g., size, concentration, surface charge, and surface

442

coating) could also be explored. In addition, both cationic and anionic Raman labels could be

443

employed to further investigate the interactions with mineral surfaces of permanent negative or

444

positive charge. The knowledge gained from such Raman-based imaging projects could raise

445

awareness with respect to the safe manufacture, processing and proper disposal of engineered

446

nanomaterials into the environment.

447 448

SUPPORTING INFORMATION

449

The Supporting Information is available free of charge on the ACS Publications website.

450

Raman spectra of micro-corundum exposed to CTAB-capped AgNPs+ with ISA and the

451

corresponding controls at different pH values, representative Raman spectra of muscovite

452

controls, a Raman image of a AgNP--macro-corundum control window, assignments of

453

the main Raman vibrational modes observed for the muscovite and corundum controls,

454

backscattered SEM images and EDX spectra of micro-corondum exposed to AgNPs- with

455

ISA and the corresponding controls.

456 457

ACKNOWLEDGEMENTS

458

The Chemistry Department at WSU and the U.S. National Science Foundation Award #1438340

459

are gratefully acknowledged for the financial support of this project. Garrett VanNess and Joseph



460

Solch are thanked for their assistance in the maintenance and operation of the ICP-OES system

461

at WSU. Dr. Dhriti Nepal is acknowledged for providing the access to the SEM-EDX system at

462

Wright Patterson Air Force Base, Dayton, Ohio. The authors are grateful to Cody Fourman and

463

Garrett VanNess for conducting BET surface area analysis on the mineral samples. The authors

464

also acknowledge support from the Wright State University Research Challenge program for

465

financial support for the surface area analysis instrumentation.

466

467

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A Raman-Based Imaging Method for Characterizing the Molecular

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