Controlled Evaluation of the Impacts of Surface Coatings on Silver

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Controlled Evaluation of the Impacts of Surface Coatings on Silver Nanoparticle Dissolution Rates Chang Liu, Weinan Leng, and Peter J. Vikesland Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05622 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on February 1, 2018

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Environmental Science & Technology

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Controlled Evaluation of the Impacts of Surface

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Coatings on Silver Nanoparticle Dissolution Rates

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Chang Liu, Weinan Leng and Peter J. Vikesland*

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Department of Civil and Environmental Engineering, Institute of Critical Technology and Applied

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Science (ICTAS), and the Center for the Environmental Implications of Nanotechnology (CEINT),

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Virginia Tech, 418 Durham Hall, Blacksburg, Virginia, 24061-0246, United States

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Abstract

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Silver nanoparticles (AgNPs) are increasingly being incorporated into a range of consumer

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products and as such there is significant potential for the environmental release of either the

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AgNPs themselves or Ag+ ions. When AgNPs are exposed to environmental systems, the

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engineered surface coating can potentially be displaced or covered by naturally abundant

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macromolecules. These capping agents, either engineered or incidental, potentially block

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reactants from surface sites and can alter nanoparticle transformation rates. We studied how

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surface functionalization affects the dissolution of uniform arrays of AgNPs fabricated by

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nanosphere lithography (NSL). Bovine serum albumin (BSA) and two molecular weights of

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thiolated polyethylene glycol (PEG; 1000 Da and 5000 Da) were tested as model capping agents.

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Dissolution experiments were conducted in air-saturated phosphate buffer containing 550 mM

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NaCl. Tapping-mode atomic force microscope (AFM) was used to measure changes in AgNP

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height over time. The measured dissolution rate for unfunctionalized AgNPs was 1.69 ± 0.23

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nm/d, while the dissolution rates for BSA, PEG1000, and PEG5000 functionalized samples were

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0.39 ± 0.05, 0.20 ± 0.10, and 0.14 ± 0.07 nm/d, respectively. PEG provides a steric barrier

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restricting mass transfer of reactants to sites on the AgNP surface and thus diminishes the

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dissolution rate. The effects of BSA, on the other hand, are more complicated with BSA initially

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enhancing dissolution, but providing protection against dissolution over extended time.

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TOC

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MAIN TEXT

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Introduction

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Silver nanoparticles (AgNPs) represent one of the most widely used nanomaterials in

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commercial and medical products.1-4 A wide range of consumer products such as textiles, food

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containers, cosmetics, and medical devices employ AgNPs as antimicrobial agents.5-7

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Unfortunately, this antimicrobial property has the potential to elicit nanotoxicity when the

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AgNPs enter the environment.6, 7 AgNP containing products have been shown to release AgNPs

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after washing or through direct use and it is expected that the input of AgNPs into aquatic

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systems will increase in the coming decades.8 Elevated human and environmental AgNP

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exposures raise concerns about potential environmental implications.9-11 Recent studies have

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explored the toxicity of AgNPs to a variety of organisms such as plants, algae, fungi,

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microorganisms, and human cells.12-14 The negative impacts of AgNPs on the environment and

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potentially humans may be long lasting and have been recently reviewed.11, 15, 16 While all of the

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mechanisms by which AgNPs elicit a toxic effect remain unclear,4, 17, 18 it is generally considered

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that the toxicity of AgNPs is at least partly driven by Ag+ ion release.16 Even if Ag+ release is only

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one of many pathways by which AgNPs elicit toxicity, dissolution remains an important process

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that alters nanoparticle properties and is thus a critical aspect of AgNP safety.

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Ag+ ions migrate from the nanoparticle surface to the bulk solution when an AgNP

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dissolves.19 This dynamic process is dependent on the particles’ chemical and surface properties,


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shape, size, and external factors such as the chemistry of the surrounding media.17 Surface

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coatings, formed by covering the surface with capping agents, can alter the dissolution rate.

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The implications of surface coating on nanoparticle reactivity are dependent on the identity of

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the surface coating and the means by which it is attached to the particle surface.17, 20, 21 When

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AgNPs enter the environment, pre-engineered surface layers may be displaced or covered by

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proteins or other naturally abundant macromolecules.17, 22 Surface coatings are expected to

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affect the reactivity of AgNPs in several ways. First, by coordinating with surface atoms,

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coatings may effectively block reactants from reaching surface sites and thus slow reaction

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rates.23 Alternatively, if organic molecules bind to the metal surface through nucleophilic

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functional groups, then they may accelerate oxidation and dissolution.24,

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hypothesized that reactive oxygen species formed during AgNP oxidation may be scavenged by

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organic coatings that slow the dissolution process by preventing these reactive agents from

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further oxidizing the metal surface.23, 26 Finally, surface functionality generally dictates AgNP

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surface charge, which in turn affects the local ionic environment near the particle surface and

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thus may alter reaction rates. 17

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It has been

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Previous studies have used polyvinylpyrrolidone (PVP)27, 28 and citrate22, 29 as coating agents

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and found that these capping agents affect AgNP dissolution. Zong et al. demonstrated the

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antimicrobial activity of polyethylene glycol (PEG)-thiol and PVP coated AgNPs, and observed

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that smaller PEG-coated particles dissolved faster than larger PEG-coated particles.30 Li et al.

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determined that AgNP dissolution was inhibited by coatings of sodium dodecyl sulfate (SDS) or

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Tween 80, but not by the initial citrate coating.31 Ostermeyer studied the influence of bovine

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serum albumin (BSA) and alginate coatings and found that while BSA prevented NH3-induced

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dissolution that alginate only weakly interacted with the AgNP surface and was unable to

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completely prevent NH3-induced dissolution.32 To date, several analytical techniques have been

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employed to study nanoparticle dissolution processes including UV-vis, DLS, TEM and ICP-MS.17,

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21, 33, 34

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attention. Some prior studies have shown that aggregation increases particle size, and

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preserves most of the surface area within the aggregate. Following aggregation the exposed

However, in most of these studies the impacts of particle aggregation were given little


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surface area of the AgNPs is reduced and this decreases the dissolution rate.22, 35 Thus it is

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important to utilize methods to evaluate the dissolution process in the absence of aggregation.

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Nanosphere lithography (NSL) has been used as a simple and cost-effective technique to

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produce metal nanoparticle arrays of controlled shape and size.36, 37 These uniform arrays of

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nanoparticles enable controlled evaluation of nanoparticle transformations in the absence of

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aggregation.38 In our previous studies, the dissolution and sulfidation of NSL-produced AgNPs

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were investigated by atomic force microscopy (AFM). Both shape and height changes were

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discussed in detail.38, 39 In this contribution, we extend our prior work and report on the

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controlled evaluation of how surface coatings affect AgNP dissolution. NSL was used to produce

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AgNPs immobilized on glass substrates and then the particles were functionalized with two

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different capping agents. BSA and PEG-thiol were chosen as coating agents due to their

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favorable binding to the AgNP surface, but differential interactions with the AgNP surface. BSA

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is commonly used as a model protein in studies of nanoparticle-protein interactions.32,

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Proteins are well known to form “coronas” around AgNPs in biological media,41, 42 which makes

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them a highly important class of surface coating from a toxicological point of view.40 Two

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different molecular weights of PEG were chosen to evaluate how the molecular weight of a

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coating agent affects dissolution. Specifically, PEG-thiols with molecular weights of 1000 Da and

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5000 Da were tested. It is reported that heavy metals associate with proteins by interacting

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with thiol groups in cysteine and acetylcysteine, so this chemical interaction enhances the

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connection between a given coating agent and the AgNPs.43, 44 AFM was used to study changes

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in the morphology of AgNPs, transmission electron microscopy (TEM) was employed to

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investigate crystal structure, and surface enhanced Raman spectroscopy (SERS) was used to

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evaluate metal-sulfur interactions.

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Materials and Methods

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Materials. Glass cover slips (60 × 24 × 0.15 mm) were purchased from Fisher Scientific. (3-

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mercaptopropyl)-trimethoxysilane and BSA were provided by Sigma-Aldrich. PEG-thiols (PEG-

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1000: MW = 1000 Da; PEG-5000: MW = 5000 Da.) were purchased from Nano CS Inc. Methanol


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was purchased from Alfa Aesar. Carboxylated polystyrene spheres were acquired from Life

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Technologies. All reagents were analytic purity and were used without further purification.

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Deionized (DI) water (>18.2 MΩ-cm) was produced by a Barnstead water purification system

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and was used throughout this study. Stainless steel specimen discs for AFM measurements

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were purchased from Ted Pella and antimony doped silicon TESPA-V2 AFM probes were

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purchased from Bruker.

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Substrate production. Glass substrates were cleaned by sequential immersion in RCA1

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solution (1 NH4OH:4 H2O2:20 H2O, v:v:v) and then in RCA2 solution (1 HCl:1 H2O2:5 H2O) at 75 °C

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for 10 min each.45 The substrates were rinsed with DI water after each cleaning step and then

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air dried. To enhance adhesion between deposited silver and the glass, the substrate was

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thiolated by immersion in 5% (3-mercaptopropyl)-trimethoxysilane in methanol for 12 h.46

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Following thiolation, glass substrates were rinsed with DI water and stored in methanol until

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use. Negatively charged carboxylated polystyrene microspheres with a diameter of 450 nm

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were deposited onto cleaned substrates by convective self-assembly (CSA).36 Specifically, the

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substrates were held horizontally on a motion stage (Thor Laboratories) below an angled plate

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in an airtight container. The distance between the substrate and the angled plate was then

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adjusted to ≈600 nm. A 4 µL aliquot of polystyrene suspension (10% w/v) was placed between

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the interspace and the substrate and was then moved at a constant velocity of 0.05 cm/s for 12

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cm. The colloidal suspension spread over the substrate and a monolayer of close-packed

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spheres formed due to solvent evaporation. Following CSA, electron beam evaporation (3-kW

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electron gun, Thermionics) was used to deposit a 45 nm thick layer of silver metal onto the

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prepared substrates. Substrates were cut into approximate squares of ≈5 mm2 and the spheres

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were removed using tape. AgNPs immobilized on glass substrates were sequentially rinsed with

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ethanol and DI water for 30 s each.

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Coating treatment. Coating solutions were prepared by dissolving PEG-thiol or BSA in DI

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water. For BSA, three different concentrations were prepared with weight: weight ratios of 0.1,

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0.5, and 1%. Coating solutions were transferred to petri dishes and five prepared substrates

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were immersed in each solution. The petri dishes were then sealed with sealing film and stored


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in the dark for a 12 h coating period. Control experiments determined that this period was

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sufficient for complete surface functionalization. The substrates were then rinsed with DI water

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and air dried prior to storage in a desiccator.

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Nanoparticle dissolution experiments. The effects of coating agent identity on AgNP

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dissolution were evaluated by immersing prepared substrates in phosphate buffered (1 mM

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NaH2PO4; 1 mM Na2HPO4) 550 mM NaCl solution with the final pH adjusted to 7.0 ± 0.1 via 0.1

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M NaOH addition. In our previous study, the relationship between NaCl concentration and

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dissolution rate was studied. With a higher NaCl concentration (varying from 10 mM to 550

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mM), the AgNP dissolution rate increased from 0.4 nm/d to 2.2 nm/d. A linear regression

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relationship was obtained between the NaCl concentration and the measured dissolution

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rate.38 Herein, a NaCl concentration of 550 mM was chosen as this value is characteristic of

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seawater and other saline solutions.47 Following the coating treatment the prepared specimens

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were submerged in 10-mL of NaCl solution in petri dishes and sealed with Parafilm. All

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dissolution experiments were conducted at room temperature (25 °C) in the dark. To quantify

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AgNP dissolution rates, each substrate was removed from solution and dried under N2 after a

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defined reaction period. For each reaction time, one specimen was used and then disposed of

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following an AFM measurement. To study the effects of coating identity on AgNP dissolution,

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the reaction period was set as 0, 1, 2, 4, 7, 10 and 14 days for each coating type. To investigate

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how the coating process itself affects dissolution, the coating experiments were conducted for

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0, 1, 2, 4, 7 and 12 h for each coating agent. The samples used for the 14 day and 12 h

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dissolution experiments are the same. The difference between these two experimental sets

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was the time period over which the samples were analyzed by AFM. All AFM images were

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measured just after removing the specimen from the reaction solution. A schematic of the

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AgNP array production and coating process is shown in Figure 1a.


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Figure 1. (a) Schematic of AgNP array production and coating process, (b) AFM image of original AgNPs, and (c) AgNP height distribution as measured by AFM. The mean height of 585 particles was 47.1 with a standard deviation of 1.5 nm.

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Analytical techniques. Samples were attached to 15 mm stainless steel specimen discs with

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wax and AFM height measurements were obtained using a Nanoscope IIIa Multimode AFM

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(Veeco) equipped with a J scanner. Antimony doped silicon TESPA-V2 AFM probes were used.

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The AFM was operated in tapping mode with a resonant frequency of 260−450 kHz. All images

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were acquired at 256 × 256 pixel resolution and a scan rate of 0.5 Hz. For each specimen, 3-5

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images were collected at different locations with the scan area of each image set at 5×5 µm2. As

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discussed elsewhere,38 some parts of the specimen exhibited irregular morphologies due to

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defects in the colloid layer, and these portions of the surface were excluded from measurement.

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A minimum of 486 particles were measured for each specimen to calculate the mean particle

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height. The “Flatten” and “Erase Scan Lines” tools of the NanoScope software were used to

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modify the collected images by correcting the baseline and removing spurious scan lines. The

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“Particle Analyze” tool was employed to measure the height of the particles and defects were

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excluded.

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Specimens for TEM measurements were attached to 0.5 mm Ni aperture grids using Loctite

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epoxy. An Allied High Tech Multiprep automated polishing system was used to thin the samples

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to 5−10 μm. Several diamond lapping films with different particle sizes were used as grinding

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media. Then the specimens were polished using a Fischione model 1010 ion mill with an

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accelerating voltage of 3.5 kV and a beam current of 5 mA. TEM images were obtained using a

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JEOL 2100 field thermionic emission TEM. Raman measurements were obtained on a WITec


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alpha500R Raman spectrometer using a 785 nm excitation laser. Raman spectra were collected

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in a 20 µm × 20 µm image scan using a 100× microscope objective (N.A. =0.9, Manufacturer：

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Olympus, model: UIS2 FN26.5) and a laser intensity of 1.0 mW. The reported spectra were

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obtained by averaging 2500 scans (integration time = 30 ms) acquired across the sample area.

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Results and Discussion

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Silver was deposited over the carboxylated latex sphere mask by electron beam evaporation to

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form AgNP arrays on the glass slides. The typical topography of a NSL-prepared AgNP array was

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determined by AFM. As shown in Figure 1, defect free domains spanned several μm2 and both

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point and line defects were evident. AFM characterization revealed that the produced AgNPs

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exhibited a truncated tetrahedral shape (Figure 1b), as expected.37, 48 The initial nanoparticle

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height was normally distributed with a mean value of 47.1 nm and a standard deviation (SD) of

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1.5 nm (Figure 1c).

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The purpose of this study was to measure changes in the height profiles of silver

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nanoparticles as a means to determine dissolution rates. In contrast to solution based Ag+

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monitoring, AFM provides a direct measurement of changes in morphology and height during

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dissolution. Fourteen-day dissolution experiments were conducted in air-saturated phosphate

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buffer (pH 7.0, 25 °C) containing 550 mM NaCl. The images in Figure 2 illustrate how AgNP

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morphology changes between day 0 (prior to addition of any coating) and after coating on day

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1 and 14. For the uncoated AgNPs, the shape of the AgNPs changed from triangular to circular

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after only one day of immersion in NaCl solution (Figure 2a). This phenomenon was not

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observed for coated AgNP samples (Figures 2b-d). After two weeks, the size of the uncoated

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AgNPs decreased substantially and there was obvious loss of individual AgNPs from the glass

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substrate (Figure 2e). In contrast, the PEG coated AgNPs exhibited no obvious changes in size

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(Figures 2f and 2g), while there was a slight decrease in size for the BSA coated AgNPs (Figure

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2h).

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morphology during surface treatment. As shown in Figure S1 in the supporting information, the

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uncoated AgNPs become more circular with many small pieces of substrate scattered around

In addition, TEM measurements were conducted to investigate changes in AgNP


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the initial AgNPs. BSA coated AgNPs maintain the triangular shape after 12 h surface treatment

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and the background is very clear with no AgNP fragments. These phenomena are in accordance

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with the results from the AFM measurements.

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AFM provides a convenient technique to measure the kinetics of AgNP dissolution. The

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“Particle Analyze” tool was employed to measure changes in particle height while excluding

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defects. Over 450 AgNPs were measured for each specimen to calculate the mean particle

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height. Changes in the heights of the AgNPs were accurately tracked and the implications of

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nanoparticle aggregation were averted. Measured height profiles are shown in Figure S2.

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Changes in shape are clearly observed in the high magnification AFM images. The height

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distribution of each coated and uncoated sample are shown in Figure S3. The height of the

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blank AgNPs was normally distributed which illustrates the uniformity of the AgNP arrays.

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Similarly, we observed a uniform distribution for all of the samples after coating.

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Figure 2. AFM images of uncoated and coated AgNPs measured after 1- and 14-days dissolution. The left panel is the AFM image for the original AgNPs without coating and dissolution; on the right side (labeled a-h) are the AFM images for uncoated and coated AgNPs after 1 and 14 days.

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The mean height of the original, uncoated AgNPs was 47.1 ± 1.5 nm, but this value increased to

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55.4 ± 1.2 nm after one day reaction with a change in nanoparticle shape. This growth was only

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observed for uncoated AgNPs. Previous work by our group has shown that the AgNP height

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increases by 6-12 nm during an initial exposure period to solutions with NaCl >10 mM and that

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this growth occurs with a concomitant steepening of the sidewalls and dissolution at the


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corners.38 Oxidation of Ag0 to Ag+ occurs at the bottom edges and corners of the AgNPs (the

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anode), while reduction of Ag+ to Ag0 occurs at the top (the cathode).38, 49, 50 This process leads

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to an Ag+ concentration gradient that favors the net flow of silver from the bottom of a

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nanoparticle to the top until the internal redox gradient is eliminated. After the initial increase,

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the mean height of the uncoated AgNPs gradually decreased to ≈30 nm during the two-week

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reaction period. The AgNPs we used in this study were produced by NSL. Due to the limitations

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of the production process some portions of the nanoparticle array are highly irregular (Figure

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S4). The amount of dissolvable Ag present within these irregular, or defect, zones is variable

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from sample to sample. For this reason, we chose not to measure Ag+ via ICP-MS as the

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measurements do not provide a reliable counterpart to the measured height changes that

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directly reflect our reported nanoparticle dissolution rates. Furthermore, our previous work

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quantified dissolved Ag+ in solution using ICP-MS, but with characteristic low Ag+ recoveries.

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The measured Ag+ values underestimated the values predicted by the AFM measurements by

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up to 40%.38 For these reasons, we did not attempt to conduct parallel ICP-MS measurements

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in this work.

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The AgNPs with a PEG1000 coating exhibited a mean height that varied between 47.0 ± 0.4

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nm and 49.4 ± 0.3 nm during the two-week experimental period. The mean height of the

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PEG5000 functionalized sample was 46.3 ± 1.2 nm after the initial coating treatment and the

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height then fluctuated between 44.3 and 46.0 nm (Figure 3). The slight, and statistically

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indistinguishable, difference between PEG1000 and PEG5000 may reflect differences in the

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initial substrates and we do not attribute this difference to differential dissolution. If polymer

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coatings provide a barrier that inhibits the mass transfer of reactants (primarily dissolved

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oxygen in this case) to sites on the AgNP surface, then longer polymer chains might have been

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expected to inhibit AgNP dissolution to a greater degree than shorter polymers. However, this

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hypothesis was not supported by the present data. One possible explanation is that PEG1000

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covered the surface of the AgNPs equally well as PEG5000. As such, no difference was observed

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for these two PEGs with different chain length. The mean height change was very slight for PEG

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coated samples and no obvious dissolution was observed. If we were to extend the reaction

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period it is possible there would be greater differences. It is notable that there was a difference


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in the initial mean heights of the differentially coated samples, (i.e. the initial mean height of

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the BSA coated sample was 42.1 ± 0.4 nm which is lower than those of the PEG coated samples).

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As discussed later, one possible reason for the difference is that the coating process has an

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effect on the height change. Such a hypothesis is discussed vida infra.

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Figure 3. (a) Mean AgNP height at different times and (b) normalized mean AgNP height at different times. Inset: Dissolution rates calculated by linear regression for the different coatings. (At least 486 particles were measured for each specimen to calculate the mean particle height. Error bars represent the standard deviation for mean heights determined by AFM for experiments performed in triplicate.)

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AgNP dissolution is typically modeled using first-order reaction kinetics;10, 21, 26, 51 however,

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solid-state reactions are dominated by interfacial interactions. Accordingly, the dissolution

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process may be more appropriately modeled by assuming that the reaction rate is proportional

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to the remaining surface area rather than the remaining mass of solid. If the solid material is

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assumed to be a sphere, the rate of change of the particle’s radius is the linear dissolution rate.

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This model, generally referred to as the contracting sphere rate law,10, 38 predicts that the

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particle’s radius will decrease at a constant rate. Because our methodology measures the mean

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height of dissolving AgNPs over time, the dissolution rate can be determined directly as the

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slope of a simple linear regression of the AFM time series data. The applicability of the

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contracting sphere model was supported by the constant linear dissolution rates observed

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herein (Figure 3b). For the uncoated AgNPs there was an initial increase in size prior to day 1

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and as noted previously we believe the dissolution process in that period differs from that

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which occurs later.38 For this reason when we normalized the data to obtain the dissolution rate,


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which is the slope of the data points, we focused on data from day 1 to day 14. By fitting the

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normalized data with a linear regression, the dissolution rate constants (k; nm/d) for both the

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uncoated and coated agents were determined. The dissolution rate for uncoated AgNPs was

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1.69 ± 0.23 nm/d with a relatively strong linear correlation with respect to time (R2 = 0.913).

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This result is comparable to that obtained previously (=2.2 nm/d) under similar conditions.38

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Following coating with BSA, the dissolution rate decreased to 0.39 ± 0.05 nm/d which suggests

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the coating layer inhibits AgNP dissolution. PEG coated samples exhibited a dissolution rate of

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0.20 ± 0.10 nm/d for PEG1000, while it was only 0.14 ± 0.07 nm/d for PEG 5000. The R2 values

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for these two agents were

Controlled Evaluation of the Impacts of Surface Coatings on Silver

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