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Characterization of Natural and Affected Environments
Real-Time Monitoring of Ligand Exchange Kinetics on Gold Nanoparticle Surfaces Enabled by Hot SpotNormalized Surface-Enhanced Raman Scattering Haoran Wei, Weinan Leng, Junyeob Song, Chang Liu, Marjorie Willner, Qishen Huang, Wei Zhou, and Peter J. Vikesland Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b03144 • Publication Date (Web): 10 Dec 2018 Downloaded from http://pubs.acs.org on December 11, 2018
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Environmental Science & Technology
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Real-Time Monitoring of Ligand Exchange Kinetics on Gold
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Nanoparticle Surfaces Enabled by Hot Spot-Normalized Surface-
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Enhanced Raman Scattering
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Haoran Wei,†,‡,§ Weinan Leng,†,‡,§ Junyeob Song,ǁ Chang Liu,†,‡,§ Marjorie R.
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Willner,†,‡,§ Qishen Huang,†,‡,§ Wei Zhou,ǁ and Peter J. Vikesland†,‡,§,*
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†
Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, Virginia.
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‡
Virginia Tech Institute of Critical Technology and Applied Science (ICTAS) Sustainable
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Nanotechnology Center (VTSuN), Blacksburg, Virginia.
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§
Center for the Environmental Implications of Nanotechnology (CEINT), Duke University,
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Durham, North Carolina.
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ǁ
Department of Electrical and Computer Engineering, Virginia Tech, Blacksburg, Virginia.
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*Corresponding author. Phone: (540) 231-3568, Email:
[email protected] 14 15 16 17 18
A manuscript prepared for Environmental Science & Technology
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TOC
Gold Nanoparticle
Exchange
Citrate Guest ligand 22
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ABSTRACT: Nanoparticle surface coatings dictate their fate, transport, and bioavailability.
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We used a gold nanoparticle-bacterial cellulose substrate and “hot spot”-normalized surface-
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enhanced Raman scattering (HSNSERS) to achieve in situ and real-time monitoring of ligand
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exchange reactions on the gold surface. This approach enables semi-quantitative determination
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of citrate surface coverage. Following exposure of the citrate-coated nanoparticles to a suite of
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guest ligands (thiolates, amines, carboxylates, inorganic ions, and proteins), the guest ligand
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signal exhibited first-order growth kinetics, while the desorption mediated decay of the citrate
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signal followed a first-order model. Guest ligand functional group chemistry dictated the kinetics
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of citrate desorption, while the guest ligand concentration played only a minor role. Thiolates
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and BSA were more efficient at ligand exchange than amine-containing chemicals, carboxylate-
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containing chemicals, and inorganic salts due to their higher binding energies with the AuNP
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surface. Amine-containing molecules overcoated rather than displaced the citrate layer via
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electrostatic interaction. Citrate exhibited low resistance to replacement at high surface
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coverages, but higher resistance at lower coverage, thus suggesting a transformation of the
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citrate-binding mode during desorption. High resistance to replacement in stream water suggests
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that the role of surface-adsorbed citrate in nanomaterial fate and transport must be better
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understood.
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INTRODUCTION
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Gold nanoparticles (AuNPs) are being manufactured for versatile biochemical applications, such
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as biosensing, labeling, catalysis, drug delivery, and photothermal therapy due to their unique
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chemical, optical, and electronic properties.1-3 In the United States, AuNPs, among other
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engineered nanomaterials, are regulated under the Toxic Substances Control Act in terms of their
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potential acute or chronic toxicity.4-6 Once released into surface waters, the interactions of
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AuNPs with natural chemicals and colloids result in either enhanced colloidal stability or
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destabilization via homo- or hetero-aggregation.7-11 Nanoparticle surface coatings interact with
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the environment and dictate nanoparticle fate and transport.7, 8, 12 AuNP surface coatings can alter
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cytotoxicity by changing biocompatibility, hydrophobicity/hydrophilicity, surface charge, or
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other factors that dictate interactions with cell membranes.12-16 For these reasons, it is important
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to have an accurate picture of the stability of AuNP surface coatings.
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Techniques that are frequently adopted for AuNP characterization include dynamic light
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scattering,
UV-VIS
spectroscopy,
transmission
or
scanning
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electrophoresis, and surface charge analysis.7,
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characterization approaches only provide information about surface coating morphologies and
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surface charge, but nothing about surface chemical composition. X-ray photoelectron
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spectroscopy enables identification of surface coatings and has been used to quantify ligand
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electron
microscopies,
Although highly useful, these
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density on the AuNP surface.17 However, it is sensitive to variations in nanoparticle geometry
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and is limited to dry powders, thus making it inappropriate for in situ characterization. Nuclear
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magnetic resonance provides high chemical and spatial resolution about ligand identity,
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arrangement, and dynamics on metal nanoparticle surfaces.18 However, it is currently only
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accurate for extremely small NPs (
