Operando electrochemical SERS monitors nanoparticle reactions by capping agent fingerprints

Abstract

Nanomaterials are frequently employed in daily life goods, including health, textile, and food industry. A comprehensive picture is lacking on the role of the capping agents, added ligand molecules, in case of nanoparticle reactions and degradation in aqueous solutions, like surface waters or biofluids. Here, we aim to elucidate the capping agent influence on nanoparticle reactivity probing two commonly employed capping agents citrate and polyvinylpyrrolidone (PVP). Their influence on silver nanoparticle (AgNP) transformation is studied, which is particularly important due to its application as an antimicrobial agent. We induce oxidation and reduction processes of AgNPs in halide solutions and we monitor the associated transformations of particles and capping agents by spectro-electrochemical surface-enhanced Raman spectroscopy (SERS). Raman bands of the capping agents are used here to track chemical changes of the nanoparticles under operando conditions. The sparingly soluble and non-plasmon active silver salts (AgBr and AgCl) are formed under potential bias. In addition, we spectroscopically observe plasmon-mediated structural changes of citrate to cis- or trans-aconitate, while PVP is unaltered. The different behavior of the capping agents implies a change in the physical properties on the surface of AgNPs, in particular with respect to the surface accessibility. Moreover, we showcase that reactions of the capping agents induced by different external stimuli, such as applied bias or laser irradiation, can be assessed. Our results demonstrate how SERS of capping agents can be exploited to operando track nanoparticle conversions in liquid media. This approach is envisaged to provide a more comprehensive understanding of nanoparticle fates in complex liquid environments and varied redox conditions.

SERS Reveals the Presence of Au–O–O–H and Enhanced Catalytic Activity of Electrochemically Dealloyed AgAu Nanoparticles

“…Figure 1a presents the spectral changes as a function of the applied potential during the dealloying of AgAuNPs on Pt WE. At a potential of −0.2 V, the characteristic Raman bands of the citrate-capped AgAuNPs are detected at 220 cm −1 (νAg−O), 1208 cm −1 (νCCO), and 1619 cm −1 (νCO 2 ), as described in detail in ref 35. The center frequencies of occurring SERS modes are tabulated in Table S1 in the SI.…”

“…Au-enriched porous nanoparticles are formed by the electrochemical dissolution of Ag from AgAuNPs in halide-free 250 mM KNO 3 (99.999%) on Pt working electrode (WE) via oxidation of Ag described by the following reaction , Ag ( s ) + NO 3 − ( aq ) ⇌ Ag + ( aq ) + normale − + NO 3 − ( aq ) Figure a presents the spectral changes as a function of the applied potential during the dealloying of AgAuNPs on Pt WE. At a potential of −0.2 V, the characteristic Raman bands of the citrate-capped AgAuNPs are detected at 220 cm –1 (νAg–O), 1208 cm –1 (νCCO), and 1619 cm –1 (νCO 2 ), as described in detail in ref . The center frequencies of occurring SERS modes are tabulated in Table S1 in the SI.…”

Characterization of Nanoparticles in Diverse Mixtures Using Localized Surface Plasmon Resonance and Nanoparticle Tracking by Dark-Field Microscopy with Redox Magnetohydrodynamics Microfluidics

“…50 RMHD has also been combined with fluorescence correlation spectroscopy 54 to attain flow profiles and diffusion coefficients of particles down to 0. silver and silica core gold shell (Ag and SiO 2 @Au), in a volume beyond the confines of the microscope's viewing window. Silver 41,55 and silica core gold shell 56,57 were chosen because they are widely studied materials and are frequently used NPs for investigation with DFM and thus serve as excellent model systems for coupled RMHD−DFM. 31 The NP types are detected and differentiated from each other based on the scattered light with blue/green and orange/red wavelengths from their LSPR, respectively, which also allows visualization of the small nanoscopic entities.…”

“…LSPR, first described by Faraday in 1857, is based on the interaction of light with nanostructures and determines the color of plasmonic NP suspensions. Because only non-transmitted light is detected in DFM, it is useful for observing individual NPs that exhibit strong LSPRs such as silver, gold, and copper, − as well as for monitoring their chemical reactions and events at their surfaces. − DFM combined with NP tracking analysis has been previously used to determine their diffusion coefficients by tracing the random paths of NPs in a static suspension . In addition to providing diffusion coefficients, single-particle tracking facilitates detailed observation of how a NP interacts with its surrounding environment.…”

Antibiotic Susceptibility Testing with Raman Biosensing

“…As a result, SERS has found numerous applications in biomedical research. A simple and efficient strategy for SERS substrate preparation is the aggregation of gold or silver nanoparticles [ 12 , 13 ], with an optional oxidation or electrochemical removal of capping agents for additional signal improvement [ 14 , 15 ]. A recent study revealed the dynamic nature of SERS signal generation in nanoparticle-based substrates [ 16 ].…”