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.
Due to their high surface-to-volume ratio and large proportion of low coordinated surface atoms, dealloyed nanoparticles have gained increasing attention as porous catalysts for the electrochemical oxygen evolution reaction (OER). Here, we characterize and rationalize the physical and chemical properties of operando gold-enriched porous nanoparticles fabricated by electrochemical dealloying of citrate-capped silver gold alloy nanoparticles in 250 mM KNO3. We combine surface-enhanced Raman spectroscopy (SERS) with electrochemistry for catalyst tracking. With SERS, we observe at 1.05 V (vs platinum quasi-reference electrode) the formation of Au–O–O–H species, a known intermediate of OER, while this is not observed for monometallic gold nanoparticles or bare electrodes. In agreement, qualitative measurements of the catalytic activity prove that appreciable OER currents are detected at lower potentials for gold-enriched porous nanoparticles compared to gold nanoparticles of the same size. Our results pave the way for the application of dealloying-derived nanoparticles as promising catalyst materials.
Cold atmospheric pressure plasma is a promising technology for surface wound healing. Its antimicrobial effect is correlated to chemical modifications of methionine (Met) caused by reactive oxygen and nitrogen species. To minimize unwanted side effects on healthy tissue it is of utmost importance to unravel the origin of the antimicrobial plasma effects. In this study, we employed confocal Raman spectroscopy on Met and Met glutathione (GSH) mixtures to obtain a chemical picture of how plasma affects Met as a function of treatment time (t = 0–600 s). We were able to observe a hitherto unknown reaction path that leads to a disulfide (MSSM) via a thiol (MSH) in addition to the well-known Met degradation route involving sulfur oxidation to methionine disulfide (Met(O)) and methionine sulfone (Met(O2)). We propose that the anti-microbial effect of plasma treatment is caused by two alternative reaction routes. The first one leads to protein damage caused by sulfur bridge formation (S-S). A second pathway is provided by MSH and dimethyl sulfoxide precursor species (detected via their characteristic Raman bands) that cause DNA damage due to strand breaks. Addition of GSH shifts the Met decay in time by 70 s while the general reaction pathways are preserved.
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