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.

Single-nanoparticle electrochemistry has been established as a tool to characterize various nanomaterials based on the charge passed during their random impact at an electrode. Here it is demonstrated that the duration and shape of the resulting current peak can be used to quantify the reaction kinetics on a single-particle basis. Both the chemical rate constant and reaction mechanism for oxidation of single nanoparticles in different electrolytes can be determined directly from the duration of the current signal recorded in high-speed, high- sensitivity current measurements. Using 29-nm-sized Ag particles in four different electrolytes as a proof of concept for this general approach, hitherto inaccessible insights into single-particle reactivity are provided. While comparable rate constants were measured for the four electrolytes at low overpotentials, transport-limited impacts at high overpotentials were found to depend strongly on the type and quantity of anions present in solution.