Adsorption-Coupled Oxidation of Single Ag Nanoparticles as Resolved by Stochastic Scanning Electrochemical Microscopy

The oxidation of Ag nanoparticles at the ultramicroelectrode (UME) has been extensively studied at the single-particle level for fundamental electrochemistry and electroanalytical sensing. The fast oxidation of a Ag nanoparticle is preceded by the adsorption of the nanoparticle, which can kinetically lower the frequency of amperometric spikes as an important analytical measure. Herein, we combine stochastic amperometry with scanning electrochemical microscopy (SECM) to quantitatively assess the adsorption kinetics of Ag nanoparticles on the Pt UME tip. We developed a theoretical model to simulate the dependence of the collision frequency on the adsorption rate constant and the distance between the tip and an insulating substrate. Experimentally, we confirm the advantage of SECM to determine the adsorption rate constant and diffusion coefficient (or concentration) of nanoparticles when the concentration (or diffusion coefficient) is known. We find that the maximum collision frequency based on the diffusion-limited adsorption of Ag nanoparticles requires a large contact area of a polished and cleaned tip with the nanoparticles. The adsorption of Ag nanoparticles is accelerated by citrate caps, which can be oxidatively chemisorbed on the Pt tip to enable a new covalent mode of nanoparticle-electrode interactions. SECM provides useful mechanistic insights into a deeper understanding of nanoparticle-electrode interactions for superior electrochemical detection.