Imaging Dynamic Collision and Oxidation of Single Silver Nanoparticles at the Electrode/Solution Interface

Dynamically imaging collision electrochemistry of single electrochemiluminescence nano-emitters

The quest for new techniques to measure single nanomaterials is a great impetus to research efforts to understand individual behaviours. Here, we develop an electrochemiluminescence (ECL) microscopy for visualization of stochastic collision electrochemistry of single nano-emitters without the interference of current and optical background. This design uses a water-immersion objective to capture the ECL emission of nanoparticles near the specular electrode surface for enhancing light collection efficiency. The approach enables us to trace the collision trajectory of multiple nanoparticles and spatially distinguish simultaneous collisions. Results reveal that collision types, frequencies and ECL intensities significantly depend on surface natures, particle concentrations, and diffusion fluxes. By recording successive collisions, we develop a "relay probe" sensing platform for long-term research. This imaging technique displays great potential for applications in single-particle electrochemical and analytical research.

Voltage-Gated Nanoparticle Transport and Collisions in Attoliter-Volume Nanopore Electrode Arrays

Single nanoparticle analysis can reveal how particle-to-particle heterogeneity affects ensemble properties derived from traditional bulk measurements. High-bandwidth, low noise electrochemical measurements are needed to examine the fast heterogeneous electron-transfer behavior of single nanoparticles with sufficient fidelity to resolve the behavior of individual nanoparticles. Herein, nanopore electrode arrays (NEAs) are fabricated in which each pore supports two vertically spaced, individually addressable electrodes. The top ring electrode serves as a particle gate to control the transport of silver nanoparticles (AgNPs) within individual attoliter volume NEAs nanopores, as shown by redox collisions of AgNPs collisions at the bottom disk electrode. The AgNP-nanoporeis system has wide-ranging technological applications as well as fundamental interest, since the transport of AgNPs within the NEA mimics the transport of ions through cell membranes via voltage-gated ion channels. A voltage threshold is observed above which AgNPs are able to access the bottom electrode of the NEAs, i.e., a minimum potential at the gate electrode is required to switch between few and many observed collision events on the collector electrode. It is further shown that this threshold voltage is strongly dependent on the applied voltage at both electrodes as well as the size of AgNPs, as shown both experimentally and through finite-element modeling. Overall, this study provides a precise method of monitoring nanoparticle transport and in situ redox reactions within nanoconfined spaces at the single particle level.

Single Ag nanoparticle collisions within a dual-electrode micro-gap cell

An adjustable width (between 600 nm and 20 μm) gap between two Au microelectrodes is used to probe the electrodissolution dynamics of single Ag nanoparticles. One Au microelectrode is used to drive the oxidation and subsequent dissolution of a single Ag nanoparticle, which displays a multi-peak oxidation behavior, while a second Au microelectrode is used to collect the Ag+ that is produced. Careful analysis of the high temporal resolution current-time traces reveals capacitive coupling between electrodes due to the sudden injection of Ag+ ions into the gap between the electrodes. The current-time traces allow measurement of the effect of citrate concentration on the electrodissolution dynamics of a single Ag nanoparticle, which reveals that the presence of 2 mM citrate significantly slows down the release of Ag+. Intriguingly, these experiments also reveal that only a portion (ca. 50%) of the oxidized Ag nanoparticle is released as free Ag+ regardless of citrate concentration.