Imaging of Electrochemically Programmed Single-Molecule Reactions

Abstract Single-molecule reactions reveal intrinsic heterogeneity that is obscured in ensemble measurements. However, most existing approaches rely on extreme dilution, ultrasmall solution volumes, or physical confinement to keep individual events sufficiently sparse. Here, we introduce electrochemically programmed single-molecule reactions (PSMR), a pulse-regulated strategy that shifts single-molecule electrochemistry from passive observation to active control. It converts continuous interfacial reactions to a sequence of short, repeatable reaction trials. By tailoring the pulse waveform, PSMR controls the driving force and duration of each reaction trial, thereby regulating the probability of single-molecule reactions, while the relaxation interval between pulses allows the interface to recover before the next trial. This coordinated control enables single-molecule reactions to be actively regulated under conventional solution volumes and reaction concentrations. By coupling individual reaction events to single-photon emission through electrogenerated chemiluminescence (ECL), we can directly count and image the reactions. The ultrashort reaction windows and relaxation periods suppress diffusion-layer buildup, thereby preserving sensitivity to local reaction kinetics, even at high overpotentials. Using this approach, individual Au nanoparticles were imaged over a broad potential range, and particles as small as 5 nm were distinguished from the substrate background. Sparse photon events were accumulated over repeated trials and further analyzed using localization- and fluctuation-based algorithms, enabling super-resolution mapping of reaction activity beyond the optical diffraction limit. PSMR therefore provides a framework for actively regulating, quantitatively imaging, and spatially mapping single-molecule electrochemical reactions at nanointerfaces, opening new opportunities for mechanistic studies and rational design across molecular electrochemistry, electrocatalysis, and energy conversion.