Abstract
Controlling surface chemistry at the nanoscale is essential for stabilizing structure and tuning function in plasmonic, catalytic and sensing systems, where even trace ligands or ions can reshape surface charge and reactivity. However, probing such dynamic interfaces under operando conditions remains challenging, limiting efforts to engineer nanomaterials with precision. Here, using in situ surface-enhanced Raman spectroscopy, we identify a transient Au-Cl adlayer that forms during electrochemical cycling at gold interfaces. The adlayer exhibits significant charge transfer between gold and chlorine, generating an outward-facing dipole that polarizes neighbouring atoms and modulates the local potential. This dipole stabilizes nanogap interfaces and directs oriented ligand rebinding, enabling reversible reconstruction of subnanometre architectures. It also alters interfacial charge distributions and mediates electron transfer between gold oxidation states, acting as a redox-active intermediate. These findings show how transient surface species shape nanoscale reactivity and stability, offering strategies for designing catalysts, sensors and nanomaterials.