Plasmonic response of electrified metal-liquid interfaces during faradaic and non-faradaic reactions by enhanced optical transmission.

Thin Au films, patterned by focused ion beam (FIB) milling to contain an array of subwavelength nanopores, exhibit enhanced optical transmission (EOT) via front-back resonance coupling. The films also serve as working electrodes capable of controlling the local potential, allowing electrochemical processes to be monitored using wavevector-resolved spectral mapping. The precise value of the surface plasmon resonance (SPR) wavevector can be extracted from the enhanced optical transmission signal and correlated with several distinct classes of electrochemical processes: double layer reorganization, faradaic adsorption/desorption, heterogeneous electron transfer, and anion adsorption. Specifically, the protonation/deprotonation reaction of an adsorbed monolayer of 4-mercaptobenzoic acid, the adsorption/desorption reaction of dodecanethiol to Au, the solution-phase reaction of ferriferrocyanide, and sulfate adsorption/desorption are investigated. A simple model is presented that encompasses both the EOT signal and electrochemical processes and produces semi-quantitative agreement with the SPR spectral wavevector mapping observed experimentally.