In-situ spectroelectrochemical insights into the EC’ mechanism: NADH oxidation catalyzed by an osmium complex

In-situ analytical techniques serve as powerful tools for visualizing reaction processes. Spectroelectrochemical methods, which integrate spectroscopy with electrochemistry, provide critical insights into electrochemical reactions. Molecules that undergo spectral changes due to oxidation- or reduction-induced alterations in their electronic structures can be monitored in real time, enabling direct confirmation of redox processes. In this study, ultraviolet-visible (UV-vis) absorption spectroscopy was combined with electrochemical analysis to investigate potential-dependent absorbance changes associated with the oxidation of NADH in the presence of an osmium complex serving as an electron transfer mediator. A distinct decrease in NADH-related absorbance was observed at potentials lower than its intrinsic onset potential, attributable to the catalytic activity of the mediator. These findings provide direct visual evidence for an EC’ (electrochemical-catalytic) reaction mechanism, wherein the mediator facilitates electron transfer by promoting NADH oxidation at reduced overpotentials.