Boosting sensitivity in electrochemically driven bilayered plasmonic platforms for optical redox detection

Conventional surface plasmon resonance (SPR) techniques often face limitations in detecting redox processes within molecular assemblies that exhibit weak optical signals and/or exist at low surface concentrations. To address this, we developed an electrochemical approach that combines a cyclic voltammetry signal modulation with an SPR reflectance interrogation that was wavelength and angle tuned for a strong signal readout and enhanced by a chemically robust silver/gold bilayer electrode. Using cytochrome <i>c</i> as a model redox probe, we systematically examined how the excitation wavelength and the effective refractive index influence the reflectance signal contrast between the oxidized and reduced electrochemical states. Comprehensive optical characterization of the gold, silver, and mercaptopropionic acid layers, along with the redox-dependent optical response of cytochrome <i>c</i>, was performed through polarized angular reflectance measurements and thin-film transfer-matrix calculations. This analysis provided accurate, wavelength-dependent optical constants across the 552-785<i>n</i><i>m</i> spectral range. Our findings show that optimal detection sensitivity occurs when measurements are conducted within the linear region of the SPR response curve and at wavelengths corresponding to the redox probe's electronic absorption band. This dual-parameter optimization strategy, which provides strong plasmonic-analyte coupling with electrochemical robustness, enables highly sensitive detection of low-concentration biomolecular assemblies and real-time probing of interfacial redox processes.