Visualizing Site-Specific Redox Potentials on the Surface of Plasmonic Nanoparticle Aggregates with Superlocalization SERS Microscopy

In this Letter, we demonstrate site-specific redox potentials for Nile Blue adsorbed to Ag nanoparticle electrodes using surface-enhanced Raman scattering (SERS) superlocalization microscopy. Nile Blue is electrochemically modulated between its oxidized and reduced form, which can be optically read out through a corresponding gain or loss in SERS intensity. SERS emission centroids are calculated by fitting the diffraction-limited SERS emission to a two-dimensional Gaussian to determine the approximate location of the emitter with 5-10 nm precision. With molecular coverage above the single molecule level, the SERS centroid trajectories shift reversibly with applied potential over multiple reduction and oxidation cycles. A mechanism is proposed to explain the centroid trajectories based on site-specific redox potentials on the nanoparticle electrode surface, where the first molecule reduced is the last to be oxidized, consistent with reversible electrochemical behavior of redox probes adsorbed to electrode surfaces.