Characterizing the Spatial Dependence of Redox Chemistry on Plasmonic Nanoparticle Electrodes Using Correlated Super-Resolution Surface-Enhanced Raman Scattering Imaging and Electron Microscopy

Super-resolution surface-enhanced Raman scattering (SERS) is used to investigate local surface potentials on plasmonic gold and silver colloidal aggregates using the redox-active reporter molecule, Nile Blue A. This molecule is electrochemically modulated between an oxidized emissive state and a reduced dark state. The diffraction-limited SERS emission from Nile Blue on the surface of a single plasmonic nanopartide aggregate is fit to a two-dimensional Gaussian to track the position of the emission centroid as a function of applied potential. Potential-dependent centroid positions are observed, consistent with molecules experiencing site-specific oxidation and reduction potentials on the nanoparticle electrode surface. Correlated structural analysis performed with scanning electron microscopy reveals that molecules residing in nanoparticle junction regions, or SERS hot spots, appear to be reduced and oxidized at the most negative applied potentials as the potential is cycled.