From Solution to a Silver Surface: Combining Raman Spectroscopy, Potential-Dependent SERS, Finite Silver-Cluster DFT, and Electromagnetic FDTD Modeling of Methylene Blue

Raman spectroscopy and surface-enhanced Raman scattering (SERS) provide complementary insights into molecular structure and molecule–metal interactions. We compare the solution Raman and potential-dependent SERS spectra of methylene blue (MB+) with calculations for free MB+ and an MB+/Ag40 adsorption model. Cyclic voltammetry on Au and Ag screen-printed electrodes distinguishes the dye’s redox response from the dominant Ag/AgCl processes accompanying electrochemical activation. The voltammograms do not directly demonstrate the formation of a particular SERS-active nanostructure on the present electrode; the proposed chloride-mediated restructuring is supported by previous microscopic studies . The experimental SERS spectra vary with potential, and potential-resolved Raman measurements reveal the evolving response of the interface. A band-position-based comparison with nine Raman-relevant MB+/Ag40 reference modes avoids quantitative use of the calculated Raman intensities. When requiring at least seven matched bands within ±30 cm−1, 23 of 29 spectra fulfilled the criterion; the smallest mean absolute wavenumber deviation (MAE = 9.38 cm−1; RMSE = 12.26 cm−1) occurred at −0.289 V versus the Ag pseudo-reference (8/9 matched bands). This minimum denotes the closest band-position agreement with the static reference, and is not interpreted as evidence for a unique adsorption geometry or oxidation state. Independent two-dimensional FDTD calculations illustrate x-polarization-selective field localization in idealized 2 nm Ag junctions; the field maps have not been demonstrated to converge, so these results are illustrative rather than predictive of experimental SERS enhancement. The main experimental observation is a potential-dependent MB SERS response occurring alongside pronounced Ag/AgCl electrochemistry; a specific surface morphology or causal enhancement mechanism cannot be established from the voltammograms alone. Together, these observations provide a complementary molecular and electromagnetic perspective on a coupled, potential-dependent Ag/AgCl–methylene blue interface, rather than separating the contributions of the molecules and the substrate. The absolute potential assignment is also restricted by the use of a printed Ag pseudo-reference, as its potential cannot be assumed to remain constant during cycling in a chloride-containing electrolyte. A significant limitation of the current EC-SERS protocol is that electrochemical activation of the Ag surface and the redox conversion of methylene blue occur within overlapping potential ranges. Consequently, the cathodic formation of the SERS-active Ag nanostructure can coincide with the reduction of MB+ to reduced methylene blue species. Consequently, spectra acquired during the cathodic scan cannot be uniquely assigned to MB+ on the basis of the electrode potential alone, and the present data do not allow molecular redox chemistry to be experimentally separated from substrate reconstruction.