Surface-enhanced Raman scattering (SERS) from a silver electrode modified with oxazine 720

Electrochemical and spectroelectrochemical data was obtained for a silver electrode modified with oxazine 720 immersed in 0.2 mol L-1 KCl solution. A quasi-reversible redox behaviour was observed for the modified electrode. The electrochemical charges, obtained during either the anodic or the cathodic processes, were higher than expected for a monolayer of flat-adsorbed oxazine 720. Surface-enhanced Raman scattering (SERS) was recorded in situ for different applied potentials. Although the tilt angle was not determined, the analysis of the most enhanced vibrational modes in the SERS data, in conjunction with results of a DFT calculation, suggest that the molecule is adsorbed with its ring perpendicular to the electrode surface. This adsorption mode allows a higher surface packing than for flat-adsorbed molecules, consistent with the electrochemically estimated surface coverage. The potential dependence of the SERS signal is consistent with the redox process observed from cyclic voltammetry. The SERS intensities remain relatively constant between -200 and -500 mV (vs. Ag | AgCl | Cl-sat(-)), but decrease drastically as the applied potential is made more negative than the onset of oxazine 720 reduction at -500 mV. The spectroelectrochemical data indicate that the molecule remains adsorbed at the SERS active sites even in its reduced form.