Surface-enhanced raman scattering from a modified silver electrode




Sanderson, Aaron Craig

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Electrochemical and spectroelectrochemical data was obtained for a silver electrode modified with oxazine 720. A quasi-reversible redox behaviour was observed for the modified electrode. Surface adsorption density, calculated from the measured electrochemical charge transfer, is higher than would be expected for a monolayer of flat-adsorbed ox¬azine 720. Surface-enhanced Raman scattering (SERS) data, in conjunction with results of a density functional theory (DFT) calculation, suggest that the molecule is adsorbed with its rings perpendicular to the electrode surface, consistent with the electrochemically estimated adsorption density. SERS was recorded in situ at different applied potentials. The SERS intensity remains relatively stable between -200 and -500 mV (versus AglAgCl Cl-sat), but decreases dramatically as the applied potential is made more negative than -500 mV. Ths is consistent with the onset of oxazine 720 reduction observed during cyclic voltammetry. The spectroelectrochemical data indicates that oxazine 720 remains adsorbed at the SERS-active sites even in its reduced form. Similar in situ SERS data was collected for rhodamine 6G and pyridine. Spectra in the Stokes and anti-Stokes regions were obtained at several applied potentials using two different laser excitation energies. Normalized ratios of the anti-Stokes to Stokes intensities were calculated for various vibrational bands of the three molecules. The measured ratios vary with changes in the excitation energy, the applied voltage and the energy of the vibrational mode being investigated. The ratios for oxazine 720 show a preferential enhancement of the Stokes scattering while the ratios for rhodamine 6G indicate an enhancement of the anti-Stokes scattering. For pyridine, the preferential enhancement changes between Stokes and anti-Stokes depending on the excitation wavelength used, the applied voltage and the vibrational band being examined. The main trends of the anti-Stokes to Stokes ratios can be satisfactorily explained using resonance models based on standard SERS theories. No evidence of a SERS-induced non-thermal population distribution among the vibrational states of the adsorbed molecules (vibrational optical pumping) was observed.



Raman effect, surface enhanced, electrodes