Electrochemical behavior of N-methyl-N'-carboxydecyl-4,4'-bipyridinium probed by surface-enhanced Raman spectroscopy.

Interfacial structure determines the activity and selectivity of a sensor and plays important roles in interfacial electrochemistry, electroanalysis, biosensing, etc. In situ electrochemical Raman spectroscopy appears to be a powerful tool to probe the electrochemical interface and surface process by providing the molecular fingerprint information. Herein, the electrochemical behaviors of N-methyl-N'-carboxydecyl-4,4'-bipyridinium (derivatives of methyl viologens, MV(2+)) with different alkyl chain lengths (n=2 and 10) on roughened Au electrodes were systematically investigated by the electrochemical surface-enhanced Raman spectroscopy (SERS). Three systems with different interfacial structures were constructed. One is to anchor the MV(2+) molecules via esterification with the 2-mercaptoethanol molecule pre-assembled on the Au surface. The second system is similar to the first one but without esterification. The third system is the direct adsorption of MV(2+) molecules on the bare roughened Au surfaces. The three systems gave different spectral response upon the change of the electrode potential. A drastically increased relative Raman intensity of 19a/8a modes of the MV(2+) molecules was observed at negative potentials. The phenomenon is attributed to the formation of the reduced form of MV(2+) molecules, which produces resonant Raman effect to enhanced the signal of 19a mode. The third system showed the highest electrochemical reduction activity towards the reduction of MV(2+) molecules, followed by the first and second systems. The result indicates that the interfacial structure can sensitively influence electrochemical activity of the electrode.