SERS Detection of Charge Transfer at Electrochemical Interfaces Using Surface-Bound Ferrocene

Abstract

We present the novel use of surface-enhanced Raman scattering spectroscopy to detect charge transfer in ferrocene/ferrocenium (Fc/Fc + ) bound to an active electrode under electrochemical working conditions. Ferrocene has a 6-carbon linker with a thiol tether (6-(ferrocenyl)hexanethiol) and produces Fc/Fc + couple voltammograms exhibiting charge transfer above 0.4 V versus Ag/AgCl. By observing in situ micro-Raman spectroscopy with a water immersion lens, we observe significant changes in the spectra that correspond to the charge state oxidation of ferrocenium (Fc + ). In particular, the Fc + state is characterized by the disappearance of the C p -breathing mode (1104 cm –1 peak) and an 11-fold increase in the hexanethiol tail mode around 1072 cm –1 . Raman spectra calculated by density functional theory exhibit predominant peaks that agree well with our experimental observations and indicate that the Fc → Fc + transition is associated with a change in the orientation, which gives rise to dramatic changes in the Raman spectra.