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

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 Cp-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.