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

We present the novel use of surface-enhanced Raman scatteringspectroscopyto 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 chargetransfer above 0.4 V versus Ag/AgCl. By observing in situ micro-Ramanspectroscopy with a water immersion lens, we observe significant changesin the spectra that correspond to the charge state oxidation of ferrocenium(Fc(+)). In particular, the Fc(+) state is characterizedby the disappearance of the C-p-breathing mode (1104 cm(-1) peak) and an 11-fold increase in the hexanethioltail mode around 1072 cm(-1). Raman spectra calculatedby density functional theory exhibit predominant peaks that agreewell with our experimental observations and indicate that the Fc & RARR;Fc(+) transition is associated with a change in the orientation,which gives rise to dramatic changes in the Raman spectra.