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.