In situ Raman spectroscopy of redox species confined in self-assembled molecular films

In situ Raman spectroscopy and electrochemical experiments are used to study different methylene blue (MB) species immobilized in molecular films (sulfur, propanethiol, dodecanethiol, mercaptopropionic acid, and mercaptoundecanoic acid monolayers), self-assembled on SERS-active Au surfaces in aqueous electrolyte under potential control. The presence of three different MB species along reduction-oxidation cycles is inferred, irrespective of the nature of the molecular film. Residual Raman intensity in the spectroscopically silent leuco methylene blue (LMB) potential range demonstrate a dissimilar degree of connectivity for electron transfer between MB molecules and substrate depending on tbe spacer. In addition, the intensity of the Raman signal depends on the monolayer chemistry where hydrophobic and/or electrostatic interactions tend to stabilize the MB species against desorption from the molecular film driven by concentration gradients.