Electrostatic binding of ions by a self-assembled monolayer of N-(6-mercapto)hexylpyridinium on the Ag electrode as probed by surface enhanced Raman spectroscopy

Abstract

The structure of a self-assembled monolayer formed from N-(6-mercapto)hexylpyridinium (MHP) on the Ag electrode and electrostatic binding of ions from a solution phase by the monolayer were studied in situ by surface enhanced Raman spectroscopy (SERS). The low frequency Ag–S stretching mode was detected at 236 cm–1 in agreement with the calculated value (230 cm–1) for the Ag3-MHP complex. The –CH2–CH2–S–Ag moiety was found to be in predominant trans conformation as was evident from the higher intensity of the C–S stretching band near 692 cm–1 comparing with the gauche conformer band at 626 cm–1. Electrostatic attraction of NH2SO3–, SO42–, NO3–, BF4–, ClO4–, and PF6– anions by the MHP self-assembled monolayer on the Ag electrode was demonstrated by SERS. It was found that frequency of the totally symmetric stretching mode of surface bound anions shifts to lower wavenumbers comparing with ions in the solution phase. The higher shift corresponds to negatively larger Gibbs hydration energy of the anion.