In situ analysis of cationic surfactants on electrode surfaces by FT-SERS microprobe spectroscopy .1. Adsorption of cetylpyridinium bromide

Surface-enhanced Raman spectra of cetylpyridinium bromide (CPB) adsorbed on a roughened polycrystalline silver electrode are reported as a function of the applied potential using a Raman microscope with a Nd:YAG laser as the excitation source. The spectra are compared both to the normal Raman spectra of the corresponding solid and solution species and to the ab initio calculated Raman spectrum of N-methylpyridinium. The assignment of molecular bands is given in the spectral range 100-3500 cm(-1). Examining the frequency shifts and the changes in relative intensities of the CPB bands due to adsorption and to potential variation reveals an enormous enhancement for the in-plane ring vibrations of the headgroup (ring breathing vibration v(1)(A(1)) and ring stretching vibration v(8a)(A(1))) while the bands assigned to vibrations of the alkane chain (symmetric and antisymmetric C-C vibrations, (CH2)(n) scissor and (CH2)(n) torsion vibrations, and symmetric and antisymmetric CH2 stretch vibrations) are nearly absent. These results suggest that the pyridinium ring is adsorbed perpendicularly with respect to the silver surface and that the hydrocarbon chain is directed away from the surface. In addition it is shown that the use of near-infrared electrode micro-SERS spectroscopy is a sensitive analytical technique for trace analysis by detecting CPB on a dried electrode in the sub-nanogram range and a powerful tool for characterizing self-assembled monolayers (SAMs). In order to reveal the optimal conditions for this kind of surface-enhanced Raman scattering spectroscopy, atomic force microscopy (AFM) was applied to investigate the surface morphology of silver electrodes exposed to different charge transfer values.