INSITU SPECTROELECTROCHEMISTRY OF ADSORBED METHYLENE-BLUE ON A SULFUR-MODIFIED GOLD ELECTRODE

The properties of a methylene blue layer adsorbed onto a sulphur-modified gold electrode have been studied by electrochemical and in-situ reflectance spectroscopy in the visible region. At saturation coverage, methylene blue monomer and dimer bands were observed in the double-layer region through the electrochromism of the adsorbed film arising from the effect of the double-layer field on the molecule transition moment. Possible dye orientations in the adsorbed state are discussed with the aid of the optical measurements. At low surface concentrations, the optical signal stems from modulation of the monomer/dimer interconversion. Repeated electrochemical reduction and oxidation of adsorbed methylene blue result in gradual desorption of the dimer species, probably due to a change in the molecular geometry upon reduction which may evolve from a planar configuration to a folded structure.