INTERFACIAL ELECTRON-TRANSFER OF FLAVIN COENZYMES AND RIBOFLAVIN ADSORBED ON TEXTURED TIO2 FILMS

Adsorption of flavins, i.e. flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD) and riboflavin (RF) on TiO2 powder was investigated and was found to obey the Langmuir isotherm. FMN and FAD gave binding constants of 1.7 x 10(5) and 1 x 10(5), respectively, indicating strong adhesion at the TiO2 surface while RF gave a value of only 1.5 x 10(4). The weaker adsorption of RF suggests an important contribution of the phosphate group to the adsorption of the flavins at the TiO2 surface. Highly textured TiO2 films covered with a monolayer of flavin were prepared and the interfacial electron transfer between adsorbed flavin and TiO2 in buffer solution (pH 6.1) was investigated by cyclic voltammetry. Facile electrochemical reduction and reoxidation of adsorbed FMN and FAD was clearly observed and standard electron transfer rate constants of 0.1 2-1 and 0.25 s-1, respectively, were derived from the dependence of the peak potential on scan rate. Photo-induced electron injection from adsorbed flavins into the conduction band of a TiO2 electrode was also observed.