ORIGINAL PAPER Electrochemical, microscopic, and EQCM studies of cathodic electrodeposition of ZnO/FAD and anodic polymerization of FAD films modified electrodes and their electrocatalytic properties S. Ashok Kumar & Shen-Ming Chen Received: 4 September 2006 / Revised: 10 October 2006 / Accepted: 25 October 2006 / Published online: 10 January 2007 # Springer-Verlag 2007 Abstract Two kinds of chemically modified electrodes were prepared. In the first type of electrodes, zinc oxide (ZnO) and flavin adenine dinucleotide (FAD) molecules were deposited onto the glassy carbon-, gold-, and SnO 2 - coated glass electrodes by using cyclic voltammetry from the bath solution containing aqueous 0.1 M zinc nitrate, 0.1 M sodium nitrate, and 1×10 -4 M FAD. It was called as ZnO/FAD modified electrodes. The second type of modi- fied electrode was prepared by the electropolymerization method. Electrochemical polymerization of FAD was carried out from the acidic solution containing 1×10 -4 M FAD monomers onto electrode surfaces. This poly(FAD)- modified electrode yields a new redox couple in addition to the monomers redox couple. The influence of the concen- trations, pH, and electrocatalytic properties of the ZnO/ FAD- and poly(FAD)-modified electrodes are investigated by means of the in situ technique electrochemical quartz– crystal microgravimetry (EQCM) combined with cyclic voltammetry and the ex situ technique scanning electron microscopy. From these studies, it appears that the cathodic deposition of ZnO/FAD-modified electrodes gives only one redox couple, and the anodically polymerized FAD film- modified electrodes gives two reversible redox couples. The pH dependence of the redox responses were investi- gated and the kinetics of electron transfer was evaluated. In addition, the EQCM technique was employed to follow the deposition process of both kinds of modified electrodes in real time as well as the characteristics of the charge transfer associated with the surface-confined redox-active couples. The electrocatalytic activity of the poly(FAD)-modified electrode towards the reduction of hydrogen peroxide and the oxidation of dopamine and ascorbic acid was explored. The important electrocatalytic properties of poly(FAD)-modified electrode were observed for simulta- neous separation of dopamine and ascorbic acid in neutral solution. This poly(FAD)-modified electrode has several advantages than the previously reported FAD-modified electrodes. Introduction Flavin is a tricyclic heteronuclear organic ring whose biochemical source is the vitamin riboflavin. The flavin moiety is often conjugated with an adenosine diphosphate to form flavin adenine dinucleotide (FAD; Scheme 1) and, in other circumstances, is found as flavin mononucleotide (FMN), a phosphorylated form of riboflavin. The flavin- containing adenine group is capable of undergoing oxida- tion–reduction reactions and can accept either one electron in a two-step process or can accept two electrons at once. In the form of FADH 2 , it is one of the cofactors that can transfer electrons to the electron transfer chain. FMN is a prosthetic group found in NADH dehydrogenase and FAD is a cofactor in the enzymes D-amino acid oxidase, glucose oxidase, and xanthine oxidase. Trace amounts of FAD play an important role in physiological processes, for example, metabolism and normal growth [1, 2]. Knowledge of the behavior of FAD near an electrode surface is necessary for the correct interpretation of the electrochemistry of FAD. J Solid State Electrochem (2007) 11:993–1006 DOI 10.1007/s10008-006-0236-6 S. A. Kumar : S.-M. Chen (*) Electroanalysis and Bioelectrochemistry Laboratory, Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, No. 1, section 3, Chung-Hsiao East Road, Taipei, Taiwan 106, Republic of China e-mail: smchen78@ms15.hinet.net