STUDIA UNIVERSITATIS BABEŞ-BOLYAI, CHEMIA, LII, 1, 2007 Dedicated to Professor Ionel Haiduc, President of The Romanian Academy at his 70 th anniversary GRAPHITE ELECTRODES MODIFIED WITH 3,7-di(m-AMINOPHENYL)-10-ETHYL-PHENOTHIAZINE VASILICA LATES a , DELIA GLIGOR a , LIANA MURESAN a , IONEL CATALIN POPESCU a *, RADU GROPEANU b , ION GROSU b ABSTRACT. The electrochemical behavior of a new functionalized phenothiazine derivative 3,7-di(m-aminophenyl)-10-ethyl-phenothiazine, adsorbed on spectrographic graphite, has been investigated. From cyclic voltammetric measurements, performed in aqueous buffer solutions of different pH values and at different potential scan rates, the rate constant of the heterogeneous electron transfer and the transfer coefficient were estimated. The electrochemical parameters of the voltammetric response and the slope of the linear regressions between the formal standard potentials and pH suggest a quasi-reversible 1e - /1H + redox process, involving a surface confined redox couple. From the dependence of peak current on the cycling time, the electrochemical stability of the modified electrode was estimated. From cyclic voltammetry measurements performed at +150 mV vs. SCE, the electrocatalytic efficiency toward NADH oxidation was evaluated and it was found ~ 67 %, recommending the new synthesized compound as a promising mediator for NADH electrocatalytic oxidation. Keywords: phenothiazine derivatives, NADH oxidation, modified electrodes. Introduction The detection of reduced β-nicotinamide adenin dinucleotide (β-NADH) is very important in enzyme assays, due to its participation in the enzymatic catalysis of more than 300 dehydrogenases, useful both in bioprocesses and in analytical applications [1,2]. The electrochemical oxidation of NADH has been studied intensively during the last two decades [3,4]. Unfortunately, the direct electrochemistry of this cofactor suffers from a high overvoltage at most solid electrodes which leads to the formation of an enzymatic inactive form of NAD + and of other side products, causing important interferences [5,6]. All these observations point to different solutions to overcome the problems of electrode fouling and overvoltage. One of the most a Department of Physical Chemistry b Department of Organic Chemistry, Babes-Bolyai University, 400028 Cluj-Napoca, ROMANIA *e-mail address: cpopescu@chem.ubbcluj.ro