Modified electrodes with new phenothiazine derivatives for electrocatylticoxidation of NADH

Phenothiazine derivatives containing two linearly condensed phenothiazine m oieties, 16H,18H-dibenzo[c,1]-7,9-dithia-16,18-diazapentacene (I), dibenzo[ e,1]-16,18-diacetyl-7,9-dithia-16,18-diazapentacene (II) dibenzo[c,1]-16,18 -dibenzoyl-7,9-dithia-16,18-diazapentacene (III), and 16H,18H-dibenzo[c,1]- 7.9-dithia-16,1 8-diazapentacene-7,7,9,9-bis-dioxide (IV), strongly adsorb on spectrographic graphite resulting in modified electrodes with electrocat alytic activity for NADH oxidation. From cyclic voltammetry measurements, p erformed in aqueous buffer solutions at different potential scan rates and pH values, the rate constants of the heterogeneous electron transfer and th e transfer coefficients were estimated. The linear dependence between the p eak current and the potential scan rate, corroborated with the slope of the formal standard potential versus pH linear regression, pointed out to a qu asi-reversible, surface confined redox process involving le(-)/1H(+). The e lectrocatalytical efficiency, evaluated from cyclic voltammetry, and the se cond order electrocatalytical rate constant (k(1)), calculated from rotatin g disk electrode experiments, revealed the same sequence of the activity de crease: III > II > IV > I. Compound III-modified electrodes were characteri zed by the highest k(1) (1.8 x 10(3) M-1 s(-1), at pH 7.0) as well as by th e highest stability, expressed by the lowest rate of the surface coverage d epleting under continuous potential cycling. (C) 2000 Elsevier Science Ltd. All rights reserved.