The electrocatalytic properties for NADH oxidation of five organic dyes, two indophenol derivatives (2,6-dichlorophenolindophenol and indophenol) and three o-quinone derivatives (phenanthrenequinone, pyrroloquinoline-quinone, and 1,2-naphthoquinone) were compared when adsorbed on zirconium phosphate entrapped in carbon paste. The electrochemical behavior of the immobilized dyes was investigated with cyclic voltammetry, performed in different aqueous MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016buffers, at different potential scan rates and pH values. The electrocatalytic efficiency for NADH oxidation was evaluated from cyclic voltammetry, and the second order electrocatalytic rate constant was calculated from rotating disk electrode experiments, at various concentrations of NADH and pH values. These studies indicate that the mechanism of such electro-oxidation proceeds via the formation of an intermediate complex. A positive effect with the addition of Ca 2þ cations to the solution was observed and the reaction rate for NADH oxidation increased. The highest second order rate constant was obtained for 2,6-dichlorophenolindophenol and found to be 4.6 Â 10 6 M À1 s À1 .

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The electrochemical and the electrocatalytic behavior for NADH oxidation of a newly synthesized phenothiazine derivative, 16H,18H-dibenzo[c,1]-7,9-dithia-16,18-diazapentacene (DDDP), adsorbed on solid spectrographic graphite and on graphite powder or on zirconium phosphate (ZP), both entrapped in carbon paste, were compared. Experiments using cyclic voltammetry and a rotating disk electrode, performed in different experimental conditions (various pH values and NADH concentrations), showed that i) in the pH range between 4 and 8, the E8× value for DDDP adsorbed on ZP becomes pH independent; ii) in contrast to other redox mediators, adsorption of DDDP on ZP induced a significant negative shift of the E8× value; iii) the reaction rate for electrocatalytic oxidation of NADH is markedly increased by adsorbing the mediator on ZP, being further improved by the presence of Ca 2 ions in the solution.