The electrochemical oxidation of dihydronicotinamide adenine dinucleotide (NADH) has drawn many attentions for use of its oxidized form (NAD + ) as an enzymatic cofactor in many dehydrogenase based biosensors and biofuel cells.
1,2At neutral pH, NADH/NAD + couple has a formal potential of −0.320 V (vs NHE), 3 but large oxidation overpotentials are generally required.2,4 In addition to this problem, direct oxidation of NADH shows unwanted side reactions or adsorption of the product.
5,6To avoid these problems, many studies using various electrocatalysts were reported. In this point of view, mediatormodified electrodes and electrochemical sensing designs were reviewed.1,2,7 The oxidation of NADH is known to follow an ECE mechanism with a rate-limiting step in the first electron transfer.2,6 Various redox mediators with quinone groups making two-electron and hydride transfer possible were reported as good candidates. 2,7,[8][9][10] Recently, carbon nanotubes were employed as a promising electrode material giving enhanced performances with less overpotentials and enhanced sensitivities.11-14 The carbon nanotubes (CNTs) in the nanocomposite films show good charge transport properties resulting in less overpotentials and good stabilities.
12,15In the present study, a redox polymer, the copolymer of polyacrylamide and poly(N-vinylimidazole) complexed with [Os(4,4'-dimethyl-2,2'-bipyridine), was employed for the electron transfer mediator for NADH oxidation. The redox polymers have been utilized for mediating electrons between the redox centers of enzymes and electrode surfaces.16,17 Some oxidase enzymes were "wired" to carbon electrodes and efficient electrocatalytic reactions such as glucose oxidation and O 2 reductions were reported. [16][17][18][19] At an electrode with a hydrogel film of the redox polymer, which is not a quinone-like mediator, catalytic electro-oxidation of NADH, but only a partial fraction, was measured. A composite film of the redox polymer and oxidatively treated single-walled carbon nanotubes (SWNTs) was applied on an edge plane graphite electrode surface. At this newly prepared electrode, an enhanced activity showing most NADH catalytically oxidized with less overpotential was achieved. As far as we know, the present redox polymer-CNT composite system, which is not a quinone type mediator, is first for the electrocatalytic NADH oxidation.Cyclic voltammograms obtained in a 2.0 mM NADH solution at bare EPG and RP/EPG electrodes for Figures 1(a) and 1(b), respectively, were shown. (For abbreviations of electrode types, consult the experimental section.) Voltammograms obtained in a pure supporting electrolyte were also shown as dashed curves. In Figure 1(a), only one oxidation wave for NADH oxidation was observed with a peak potential of 0.34 V. In Figure 1(b), in the absence of .