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Conductive boron-doped chemical vapor-deposited diamond thin films, already known to have superior properties for general electroanalysis, including low background current and a wide potential window, are here shown to have additional advantages with respect to electrochemical oxidation of nicotinamide adenine dinucleotide (NADH), including high resistance to deactivation and insensitivity to dissolved oxygen. Cyclic voltammetry, amperometry, and the rotating disk electrode technique were used to study the reaction in neutral pH solution. Highly reproducible cyclic voltammograms for NADH oxidation were obtained at as-deposited diamond electrodes. The response was stable over several months of storage in ambient air, in contrast to glassy carbon electrodes, which deactivated within 1 h. The diamond electrode exhibited very high sensitivity for NADH, with an amperometric detection limit of 10 nM (S/N = 7). The response remained stable, even in the very low concentration range, for several months. In addition, interference effects due to ascorbic acid were minimal when the concentrations of NADH and ascorbic acid were comparable. An NADH-mediated dehydrogenese-based ethanol biosensor incorporating an unmodified diamond electrode is demonstrated. The present results indicate that diamond is a useful electrode material for the analytical detection of NADH, making it attractive for use in sensors based on enzyme-catalyzed reactions involving NADH as a cofactor.

The self-assembling process of ferrocenylundecanethiol on a gold surface has been measured in situ using the electrochemical quartz crystal microbalance (EQCM). Initial adsorption was fast and was followed by the slow adsorption whose rate was 2 orders of magnitude smaller. The number of adsorbed thiol molecules, estimated from the frequency change, suggests the formation of the monomolecular layer of the thiol. This number agrees well with that calculated from the electrochemical charge associated with oxidationtreduction of the monolayer in 1 M HClOd. The EQCM was also used to follow the mass change during the oxidationtreduction of the ferrocenylundecanethiol monolayer. It was confirmed that anions are incorporated into the self-assembled layer upon oxidation of the ferrocene group. IntroductionControlled modification of electrode surfaces with monolayer film of organic molecules has attracted much attention because of the relevance to the construction of molecular devices. The modified electrode is also usefui for the understanding of electron transfer mechanism at electrode/electrolyte interfaces at a molecular level. Oriented monolayer assembly can be created by both the Langmuir-Blodgett and the self-assembling methods using various alkane derivatives and substrates.' The selfassembled monolayers of alkane derivatives with sulfurcontaining head groups on gold substrates have been widely examined recently, since the binding between S atoms and Au surface is strong and the S-anchored monolayers thus formed are in well-oriented ~tructure.~-~ Structure and orientation of these monolayer assemblies have been examined by infrared reflectance spectroscopy? ellipsometry? X-ray photoelectron spectroscopy,lO etc. Although these techniques give useful information, these studies have been conducted only under ex situ conditions. To understand the structure of the assembled monolayer and electron-transfer mechanism, it is essential to conduct quantitative in situ monitoring of mass transport both at an assembling process and at an electron transfer process so that we can quantify the number of adsorbed molecules and the movement of ion and solvent accompanying an electron transfer process. A recently developed electrochemical quartz crystal microbalance (EQCM)ll is one of the best techniques to obtain this quantitative information. So far, the EQCM has been used in only a few works as far as self-assembled monolayers are concerned. Buttry and his co-workers have examined mass transport during the redox of self-assembled layers of viologen derivatives12

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