Can. J. Chem. 66, 86 (1988) A detailed investigation of the electrochemical behavior of flavin adenine dinucleotide (FAD) in neutral solutions has been carried out at Hg and glassy carbon electrodes. At FAD concentrations of about M, cyclic voltammetiy (CV) shows apair of anodic and cathodic peaks having a peak separation at low sweep rates indicative of a two-electron transfer process and yielding a formal redox potential for FAD of -0.206 k 0.003 V vs. NHE at pH 7. Evidence for FAD adsorption was obtained in experiments at high sweep rates, from the effect of time of exposure of the electrode surface to FAD in solution and from the effect of the potential limits on the cyclic voltammetric response. The process of FAD adsorption was studied in detail in dilute FAD solutions (ca M ) using a hanging mercury drop electrode and the techniques of CV and ac voltammetry. Three distinct stages of FAD adsorption were observed and a model of the orientation of FAD on the electrode surface as a function of time and potential is presented. In addition, the kinetics of oxidation and reduction of adsorbed FAD was studied for each of the stages of FAD deposition, and a surface standard rate constant of ca.

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The adsorption of flavin adenine dinucleotide (FAD) at a mercury electrode surface has been investigated in dilute FAD solutions from pH ca. 2-7. FAD is found to adsorb initially in an orientation in which the isoalloxazine and adenine moieties lie parallel to the electrode surface (Stage I) at all acidic pH values and potentials. At low coverage in Stage I, a persistent separation of the anodic and cathodic peaks is present, independent of sweep rate, perhaps indicative of the expected planar (oxidized) and nonplanar (reduced) conformations of the isoalloxazine ring system. As the surface coverage increases, this difference diminishes and a sudden reorientation occurs to a structure in which it is suggested that alternating isoalloxazine and adenine moieties are adsorbed perpendicularly to the electrode surface (Stage II). At pH < 4.5 this structure is stable over a very wide potential range. Also, a second vertical highly condensed orientation is observed at positive potentials in these more acidic solutions. These observations may reflect the effects of adenine protonation and the consequent coulombic interactions between it and the charged electrode surface. A third FAD orientation is observed (Stage III) after some time in acidic solutions, possibly involving a bilayer or a multilayer adsorbed on the mercury electrode surface.

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Ir oxide (IrOx) films, formed electrochemically on bulk Ir metal (Ir/IrOx) and also on sol‐gel (SG) derived non‐silica based nanoparticulate Ir, have been studied as material useful for the detection of hydrogen peroxide, with possible application as a glucose biosensor. H2O2 reduction and oxidation on Ir/IrOx and SG‐derived IrOx films, deposited on various substrates such as Pt, Ir and GC, have been compared to the H2O2 behavior at the bare substrate. It was found that H2O2 reduction proceeds on the underlying electrode substrate, while H2O2 oxidation is independent of the nature of the substrate, therefore occurring via the IrOx film. The reactivity of IrOx towards H2O2 oxidation is similar to that seen at Pt, although IrOx has the additional advantages of excellent stability, insensitivity to common interfering substances, biocompatibility and a linear range of detection, up to at least 12 mM H2O2. At micromolar concentrations of H2O2, a second mode of detection, involving the catalyzed growth of IrOx films at Ir substrates, can be employed. These two methods of H2O2 analysis (oxidation/reduction and enhanced IrOx growth) can also be employed for glucose detection using IrOx‐based glucose biosensors.