engleski

Interactive Adsorption Behavior of NAD^+ at a Gold Electrode Surface

The adsorption of an oxidized form of nicotinamide adenine dinucleotide, NAD^+, on a polycrystalline gold electrode surface and the subsequent surface conformation of the molecule was investigated over a wide temperature and potential range, using electrochemical differential capacitance and PM-IRRAS techniques. The adsorption process was described by the Langmuir adsorption isotherm. The corresponding thermodynamic parameters were determined: the Gibbs energy, enthalpy and entropy of adsorption. The large negative Gibbs energy of adsorption confirms that the NAD^+ adsorption process is highly spontaneous, while the large entropy gain on a positively and negatively charged surface, respectively) was found to represent the adsorption driving force. It was demonstrated that the energetics of the adsorption process is surface-charge controlled, while its kinetics is both mass-transport and surface-charge controlled. A surface-charge dependent conformation model for the adsorbed NAD+ molecule is proposed. These findings suggest that the origin of the NAD^+ reduction overpotential is related to the surface conformation of the adsorbed NAD^+ molecule, rather than to the electrode Fermi level position.

gold; nicotinamide adenine dinucleotide; Langmuir adsorption isotherm

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