Adsorption\nBehavior of the Coenzyme NADH at the Carbon/Electrolyte\nInterface Determined by Neutron Reflectometry

The adsorption behavior of β-nicotinamide adenine\ndinucleotide\n(NADH) at the carbon/electrolyte interface has been studied using\na combination of neutron reflectometry (NR) and solution depletion\nisotherms. Coupling the NR technique with an electrochemical cell\nallowed <i>in situ</i> observation of the reversible adsorption\nand desorption of the molecule at the electrode surface over a range\nof applied potentials. The overall surface coverage was low (30–50%),\nsuggesting adsorption only at specific defect sites on the surface.\nIsotherms conducted over a range of temperatures were used to extract\nthermodynamic parameters, which implied strong physisorption via electrostatic\ninteractions. In addition, changes in the outermost layer of the carbon\nelectrode were observed as the applied potential was varied, which\nwere confirmed with <i>ex situ</i> X-ray reflectivity measurements\n(XRR). X-ray photoelectron spectroscopy (XPS) measurements of the\ncarbon surface demonstrated the majority of carbon atoms were in an\nsp<sup>2</sup> state.