Thermodynamics and kinetics of NAD+ adsorption on a glassy carbon electrode

Thermodynamics and kinetics of nicotinamide adenine dinucleotide (NAD(+)) adsorption on a glassy carbon (GC) electrode surface was investigated at various electrode potentials and NAD(+) concentrations using differential capacitance (DC) and attenuated total reflection Fourier transform infrared (ATR-FTIR) techniques. Equilibrium adsorption measurements confirmed that NAD(+) spontaneously and strongly adsorbs on the GC electrode surface. The affinity of NAD(+) towards adsorption on the GC electrode surface was found to increase with an increase in electrode potential (charge) to more positive values; the corresponding apparent Gibbs free energy of adsorption was -32.80 +/- 0.25, -35.61 +/- 0.86, and -38.02 +/- 0.40 kJ mol(-1) on negatively, neutral, and positively charged electrode surfaces, respectively. The kinetics of NAD(+) adsorption is also found to be highly dependent on the electrode surface potential (charge), and it increases with an increase in electrode potential (charge) to positive values. The adsorption process was modeled using a two-step kinetic model, in which the adsorption process involves the formation of two forms of NAD(+) on the surface: the thermodynamically unstable (NAD(+) (ads,rev)) and stable (NAD(+) (ads,stable)) forms. ATR-FTIR further confirmed that NAD(+), indeed, adsorbed on the GC electrode surface.