Methylene Green Electrodeposited on SWNTs-Based “Bucky” Papers for NADH and l-Malate Oxidation

Abstract

This research introduces a cavity anode design based on new single-walled nanotube (SWNTs) papers, "bucky" papers, used for the oxidation (and regeneration) of nicotinamide adenine dinucleotide (NADH) and the oxidation of l-malate. The materials designed are paper-like processed composites containing also additives: BP11 sample contains SWNTs and isopropanol (IPA); the BPMG sample contains SWNTs, IPA, and methylene green (MG). NADH/NAD(+) is the cofactor responsible for the oxidation of l-malate by malate dehydrogenase (MDH), in the Krebs' cycle. Because of the high overpotential of NADH oxidation, poly methylene green (PMG) was utilized as the electrocatalyst to produce NAD(+). The electrocatalyst was deposited on the surface of the "bucky" papers by electropolymerization by means of 10 voltammetric cycles in a range of -0.5 V and +1.3 V (vs Ag/AgCl reference electrode) at a scan rate of 5 mV/s. The catalytic performance of PMG was evaluated by chronoamperometric measurements of NADH oxidation at 0.3 V in phosphate buffer and l-malate oxidation at 0.1 V in the presence of MDH. For both "bucky" papers, the chronoamperometric curves of PMG, current vs NADH concentration, show a linear relationship demonstrating to have a first order Fick's law behavior for concentrations of NADH lower than 6 mM. The chronoamperometric curves in the presence of MDH, current against l-malate concentration, show a Michaelis-Menten behavior where no inhibition or competitive reaction are detected. Additionally, the anodic materials were characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), the polymerization of MG is effectively observed in the form of particles nucleation. The anodes show an excellent electrocatalytic activity toward NADH oxidation. The electrode design is feasible, reproducible, and overall stable.