Electrochemical oxidation of NADH at a bare glassy carbon electrode in different supporting electrolytes

Amperometric determination of NADH and dehydrogenase enzymes at a redox-active nanocomposite

Aminated carbon nanotubes, CNTNH2, were covalently modified with glutardialdehyde (GDI) and the redox dye Azure to form a new electrode material CNTNH2-GDI-Azure (CGA). The nanocomposite of CGA and polysaccharide chitosan was used for the anodic determination of NADH. Compared to conventional carbon and metal electrodes, the CGA electrode drastically lowered the overpotential for NADH oxidation (by > 0.40 V), which minimized the interferences from the matrix of real-life samples (human serum). At -0.10 and 0.00 V, the CGA detected NADH and two NADH-dependent enzymes (lactate and malate dehydrogenase, LDH and MDH) down to 0.6 μM, 1.5 U L-1, and 0.5 U L-1, respectively. The activity of enzymes was quantified in 200-μL samples of human serum by the rapid (5 min) internally calibrated electrochemical continuous enzyme assay (ICECEA) at CGA within the clinically relevant linear ranges of 1.5-30 U L-1 (R2 = 0.994) for LDH and 0.5-8.1 U L-1 (R2 = 0.996) for MDH. Another attractive feature of CGA was that it maintained a stable NADH current (∼103 %) during a continuous 10-h long oxidation of 0.10 mM NADH, which sharply contrasted with a decaying NADH current observed at conventional electrodes. The CGA is a stable material that can be used in the form of a film or renewable paste electrode for the determination of NADH and hundreds of NADH-dependent dehydrogenases as biomarkers of human diseases.

Adsorption Behavior of the Coenzyme NADH at the Carbon/Electrolyte Interface Determined by Neutron Reflectometry

The adsorption behavior of β-nicotinamide adenine dinucleotide (NADH) at the carbon/electrolyte interface has been studied using a combination of neutron reflectometry (NR) and solution depletion isotherms. Coupling the NR technique with an electrochemical cell allowed in situ observation of the reversible adsorption and desorption of the molecule at the electrode surface over a range of applied potentials. The overall surface coverage was low (30-50%), suggesting adsorption only at specific defect sites on the surface. Isotherms conducted over a range of temperatures were used to extract thermodynamic parameters, which implied strong physisorption via electrostatic interactions. In addition, changes in the outermost layer of the carbon electrode were observed as the applied potential was varied, which were confirmed with ex situ X-ray reflectivity measurements (XRR). X-ray photoelectron spectroscopy (XPS) measurements of the carbon surface demonstrated the majority of carbon atoms were in an sp² state.

Modified carbon paste electrodes for flow injection amperometric determination of isocitrate dehydrogenase activity in serum

A carbon paste electrode modified with the adsorbed products of the electrochemical oxidation of adenosine triphosphate is described. The electrode was applied to the amperometric electrocatalytic detection of the reduced form of both nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate. The catalytic oxidation current shows a linear dependence on the concentration of the reduced form of nicotinamide adenine dinucleotide up to 1x10(-4)M, with a detection limit of 5x10(-9)M. Modified carbon paste electrodes were coated with an electrogenerated film of nonconducting poly(o-phenylenediamine) to obtain a stable amperometric response for at least 150h. In addition to static measurements, determination of both reduced cofactors was carried out in a flow injection analysis system with a thin-layer amperometric detection cell. The electrocatalytic monitoring of reduced nicotinamide adenine dinucleotide phosphate was applied to flow injection measurement of isocitrate dehydrogenase activity in serum. The results were in good agreement with those for the standard spectrophotometric test kit. The proposed method consumed less time and reagents and provided better precision than the standard method.