Electrocatalytic oxidation of NADH in flow analysis by graphite electrode modified with 2,6-dichlorophenolindophenol salts

The preparation of a 2,6-dichlorophenolindophenol (DCPI) modified graphite electrode is described. DCPI was successfully immobilized by physical adsor ption onto a plain graphite electrode (DCPI-CME) and onto graphite electrod es pretreated with lanthanum nitrate (DCPI-La-CME) or thorium nitrate (DCPI -Th-CME). The electrochemical behavior of DCPI-CME was extensively studied using cyclic voltammetry. The electrochemical redox reaction of DCPI was fo und to be fairly reversible at low coverage with E degrees' = + 55 mV (vs. Ag/AgCl/3M KCl) at pH 6.5. A pK(a) value of 5.8 +/- 0.1 for immobilized for m of DCPI is determined from the intersection of the lines in the plot E de grees' vs. pH. The current I-p has a linear relationship with the scan rate up to 1200 mV s(-1), which is indicative for very fast electron transfer k inetics. The calculated value of the standard rate constant is k degrees = 18 +/- 4 s(-1). No decrease of either the anodic or the cathodic current of the cyclic voltammogram was observed after 500 runs of successive sweeps. The influence of the morphology of the electrode surface on the electrochem ical behavior of the DCPI-CME was studied and a mathematical model is propo sed, which partly describes the dependence of the geometrical area of the e lectrode surface on the grid of the emery paper. The modified electrodes we re mounted in a flow-injection manifold, poised at +60 mV (vs. Ag/AgCl/3M K Cl) and a catalytic current due to the oxidation of NADH was observed reduc ing thus the oxidation overpotential of NADH for about 400 mV. Interference from various reductive species present in real samples was investigated. T he repeatability was 1.2% RSD (n = 10 for 0.1 mM NADH). The sensor showed g ood operational and storage stability.