Mediatorless voltammetric oxidation of NADH and sensing of ethanol

A simple, selective and sensitive method for the detection of NADH and ethanol is presented. Self-assembled monolayers (SAMs) of mercaptopyrimidine (MPM) and their derivatives, thiocytosine (TC) and 4,6-diamino-2-mercaptopyrimidine (DMP) on gold (Au) electrode are used for the voltammetric detection of NADH and ethanol in neutral aqueous solution. A decrease of 200-300 mV in the overpotential associated with an observable increase in the peak current was obtained for the oxidation of NADH on MPM and TC monolayer-modified electrodes without any redox mediator. The facilitated electron transfer for the oxidation of NADH at the TC monolayer is ascribed to the existence of stable cationic p-quinonoid form of TC. The electrode modified with DMP monolayer could not exhibit stable response for NADH owing to the fouling of electrode surface. The MPM and TC monolayer-modified electrodes show high selectivity and excellent sensitivity (MPM: 0.633 ± 0.005 μA cm−2 μM−1; TC: 0.658 ± 0.008 μA cm−2 μM−1) towards NADH with detection limit (3σ) of 2.5 and 0.5 μM, respectively. Presence of large excess of ascorbate (AA) does not interfere the detection of NADH and the monolayer-modified electrode shows individual voltammetric peaks for AA and NADH. Voltammetric sensing of ethanol using alcohol dehydrogenase on MPM and TC monolayer-modified electrode is successfully demonstrated and these electrode can detect as low as 0.5 mM ethanol in neutral pH. The sensitivity of the MPM and TC monolayer-modified electrodes toward ethanol was found to be 3.24 ± 0.03 and 3.435 ± 0.04 μA cm−2 mM−1, respectively.