Detection of NADH via electrocatalytic oxidation at single-walled carbon nanotubes modified with Variamine blue

Screen-printed electrodes (SPEs) modified with Variamine blue (VB), covalently attached to the oxidized single-walled carbon nanotubes (SWCNTs-COOH), were developed and used as chemical sensors for the detection of the reduced nicotinamide adenine dinucleotide (NADH). The Variamine blue redox mediator was covalently linked to the SWCNTs-COOH by the N,N′-dicyclohexylcarbodiimide (DCC) and N-hydroxysuccinimide (NHS) chemistry. Infrared Fourier transform (FT-IR) spectroscopy revealed the presence of the amide bands situated at 1623 cm⁻¹ (I band), 1577 cm⁻¹ (II band) and 1437 cm⁻¹ (III band) demonstrating the covalent linkage of Variamine blue to SWCNTs-COOH. The heterogeneous electron transfer rate, k_obs., was 13,850 M⁻¹ s⁻¹, and the k_s and α were 0.8 s⁻¹ and 0.56, respectively. The pH dependence was also investigated. SPEs modified with Variamine blue by using the DCC/NHS conjugation method, showed a variation of −36 mV per pH unit.A successful application was the development of a lactate biosensor obtained by the immobilization of the l-lactate dehydrogenase on the NADH sensor.