Electro-oxidative polymerization of phenothiazine dyes into a multilayer-containing carbon nanotube on a glassy carbon electrode for the sensitive and low-potential detection of NADH

An electrochemical sensing platform was developed for the amperometric detection of β-nicotinamide adenine dinucleotide (NADH) through the integration of a multi-walled carbon nanotube (MWCNT) into electropolymerized phenothiazine dyes. The composite containing MWCNT and poly(phenothiazine) was prepared by electro-oxidative polymerization of phenothiazine derivatives, Azure B, Azure A and thionine, into an MWCNT/ poly(diallyldimethylammonium chloride) (PDDA) multilayer, which was constructed by electrostatic layer-by-layer assembly on a glassy carbon electrode. The three phenothiazine monomers used in this study exhibited similar electrochemical behaviors. Azure B was used extensively as a model monomer for the investigation. Electrochemical techniques and scanning electron microscopy were used to demonstrate that the porous composite was formed and that the carbon nanotube served as a nano-sized backbone for the loading of polymeric phenothiazine. The electrocatalytic current for NADH oxidation was enhanced as the number of layers increased, implying that the increase of NADH-accessible poly(phenothiazine) and the three-dimensional arrangement of the poly(phenothiazine)-coated MWCNT in the composite facilitated electron and NADH transfer. Under optimal conditions, the detection limit for NADH decreases to 7.0 × 10^?8?M at a potential of 0.1 V (versus Ag/AgCl) using a {MWCNT/PDDA}₈?poly(Azure B) composite modified glassy carbon electrode, with a response time of about 5 s. This work demonstrates that the electropolymerization of the phenothiazine monomer into a pre-formed multilayer containing MWCNT can be used for the controllable preparation of stable MWCNT/poly(phenothiazine) composites on electrode surfaces, which have the potential to provide a platform for electrochemical biosensors based on NAD⁺-dependent dehydrogenase enzymes.