Carbon nanotubes (CNTs) have been established as an electrocatalytic material for the oxidation of dihydronicotinamide adenine dinucleotide (NADH). Heteroatom doped CNTs, either boron or nitrogen, have been shown to enhance the electrocatalytic oxidation. Methylene green (MG), a well-established redox mediator for NADH oxidation, is shown here to further enhance the electrocatalytic oxidation of NADH at CNTs and nitrogen-doped CNTs (N-CNTs), but the standard technique of electropolymerizing MG on the surface of the electrode attenuates the natural reactivity of the nanotube/MG couple as formed through spontaneous adsorption. Additionally, MG coupled CNT/N-CNTs produces a leveling effect on N-CNTs, eliminating their electrocatalytic enhancement due to nitrogen doping. Nondoped CNTs display a slightly more efficient oxidation of NADH after adsorption of MG, due to an increased hydrophobic character which causes the adsorbed MG to reside slightly closer to the nanotube surface, and thus, facilitates more facile electron transfer between the nanotube/MG redox couple. Investigation of the influence of potential on MG adsorbed onto CNT/N-CNT electrodes allows the detection of MG polymerization, which is initiated at 0.05 V (vs. Hg/Hg2SO4) as an anodic stripping peak, along with the appearance of a reversible surface wave with E-1/2 at -1.0 V (0.1 M sodium phosphate buffer pH 7.0). (C) 2014 The Electrochemical Society. All rights reserved.