Full Paper Structure and Electrochemical Properties of Electrocatalysts for NADH Oxidation Rosalba A. Rinco ´n, a Kateryna Artyushkova, a Mike Mojica, a Marguerite N. Germain, b Shelley D. Minteer, b * Plamen Atanassov a * a Department of Chemical & Nuclear Engineering, Center for Emerging Energy Technologies, University of New Mexico, Albuquerque, NM 87131, USA b Chemistry Department, Saint Louis University, St. Louis, MO 63103, USA *e-mail: minteers@slu.edu; plamen@unm.edu Received: June 15, 2008 Accepted: October 1, 2009 Abstract NADH electrocatalysts have been an area of study for over 3 decades. Polyazines have been popular electrocatalysts of choice for NADH oxidation for both sensors and biofuel cell applications. However, little is known about the structure and function relationship between these polyazines and their ability for NADH oxidation. In this paper, we utilize XPS, SEM, and NMR to evaluate the structure of polyazines and relate that to their electrochemical properties. Keywords: Electropolymerization, Methylene green, Methylene blue, NADH oxidation, XPS, NMR, Fuel cells DOI: 10.1002/elan.200880008 1. Introduction The interest in energy harvesting devices research has increased over the past decade. Among these types of devices, enzymatic biofuel cells are those that employ enzymes as the catalysts for either anodic or cathodic processes. Sugars and alcohols can serve as biofuels to power these devices, but in order to fully oxidize complex fuels like sugars and obtain the largest energy density possible, there is a need to mimic their pathways for enzymatic breakdown like glycolysis and the Krebs cycle [1, 2]. Dehydrogenase enzymes play an essential role in such pathways, and over 300 dehydrogenases are known to be dependent on the nicotinamide adenine dinucleotide cofactor in its reduced (NADH) and oxidized (NAD þ ) forms. However, regener- ation of NADH has been a major challenge in practical biotechnology over the past couple decades. Oxidation of NADH (reductive mediator) at ordinary electrodes like bare glassy carbon or gold is irreversible and requires high overvoltages, sometimes exceeding 1 V [3, 4]. By overcom- ing this high overpotential, one could possibly use any NAD-dependent enzyme for a biofuel cell anode and create a cascade of enzymes that could achieve deep oxidation of complex fuels, giving us a wide variety of these biological fuels where to choose from. Hence, finding ways of regenerating the NADH active site will broaden the opportunities for enzymatic biofuel cells. The use of mediators to reduce the overpotential of NADH oxidation and improve its kinetics has been extensively studied by various researchers. Some of the structures that have been investigated for electrocatalytic oxidation of NADH are among the o-quinones, p-quinones, phenothiazine and phenoxazine derivatives [5]. The two latter being part of the family of azines. Formerly, the catalyst (monomer) was adsorbed onto the surface of the electrodes [6 – 8], which limited its long-term stability due to leaking or desorption of the catalyst from the electrode surface[9]. Another limitation of having the mediator physically attached to the electrode is that one could only work in quiescent conditions (biobattery [10]) and not under a flow through regime (biofuel cell). This issue has been targeted by others previously [5, 11 – 16] by depositing through electrochemical polymerization films of polyazines on the surface of electrodes, which has proved to be the most reliable strategy for immobilization concerning the stability issue. Dai et al. have reported that a methylene green- modified electrode can oxidize NADH by a simultaneous two-electron reaction and reduce the overpotential of NADH oxidation by about 650 mV [5]. Karyakin et al. have made a considerable effort in studying these electro- active polymer films and characterizing them, particularly methylene blue [11, 12]. Svoboda et al. [17] have studied the growth of poly(methylene green) on platinum electrodes and performed in situ and ex situ characterization studies of the electrochemistry and growth of the thin films. Despite all of the effort by many researchers, there is still discrepancy when defining the structure of such electro- active polymers. The lack of understanding of the structure of such films is stopping us from identifying the properties that make them a good catalyst for NADH regeneration and hence from developing a better one by understanding the relationship between chemical structure and material function. Full Paper Electroanalysis 2010, 22, No. 7-8, 799 – 806  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 799