Electrochemical behavior of nitrate reductase immobilized in self-assembled structures with redox polyviologen

We report on a novel bioelectrode based on self-assembled multilayers of nitrate reductase (NR) intercalated with a cationic viologen-functionalized polyvinylpyridinium (PV) polymer specially synthesized for this purpose. Different samples of polymer proved to have varying substitution ratios, according to NMR experiments. The electrostatic self-assemblies were built up on a thiol-modified gold surface using a strategy previously described by Hodak et al. The electrochemical behavior of PV and the response of the electrodes toward nitrate were followed by cyclic voltammetry. The catalytic currents obtained were proportional to the number of NR-immobilized layers, as well as to the polymer substitution degree, proving that the enzyme remained active under immobilization conditions and that it can be regenerated by PV redox moieties. Experiments with quartz crystal microbalance electroacoustic impedance were carried out to follow quantitatively the multilayer film formation. The kinetics of the adsorption processes and the influence of the ionic strength are investigated. The evaluation of water and ion exchange during the reduction of PV is also achieved from electroacoustic and electrochemical experiments. Results indicate an important water exchange associated with the polymer redox behavior.