Wiring of Enzymes to Electrodes by Ultrathin Conductive Polyion Underlayers: Enhanced Catalytic Response to Hydrogen Peroxide

Stable electroactive films were grown layer by layer on rough pyrolytic graphite electrodes featuring 4-nm underlayers of sulfonated polyaniline (SPAN) covered with a film containing myoglobin or horseradish peroxidase grown in alternating layers with poly(styrenesulfonate). The self-doped polyanionic SPAN layer, grown on a 2nm polycation layer, was conductive between about 0.1 and -0.4 V vs SCE at pH 4.5. The enzyme films had the architecture PDDA/SPAN/(enzyme/PSS)3, where PDDA is poly(diallyldimethylammonium) ion. Comparisons of voltammetric measurements of electroactive protein with quartz crystal microbalance measurements of total protein showed that 90% or more of the protein was coupled to the electrode when the SPAN underlayer was present, as opposed to ~40% protein electroactivity when SPAN was absent. As a consequence of the highly efficient coupling between enzymes and electrode, the PDDA/SPAN/(enzyme/PSS)3 films exhibited a higher sensitivity for the electrochemical catalytic reduction of hydrogen peroxide. Amperometry at a rotating disk electrode at 0 V gave sensitivity for hydrogen peroxide up to 14 (mu)A (mu)M-1 cm-2 in the submicromolar concentration range and a detection limit of ~3 nM. Results suggest the future utility of ultrathin layers of conductive self-doping polyions in improving sensitivity of enzyme biosensors.