Bacterial-biofilm enhanced design for improved electrocatalytic reduction of oxygen in neutral medium

Abstract

The specific reactivity and ability of biofilms to form stable polymer-like hydrogel aggregates of microorganisms adhering to common solid (including the glassy carbon electrode) surfaces have been explored here to form systems analogous to electrocatalytic redox-polymer modified electrodes. Growth of biofilms has been demonstrated with use of Yersinia enterocolitica, a robust Gram-negative rod-shaped bacteria known to be resistive to pH changes (4-10) and temperature variations (0–40°C). Charge distribution and propagation within the biofilm have been enhanced by introduction of multi-walled carbon nanotubes. The fact that carbon nanotubes are derivatized with the carboxyl-group containing 4-(pyrrole-l-yl) benzoic acid has facilitated the hybrid material integrity and stability, namely through electrostatic attractive interactions between anionic carboxyl sites and positively charged domains of bacterial aggregates. In neutral media, the biofilm-based composite (hybrid) matrices have exhibited themselves electrocatalytic activity during electroreductions of oxygen and hydrogen peroxide (with possibility of its sensing in a broad range of concentrations). By immobilizing additional catalytic (cobalt porphyrin) sites, a truly bifunctional redox-polymer-like electrocatalytic system capable of significantly enhancing oxygen reduction currents has been produced. Apparently, the reduction of oxygen (to hydrogen peroxide) is initiated at cobalt porphyrin centers, and the second step (decomposition of hydrogen peroxide intermediate to water) is pursued at reactive sites (perhaps c-cytochrome) existing within biofilm matrix. Comparative measurements have been performed with the biofilm-supported platinum nanoparticles as well as with such a model catalytic system as platinized carbon nanotubes. The proposed electrode designs are relevant to biosensing and to the development of alternate cathode materials for biofuel cells or biobatteries.