(Keynote) Biofilm-Supported Redox-Polymer-Type Materials for Electrocatalytic Oxygen Reduction of Importance to Biosensing and Bioenergetics

Unique properties of biofilms, which include possibility of extracellular electron transport, specific redox activity and capability to form stable polymeric or hydro-gel like microorganism aggregates adhering to common surfaces (including the glassy carbon electrode substrate), were explored here to form systems analogous to redox-polymer modified electrodes. Growth of biofilms was 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^oC). The dynamics of bioelectronic transfers originating from the presence of c-type cytochromes, i.e. heme-containing proteins within the biofilm, were further enhanced by introduction of multi-walled carbon nanotubes. Long-term stability of the biofim-based materials was achieved by impregnating them with porous but rigid conducting polymer, poly(3,4-ethylenodioxythiophene) or PEDOT. The fact that carbon nanotubes were derivatized with the carboxyl-group containing 4-(pyrrole-l-yl) benzoic acid facilitated the materials integrity through electrostatic attractive interactions between anionic carboxyl sites and positively charged domains of conducting polymer (PEDOT) structures. In neutral media, the biofilm-based composite (hybrid) matrices exhibited themselves electrocatalytic activity toward reduction of hydrogen peroxide (with possibility of sensing in the broad range of concentrations) and some activity during electroreduction of oxygen. By immobilization of additional catalytic (cobalt porphyrin) sites, a truly bifunctional electrocatalytic interface capable of significantly enhancing oxygen electroreduction currents was produced. Apparently, the reduction of oxygen (to hydrogen peroxide) was initiated at cobalt porphyrin centers and the further reaction (decomposition of hydrogen peroxide intermediate to water) was continued at c-cytochrome–containing biofilm matrix. These features are of importance to bioenergetics, i.e. to the development of cathodes for biofuel cells or biobatteries.