Microbial biofilms grown utilizing electrodes as metabolic electron acceptors or donors are a new class of biomaterials with distinct electronic properties. Here we report that electron transport through living electrode-grown Geobacter sulfurreducens biofilms is a thermally activated process with incoherent redox conductivity. The temperature dependency of this process is consistent with electron-transfer reactions involving hemes of c-type cytochromes known to play important roles in G. sulfurreducens extracellular electron transport. While incoherent redox conductivity is ubiquitous in biological systems at molecular-length scales, it is unprecedented over distances it appears to occur through living G. sulfurreducens biofilms, which can exceed 100 microns in thickness.
This research introduces a method for fabrication of conductive electrode materials with hierarchical structure from porous polymer/carbon composite materials. We describe the fabrication of (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) scaffolds doped with carbon materials that provide a conductive three-dimensional architecture that was demonstrated for application in microbial fuel cell (MFC) anodes. Composite electrodes from PHBV were fabricated to defined dimensions by solvent casting and particulate leaching of a size-specific porogen (in this case, sucrose). The cellular biocompatibility of the resulting composite material facilitated effective immobilization of a defined preparation of Shewanella oneidensis DSP-10 as a model microbial catalyst. Bacterial cells were immobilized via chemical vapor deposition (CVD) of silica to create an engineered biofilm that exhibits efficient bioelectrocatalysis of a simple-carbon fuel in a MFC. The functionalized PHBV electrodes demonstrate stable and reproducible anodic open circuit potentials of -320 ± 20 mV (vs Ag/AgCl) with lactate as the electron donor. Maximum power densities achieved by the hierarchically structured electrodes (~5 mW cm(3)) were significantly higher than previously observed for graphite-felt electrodes. The methodology for fabrication of scalable electrode materials may be amenable to other bioelectrochemical applications, such as enzyme fuel cells and biosensors, and could easily be adapted to various design concepts.
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