Gold line array electrodes increase substrate affinity and current density of electricity-producing G. sulfurreducens biofilms

Abstract

During growth of Geobacter species able to transfer electrons to electrodes, biofilms consisting of multiple cell layers accumulate on surfaces. These biofilms require pathways for efficient electron relay towards the electrode, and diffusion of protons and end products away from the electrode. We hypothesized that altering the geometry of current-collecting electrodes would improve diffusion of substrates into electricity-producing biofilms, and allow testing of hypotheses related to the limits of long-range electron transfer. Two designs exposing equal gold surface areas to cultures of Geobacter sulfurreducens were compared: one consisting of a rectangular gold electrode and the other an array of 10 µm wide lines separated by 100 µm of non-conductive material. In all experiments, the line array electrode stabilized at a current density 4-fold higher (per unit electrode surface area) after 140 h of growth (1600 µA cm⁻²vs. 400 µA cm⁻²). Confocal imaging and cyclic voltammetry analysis demonstrated that because cells could grow at least 15 µm outward in a semicylinder from the gold lines, 4-fold more biomass could share each line electrode, compared to the rectangular geometry. The semicylinder-shaped biofilms did not fill in gaps between the electrodes after 300 h of growth, suggesting a limitation to the distance of useful between-cell electron transfer. The wider spacing of biofilms also improved the affinity of cells for acetate, especially under quiescent conditions.