Promoting Shewanella Bidirectional Extracellular Electron Transfer for Bioelectrocatalysis by Electropolymerized Riboflavin Interface on Carbon Electrode

Abstract

The extracellular electron transfer (EET) that connects the intracellular metabolism of electroactive microorganisms to external electron donors/acceptors, is the foundation to develop diverse microbial electrochemical technologies. For a particular microbial electrochemical device, the surface chemical property of an employed electrode material plays a crucial role in the EET process owing to the direct and intimate biotic-abiotic interaction. The functional modification of an electrode surface with redox mediators has been proposed as an effectual approach to promote EET, but the underlying mechanism remains unclear. In this work, we investigated the enhancement of electrochemically polymerized riboflavin interface on the bidirectional EET of Shewanella putrefaciens CN32 for boosting bioelectrocatalytic ability. An optimal polyriboflavin functionalized carbon cloth electrode achieved about 4.3-fold output power density (∼707 mW/m²) in microbial fuel cells and 3.7-fold cathodic current density (∼0.78 A/m²) for fumarate reduction in three-electrode cells compared to the control, showing great increases in both outward and inward EET rates. Likewise, the improvement was observed for polyriboflavin-functionalized graphene electrodes. Through comparison between wild-type strain and outer-membrane cytochrome (MtrC/UndA) mutant, the significant improvements were suggested to be attributed to the fast interfacial electron exchange between the polyriboflavin interface with flexible electrochemical activity and good biocompatibility and the outer-membrane cytochromes of the Shewanella strain. This work not only provides an effective approach to boost microbial electrocatalysis for energy conversion, but also offers a new demonstration of broadening the applications of riboflavin-functionalized interface since the widespread contribution of riboflavin in various microbial EET pathways together with the facile electropolymerization approach.

Keywords: Shewanella; bioelectrocatalysis; electropolymerization; extracellular electron transfer; riboflavin.

FIGURE 1

Electrochemical characterization of electrodes. (A) The CV curves of the PRF@CC-30, RF/CC and bare CC electrodes at a scanning speed of 30 mV/s in 0.1 M PBS solution, (B) and the variation of peak current density of riboflavin redox reaction for the PRF@CC-30 and RF/CC electrodes (each value is the mean of three replicates).

FIGURE 2

Bioelectricity production in dual-chamber MFCs equipped with different electrodes. (A) The output voltage curves under the external loading resistance of 1500 Ω, (B) the polarization curves and power density plots (each value is the mean of three replicates).

FIGURE 3

Electrochemical behaviors of the PRF@CC-30 and bare CC electrodes in three-electrode cells inoculated with either wild-type (WT) Shewanella putrefaciens CN32 or its ΔmtrC/undA mutant strain, and the microbial biofilm growth. (A) The amperometric I-T curves when a poised potential of +0.2 V, (B) the CV curves under either turnover condition (lactate consumption) or non-turnover condition (lactate depletion) and (C) the corresponding first-order derivatives, (D) the Nyquist plots under an initial potential of -0.45 V, (E) a representative SEM image of microbial cells grown on electrode and (F) the total protein content attached on electrodes.

FIGURE 4

Bioelectrocatalytic fumarate reduction for the PRF@CC-30 and bare CC electrodes in three-electrode cells. (A) The CV curves of fumarate responses for different electrodes and inoculums (dashed line, no addition; solid line, 20 mM fumarate), (B) the amperometric I-T curves where electrodes were poised at –0.6 V and 20 mM fumarate was added after 30 min.