Electrochemical Study of Quinone-Mediated Extracellular Electron Transfer in <i>Escherichia coli</i> during Glucose Oxidation Metabolism

Establishing efficient extracellular electron transfer (EET) between bacteria and electrode surfaces is critical for the development of sustainable microbial electrochemical technologies. In this context, soluble, redox-active quinones are frequently employed as exogenous electron shuttles (i.e., mediators) to facilitate EET from microbial respiration processes to the electrode surface. However, there is limited information on the mediated bioelectrocatalytic performance of quinone-mediated EET with regard to mediator properties. In this study, we show a quinone-based mediator system designed to facilitate EET in the model microorganism Escherichia coli during glucose metabolism. A library of 12 quinone redox mediators was experimentally evaluated through electrochemical measurements, revealing distinct mediated current densities, dependent on mediator structure and concentration. Among the quinones tested, tetrahydroxy-1,4-benzoquinone achieved the highest mediated current density of 11.7 ± 1.1 μA cm–2. Further electrochemical characterization of the formal reduction potentials of each quinone mediator in both aqueous and aprotic media was performed. Our results show that the redox properties of quinone mediators in aqueous environments correlate with the measured mediated current densities in E. coli, suggesting that the critical electron transfer step occurs either within the cytoplasm (or periplasm) of bacterial cells or outside of the cell. This study offers valuable insight into EET for rationally designing mediated bioelectrocatalysis systems. As such, it highlights the importance of independently understanding the microorganism type, metabolic processes, and redox behavior of mediators in both aqueous and aprotic media.