Electrochemical\nStudy of Quinone-Mediated Extracellular\nElectron Transfer in <i>Escherichia coli</i> during Glucose\nOxidation Metabolism

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