Probing of the single-cell level extracellular electron transfer highlights the maximum output current for microbial fuel cells (MFCs) at hundreds of femtoampere per cell, which is difficult to achieve by existing devices. Past studies focus on the external factors for boosting charge-extraction efficiency from bacteria. Here, we elucidate the intracellular factors that determine this output limit by monitoring the respiratory-driven shrinking kinetics of a single magnetite nanoprobe immobilized on a single Shewanella oneidensis MR-1 cell with plasmonic imaging. Quantified dissolving of nanoprobes unveils a previously undescribed bio-current fluctuation between 0 and 2.7 fA on a ∼40 min cycle. Simultaneously tracing of endogenous oscillations indicates that the bio-current waves are correlated with the periodic cellular electrokinesis. The unsynchronized electron transfer capability in the cell population results in the mean current of 0.24 fA per cell, significantly smaller than in single cells. It explains why the averaged output current of MFCs cannot reach the measured single-cell currents. This work offers a different perspective to improve the power output by extending the active episodes of the bio-current waves.
Keywords: cellular metabolism; electron transfer; microbial fuel cells; nanoprobes; plasmonic imaging.