In Situ Optical Quantification of Extracellular Electron Transfer from Shewanella oneidensis using Plasmonic Metal Oxide Nanocrystals
Extracellular electron transfer (EET) is a critical form of microbial metabolism that enables respiration on a variety of inorganic substrates, including metal oxides. For this reason, engineering EET processes has garnered significant interest for applications ranging from bioelectronics to materials synthesis. These applications require a strong understanding of electron flux from EET-relevant microbes. However, quantifying current generated by electroactive bacteria has been predominately
... en predominately limited to biofilms formed on electrodes, which require long incubation times, electrode colonization, and convolute contributions to EET from planktonic cells. To address this, we developed a platform for quantifying time-resolved EET flux from cell suspensions using aqueous dispersions of plasmonic tin-doped indium oxide nanocrystals. Tracking the change in optical extinction during electron transfer and fitting the optical response to a free electron model enabled quantification of current generation and electron transfer rate constants from planktonic Shewanella oneidensis cultures. Using this method, we differentiated between starved and actively respiring S. oneidensis, and between cells of varying genotype using an EET knockout strain. In addition, we quantified current production ranging from 0.12 - 0.68 fA · cell-1 from S. oneidensis cells engineered to differentially express a key EET gene using an inducible genetic circuit. Overall, our results validate the utility of colloidally stable plasmonic metal oxide nanocrystals as quantitative biosensors in native biological environments and contribute to a fundamental understanding of planktonic S. oneidensis electrophysiology using simple in situ spectroscopy.