In SituOptical Quantification of Extracellular Electron Transfer using Plasmonic Metal Oxide Nanocrystals

Abstract

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 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.

How to Quantify Electrons in Plasmonic Colloidal Metal Oxide Nanocrystals

“…for example, electron transfer has been tracked from electroactive bacteria 36 and as a function of chemical reduction. 25 Alternatively, titrating the NC dispersion with an oxidizing agent and tracking the decrease in LSPR extinction as a function of oxidation uses the extinction coefficient per free electron to extrapolate the total number of free electrons per NC.…”

Depletion-driven assembly of polymer-coated nanocrystals

“…18 For the case of copolymer-grafted particles, macromolecular design has been leveraged to optimize thermoresponsive behavior for drug delivery, 19 to tailor mechanical robustness in composites, 20 and to modify ion content and conductivity in membranes. 21 Likewise, polymer wrapping has been used to direct nanoparticle surface charge, 22 to compatibilize NCs with cell culture medium, 23,24 and to prepare mechanically robust hydrogel composites. 25 By tuning the arrangement of nanoparticles and thereby their properties as well, assembly provides another degree of control over material design and functionality.…”

Highly Responsive Plasmon Modulation in Dopant-Segregated Nanocrystals

“…Furthermore, the change in LSPR absorption due to post-synthetic modulation of electronic charge in metal oxide NCs is enabling for electrochromic smart windows, [14][15][16][17][18] and redox sensing. [19][20][21][22][23] The potential of doped metal oxide NCs for such applications is dependent on the near-field enhancement (NFE), which quantifies the intensity with which light is concentrated near the NC surface, and on the dynamic range of modulation of the LSPR.…”