Enhancing Bioelectrochemical Conversion: Molecular Modifications for Amplified Transmembrane Electron Transfer
In Bioelectronics—the confluence of Biology and Electronics—living biological entities are interfaced with electrical components for applications in bioenergy conversion and catalysis, biosensing, medical diagnostics and drug delivery, neural and tissue interrogation, and more. Improving the contacts at biotic-abiotic charge transfer interfaces is therefore of fundamental importance for improving these various bioelectrochemical systems. Here, specific attention is drawn to chemically modifying electrically insulating lipid bilayer membrane interfaces so that biologically-derived electrons may be more readily collected at an electrode. This research is of fundamental scientific interest from a biophysical perspective as well as immense practical importance for bioelectrochemical conversion technologies that interconvert organic fuels and electrical current.
Consider microbial bioelectrochemical conversion systems wherein certain bacterial species are commonly employed that have the evolved capacity to directly produce electrical current as a metabolic product. A unifying feature of these species is that they construct conductive membrane-bound redox-protein/cofactor nanostructures for transmembrane electron transport. Drawing inspiration from this molecular functionality, one may envision and synthesize organic semiconducting molecules designed for biological/membrane affinity. The implementation of these materials in living devices for the purpose of amplifying biological transmembrane electron transport is the subject of this dissertation.
p-Phenylenevinylene-based conjugated oligoelectrolytes (PPV-COEs) are a class of organic semiconducting molecules designed for membrane modification. Early experiments indicated that PPV-COEs will spontaneously intercalate into lipid bilayer membranes and improve biocurrent outputs, suggesting that PPV-COEs act as transmembrane “molecular wires” for electron transmission. In order to test this hypothesized mechanism, the model lactate-consuming electrogenic bacterium Shewanella oneidensis MR-1 was cultivated and modified with PPV-COEs in microbial three-electrode electrochemical reactors (M3Cs). Because S. oneidensis MR-1 utilizes direct electron transfer (DET) and mediated electron transfer (MET) at distinct potentials, perturbations to the DET and MET current signals in M3Cs provide a view into the mechanism of PPV-COE biocurrent amplification. Results indicate that PPV-COEs statistically improve the coulombic efficiency of S. oneidensis MR-1 lactate-to-current conversion from 51 ± 10% to an exceptional 84 ± 7% (P = 0.0098) by amplifying the native bacterial DET pathway and increasing colonization of the electrode, but PPV-COEs do not appear to act as “molecular wires.”
PPV-COEs were next applied to an anaerobic, obligately-crossfeeding (syntrophic) cultures of Pelobacter acetylenicus and Acetobacterium woodii and then to photobioelectrochemical devices based on photosynthetic green plant thylakoid membranes, and these were biochemically and electrochemically characterized. In the former experiments, it was found that PPV-COEs improve reaction rates and intercellular exchange of electron equivalents as a function of molecular length, while in the latter, interfacial contacts and photocurrent were improved as a function of molecular structure and charge distribution; however, direct “molecular wiring” of the organisms to each other and thylakoids to electrodes were again ruled out. Two primary considerations rationalize this result: (a) mismatch of the PPV-COE frontier orbital energies with biological frontier orbital energies and the electrode Fermi energy and (b) the absence of direct electrode contacts.
Following this mechanistic insight, a similar experimental approach was extended to two different materials systems. First, a COE with membrane affinity containing a redox-active ferrocene moiety, DSFO+, was synthesized and applied to M3Cs. The frontier orbitals of DSFO+ are energetically aligned with physiological potentials, so DSFO+ catalytically couples to biocurrent production via ferrocene redox activity, remarkably also enabling partial recovery of biocurrent production in non-electrogenic mutant strains of S. oneidensis MR-1. Second, a set of four conjugated polyelectrolytes (CPEs) with systematic variations in backbone structure, pendant ionic functionalities, and the ability to remain doped at neutral pH in aqueous media were synthesized and applied in M3Cs. The self-doped p-type anionic derivative CPE-K is highly conductive and statistically significantly increases steady-state biocurrent output from S. oneidensis MR-1 by 2.7 ± 0.7-fold relative to unmodified controls (P = 0.002). Important structure-property relationships are revealed in these experiments suggesting that anionic pendant groups and the ability to be doped in aqueous media are necessary for CPE biocurrent enhancement. By absorbance spectroscopy, it appears that S. oneidensis MR-1 may de-dope (neutralize) CPE-K, allowing the electrode to re-dope (re-oxidize) it, creating an electronic extension of the electrode. This helps explain the increase in electrode cell colonization from CPE-K. These results provide a foundation for continued improvement of biotic-abiotic contacts with organic semiconductors in Bioelectronic devices.