Tuning Biotic/Abiotic Interfaces for Enhanced Bioelectrochemical Conversion: Redox-Active Conjugated Oligo- and Polyelectrolytes
Bioelectronics concerns the integration of electronic elements with biological systems to form functional devices including biosensors for detection and analysis, neuroelectronic junctions, and biofuel cells for energy conversion and catalysis. A fundamental part of any bioelectronic system is the electronic coupling and transport at the various biotic/abiotic interfaces between biomaterials and electronic supports. The development and study of materials that modulate charge transport across these interfaces is therefore an area of interest. Advances in increasing the synergy between electronics and biology will see impacts in not only our understanding of system, cell, and molecular biology, but also more efficient electronics for conversion of bioenergy. In this development, we have focused on the implementation of redox-active conjugated electrolytes at the biotic/abiotic interface for amplifying biocurrent collection.
In specifically-designed microbial bioelectrochemical systems, bacteria interface with electrodes to catalyze the interconversion of chemical and electrical energy. Membrane modifiers called conjugated oligoelectrolytes (COEs), defined by their conjugated core and peripheral ionic pendant groups, have been shown to increase the ability of electrogenic microbes to electronically communicate with abiotic components through a number of indirect factors including increasing electrode colonization and coulombic efficiency. However, precise control over the mechanism of current enhancement remained to be seen. Two ferrocene-containing COEs called DVFBO and F4-DVFBO were designed to test voltammetric control over biocatalytic current production. Both COEs have a π-delocalized core capped on each end by ferrocene units and show similar optical properties, affinity for the membrane, and toxicity, but fluorination of the core (F4-DVFBO) results in a higher redox potential (422 ± 5 mV compared to 365 ± 4 mV vs Ag/AgCl for DVFBO). The COEs were tested in anaerobic microbial three-electrode electrochemical cells (M3Cs) containing the model electrogenic bacterium Shewanella oneidensis MR-1. At a low electrode potential, the addition of the COEs produced negligible current enhancement compared to controls. However, at E = 365 mV, DVFBO increased steady-state biocurrent 67 ± 12% relative to controls while F4-DVFBO only increased biocurrent by 30 ± 5%. With no change in electrode colonization, cyclic voltammetry supports that DVFBO is the primary conduit of the increased catalytic current and shows F4-DVFBO has less activity at this poised potential. Overall, this work demonstrates the ability to modulate electron transfer from microbial species exclusively via the oxidation potential of the COE.
While microbial membrane modification with redox-active COEs led to improvement in current extraction, a two-dimensional electrode structure limits the surface area for electroactive bacteria attachment and restricts efficient substrate and buffer diffusion. Conjugated polyelectrolytes represent a complementary interfacial materials strategy to enhance biotic/abiotic electronic coupling. First, the mixed ionic-electronic conduction properties of the self-doped conjugated polyelectrolyte CPE-K were investigated with respect to hydration. Films of CPE-K showed both increasing ionic and electronic conduction with hydration. Results indicate that the increase in electronic conduction was due to both increased number of free charge carriers and closer π-π stacking.
Following this characterization, CPE-K was used as a conductive matrix to electronically connect a three-dimensional network of Shewanella oneidensis MR-1 to a gold electrode. At critical CPE-K concentrations, these biocomposites spontaneously assemble from solution into an intricate arrangement of cells within a conductive polymer matrix, thereby increasing biocurrent ~150-fold over control biofilms in M3C tests. While increased biocurrent is due to more cells in communication with the electrode, current extracted per cell is also enhanced indicating efficient long-range electron transport. Further, biocomposites show almost an order-of-magnitude lower charge transfer resistance than CPE-K alone, supporting that the electroactive bacteria and the conjugated polyelectrolyte work synergistically towards an effective electronic biocomposite. Examination of these biocomposites in microbial fuel cell configurations reinforces the need for continued pursuit of synthetic designs to improve the biotic/abiotic interface in bioelectronics.