Imagine if the energy required to treat wastewater could be derived directly from wastewater itself. The concept is not a distant thought. The bacterial community natively found in wastewater naturally breaks down biomass present in wastewater to derive energy for growth, liberating electrons that may be captured to do useful work (i.e. provide electricity). Demonstrations of such a concept are far past the point of proof-of-principle. Bioelectrochemical devices such as microbial fuel cells (MFCs) have been developed for this very purpose. However, low efficiency of charge extraction from microorganisms remains one of the major challenges limiting widespread implementation of such renewable, bio-based technologies. Using a class of small molecules (termed conjugated oligoelectrolytes (COEs)) that spontaneously insert into microbial membranes and facilitate charge transport across these typically insulating cellular constructs, researchers in the Bazan Research Group at the University of California at Santa Barbara in the US have demonstrated a new approach to improving charge collection in bioelectrochemical devices such as MFCs.
COEs developed by the Bazan Research Group are water-soluble, wire-like molecules that have a pi-conjugated, semiconducting core and charged end groups. This molecular architecture is biocompatible, allowing spontaneous uptake of the molecules into cellular lipid bilayers after simply adding the molecules to the aqueous medium surrounding the cells. Fluorescence imaging of COE-modified microorganisms (COE added at low micromolar concentrations) provides confirmation of the localization of these molecules to the cellular membrane. When implemented in MFCs, COE-modified microorganisms demonstrate up to 5-fold improvement in current generation under steady-state operating conditions and up to 25-fold enhancement in power density, depending on the molecular structure of the COE employed. Such enhancements in device performance have not only been demonstrated in model systems comprised of pure cultures of a single bacterial species, but have also been shown for systems employing raw wastewater comprised of complex, unidentified microbial communities. In the case of wastewater MFCs, increased degradation of organic contaminants was concomitantly observed in COE-modified systems relative to the unmodified, control systems. More recently, COEs have been shown to facilitate charge injection into microorganisms to affect microbial metabolite production, which will have interesting implications for future applications in bioelectrosynthesis.
These results are noteworthy as there is significant interest in conferring extracellular electron transfer capability to a variety of different microorganisms to increase the breadth of bioelectronic technologies. Some microorganisms have naturally evolved as highly efficient ‘electricigens’ and are able to directly transfer electrons to the extracellular environment. This is accomplished via specialized proteins located on the outer membrane surface or via conductive cellular appendages that facilitate extracellular electron transfer to a charge-collecting surface to which the microbes are directly attached. Microorganisms may also secrete soluble electron shuttles that allow for charge equivalents to leave the cell. However, the abundance of known, naturally occurring electrigens is low. The addition of exogenous, soluble redox shuttles to improve the performance of MFCs has been previously demonstrated in the bioelectrochemical literature. Although effective, such an approach requires that the redox shuttles require continuous replenishment, which can be costly. Furthermore, redox shuttles may be toxic to the microorganisms at elevated concentrations. COEs developed by the Bazan Research Group circumvent some of these limitations. It is anticipated that ongoing studies into the mechanism by which COEs act to improve charge transport across biological membranes will lead to greater insight as to how COE molecular structure and chemical functionality may be tuned, yielding further improvements in bioelectrochemical device performance. Such findings will be of relevance to advancing bio-based technologies to meet future demands related to energy, chemical production, waste remediation, and diagnostics.