Electron transfer between microorganisms and an electrode — even across long distances — enables the former to live by coupling to an electronic circuit. Such a system integrates biological metabolism with artificial electronics; studying these systems adds to our knowledge of charge transport in the chemical species involved, as well as, perhaps most importantly, to our knowledge of charge transport and chemistry at the cell–electrode interfaces. This understanding may lead to microbial electrochemical systems finding widespread application, particularly in the energy sector. Bioelectrochemical systems have already shown promise for electricity generation, as well as for the production of biochemical and chemical feedstocks, and with improvement are likely to give rise to viable applications.

Redox-active organic compounds have been studied as corrosion inhibitors for steel. Even though it is clear that chemical interactions at the organic–metal oxide interface are behind this inhibitive process, the detailed mechanism is not yet fully understood. Using phenyl-capped aniline tetramer (PCAT), we have elucidated the interactions at the interface with iron oxide. We demonstrate the partial reduction of fully oxidized PCAT and the partial oxidation of fully reduced PCAT upon interaction with iron oxide. X-ray photoelectron spectroscopy reveals the appearance of charged nitrogen structures in PCAT deposited on hematite. Iron oxide films in contact with reduced PCAT show a higher conductance due to the introduction of defects, resulting in n-doping. In contrast, the iron oxide film in contact with oxidized PCAT shows a lower conductance, indicating that defects in the film are removed via oxidation, thus reducing the doping level. This is consistent with accepted models for corrosion inhibition, in which PCAT assists in the formation of a passive oxide film. These results are indicative of interfacial charge transfer between PCAT and iron oxide. The extent of the charge transfer and the direction of redox processes depend on the oxidation state of the molecules, enabling the construction of redox-active devices, including sensors and switches.

Free chlorine from dissolved chlorine gas is widely used as a disinfectant for drinking water. The residual chlorine concentration has to be continuously monitored and accurately controlled in a certain range around 0.5–2 mg/l to ensure drinking water safety and quality. However, simple, reliable, and reagent free monitoring devices are currently not available. Here, we present a free chlorine sensor that uses oxidation of a phenyl-capped aniline tetramer (PCAT) to dope single wall carbon nanotubes (SWCNTs) and to change their resistance. The oxidation of PCAT by chlorine switches the PCAT-SWCNT system into a low resistance (p-doped) state which can be detected by probing it with a small voltage. The change in resistance is found to be proportional to the log-scale concentration of the free chlorine in the sample. The p-doping of the PCAT-SWCNT film then can be electrochemically reversed by polarizing it cathodically. This sensor not only shows good sensing response in the whole concentration range of free chlorine in drinking water but is also able to be electrochemically reset back many times without the use of any reagents. This simple sensor is ideally suited for measuring free chlorine in drinking water continuously.

Functional hydrogels show great potential for promoting the structural integration and signal transduction at bio/electronic interfaces.

Extracellular electron transfer (EET) is a process performed by electrochemically active bacteria (EAB) to transport metabolically-generated electrons to external solid-phase acceptors through specific molecular pathways. Naturally bridging biotic and abiotic charge transport systems, EET offers ample opportunities in a wide range of bio-interfacing applications, from renewable energy conversion, resource recovery, to bioelectronics. Full exploration of EET fundamentals and applications demands technologies that could seamlessly interface and interrogate with key components and processes at relevant length scales. In this review, we will discuss the recent development of nanoscale platforms that enabled EET investigation from single-cell to network levels. We will further overview research strategies for utilizing rationally designed and integrated nanomaterials for EET facilitation and efficiency enhancement. In the future, EET components such as c-cytochrome based outer membranes and bacterial nanowires along with their assembled structures will present themselves as a whole new category of biosynthetic electroactive materials with genetically encoded functionality and intrinsic biocompatibility, opening up possibilities to revolutionize the way electronic devices communicate with biological systems.