Search for: All records

Electrically conductive, genetically tunable, pilin-based protein nanowires (ePNs) expressed in Escherichia coli grown on the biodiesel byproduct glycerol are a sustainable electronic material. They were previously shown to effectively function as sensing components for volatile analytes when deployed as thin films in electronic devices. However, thin-film devices are not suitable for analyzing components dissolved in water. To evaluate the possibility of fabricating a water-stable ePN matrix, ePNs purified from cells were mixed with polyvinyl butyral (PVB) to produce a transparent, electrically conductive, water-stable composite. ePN/PVB composite conductivity was tuned by changing the concentration of ePNs in the composite or genetically tailoring ePNs for different conductivities. Devices with an ePN/PVB sensing component rapidly responded in a linear fashion to changes in concentrations of dissolved ammonia or acetate. Genetically modifying nanowires to display an analyte-binding peptide on the ePN outer surface that was specific for ammonia or acetate increased sensing sensitivity and specificity. Composites comprised of whole cells of E. coli expressing ePNs and PVB were also electrically conductive. They functioned as sensing components whose sensitivity could also be tuned with the expression of ePNs displaying specific analyte-binding peptides. This approach avoids the laborious and time-consuming purification of protein nanowires from cells. The simplicity of sustainably fabricating an electronic sensing component with ePN-expressing E. coli mixed with a polymer, coupled with the potential of exquisitely tuning sensing specificity with facile ‘plug and play’ nanowire design, demonstrates the possibility of simply and inexpensively producing sensing devices for detecting a broad range of analytes. 

Nanowires have substantial potential as the sensor component in electronic sensing devices. However, surface functionalization of traditional nanowire and nanotube materials with short peptides that increase sensor selectivity and sensitivity requires complex chemistries with toxic reagents. In contrast, microorganisms can assemble pilin monomers into protein nanowires with intrinsic conductivity from renewable feedstocks, yielding an electronic material that is robust and stable in applications, but also biodegradable. Here we report that the sensitivity and selectivity of protein nanowire-based sensors can be modified with a simple plug and play genetic approach in which a short peptide sequence, designed to bind the analyte of interest, is incorporated into the pilin protein that is microbially assembled into nanowires. We employed a scalable Escherichia coli chassis to fabricate protein nanowires that displayed either a peptide previously demonstrated to effectively bind ammonia, or a peptide known to bind acetic acid. Sensors comprised of thin films of the nanowires amended with the ammonia-specific peptide had a ca. 100-fold greater response to ammonia than sensors made with unmodified protein nanowires. Protein nanowires with the peptide that binds acetic acid yielded a 4-fold higher response than nanowires without the peptide. The protein nanowire-based sensors had greater responses than previously reported sensors fabricated with other nanomaterials. The results demonstrate that protein nanowires with enhanced sensor response for analytes of interest can be fabricated with a flexible genetic strategy that sustainably eliminates the energy, environmental, and health concerns associated with other common nanomaterials. 

Abstract Employing renewable materials for fabricating clean energy harvesting devices can further improve sustainability. Microorganisms can be mass produced with renewable feedstocks. Here, we demonstrate that it is possible to engineer microbial biofilms as a cohesive, flexible material for long-term continuous electricity production from evaporating water. Single biofilm sheet (~40 µm thick) serving as the functional component in an electronic device continuously produces power density (~1 μW/cm 2 ) higher than that achieved with thicker engineered materials. The energy output is comparable to that achieved with similar sized biofilms catalyzing current production in microbial fuel cells, without the need for an organic feedstock or maintaining cell viability. The biofilm can be sandwiched between a pair of mesh electrodes for scalable device integration and current production. The devices maintain the energy production in ionic solutions and can be used as skin-patch devices to harvest electricity from sweat and moisture on skin to continuously power wearable devices. Biofilms made from different microbial species show generic current production from water evaporation. These results suggest that we can harness the ubiquity of biofilms in nature as additional sources of biomaterial for evaporation-based electricity generation in diverse aqueous environments. 

Abstract Incorporating neuromorphic electronics in bioelectronic interfaces can provide intelligent responsiveness to environments. However, the signal mismatch between the environmental stimuli and driving amplitude in neuromorphic devices has limited the functional versatility and energy sustainability. Here we demonstrate multifunctional, self-sustained neuromorphic interfaces by achieving signal matching at the biological level. The advances rely on the unique properties of microbially produced protein nanowires, which enable both bio-amplitude (e.g., <100 mV) signal processing and energy harvesting from ambient humidity. Integrating protein nanowire-based sensors, energy devices and memristors of bio-amplitude functions yields flexible, self-powered neuromorphic interfaces that can intelligently interpret biologically relevant stimuli for smart responses. These features, coupled with the fact that protein nanowires are a green biomaterial of potential diverse functionalities, take the interfaces a step closer to biological integration. 

ABSTRACT Geobacter sulfurreducens is a model microbe for elucidating the mechanisms for extracellular electron transfer in several biogeochemical cycles, bioelectrochemical applications, and microbial metal corrosion. Multiple lines of evidence previously suggested that electrically conductive pili (e-pili) are an essential conduit for long-range extracellular electron transport in G. sulfurreducens . However, it has recently been reported that G. sulfurreducens does not express e-pili and that filaments comprised of multi-heme c -type cytochromes are responsible for long-range electron transport. This possibility was directly investigated by examining cells, rather than filament preparations, with atomic force microscopy. Approximately 90% of the filaments emanating from wild-type cells had a diameter (3 nm) and conductance consistent with previous reports of e-pili harvested from G. sulfurreducens or heterologously expressed in Escherichia coli from the G. sulfurreducens pilin gene. The remaining 10% of filaments had a morphology consistent with filaments comprised of the c -type cytochrome OmcS. A strain expressing a modified pilin gene designed to yield poorly conductive pili expressed 90% filaments with a 3-nm diameter, but greatly reduced conductance, further indicating that the 3-nm diameter conductive filaments in the wild-type strain were e-pili. A strain in which genes for five of the most abundant outer-surface c -type cytochromes, including OmcS, were deleted yielded only 3-nm-diameter filaments with the same conductance as in the wild type. These results demonstrate that e-pili are the most abundant conductive filaments expressed by G. sulfurreducens , consistent with previous functional studies demonstrating the need for e-pili for long-range extracellular electron transfer. IMPORTANCE Electroactive microbes have significant environmental impacts, as well as applications in bioenergy and bioremediation. The composition, function, and even existence of electrically conductive pili (e-pili) has been one of the most contentious areas of investigation in electromicrobiology, in part because e-pili offer a mechanism for long-range electron transport that does not involve the metal cofactors common in much of biological electron transport. This study demonstrates that e-pili are abundant filaments emanating from Geobacter sulfurreducens , which serves as a model for long-range extracellular electron transfer in direct interspecies electron transfer, dissimilatory metal reduction, microbe-electrode exchange, and corrosion caused by direct electron uptake from Fe(0). The methods described in this study provide a simple strategy for evaluating the distribution of conductive filaments throughout the microbial world with an approach that avoids artifactual production and/or enrichment of filaments that may not be physiologically relevant. 

ABSTRACT Nitrogenase iron (Fe) proteins reduce CO 2 to CO and/or hydrocarbons under ambient conditions. Here, we report a 2.4-Å crystal structure of the Fe protein from Methanosarcina acetivorans ( Ma NifH), which is generated in the presence of a reductant, dithionite, and an alternative CO 2 source, bicarbonate. Structural analysis of this methanogen Fe protein species suggests that CO 2 is possibly captured in an unactivated, linear conformation near the [Fe 4 S 4 ] cluster of Ma NifH by a conserved arginine (Arg) pair in a concerted and, possibly, asymmetric manner. Density functional theory calculations and mutational analyses provide further support for the capture of CO 2 on Ma NifH while suggesting a possible role of Arg in the initial coordination of CO 2 via hydrogen bonding and electrostatic interactions. These results provide a useful framework for further mechanistic investigations of CO 2 activation by a surface-exposed [Fe 4 S 4 ] cluster, which may facilitate future development of FeS catalysts for ambient conversion of CO 2 into valuable chemical commodities. IMPORTANCE This work reports the crystal structure of a previously uncharacterized Fe protein from a methanogenic organism, which provides important insights into the structural properties of the less-characterized, yet highly interesting archaeal nitrogenase enzymes. Moreover, the structure-derived implications for CO 2 capture by a surface-exposed [Fe 4 S 4 ] cluster point to the possibility of developing novel strategies for CO 2 sequestration while providing the initial insights into the unique mechanism of FeS-based CO 2 activation.