Microbial biofilms as living photoconductors due to ultrafast electron transfer in cytochrome OmcS nanowires

Controlling heme redox properties in peptide amphiphile fibers with sequence and heme loading ratio

Controlling the reduction midpoint potential of heme B is a key factor in many bioelectrochemical reactions, including long-range electron transport. Currently, there are a number of globular model protein systems to study this biophysical parameter; however, there are none for large polymeric protein model systems (e.g., the OmcS protein from G. sulfurreducens). Peptide amphiphiles, short peptides with a lipid tail that polymerize into fibrous structures, fill this gap. Here, we show a peptide amphiphile model system where one can tune the electrochemical potential of heme B by changing the loading ratio and peptide sequence. Changing the loading ratio resulted in the most significant increase, with values as high as -22 mV down to -224 mV. Circular dichroism spectra of certain sequences show Cotton effects at lower loading ratios that disappear as more heme B is added, indicating an ordered environment that becomes disrupted if heme B is overpacked. These findings can contribute to the design of functional self-assembling biomaterials.

Influence of support materials on the electroactive behavior, structure and gene expression of wild type and GSU1771-deficient mutant of Geobacter sulfurreducens biofilms

Geobacter sulfurreducens DL1 is a metal-reducing dissimilatory bacterium frequently used to produce electricity in bioelectrochemical systems (BES). The biofilm formed on electrodes is one of the most important factors for efficient electron transfer; this is possible due to the production of type IV pili and c-type cytochromes that allow it to carry out extracellular electron transfer (EET) to final acceptors. In this study, we analyzed the biofilm formed on different support materials (glass, hematite (Fe2O3) on glass, fluorine-doped tin oxide (FTO) semiconductor glass, Fe2O3 on FTO, graphite, and stainless steel) by G. sulfurreducens DL1 (WT) and GSU1771-deficient strain mutant (Δgsu1771). GSU1771 is a transcriptional regulator that controls the expression of several genes involved in electron transfer. Different approaches and experimental tests were carried out with the biofilms grown on the different support materials including structure analysis by confocal laser scanning microscopy (CLSM), characterization of electrochemical activity, and quantification of relative gene expression by RT-qPCR. The gene expression of selected genes involved in EET was analyzed, observing an overexpression of pgcA, omcS, omcM, and omcF from Δgsu1771 biofilms compared to those from WT, also the overexpression of the epsH gene, which is involved in exopolysaccharide synthesis. Although we observed that for the Δgsu1771 mutant strain, the associated redox processes are similar to the WT strain, and more current is produced, we think that this could be associated with a higher relative expression of certain genes involved in EET and in the production of exopolysaccharides despite the chemical environment where the biofilm develops. This study supports that G. sulfurreducens is capable of adapting to the electrochemical environment where it grows.

Widespread extracellular electron transfer pathways for charging microbial cytochrome OmcS nanowires via periplasmic cytochromes PpcABCDE

Extracellular electron transfer (EET) via microbial nanowires drives globally-important environmental processes and biotechnological applications for bioenergy, bioremediation, and bioelectronics. Due to highly-redundant and complex EET pathways, it is unclear how microbes wire electrons rapidly (>106 s-1) from the inner-membrane through outer-surface nanowires directly to an external environment despite a crowded periplasm and slow (<105 s-1) electron diffusion among periplasmic cytochromes. Here, we show that Geobacter sulfurreducens periplasmic cytochromes PpcABCDE inject electrons directly into OmcS nanowires by binding transiently with differing efficiencies, with the least-abundant cytochrome (PpcC) showing the highest efficiency. Remarkably, this defined nanowire-charging pathway is evolutionarily conserved in phylogenetically-diverse bacteria capable of EET. OmcS heme reduction potentials are within 200 mV of each other, with a midpoint 82 mV-higher than reported previously. This could explain efficient EET over micrometres at ultrafast (<200 fs) rates with negligible energy loss. Engineering this minimal nanowire-charging pathway may yield microbial chassis with improved performance.

Observation of Ultrahigh Photoconductivity in DNA-MoS2 Nano-Biocomposite

A nano-biocomposite film with ultrahigh photoconductivity remains elusive and critical for bio-optoelectronic applications. A uniform, well-connected, high-concentration nanomaterial network in the biological matrix remains challenging to achieve high photoconductivity. Wafer-scale continuous nano-biocomposite film without surface deformations and cracks plays another major obstacle. Here ultrahigh photoconductivity is observed in deoxyribonucleic acid-molybdenum disulfide (DNA-MoS2) nano-biocomposite film by incorporating a high-concentration, well-percolated, and uniform MoS2 network in the ss-DNA matrix. This is achieved by utilizing DNA-MoS2 hydrogel formation, which results in crack-free, wafer-scale DNA-MoS2 nano-biocomposite films. Ultra-high photocurrent (5.5 mA at 1 V) with a record-high on/off ratio (1.3 × 106) is observed, five orders of magnitude higher than conventional biomaterials (≈101) reported so far. The incorporation of the Wely semimetal (Bismuth) as an electrical contact exhibits ultrahigh photoresponsivity (2.6 × 105 A W-1). Such high photoconductivity in DNA-MoS2 nano-biocomposite could bridge the gap between biology, electronics, and optics for innovative biomedicine, bioengineering, and neuroscience applications.

Dynamically Reconstructed Triple-Copper-Vacancy Associates Confined in Cu Nanowires Enabling High-Rate and Selective CO2 Electroreduction to C2+ Products

Electrochemically reconstructed Cu-based catalysts always exhibit enhanced CO2  electroreduction performance; however, it still remains ambiguous whether the reconstructed Cu vacancies have a substantial impact on CO2 -to-C2+ reactivity. Herein, Cu vacancies are first constructed through electrochemical reduction of Cu-based nanowires, in which high-angle annular dark-field scanning transmission electron microscopy image manifests the formation of triple-copper-vacancy associates with different concentrations, confirmed by positron annihilation lifetime spectroscopy. In situ attenuated total reflection-surface enhanced infrared absorption spectroscopy discloses the triple-copper-vacancy associates favor *CO adsorption and fast *CO dimerization. Moreover, density-functional-theory calculations unravel the triple-copper-vacancy associates endow the nearby Cu sites with enriched and disparate local charge density, which enhances the *CO adsorption and reduces the CO-CO coupling barrier, affirmed by the decreased *CO dimerization energy barrier by 0.4 eV. As a result, the triple-copper-vacancy associates confined in Cu nanowires achieve a high Faradaic efficiency of over 80% for C2+ products in a wide current density range of 400-800 mA cm-2 , outperforming most reported Cu-based electrocatalysts.

Interfacial Electron Transfer in Chemical and Biological Transformation of Pollutants in Environmental Catalysis

Interfacial electron transfer (IET) is essential for chemical and biological transformation of pollutants, operative across diverse lengths and time scales. This Perspective presents an array of multiscale molecular simulation methodologies, supplemented by in situ monitoring and imaging techniques, serving as robust tools to decode IET enhancement mechanisms such as interface molecular modification, catalyst coordination mode, and atomic composition regulation. In addition, three IET-based pollutant transformation systems, an electrocatalytic oxidation system, a bioelectrochemical spatial coupling system, and an enzyme-inspired electrocatalytic system, were developed, demonstrating a high effect in transforming and degrading pollutants. To improve the effectiveness and scalability of IET-based strategies, the refinement of these systems is necessitated through rigorous research and theoretical exploration, particularly in the context of practical wastewater treatment scenarios. Future endeavors aim to elucidate the synergy between biological and chemical modules, edit the environmental functional microorganisms, and harness machine learning for designing advanced environmental catalysts to boost efficiency. This Perspective highlights the powerful potential of IET-focused environmental remediation strategies, emphasizing the critical role of interdisciplinary research in addressing the urgent global challenge of water pollution.

Long-Range Electron Transport Rates Depend on Wire Dimensions in Cytochrome Nanowires

The ability to redirect electron transport to new reactions in living systems opens possibilities to store energy, generate new products, or probe physiological processes. Recent work by Huang et al. showed that 3D crystals of small tetraheme cytochromes (STC) can transport electrons over nanoscopic to mesoscopic distances by an electron hopping mechanism, making them promising materials for nanowires. However, fluctuations at room temperature may distort the nanostructure, hindering efficient electron transport. Classical molecular dynamics simulations of these fluctuations at the nano- and mesoscopic scales allowed us to develop a graph network representation to estimate maximum electron flow that can be driven through STC wires. In longer nanowires, transient structural fluctuations at protein-protein interfaces tended to obstruct efficient electron transfer, but these blockages are ameliorated in thicker crystals where alternative electron transfer pathways become more efficient. The model implies that more flexible proteinprotein interfaces limit the required minimum diameter to carry currents commensurate with conventional electronics.

Mammalian Fuel Cells Produce Electric Current

The increasing concern about climate change has led scientists around the world to develop clean energy technologies that may replace the traditional use of fossil fuels. A promising approach is the utilization of unicellular organisms as electron donors in bio-fuel cells. To date, this method has been limited to microorganisms such as bacteria, yeast, and microalgae. In this work, we show for the first time the concept of using mammalian cell cultures and organoids as electron donors in biofuel cells. We apply cyclic voltammetry to show that upon association of ARPE19 cells with the anode, they release reducing molecules to produce electricity. Furthermore, we apply 2D-fluorescence measurements and show that upon illumination, photosensitive stem cell-derived retinal organoids, which consist of rod photoreceptors and interneurons, secrete NADH and NADPH molecules that can donate electrons at the anode to produce photocurrent.

Enhanced photocatalytic CO2 reduction on biomineralized CdS via an electron conduit in bacteria

There is an increasing trend in semi-artificial photosynthesis systems that combine living cells with inorganic semiconductors to activate a bacterial catalytic network. However, these systems face various challenges, including electron-hole recombination, photocorrosion, and the generation of photoexcited radicals by semiconductors, all of which impair the efficiency, stability, and sustainability of biohybrids. We first focus on a reverse strategy to improve highly efficient CO₂ photoreduction on biosynthesized inorganic semiconductors using an electron conduit in the electroactive bacterium S. oneidensis MR-1. Due to the suppressed charge recombination and photocorrosion on CdS, the maximum photocatalytic production rate of formate in water was 2650 μmol g^-1 h^-1 (with a selectivity of ca.100%), which ranks high among all photocatalysts and is the highest for inorganic-biological hybrid systems in an all-inorganic aqueous environment. The reverse enhancement effect of electrogenic bacteria on photocatalysis on semiconductors inspires new insight to develop a new generation of bio-semiconductor catalysts for solar chemical production.

Microplastics degradation stimulated by in-situ bioelectric field in agricultural soils

Bioelectric field is a stimulated force to degrade xenobiotic pollutants in soils. However, the effect of bioelectric field on microplastics (MPs) aging is unclear. The degradation behavior of polyvinyl chloride (PVC), polyethylene (PE) and polylactic acid (PLA) was investigated in an agricultural soil microbial electrochemical system in which bioelectric field was generated in-situ by native microbes. Based on the density function theory, the energy gaps between the highest and the lowest occupied molecular orbitals of the three polymers with periodic structure were 4.20, 7.24 and 10.09 eV respectively, and further decreased under the electric field, indicating the higher hydrolysis potential of PLA. Meanwhile, the mass loss of PLA in the closed-circuit group (CC) was the highest on day 120, reaching 8.94%, which was 3.01-3.54 times of that without bioelectric field stimulation. This was mainly due to the enrichment of plastic-degrading bacteria and a robust co-occurrence network as the deterministic assembly process, e.g., the abundance of potential plastic-degrading bacteria on the surface of PLA and PVC in the CC increased by 1.92 and 1.30 times, respectively, compared to the open-circuit group. In terms of functional genes, the xenobiotic biodegradation and metabolism capacity of plasticsphere in the CC were stronger than that in soil, and determined by the bioaccessibility of soil nitrogen and carbon. Overall, this study explored the promoting effect of bioelectric field on the degradation of MPs and reveled the mechanism from quantum chemical calculations and microbial community analysis, which provides a novel perception to the in-situ degradation of MPs.

Emerging high-ammonia‑nitrogen wastewater remediation by biological treatment and photocatalysis techniques

The bacterial and photocatalysis techniques have been widely applied into the remediation of ammonia nitrogen wastewater. Although traditional microbial methods had been verified useful; more efficient, energy-saving and controllable candidate treatment methods are still urgently needed to cover the increasingly diverse ammonia nitrogen pollution cases. The bacterial treatment technique for ammonia nitrogen mainly depends on the ammonia nitrogen oxidation-reduction (e.g. nitrification, denitrification) by nitrifying bacteria and denitrifying bacteria, but these reactions suffer from slow denitrifying kinetic process and uncontrolled disproportionation reaction. In comparison, the photocatalysis technique based on photoelectrons is more efficient and has some advantages, such as low temperature reaction and long life, while the photocatalysis technique can not perform multiple complex biochemical reactions. Despite much scientific knowledge obtained about this issue recently, such research has yet not been widely adopted in the industry because of many concerns about subsequent catalyst stability and economic feasibility. This review summarized and discussed the very recent achievements and key problems on remediation of high-ammonia‑nitrogen wastewater and oxidation driven by bacterial treatment and photocatalysis techniques, as well as the most promising future directions for these two techniques, especially the potential of jointly bacterial-photocatalysis techniques.

Electron transfer in heterojunction catalysts

Heterojunction catalysis, the cornerstone of the modern chemical industry, shows potential to tackle the growing energy and environmental crises. Electron transfer (ET) is ubiquitous in heterojunction catalysts, and it holds great promise for improving the catalytic efficiency by tuning the electronic structures or building internal electric fields at interfaces. This perspective summarizes the recent progress of catalysis involving ET in heterojunction catalysts and pinpoints its crucial role in catalytic mechanisms. We specifically highlight the occurrence, driving forces, and applications of ET in heterojunction catalysis. For corroborating the ET processes, common techniques with measurement principles are introduced. We end with the limitations of the current study on ET, and envision future challenges in this field.

Engineered Living Materials For Sustainability

Recent advances in synthetic biology and materials science have given rise to a new form of materials, namely engineered living materials (ELMs), which are composed of living matter or cell communities embedded in self-regenerating matrices of their own or artificial scaffolds. Like natural materials such as bone, wood, and skin, ELMs, which possess the functional capabilities of living organisms, can grow, self-organize, and self-repair when needed. They also spontaneously perform programmed biological functions upon sensing external cues. Currently, ELMs show promise for green energy production, bioremediation, disease treatment, and fabricating advanced smart materials. This review first introduces the dynamic features of natural living systems and their potential for developing novel materials. We then summarize the recent research progress on living materials and emerging design strategies from both synthetic biology and materials science perspectives. Finally, we discuss the positive impacts of living materials on promoting sustainability and key future research directions.

Photocurrent Production from Cherries in a Bio-Electrochemical Cell

In recent years, clean energy technologies that meet ever-increasing energy demands without the risk of environmental contamination has been a major interest. One approach is the utilization of plant leaves, which release redox-active NADPH as a result of photosynthesis, to generate photocurrent. In this work, we show for the first time that photocurrent can be harvested directly from the fruit of a cherry tree when associated with a bio-electrochemical cell. Furthermore, we apply electrochemical and spectroscopic methods to show that NADH in the fruit plays a major role in electric current production.

Structure of Geobacter cytochrome OmcZ identifies mechanism of nanowire assembly and conductivity

OmcZ nanowires produced by Geobacter species have high electron conductivity (>30 S cm-1). Of 111 cytochromes present in G. sulfurreducens, OmcZ is the only known nanowire-forming cytochrome essential for the formation of high-current-density biofilms that require long-distance (>10 µm) extracellular electron transport. However, the mechanisms underlying OmcZ nanowire assembly and high conductivity are unknown. Here we report a 3.5-Å-resolution cryogenic electron microscopy structure for OmcZ nanowires. Our structure reveals linear and closely stacked haems that may account for conductivity. Surface-exposed haems and charge interactions explain how OmcZ nanowires bind to diverse extracellular electron acceptors and how organization of nanowire network re-arranges in different biochemical environments. In vitro studies explain how G. sulfurreducens employ a serine protease to control the assembly of OmcZ monomers into nanowires. We find that both OmcZ and serine protease are widespread in environmentally important bacteria and archaea, thus establishing a prevalence of nanowire biogenesis across diverse species and environments.

New Perspective on Electron Transfer through Molecules

Motivated by experiments which display unusual length and temperature effects for electron transfer in the nanometer length regime, we propose a new approach for describing long-range electron transfer (ET) processes through molecules. We posit that the electron reorganization in the molecules (e.g., the electronic polarization of a macromolecule or organic film by an applied electric potential, or the injected charge generating a dipole moment) should be included in the description. We numerically solve a one-dimensional model for the electron transport, which includes electron-electron interactions explicitly, and we show that it generates a power law distance dependence for electron transport similar to that observed in experiments. The model does not include vibrations explicitly and should be consistent with the weak temperature dependences observed experimentally. This approach emphasizes the need to treat the electronic changes in the molecule(s) more explicitly to understand the behavior.

Self-Enclosed Bio-Photoelectrochemical Cell in Succulent Plants

Harvesting an electrical current from biological photosynthetic systems (live cells or isolated complexes) is typically achieved by immersion of the system into an electrolyte solution. In this study, we show that the aqueous solution found in the tissues of succulent plants can be used directly as a natural bio-photo electrochemical cell. Here, the thick water-preserving outer cuticle of the succulent Corpuscularia lehmannii serves as the electrochemical container, the inner water content as the electrolyte into which an iron anode and platinum cathode are introduced. We produce up to 20 μA/cm² bias-free photocurrent. When 0.5 V bias is added to the iron anode, the current density increases ∼10-fold, and evolved hydrogen gas can be collected with a Faradaic efficiency of 2.1 and 3.5% in dark or light, respectively. The addition of the photosystem II inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea inhibits the photocurrent, indicating that water oxidation is the primary source of electrons in the light. Two-dimensional fluorescence measurements show that NADH and NADPH serve as the major mediating electron transfer molecules, functionally connecting photosynthesis to metal electrodes. This work presents a method to simultaneously absorb CO₂ while producing an electrical current with minimal engineering requirements.