Structure-Activity Relationships of Hierarchical Three-Dimensional Electrodes with Photosystem II for Semiartificial Photosynthesis

Ultrathin Film Antimony-Doped Tin Oxide Prevents [NiFe] Hydrogenase Inactivation at High Electrode Potentials

For hydrogenases to serve as effective electrocatalysts in hydrogen biotechnological devices, such as enzymatic fuel cells, it is imperative to design electrodes that facilitate stable and functional enzyme immobilization, efficient substrate accessibility, and effective interfacial electron transfer. Recent years have seen considerable advancements in this area, particularly concerning hydrogenases. However, a significant limitation remains: the inactivation of hydrogenases at high oxidative potentials across most developed electrodes. Addressing this issue necessitates a thorough understanding of the interactions between the enzyme and the electrode surface. In this study, we employ ATR-IR spectroscopy combined with electrochemistry in situ to investigate the interaction mechanisms, electrocatalytic behavior, and stability of the oxygen-tolerant membrane-bound [NiFe] hydrogenase from Cupriavidus necator (MBH), which features a His-tag on its small subunit C-terminus. Antimony-doped tin oxide (ATO) thin films were selected as electrodes due to their protein compatibility, suitable potential window, conductivity, and transparency, making them an ideal platform for spectroelectrochemical measurements. Our comprehensive examination of the physiological and electrochemical processes of [NiFe] MBH on ATO thin film electrodes demonstrates that by tuning the electron transport properties of the ATO thin film, we can prevent MBH inactivation at extended oxidative potentials while maintaining direct electron transfer between the enzyme and the electrode.

Operando film-electrochemical EPR spectroscopy tracks radical intermediates in surface-immobilized catalysts

The development of surface-immobilized molecular redox catalysts is an emerging research field with promising applications in sustainable chemistry. In electrocatalysis, paramagnetic species are often key intermediates in the mechanistic cycle but are inherently difficult to detect and follow by conventional in situ techniques. We report a new method, operando film-electrochemical electron paramagnetic resonance spectroscopy (FE-EPR), which enables mechanistic studies of surface-immobilized electrocatalysts. This technique enables radicals formed during redox reactions to be followed in real time under flow conditions, at room temperature and in aqueous solution. Detailed insight into surface-immobilized catalysts, as exemplified here through alcohol oxidation catalysis by a surface-immobilized nitroxide, is possible by detecting active-site paramagnetic species sensitively and quantitatively operando, thereby enabling resolution of the reaction kinetics. Our finding that the surface electron-transfer rate, which is of the same order of magnitude as the rate of catalysis (accessible from operando FE-EPR), limits catalytic efficiency has implications for the future design of better surface-immobilized catalysts.

Amine-Rich Hydrogels for Molecular Nanoarchitectonics of Photosystem II and Inverse Opal TiO2 toward Solar Water Oxidation

Solar water oxidation is a crucial process in light-driven reductive synthesis, providing electrons and protons for various chemical reductions. Despite advances in light-harvesting materials and cocatalysts, achieving high efficiency and stability remains challenging. In this study, we present a simple yet effective strategy for immobilizing natural photosystems (PS) made of abundant and inexpensive elements, using amine-rich polyethylenimine (PEI) hydrogels, to fabricate organic/inorganic hybrid photoanodes. Natural PS II extracted from spinach was successfully immobilized on inverse opal TiO2 photoanodes in the presence of PEI hydrogels, leading to greatly enhanced solar water oxidation activity. Photoelectrochemical (PEC) analyses reveal that PS II can be immobilized in specific orientations through electrostatic interactions between the positively charged amine groups of PEI and the negatively charged stromal side of PS II. This specific orientation ensures efficient photogenerated charge separation and suppresses undesired side reactions such as the production of reactive oxygen species. Our study provides an effective immobilization platform and sheds light on the potential utilization of PS II in PEC water oxidation.

Thermophilic cyanobacteria-exciting, yet challenging biotechnological chassis

Thermophilic cyanobacteria are prokaryotic photoautotrophic microorganisms capable of growth between 45 and 73 °C. They are typically found in hot springs where they serve as essential primary producers. Several key features make these robust photosynthetic microbes biotechnologically relevant. These are highly stable proteins and their complexes, the ability to actively transport and concentrate inorganic carbon and other nutrients, to serve as gene donors, microbial cell factories, and sources of bioactive metabolites. A thorough investigation of the recent progress in thermophilic cyanobacteria reveals a significant increase in the number of newly isolated and delineated organisms and wide application of thermophilic light-harvesting components in biohybrid devices. Yet despite these achievements, there are still deficiencies at the high-end of the biotechnological learning curve, notably in genetic engineering and gene editing. Thermostable proteins could be more widely employed, and an extensive pool of newly available genetic data could be better utilised. In this manuscript, we attempt to showcase the most important recent advances in thermophilic cyanobacterial biotechnology and provide an overview of the future direction of the field and challenges that need to be overcome before thermophilic cyanobacterial biotechnology can bridge the gap with highly advanced biotechnology of their mesophilic counterparts. KEY POINTS: • Increased interest in all aspects of thermophilic cyanobacteria in recent years • Light harvesting components remain the most biotechnologically relevant • Lack of reliable molecular biology tools hinders further development of the chassis.

Connecting Biological and Synthetic Approaches for Electrocatalytic CO2 Reduction

Electrocatalytic CO2 reduction has developed into a broad field, spanning fundamental studies of enzymatic 'model' catalysts to synthetic molecular catalysts and heterogeneous gas diffusion electrodes producing commercially relevant quantities of product. This diversification has resulted in apparent differences and a disconnect between seemingly related approaches when using different types of catalysts. Enzymes possess discrete and well understood active sites that can perform reactions with high selectivity and activities at their thermodynamic limit. Synthetic small molecule catalysts can be designed with desired active site composition but do not yet display enzyme-like performance. These properties of the biological and small molecule catalysts contrast with heterogeneous materials, which can contain multiple, often poorly understood active sites with distinct reactivity and therefore introducing significant complexity in understanding their activities. As these systems are being better understood and the continuously improving performance of their heterogeneous active sites closes the gap with enzymatic activity, this performance difference between heterogeneous and enzymatic systems begins to close. This convergence removes the barriers between using different types of catalysts and future challenges can be addressed without multiple efforts as a unified picture for the biological-synthetic catalyst spectrum emerges.

Photosystem II as a chemiluminescence-induced photosensitizer for photoelectrochemical biofuel cell-type biosensing system

Herein, photosystem II (PSII), extracted from spinach, is used for the first time as an efficient and green sensitizer for a photobioanode in a photoelectrochemical glucose biofuel cell (PBFC) setup. The concept is based on the formation of hemin-catalyzed luminol chemiluminescence (CL) after the enzymatic oxidation of glucose and the simultaneous production of hydrogen peroxide by glucose oxidase. The photosynthetic enzyme PSII, combined with an osmium polymer serving as mediator and photosensitizer, is immobilized and wired on microporous carbonaceous material (MC) for the chemiluminescence-induced oxidation of water to O₂ at the photobioanode (GCE|MC|Os polymer|PSII). Also, bilirubin oxidase immobilized on multiwalled carbon nanotubes (MWCNTs) coated electrode (GCE|MWCNT|BOx) serves as a biocathode. The photoelectrochemical biofuel cell (PBFC) is applied to a biosensor model system to validate the appropriateness of such a bioanode operating in a self-powered mode. Os redox polymer attached to MCs provides abundant PSII immobilization and a reliable electron transfer pathway. The well-matching energy levels of photosensitive entities reduce recombination phenomena while MC enhances the charge collection. Substantial photocatalytic water oxidation was observed under CL due to the well-matched CL emission and PSII absorption. The electrode is rationally designed to gain the maximum luminol CL power for the photobioanode. The open circuit potential of PBFC linearly increased with the CL power intensity and, in turn, glucose concentrations in the range of 0-6 mmol L^-1. The PBFC yielded an OCP of 0.531 V in 30 mmolL^-1 glucose. The study may open a new horizon to the green and pioneering PEC biosensing realm.

Closing the green gap of photosystem I with synthetic fluorophores for enhanced photocurrent generation in photobiocathodes

One restriction for biohybrid photovoltaics is the limited conversion of green light by most natural photoactive components. The present study aims to fill the green gap of photosystem I (PSI) with covalently linked fluorophores, ATTO 590 and ATTO 532. Photobiocathodes are prepared by combining a 20 μm thick 3D indium tin oxide (ITO) structure with these constructs to enhance the photocurrent density compared to setups based on native PSI. To this end, two electron transfer mechanisms, with and without a mediator, are studied to evaluate differences in the behavior of the constructs. Wavelength-dependent measurements confirm the influence of the additional fluorophores on the photocurrent. The performance is significantly increased for all modifications compared to native PSI when cytochrome c is present as a redox-mediator. The photocurrent almost doubles from -32.5 to up to -60.9 μA cm^-2. For mediator-less photobiocathodes, interestingly, drastic differences appear between the constructs made with various dyes. While the turnover frequency (TOF) is doubled to 10 e^-/PSI/s for PSI-ATTO590 on the 3D ITO compared to the reference specimen, the photocurrents are slightly smaller since the PSI-ATTO590 coverage is low. In contrast, the PSI-ATTO532 construct performs exceptionally well. The TOF increases to 31 e^-/PSI/s, and a photocurrent of -47.0 μA cm^-2 is obtained. This current is a factor of 6 better than the reference made with native PSI in direct electron transfer mode and sets a new record for mediator-free photobioelectrodes combining 3D electrode structures and light-converting biocomponents.

Facile fabrication of hierarchically nanostructured gold electrode for bio-electrochemical applications

Nanoporous gold (NPG) is one of the most extensively investigated nanomaterials owing to its tunable pore size, ease of surface modification, and range of applications from catalysis, actuation, and molecular release to the development of electrochemical sensors. In an effort to improve the usefulness of NPG, a simple and robust method for the fabrication of hierarchical and bimodal nanoporous gold electrodes (hb-NPG) containing both macro-and mesopores is reported using electrochemical alloying and dealloying processes to engineer a bicontinuous solid/void morphology. Scanning electron microscopy (color SEM) images depict the hierarchical pore structure created after the multistep synthesis with an ensemble of tiny pores below 100 nm in size located in ligaments spanning larger pores of several hundred nanometers. Smaller-sized pores are exploited for surface modification, and the network of larger pores aids in molecular transport. Cyclic voltammetry (CV) was used to compare the electrochemically active surface area of the hierarchical bimodal structure with that of the regular unimodal NPG with an emphasis on the critical role of both dealloying and annealing in creating the desired structure. The adsorption of different proteins was followed using UV-vis absorbance measurements of solution depletion revealing the high loading capacity of hb-NPG. The surface coverage of lipoic acid on the hb-NPG was analyzed using thermogravimetric analysis (TGA) and reductive desorption. The roughness factor determinations suggest that the fabricated hb-NPG electrode has tremendous potential for biosensor development by changing the scaling relations between volume and surface area which may lead to improved analytical performance. We have chosen to take advantage of the surface architectures of hb-NPG due to the presence of a large specific surface area for functionalization and rapid transport pathways for faster response. It is shown that the hb-NPG electrode has a higher sensitivity for the amperometric detection of glucose than does an NPG electrode of the same geometric surface area.

3D-printed hierarchical pillar array electrodes for high-performance semi-artificial photosynthesis

The rewiring of photosynthetic biomachineries to electrodes is a forward-looking semi-artificial route for sustainable bio-electricity and fuel generation. Currently, it is unclear how the electrode and biomaterial interface can be designed to meet the complex requirements for high biophotoelectrochemical performance. Here we developed an aerosol jet printing method for generating hierarchical electrode structures using indium tin oxide nanoparticles. We printed libraries of micropillar array electrodes varying in height and submicrometre surface features, and studied the energy/electron transfer processes across the bio-electrode interfaces. When wired to the cyanobacterium Synechocystis sp. PCC 6803, micropillar array electrodes with microbranches exhibited favourable biocatalyst loading, light utilization and electron flux output, ultimately almost doubling the photocurrent of state-of-the-art porous structures of the same height. When the micropillars' heights were increased to 600 µm, milestone mediated photocurrent densities of 245 µA cm^-2 (the closest thus far to theoretical predictions) and external quantum efficiencies of up to 29% could be reached. This study demonstrates how bio-energy from photosynthesis could be more efficiently harnessed in the future and provide new tools for three-dimensional electrode design.

Stabilisierung von Elektronentransferwegen erlaubt Stabilität von Biohybrid-Photoelektroden über Jahre

Die Nutzung natürlicher photosynthetischer Enzyme in biohybriden Anwendungen stellt eine attraktive und potenziell nachhaltige Möglichkeit zur Umwandlung von solarer Energie in Elektrizität und Brennstoffe dar. Jedoch begrenzt die Stabilität von photosynthetisch aktiven Proteinen nach der Implementierung in biohybride Anwendungsdesigns die operative Lebensdauer von Biophotoelektroden auf bisher wenige Stunden. In dieser Publikation demonstrieren wir, wie sich die Stabilität einer mesoporösen Elektrode, welche mit dem Photoprotein RC‐LH1 aus Rhodobacter sphaeroides beschichtet ist, erheblich steigern lässt. Durch die Aufrechterhaltung der Elektronenübertragungswege konnte die operative Lebensdauer unter Dauerlicht auf 33 Tage gesteigert werden und die operative Funktionalität nach einer Lagerung über mehr zwei Jahre hinweg demonstriert werden. Kombiniert mit hohen Photoströmen, die Spitzenwerte von 4.6 mA cm−2 erreichten, erzeugte die optimierte Biophotoelektrode eine kumulative Leistung von 86 C cm−2, die höchste bisher berichtete Leistung für diese Art von Elektroden. Unsere Ergebnisse zeigen, dass der Faktor, welcher die Stabilität einschränkt, die Architektur der Struktur ist, die das Photoprotein umgibt, sowie das entsprechende biohybride Sensoren und photovoltaische Geräte mit einer Betriebsdauer von mehreren Jahren möglich sind.

Sustaining Electron Transfer Pathways Extends Biohybrid Photoelectrode Stability to Years

The exploitation of natural photosynthetic enzymes in semi-artificial devices constitutes an attractive and potentially sustainable route for the conversion of solar energy into electricity and solar fuels. However, the stability of photosynthetic proteins after incorporation in a biohybrid architecture typically limits the operational lifetime of biophotoelectrodes to a few hours. Here, we demonstrate ways to greatly enhance the stability of a mesoporous electrode coated with the RC-LH1 photoprotein from Rhodobacter sphaeroides. By preserving electron transfer pathways, we extended operation under continuous high-light to 33 days, and operation after storage to over two years. Coupled with large photocurrents that reached peak values of 4.6 mA cm-2 , the optimized biophotoelectrode produced a cumulative output of 86 C cm-2 , the largest reported performance to date. Our results demonstrate that the factor limiting stability is the architecture surrounding the photoprotein, and that biohybrid sensors and photovoltaic devices with operational lifetimes of years are feasible.

Graphene oxide decorated with gold enables efficient biophotovolatic cells incorporating photosystem I

This paper describes the use of reduced graphene oxide decorated with gold nanoparticles as an efficient electron transfer layer for solid-state biophotovoltic cells containing photosystem I as the sole photo-active component. Together with polytyrosine–polyaniline as a hole transfer layer, this device architecture results in an open-circuit voltage of 0.3 V, a fill factor of 38% and a short-circuit current density of 5.6 mA cm⁻² demonstrating good coupling between photosystem I and the electrodes. The best-performing device reached an external power conversion efficiency of 0.64%, the highest for any solid-state photosystem I-based photovoltaic device that has been reported to date. Our results demonstrate that the functionality of photosystem I in the non-natural environment of solid-state biophotovoltaic cells can be improved through the modification of electrodes with efficient charge-transfer layers. The combination of reduced graphene oxide with gold nanoparticles caused tailoring of the electronic structure and alignment of the energy levels while also increasing electrical conductivity. The decoration of graphene electrodes with gold nanoparticles is a generalizable approach for enhancing charge-transfer across interfaces, particularly when adjusting the levels of the active layer is not feasible, as is the case for photosystem I and other biological molecules.

This paper describes the use of reduced graphene oxide decorated with gold nanoparticles as an efficient electron transport layer for solid-state biophotovoltic cells containing photosystem I as the sole photo-active component.

Fast CO2 hydration kinetics impair heterogeneous but improve enzymatic CO2 reduction catalysis

The performance of heterogeneous catalysts for electrocatalytic CO₂ reduction (CO₂R) suffers from unwanted side reactions and kinetic inefficiencies at the required large overpotential. However, immobilised CO₂R enzymes — such as formate dehydrogenase — can operate with high turnover and selectivity at a minimal overpotential and are therefore ‘ideal’ model catalysts. Here, through the co-immobilisation of carbonic anhydrase, we study the effect of CO₂ hydration on the local environment and performance of a range of disparate CO₂R systems from enzymatic (formate dehydrogenase) to heterogeneous systems. We show that the co-immobilisation of carbonic anhydrase increases the kinetics of CO₂ hydration at the electrode. This benefits enzymatic CO₂ reduction — despite the decrease in CO₂ concentration — due to a reduction in local pH change, whereas it is detrimental to heterogeneous catalysis (on Au), because the system is unable to suppress the H₂ evolution side reaction. Understanding the role of CO₂ hydration kinetics within the local environment on the performance of electrocatalyst systems provides important insights for the development of next generation synthetic CO₂R catalysts.

Understanding the local chemical environment of bioelectrocatalysis

Bioelectrochemistry employs an array of high-surface-area meso- and macroporous electrode architectures to increase protein loading and the electrochemical current response. While the local chemical environment has been studied in small-molecule and heterogenous electrocatalysis, conditions in enzyme electrochemistry are still commonly established based on bulk solution assays, without appropriate consideration of the nonequilibrium conditions of the confined electrode space. Here, we apply electrochemical and computational techniques to explore the local environment of fuel-producing oxidoreductases within porous electrode architectures. This improved understanding of the local environment enabled simple manipulation of the electrolyte solution by adjusting the bulk pH and buffer pKa to achieve an optimum local pH for maximal activity of the immobilized enzyme. When applied to macroporous inverse opal electrodes, the benefits of higher loading and increased mass transport were employed, and, consequently, the electrolyte adjusted to reach −8.0 mA ⋅ cm−2 for the H2 evolution reaction and −3.6 mA ⋅ cm−2 for the CO2 reduction reaction (CO2RR), demonstrating an 18-fold improvement on previously reported enzymatic CO2RR systems. This research emphasizes the critical importance of understanding the confined enzymatic chemical environment, thus expanding the known capabilities of enzyme bioelectrocatalysis. These considerations and insights can be directly applied to both bio(photo)electrochemical fuel and chemical synthesis, as well as enzymatic fuel cells, to significantly improve the fundamental understanding of the enzyme–electrode interface as well as device performance.

Rational Utilization of Photoelectrochemistry of Photosystem II for Self-Powered Photocathodic Detection of MicroRNA in Cells

The photoanode, photosystem II (PSII)/hierarchical inverse opal (IO) TiO₂, is coupled to the complementary photocathode, PbS quantum dots (QDs)/DNA probes, which is then integrated into a two-compartment photoelectrochemical (PEC) cell to achieve a self-powered system to enable photocathodic detection of microRNA-10b from HeLa cells. In such a system, all of the PSII catalytic products, i.e., electrons, protons, and O₂, were rationally utilized and could overcome the general issue of varied O₂ levels in photocathodic detection. The correlation between the target-triggered formation of the DNA complexes and the catalytic reduction of the dissolved O₂ makes possible the steady microRNA-10b detection with good sensitivity and selectivity. This work has unveiled the ability of PSII to construct self-powered detecting devices and shed light on its application in new arenas.

Photosynthetic Nanomaterial Hybrids for Bioelectricity and Renewable Energy Systems

Harvesting solar energy in the form of electricity from the photosynthesis of plants, algal cells, and bacteria has been researched as the most environment-friendly renewable energy technology in the last decade. The primary challenge has been the engineering of electrochemical interfacing with photosynthetic apparatuses, organelles, or whole cells. However, with the aid of low-dimensional nanomaterials, there have been many advances, including enhanced photon absorption, increased generation of photosynthetic electrons (PEs), and more efficient transfer of PEs to electrodes. These advances have demonstrated the possibility for the technology to advance to a new level. In this article, the fundamentals of photosynthesis are introduced. How PE harvesting systems have improved concerning solar energy absorption, PE production, and PE collection by electrodes is discussed. The review focuses on how different kinds of nanomaterials are applied and function in interfacing with photosynthetic materials for enhanced PE harvesting. Finally, the review analyzes how the performance of PE harvesting and stand-alone systems have evolved so far and its future prospects.

The Interaction of Water-Soluble Nitroxide Radicals with Photosystem II

In this work, we investigated the redox transients of a number of water-soluble spin labels upon their interactions with Photosystem II (PS II) core complexes isolated from spinach leaves. We have found that the reactivity of nitroxide radicals, determined by the rate of their reduction upon illumination of PS II, depends on the chemical structure of radicals and the capability of their coming close to low-potential redox centers of photoactive PS II complexes. An enhanced capability of nitroxide radicals to accept electrons from PS II correlates with their chemical structure. Nitroxide radicals NTI (2,2,5,5-tetramethyl-4-nitromethylene-3-imidazolidine-N-oxyl) and Tacet (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl-acetate), containing polar groups, appear to be most efficient acceptors of electrons donated by PS II compared to neutral (TEMPOL, 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl) or positively charged (Tamine, 4-amino-2,2,6,6-tetramethylpiperidine-l-oxyl) spin labels. We assume that enhanced reactivities of polar nitroxide radicals, NTI and Tacet, are determined (1) by their relatively high redox potentials, providing the possibility to accept electrons from PS II, and (2) by their affinities to the closest binding sites on the surface of PS II in the vicinity of the primary plastoquinone acceptor PQA (12-14 Å) or/and in the intraprotein cavity for the secondary plastoquinone PQB (~ 22 Å).

A Semi-artificial Photoelectrochemical Tandem Leaf with a CO2 -to-Formate Efficiency Approaching 1

Semi-artificial photoelectrochemistry can combine state-of-the-art photovoltaic light-absorbers with enzymes evolved for selective fuel-forming reactions such as CO₂ reduction, but the overall performance of such hybrid systems has been limited to date. Here, the electrolyte constituents were first tuned to establish an optimal local environment for a W-formate dehydrogenase to perform electrocatalysis. The CO₂ reductase was then interfaced with a triple cation lead mixed-halide perovskite through a hierarchically structured porous TiO₂ scaffold to produce an integrated photocathode achieving a photocurrent density of -5 mA cm^-2 at 0.4 V vs. the reversible hydrogen electrode during simulated solar light irradiation. Finally, the combination with a water-oxidizing BiVO₄ photoanode produced a bias-free integrated biophotoelectrochemical tandem device (semi-artificial leaf) with a solar CO₂ -to-formate energy conversion efficiency of 0.8 %.

Photosystem II-based biomimetic assembly for enhanced photosynthesis

Photosystem II (PSII) is a fascinating photosynthesis-involved enzyme, participating in sunlight-harvest, water splitting, oxygen release, and proton/electron generation and transfer. Scientists have been inspired to couple PSII with synthetic hierarchical structures via biomimetic assembly, facilitating attainment of natural photosynthesis processes, such as photocatalytic water splitting, electron transfer and ATP synthesis, in vivo. In the past decade, there has been significant progress in PSII-based biomimetic systems, such as artificial chloroplasts and photoelectrochemical cells. The biomimetic assembly approach helps PSII gather functions and properties from synthetic materials, resulting in a complex with partly natural and partly synthetic components. PSII-based biomimetic assembly offers opportunities to forward semi-biohybrid research and synchronously inspire optimization of artificial light-harvest micro/nanodevices. This review summarizes recent studies on how PSII combines with artificial structures via molecular assembly and highlights PSII-based semi-natural biosystems which arise from synthetic parts and natural components. Moreover, we discuss the challenges and remaining problems for PSII-based systems and the outlook for their development and applications. We believe this topic provides inspiration for rational designs to develop biomimetic PSII-based semi-natural devices and further reveal the secrets of energy conversion within natural photosynthesis from the molecular level.

Multifunctional Textiles Based on Three-Dimensional Hierarchically Structured TiO2 Nanowires

The development of three-dimensional (3D) micro-/nanostructures with multiscale hierarchy offers new potential for the improvement of the pristine textile properties. In this work, a polyester fabric coated with 3D hierarchically structured rutile TiO₂ nanowires (THNWP) was fabricated by a facile hydrothermal strategy. The THNWP samples exhibit markedly improved photocatalytic activities and antibacterial properties owing to their 3D hierarchical architecture constructed by one-dimensional nanowire structures, good crystallinity, excellent light-harvesting capability, and fast electron-transfer rate. Furthermore, the unique 3D hierarchical nanostructures also combine with the monofilament to produce ternary-scale hierarchy, which endows the fabric surface with outstanding superamphiphobicity after further facile fluorination treatment. The supportive air-pockets trapped within the unique ternary-scale architectures are proved to be the crucial factor in the achievement of high liquid repellency, and the highest performing superamphiphobic surface is capable of repelling liquids down to a minimal surface tension of 23.4 mN m^-1. We envision that our findings may possess great potential in the bottom-up design of high-performance textiles.

Symbiotic integration of bioprocesses to design a self-sustainable life supporting ecosystem in a circular economy framework

Climate change, resource depletion and unsustainable crop productivity are major challenges that mankind is currently facing. Natural ecosystems of earth's biosphere are becoming vulnerable and there is a need to design Bioregenerative Life Support Systems (BLSS) which are ecologically engineered microcosms that could effectively deal with problems associated with urbanization and industrialization in a sustainable manner. The principles of BLSS could be integrated with waste fed biorefineries and solar energy to create a self-sustainable bioregenerative ecosystem (SSBE). Such engineered ecosystems will have potential to fulfil urban life essentials and climate change mitigation thus generating ecologically smart and resilient communities which can strengthen the global economy. This article provides a detailed overview on SSBE framework and its improvement in the contemporary era to achieve circular bioeconomy by means of effective resource recycling.

Scalable Three-Dimensional Photobioelectrodes Made of Reduced Graphene Oxide Combined with Photosystem I

Photobioelectrodes represent one of the examples where artificial materials are combined with biological entities to undertake semi-artificial photosynthesis. Here, an approach is described that uses reduced graphene oxide (rGO) as an electrode material. This classical 2D material is used to construct a three-dimensional structure by a template-based approach combined with a simple spin-coating process during preparation. Inspired by this novel material and photosystem I (PSI), a biophotovoltaic electrode is being designed and investigated. Both direct electron transfer to PSI and mediated electron transfer via cytochrome c from horse heart as redox protein can be confirmed. Electrode preparation and protein immobilization have been optimized. The performance can be upscaled by adjusting the thickness of the 3D electrode using different numbers of spin-coating steps during preparation. Thus, photocurrents up to ∼14 μA/cm² are measured for 12 spin-coated layers of rGO corresponding to a turnover frequency of 30 e^- PSI^-1 s^-1 and external quantum efficiency (EQE) of 0.07% at a thickness of about 15 μm. Operational stability has been analyzed for several days. Particularly, the performance at low illumination intensities is very promising (1.39 μA/cm² at 0.1 mW/cm² and -0.15 V vs Ag/AgCl; EQE 6.8%).

Understanding active sites in molecular (photo)electrocatalysis through complementary vibrational spectroelectrochemistry

Highlighting vibrational spectroelectrochemistry for the investigation of synthetic molecular (photo) electrocatalysts for key energy conversion reactions.

Electrosprayed Thylakoid-Alginate Film on a Micro-Pillar Electrode for Scalable Photosynthetic Energy Harvesting

Direct harvesting of electricity from photosynthesis is highly desired as an eco-friendly and sustainable energy harvesting technology. Photosynthetic apparatuses isolated from plants, such as thylakoid membranes (TMs), are deposited on an electrode by which photosynthetic electrons (PEs) are collected from water splitting. To enhance PE collection efficiency, it is critical to increase the electrochemical interfaces between TMs and the electrode. Considering the size of TMs to be around a few hundred nanometer, we hypothesize that an array of micropillar-shaped (MP) electrode can maximize the TM/electrode interface area. Thus, we developed MP electrodes with different heights and investigated the electrospraying of TM-alginate mixtures to fill the gaps between MPs uniformly and conformally. The uniformity of the TM-alginate film and the interaction between the TM and the MP electrode were evaluated to understand how the MP heights and film quality influenced the magnitude of the PE currents. PE currents increased up to 2.4 times for an MP electrode with an A/R of 1.8 compared to a flat electrode, indicating increased direct contact interface between TMs and the electrode. Furthermore, to demonstrate the scalability of this approach, an array of replicated SU-8 MP electrodes was prepared and PE currents of up to 3.2 μA were monitored without a mediator under 68 mW/cm². Finally, the PE current harvesting was sustained for 14 days without decay, demonstrating the long-term stability of the TM-alginate biophotoanodes.

Photovoltage generation by photosystem II core complexes immobilized onto a Millipore filter on an indium tin oxide electrode

The light-induced functioning of photosynthetic pigment-protein complex of photosystem II (PSII) is linked to the vectorial translocation of charges across the membrane, which results in the formation of voltage. Direct measurement of the light-induced voltage (∆V) generated by spinach oxygen-evolving PSII core complexes adsorbed onto a Millipore membrane filter (MF) on an indium tin oxide (ITO) electrode under continuous illumination has been performed. PSII was shown to participate in electron transfer from water to the ITO electrode, resulting in ∆V generation. No photovoltage was detected in PSII deprived of the water-oxidizing complex. The maximal and stable photoelectric signal was observed in the presence of disaccharide trehalose and 2,6-dichloro-1,4-benzoquinone, acting as a redox mediator between the primary quinone acceptor Q_A of PSII and electrode surface. Long time preservation of the steady-state photoactivity at room temperature in a simple in design ITO|PSII-MF|ITO system may be related to the retention of water molecules attached to the PSII surface in the presence of trehalose.

His/Met heme ligation in the PioA outer membrane cytochrome enabling light-driven extracellular electron transfer by Rhodopseudomonas palustris TIE-1

A growing number of bacterial species are known to move electrons across their cell envelopes. Naturally this occurs in support of energy conservation and carbon-fixation. For biotechnology it allows electron exchange between bacteria and electrodes in microbial fuel cells and during microbial electrosynthesis. In this context Rhodopseudomonas palustris TIE-1 is of much interest. These bacteria respond to light by taking electrons from their external environment, including electrodes, to drive CO₂-fixation. The PioA cytochrome, that spans the bacterial outer membrane, is essential for this electron transfer and yet little is known about its structure and electron transfer properties. Here we reveal the ten c-type hemes of PioA are redox active across the window +250 to -400 mV versus Standard Hydrogen Electrode and that the hemes with most positive reduction potentials have His/Met and His/H₂O ligation. These chemical and redox properties distinguish PioA from the more widely studied family of MtrA outer membrane decaheme cytochromes with ten His/His ligated hemes. We predict a structure for PioA in which the hemes form a chain spanning the longest dimension of the protein, from Heme 1 to Heme 10. Hemes 2, 3 and 7 are identified as those most likely to have His/Met and/or His/H₂O ligation. Sequence analysis suggests His/Met ligation of Heme 2 and/or 7 is a defining feature of decaheme PioA homologs from over 30 different bacterial genera. His/Met ligation of Heme 3 appears to be less common and primarily associated with PioA homologs from purple non-sulphur bacteria belonging to the alphaproteobacteria class.

Disparity of Cytochrome Utilization in Anodic and Cathodic Extracellular Electron Transfer Pathways of Geobacter sulfurreducens Biofilms

Extracellular electron transfer (EET) in microorganisms is prevalent in nature and has been utilized in functional bioelectrochemical systems. EET of Geobacter sulfurreducens has been extensively studied and has been revealed to be facilitated through c-type cytochromes, which mediate charge between the electrode and G. sulfurreducens in anodic mode. However, the EET pathway of cathodic conversion of fumarate to succinate is still under debate. Here, we apply a variety of analytical methods, including electrochemistry, UV-vis absorption and resonance Raman spectroscopy, quartz crystal microbalance with dissipation, and electron microscopy, to understand the involvement of cytochromes and other possible electron-mediating species in the switching between anodic and cathodic reaction modes. By switching the applied bias for a G. sulfurreducens biofilm coupled to investigating the quantity and function of cytochromes, as well as the emergence of Fe-containing particles on the cell membrane, we provide evidence of a diminished role of cytochromes in cathodic EET. This work sheds light on the mechanisms of G. sulfurreducens biofilm growth and suggests the possible existence of a nonheme, iron-involving EET process in cathodic mode.

A three-dimensional hybrid electrode with electroactive microbes for efficient electrogenesis and chemical synthesis

Addressing the global challenge of sustainability calls for cost-effective and eco-friendly pathways to go beyond the existing energy-intense synthetic routes. Biohybrid electrochemical systems integrate electroactive bacteria with synthetic electrodes to leverage the power of biocatalysis for energy conversion and chemical synthesis. This work presents a three-dimensional electrode scaffold to couple the intracellular metabolism with extracellular redox transformations by means of electrochemistry. The large population of bacteria actively metabolizing within the electrode scaffold produces a benchmark current density. The biohybrid electrode can also carry out synthetic reactions within or beyond biochemical pathways driven by solar light. This hierarchical electrode provides a robust and versatile platform to wire bacteria’s intrinsic physiological functionalities with artificial electronics for sustainable energy conversion and chemical production.

Integration of electroactive bacteria into electrodes combines strengths of intracellular biochemistry with electrochemistry for energy conversion and chemical synthesis. However, such biohybrid systems are often plagued with suboptimal electrodes, which limits the incorporation and productivity of the bacterial colony. Here, we show that an inverse opal-indium tin oxide electrode hosts a large population of current-producing Geobacter and attains a current density of 3 mA cm⁻² stemming from bacterial respiration. Differential gene expression analysis revealed Geobacter’s transcriptional regulations to express more electron-relaying proteins when interfaced with electrodes. The electrode also allows coculturing with Shewanella for syntrophic electrogenesis, which grants the system additional flexibility in converting electron donors. The biohybrid electrode containing Geobacter can also catalyze the reduction of soluble fumarate and heterogenous graphene oxide, with electrons from an external power source or an irradiated photoanode. This biohybrid electrode represents a platform to employ live cells for sustainable power generation and biosynthesis.

Coupling biology to synthetic nanomaterials for semi-artificial photosynthesis

Biohybrid artificial photosynthesis aims to combine the advantages of biological specificity with a range of synthetic nanomaterials to create innovative semi-synthetic systems for solar-to-chemical conversion. Biological systems utilize highly efficient molecular catalysts for reduction-oxidation reactions. They can operate with minimal overpotentials while selectively channeling reductant energy into specific transformation chemistries and product forming pathways. Nanomaterials can be synthesized to have efficient light-absorption capacity and tuneability of charge separation by manipulation of surface chemistries and bulk compositions. These complementary aspects have been combined in a variety of ways, for example, where biological light-harvesting complexes function as antenna for nanoparticle catalysts or where nanoparticles function as light capture, charge separation components for coupling to chemical conversion by redox enzymes and whole cells. The synthetic diversity that is possible with biohybrids is still being explored. The progress arising from creative approaches is generating new model systems to inspire scale-up technologies and generate understanding of the fundamental mechanisms that control energy conversion at the molecular scale. These efforts are leading to discoveries of essential design principles that can enable the development of scalable artificial photosynthesis systems.

Semi-biological approaches to solar-to-chemical conversion

This review provides an overview of the cross-disciplinary field of semi-artificial photosynthesis, which combines strengths of biocatalysis and artificial photosynthesis to develop new concepts and approaches for solar-to-chemical conversion.

Photolyase-Like Catalytic Behavior of CeO2

Nanomaterials with intrinsic enzyme-like characteristics exhibit their great potentials as alternatives to natural enzymes. Among various enzymes, the finding of substitutes of DNA photolyases, a family of photoenzymes for repairing the ultraviolet (UV)-induced DNA damage by forming cyclobutane pyrimidine dimers (CPDs) between two adjacent thymines in a DNA strand, is still unsuccessful. CPDs raise significant health concerns in various skin diseases. Essentially, DNA photolyases selectively split dimers into monomers by photoelectrons under visible-light irradiation, and this is a photocatalytic process. However, the majority of semiconductors are unprosperous due to the accompanied photogenerated reactive oxygen species (ROS), which decompose CPDs into fragments and thereby lead to a nonselective photocatalysis. Fortunately, CeO₂ as a semiconductor might deliver the selectively photocatalytic repair of UV-induced DNA damages, where the photoelectrons are used for the CPD cleavage, and the photogenerated ROS are locally suppressed for its antioxidant nature. Herein, we reported the defective porous CeO₂ delivered the photolyase-like activity by enhancing visible-light absorption, enabling the effective interaction between CPDs and catalysts, and subsequently triggering the selective photocleavage of CPDs into monomers. Further, in vitro cellular and in vivo animal evaluations illustrated its high potentials as alternatives to DNA photolyases.

Inverse Opal CuCrO2 Photocathodes for H2 Production Using Organic Dyes and a Molecular Ni Catalyst

Dye-sensitized photoelectrochemical (DSPEC) cells are an emerging approach to producing solar fuels. The recent development of delafossite CuCrO2 as a p-type semiconductor has enabled H2 generation through the coassembly of catalyst and dye components. Here, we present a CuCrO2 electrode based on a high-surface-area inverse opal (IO) architecture with benchmark performance in DSPEC H2 generation. Coimmobilization of a phosphonated diketopyrrolopyrrole (DPP-P) or perylene monoimide (PMI-P) dye with a phosphonated molecular Ni catalyst (NiP) demonstrates the ability of IO-CuCrO2 to photogenerate H2. A positive photocurrent onset potential of approximately +0.8 V vs RHE was achieved with these photocathodes. The DPP-P-based photoelectrodes delivered photocurrents of -18 μA cm-2 and generated 160 ± 24 nmol of H2 cm-2, whereas the PMI-P-based photocathodes displayed higher photocurrents of -25 μA cm-2 and produced 215 ± 10 nmol of H2 cm-2 at 0.0 V vs RHE over the course of 2 h under visible light illumination (100 mW cm-2, AM 1.5G, λ > 420 nm, 25 °C). The high performance of the PMI-constructed system is attributed to the well-suited molecular structure and photophysical properties for p-type sensitization. These precious-metal-free photocathodes highlight the benefits of using bespoke IO-CuCrO2 electrodes as well as the important role of the molecular dye structure in DSPEC fuel synthesis.

A precursor-approach in constructing 3D ITO electrodes for the improved performance of photosystem I-cyt c photobioelectrodes

In recent years the use of photoelectrodes based on conductive metal oxides has become very popular in the field of photovoltaics. The application of 3D electrodes holds great promise since they can integrate large amounts of photoactive proteins. In this study photosystem I (PSI) from the thermophilic cyanobacterium Thermosynechococcus elongatus was immobilized on 3D ITO electrodes and electrically wired via the redox protein cytochrome c (cyt c). The main goal, however, was the investigation of construction parameters of such electrodes for achieving a high performance. For this, ITO electrodes were constructed from liquid precursors resulting in improved transmission compared to previous nanoparticle-based preparation protocols. First, the doping level of Sn was varied for establishing suitable conditions for a fast cyt c electrochemistry on such 3D electrodes. In a second step the pore diameter was varied in order to elucidate optimal conditions. Third, the scalability of the template-based preparation was studied from 3 to 15 layers during spin coating and the subsequent baking step. In the thickness range from 3 to 17 μm no limitation in the protein immobilization and also in the photocurrent generation was found. Consequently, a photocurrent of about 270 μA cm-2 and a turnover number (Te) of 30 e- s-1 at PSI were achieved. Because of the high current flow the withdrawal of electrons at the stromal side of PSI becomes clearly rate limiting. Here improved transport conditions and alternative electron acceptors were studied to overcome this limitation.

Protein Cross-Interactions for Efficient Photosynthesis in the Cassava Cultivar SC205 Relative to Its Wild Species

The underlying mechanisms of the higher photosynthetic efficiency of cultivated cassava relative to its wild species are poorly understood. In the present study, proteins in leaves and chloroplasts were analyzed to compare the differences among the cultivar SC205, its wild ancestor W14, and the related species Glaziovii. The functions of differential proteins are associated with 10 ontology groups including photosynthesis, carbohydrate and energy metabolism, as well as potential signal pathway. The protein-protein networks among 41 differential proteins showed that PGK1 is a hub protein and protein cross-interactions affected the differentiation of photosynthetic rate. Anatomy patterns and PEPC detection suggested that SC205 has more C₄ photosynthesis characteristics than Glaziovii and W14. Finally, a mechanism model of the efficient photosynthesis was proposed based on the remarkable variations in photosynthetic parameters and protein functions in the domestic cultivars.

Advancing Techniques for Investigating the Enzyme-Electrode Interface

Enzymes are the essential catalytic components of biology and adsorbing redox-active enzymes on electrode surfaces enables the direct probing of their function. Through standard electrochemical measurements, catalytic activity, reversibility and stability, potentials of redox-active cofactors, and interfacial electron transfer rates can be readily measured. Mechanistic investigations on the high electrocatalytic rates and selectivity of enzymes may yield inspiration for the design of synthetic molecular and heterogeneous electrocatalysts. Electrochemical investigations of enzymes also aid in our understanding of their activity within their biological environment and why they evolved in their present structure and function. However, the conventional array of electrochemical techniques (e.g., voltammetry and chronoamperometry) alone offers a limited picture of the enzyme–electrode interface.

How many enzymes are loaded onto an electrode? In which orientation(s) are they bound? What fraction is active, and are single or multilayers formed? Does this static picture change over time, applied voltage, or chemical environment? How does charge transfer through various intraprotein cofactors contribute to the overall performance and catalytic bias? What is the distribution of individual enzyme activities within an ensemble of active protein films? These are central questions for the understanding of the enzyme–electrode interface, and a multidisciplinary approach is required to deliver insightful answers.

Complementing standard electrochemical experiments with an orthogonal set of techniques has recently allowed to provide a more complete picture of enzyme–electrode systems. Within this framework, we first discuss a brief history of achievements and challenges in enzyme electrochemistry. We subsequently describe how the aforementioned challenges can be overcome by applying advanced electrochemical techniques, quartz-crystal microbalance measurements, and spectroscopic, namely, resonance Raman and infrared, analysis. For example, rotating ring disk electrochemistry permits the simultaneous determination of reaction kinetics and quantification of generated products. In addition, recording changes in frequency and dissipation in a quartz crystal microbalance allows to shed light into enzyme loading, relative orientation, clustering, and denaturation at the electrode surface. Resonance Raman spectroscopy yields information on ligation and redox state of enzyme cofactors, whereas infrared spectroscopy provides insights into active site states and the protein secondary and tertiary structure. The development of these emerging methods for the analysis of the enzyme–electrode interface is the primary focus of this Account. We also take a critical look at the remaining gaps in our understanding and challenges lying ahead toward attaining a complete mechanistic picture of the enzyme–electrode interface.

Protein film electrochemical EPR spectroscopy as a technique to investigate redox reactions in biomolecules

Direct potential control of protein redox centres for both electrochemical and EPR characterisation.