Characterization of photochemical processes for H2 production by CdS nanorod-[FeFe] hydrogenase complexes

Photosynthetic biohybrid systems for solar fuels catalysis

Photosynthetic reaction center (RC) proteins are finely tuned molecular systems optimized for solar energy conversion. RCs effectively capture and convert sunlight with near unity quantum efficiency utilizing light-induced directional electron transfer through a series of molecular cofactors embedded within the protein core to generate a long-lived charge separated state with a useable electrochemical potential. Of current interest are new strategies that couple RC chemistry to the direct synthesis of energy-rich compounds. This Feature Article highlights recent work from our lab on RC and RC-inspired hybrid systems that capture the Sun's energy and convert it to chemical energy in the form of H2, a carbon-neutral energy source derived from water. Biohybrids made from the Photosystem I (PSI) RC are among the best photocatalytic H2-producing protein hybrids to date. Targeted self-assembly strategies that couple abiotic catalysts to PSI translate to catalyst incorporation at intrinsic PSI sites within thylakoid membranes to achieve complete solar water-splitting systems. RC-inspired biohybrids interface synthetic photosensitizers and molecular catalysts with small proteins to create photocatalytic systems and enable the spectroscopic discernment of the structural features and electron transfer processes that underpin solar-driven proton reduction. In total, these studies showcase the incredible scientific opportunities photosynthetic biohybrid research provides for harnessing the optimal qualities of both artificial and natural photosynthetic systems and developing materials that capture, convert, and store solar energy as a fuel.

The missing pieces in the catalytic cycle of [FeFe] hydrogenases

Hydrogen could provide a suitable means for storing energy from intermittent renewable sources for later use on demand. However, many challenges remain regarding the activity, specificity, stability and sustainability of current hydrogen production and consumption methods. The lack of efficient catalysts based on abundant and sustainable elements lies at the heart of this problem. Nature's solution led to the evolution of hydrogenase enzymes capable of reversible hydrogen conversion at high rates using iron- and nickel-based active sites. Through a detailed understanding of these enzymes, we can learn how to mimic them to engineer a new generation of highly active synthetic catalysts. Incredible progress has been made in our understanding of biological hydrogen activation over the last few years. In particular, detailed studies of the [FeFe] hydrogenase class have provided substantial insight into a sophisticated, optimised, molecular catalyst, the active site H-cluster. In this short perspective, we will summarise recent findings and highlight the missing pieces needed to complete the puzzle.

Microscale Thermophoresis (MST) as a Tool to Study Binding Interactions of Oxygen-Sensitive Biohybrids

Microscale thermophoresis (MST) is a technique used to measure the strength of molecular interactions. MST is a thermophoretic-based technique that monitors the change in fluorescence associated with the movement of fluorescent-labeled molecules in response to a temperature gradient triggered by an IR LASER. MST has advantages over other approaches for examining molecular interactions, such as isothermal titration calorimetry, nuclear magnetic resonance, biolayer interferometry, and surface plasmon resonance, requiring a small sample size that does not need to be immobilized and a high-sensitivity fluorescence detection. In addition, since the approach involves the loading of samples into capillaries that can be easily sealed, it can be adapted to analyze oxygen-sensitive samples. In this Bio-protocol, we describe the troubleshooting and optimization we have done to enable the use of MST to examine protein-protein interactions, protein-ligand interactions, and protein-nanocrystal interactions. The salient elements in the developed procedures include 1) loading and sealing capabilities in an anaerobic chamber for analysis using a NanoTemper MST located on the benchtop in air, 2) identification of the optimal reducing agents compatible with data acquisition with effective protection against trace oxygen, and 3) the optimization of data acquisition and analysis procedures. The procedures lay the groundwork to define the determinants of molecular interactions in these technically demanding systems. Key features • Established procedures for loading and sealing tubes in an anaerobic chamber for subsequent analysis. • Sodium dithionite (NaDT) could easily be substituted with one electron-reduced 1,1'-bis(3-sulfonatopropyl)-4,4'-bipyridinium [(SPr)2V•] to perform sensitive biophysical assays on oxygen-sensitive proteins like the MoFe protein. • Established MST as an experimental tool to quantify binding affinities in novel enzyme-quantum dot biohybrid complexes that are extremely oxygen-sensitive.

Electron Donor-Specific Surface Interactions Promote the Photocatalytic Activity of Metal-Semiconductor Nanohybrids

In the past two decades, the application of colloidal semiconductor-metal nanoparticles (NPs) as photocatalysts for the hydrogen generation from water has been extensively studied. The present body of literature studies agrees that the photocatalytic yield strongly depends on the electron donating agent (EDA) added for scavenging the photogenerated holes. The highest reported hydrogen production rates are obtained in the presence of ionic EDAs and at high pH. The large hydrogen production rates are attributed to fast hole transfer from the NP onto the EDAs. However, the present discussions do not treat the influence of EDA-specific surface interactions. This systematic study focuses on that aspect by combining steady-state hydrogen production measurements with time-resolved and static optical spectroscopy, employing 11-mercaptoundecanoic acid-capped, Pt-tipped CdSe/CdS dot-in-rods in the presence of a large set of EDAs. Based on the experimental results, two distinct EDA groups are identified: surface-active and diffusion-limited EDAs. The largest photocatalytic efficiencies are obtained in the presence of surface-active EDAs that induce an agglomeration of the NPs. This demonstrates that the introduction of surface-active EDAs can significantly enhance the photocatalytic activity of the NPs, despite reducing their colloidal stability and inducing the formation of NP networks.

Engineering of bespoke photosensitiser-microbe interfaces for enhanced semi-artificial photosynthesis

Biohybrid systems for solar fuel production integrate artificial light-harvesting materials with biological catalysts such as microbes. In this perspective, we discuss the rational design of the abiotic-biotic interface in biohybrid systems by reviewing microbes and synthetic light-harvesting materials, as well as presenting various approaches to coupling these two components together. To maximise performance and scalability of such semi-artificial systems, we emphasise that the interfacial design requires consideration of two important aspects: attachment and electron transfer. It is our perspective that rational design of this photosensitiser-microbe interface is required for scalable solar fuel production. The design and assembly of a biohybrid with a well-defined electron transfer pathway allows mechanistic characterisation and optimisation for maximum efficiency. Introduction of additional catalysts to the system can close the redox cycle, omitting the need for sacrificial electron donors. Studies that electronically couple light-harvesters to well-defined biological entities, such as emerging photosensitiser-enzyme hybrids, provide valuable knowledge for the strategic design of whole-cell biohybrids. Exploring the interactions between light-harvesters and redox proteins can guide coupling strategies when translated into larger, more complex microbial systems.

Liquid-Liquid Phase Separation-Mediated Photocatalytic Subcellular Hybrid System for Highly Efficient Hydrogen Production

Plant chloroplasts have a highly compartmentalized interior, essential for executing photocatalytic functions. However, the construction of a photocatalytic reaction compartment similar to chloroplasts in inorganic-biological hybrid systems (IBS) has not been reported. Drawing inspiration from the compartmentalized chloroplast and the phenomenon of liquid-liquid phase separation, herein, a new strategy is first developed for constructing a photocatalytic subcellular hybrid system through liquid-liquid phase separation technology in living cells. Photosensitizers and in vivo expressed hydrogenases are designed to coassemble within the cell to create subcellular compartments for synergetic photocatalysis. This compartmentalization facilitates efficient electron transfer and light energy utilization, resulting in highly effective H2 production. The subcellular compartments hybrid system (HM/IBSCS) exhibits a nearly 87-fold increase in H2 production compared to the bare bacteria/hybrid system. Furthermore, the intracellular compartments of the photocatalytic reactor enhance the system's stability obviously, with the bacteria maintaining approximately 81% of their H2 production activity even after undergoing five cycles of photocatalytic hydrogen production. The research brings forward visionary prospects for the field of semi-artificial photosynthesis, offering new possibilities for advancements in areas such as renewable energy, biomanufacturing, and genetic engineering.

Concurrent Ammonia Synthesis and Alcohol Oxidation Boosted by Glutathione-Capped Quantum Dots under Visible Light

Mother nature accomplishes efficient ammonia synthesis via cascade N2 oxidation by lightning strikes followed with enzyme-catalyzed nitrogen oxyanion (NOx -, x = 2,3) reduction. The protein environment of enzymatic centers for NOx --to-NH4 + process greatly inspires the design of glutathione-capped (GSH) quantum dots (QDs) for ammonia synthesis under visible light (440 nm) in tandem with plasma-enabled N2 oxidation. Mechanistic studies reveal that GSH induces positive shift of surface charge to strengthen the interaction between NOx - and QDs. Upon visible light irradiation of QDs, the balanced and rapid hole and electron transfer furnish GS·radicals for 2e-/2H+ alcohol oxidation and H·for 8e-/10H+ NO3 --to-NH4 + reduction simultaneously. For the first time, mmol-scale ammonia synthesis is realized with apparent quantum yields of 5.45% ± 0.64%, and gram-scale synthesis of value-added acetophenone and NH4Cl proceeds with 1:4 stoichiometry and stability, demonstrating promising multielectron and multiproton ammonia synthesis efficiency and sustainability with nature-inspired artificial photocatalysts.

Charge Transfer from Quantum-Confined 0D, 1D, and 2D Nanocrystals

The properties of colloidal quantum-confined semiconductor nanocrystals (NCs), including zero-dimensional (0D) quantum dots, 1D nanorods, 2D nanoplatelets, and their heterostructures, can be tuned through their size, dimensionality, and material composition. In their photovoltaic and photocatalytic applications, a key step is to generate spatially separated and long-lived electrons and holes by interfacial charge transfer. These charge transfer properties have been extensively studied recently, which is the subject of this Review. The Review starts with a summary of the electronic structure and optical properties of 0D-2D nanocrystals, followed by the advances in wave function engineering, a novel way to control the spatial distribution of electrons and holes, through their size, dimension, and composition. It discusses the dependence of NC charge transfer on various parameters and the development of the Auger-assisted charge transfer model. Recent advances in understanding multiple exciton generation, decay, and dissociation are also discussed, with an emphasis on multiple carrier transfer. Finally, the applications of nanocrystal-based systems for photocatalysis are reviewed, focusing on the photodriven charge separation and recombination processes that dictate the function and performance of these materials. The Review ends with a summary and outlook of key remaining challenges and promising future directions in the field.

Cascade reactions with two non-physiological partners for NAD(P)H regeneration via renewable hydrogen

An attractive application of hydrogenases, combined with the availability of cheap and renewable hydrogen (i.e., from solar and wind powered electrolysis or from recycled wastes), is the production of high-value electron-rich intermediates such as reduced nicotinamide adenine dinucleotides. Here, the capability of a very robust and oxygen-resilient [FeFe]-hydrogenase (CbA5H) from Clostridium beijerinckii SM10, previously identified in our group, combined with a reductase (BMR) from Bacillus megaterium (now reclassified as Priestia megaterium) was tested. The system shows a good stability and it was demonstrated to reach up to 28 ± 2 nmol NADPH regenerated s-1 mg of hydrogenase-1 (i.e., 1.68 ± 0.12 U mg-1, TOF: 126 ± 9 min-1) and 0.46 ± 0.04 nmol NADH regenerated s-1 mg of hydrogenase-1 (i.e., 0.028 ± 0.002 U mg-1, TOF: 2.1 ± 0.2 min-1), meaning up to 74 mg of NADPH and 1.23 mg of NADH produced per hour by a system involving 1 mg of CbA5H. The TOF is comparable with similar systems based on hydrogen as regenerating molecule for NADPH, but the system is first of its kind as for the [FeFe]-hydrogenase and the non-physiological partners used. As a proof of concept a cascade reaction involving CbA5H, BMR and a mutant BVMO from Acinetobacter radioresistens able to oxidize indole is presented. The data show how the cascade can be exploited for indigo production and multiple reaction cycles can be sustained using the regenerated NADPH.

Hole-scavenging in photo-driven N2 reduction catalyzed by a CdS-nitrogenase MoFe protein biohybrid system

The light-driven reduction of dinitrogen (N2) to ammonia (NH3) catalyzed by a cadmium sulfide (CdS) nanocrystal‑nitrogenase MoFe protein biohybrid is dependent on a range of different factors, including an appropriate hole-scavenging sacrificial electron donor (SED). Here, the impact of different SEDs on the overall rate of N2 reduction catalyzed by a CdS quantum dot (QD)-MoFe protein system was determined. The selection of SED was guided by several goals: (i) molecules with standard reduction potentials sufficient to reduce the oxidized CdS QD, (ii) molecules that do not absorb the excitation wavelength of the CdS QD, and (iii) molecules that could be readily reduced by sustainable processes. Earlier studies utilized buffer molecules or ascorbic acid as the SED. The effectiveness of ascorbic acid as SED was compared to dithionite (DT), triethanolamine (TEOA), and hydroquinone (HQ) across a range of concentrations in supporting N2 reduction to NH3 in a CdS QD-MoFe protein photocatalytic system. It was found that TEOA supported N2 reduction rates comparable to those observed for dithionite and ascorbic acid. HQ was found to support significantly higher rates of N2 reduction compared to the other SEDs at a concentration of 50 mM. A comparison of the rates of N2 reduction by the biohybrid complex to the standard reduction potential (Eo) of the SEDs reveals that Eo is not the only factor impacting the efficiency of hole-scavenging. These findings reveal the importance of the SED properties for improving the efficiency of hole-scavenging in the light-driven N2 reduction reaction catalyzed by a CdS QD-MoFe protein hybrid.

Photo-Electro-Biochemical H2 Production Using the Carbon Material-Based Cathode Combined with Genetically Engineered Escherichia coli Whole-Cell Biocatalysis

Abio/bio hybrids, which incorporate biocatalysts that promote efficient and selective material conversions under mild conditions into existing catalytic reactions, have attracted considerable attention for developing new catalytic systems. This study constructed a H2 -forming biocathode based on a carbon material combined with whole-cell biocatalysis of genetically-engineered-hydrogenase-overproducing Escherichia coli for the photoelectrochemical water splitting for clean H2 production. Low-cost and abundant carbon materials are generally not suitable for H2 -forming cathode due to their high overpotential for proton reduction; however, the combination of the reduction of an organic electron mediator on the carbon electrode and the H2 formation with the reduced mediator by the redox enzyme hydrogenase provides a H2 -forming cathodic reaction comparable to that of the noble metal electrode. The present study demonstrates that the recombinant E. coli whole cell can be employed as a part of the H2 -forming biocathode system, and the biocathode system wired with TiO2 photoanode can be a photoelectrochemical water-splitting system without external voltage assistance under natural pH. The findings of this study expand the feasibility of applications of whole-cell biocatalysis and contribute to obtaining solar-to-chemical conversions by abio/bio hybrid systems, especially for low-cost, noble-metal-free, and clean H2 production.

High Affinity Electrostatic Interactions Support the Formation of CdS Quantum Dot:Nitrogenase MoFe Protein Complexes

Nitrogenase MoFe protein can be coupled with CdS nanocrystals (NCs) to enable photocatalytic N2 reduction. The nature of interactions that support complex formation is of paramount importance in intermolecular electron transfer that supports catalysis. In this work we have employed microscale thermophoresis to examine binding interactions between 3-mercaptopropionate capped CdS quantum dots (QDs) and MoFe protein over a range of QD diameters (3.4-4.3 nm). The results indicate that the interactions are largely electrostatic, with the strength of interactions similar to that observed for the physiological electron donor. In addition, the strength of interactions is sensitive to the QD diameter, and the binding interactions are significantly stronger for QDs with smaller diameters. The ability to quantitatively assess NC protein interactions in biohybrid systems supports strategies for understanding properties and reaction parameters that are important for obtaining optimal rates of catalysis in biohybrid systems.

Utilizing Atmospheric Carbon Dioxide and Sunlight in Graphene Quantum Dot-Based Nano-Biohybrid Organisms for Making Carbon-Negative and Carbon-Neutral Products

Increasing emissions of greenhouse gases compounded with legacy emissions in the earth's atmosphere poses an existential threat to human survival. One potential solution is creating carbon-negative and carbon-neutral materials, specifically for commodities used heavily throughout the globe, using a low-cost, scalable, and technologically and economically feasible process that can be deployed without the need for extensive infrastructure or skill requirements. Here, we demonstrate that nickel-functionalized graphene quantum dots (GQDs) can effectively couple to nonphotosynthetic bacteria at a cellular, molecular, and optoelectronic level, creating nanobiohybrid organisms (nanorgs) that enable the utilization of sunlight to convert carbon dioxide, air, and water into high-value-added chemicals such as ammonia (NH3), ethylene (C2H4), isopropanol (IPA), 2,3-butanediol (BDO), C11-C15 methyl ketones (MKs), and degradable bioplastics poly hydroxybutyrate (PHB) with high efficiency and selectivity. We demonstrate a high turnover number (TON) of up to 108 (mol of product per mol of cells), ease of application, facile scalability (demonstrated using a 30 L tank in a lab), and sustainable generation of carbon nanomaterials from recovered bacteria for creating nanorgs without the use of any toxic chemicals or materials. These findings can have important implications for the further development of sustainable processes for making carbon-negative materials using nanorgs.

Extracellular self-photosensitizer combined with metal oxide-based nano bio-hybrid system encapsulated by alginate improves hydrogen production in the presence of oxygen

The photocatalytic nano-biohybrid systems have great potential for the conversion of solar energy to fermentative hydrogen production. Herein, a whole-cell nano-biohybrid system consisting of biosynthesized cadmium sulfide, Enterobacter aerogenes cells, and metal oxide nanoparticles was constructed. The system was encapsulated with sodium alginate and used for light-driven biohydrogen production under anaerobic and in the presence of oxygen conditions. After 48 h incubation in the presence of oxygen, the E. aerogenes cells with the encapsulated hybrid system yielded 2.7 mmol H2/mmol glucose, a 13.5-fold higher than that of the E. aerogenes cells without encapsulation. The encapsulated hybrid system could produce hydrogen for up to 96 h and could produce hydrogen even under natural sunlight conditions. These results revealed that efficient hydrogen production is possible in the presence of oxygen. Overall, the present study demonstrated the potential of using proper nano-biohybrid system with encapsulation for the production of hydrogen under ambient air condition.

Efficient and Selective Photogeneration of Stable N-Centered Radicals via Controllable Charge Carrier Imbalance in Cesium Lead Halide Nanocrystals

Despite the versatility of semiconductor nanocrystals (NCs) in photoinduced chemical processes, the generation of stable radicals has been more challenging due to reverse charge transfer or charge recombination even in the presence of sacrificial charge acceptors. Here, we show that cesium lead halide (CsPbX3) NCs can selectively photogenerate either aminium or aminyl radicals from amines, taking advantage of the controllable imbalance of the electron and hole populations achieved by varying the solvent composition. Using dihalomethane as the solvent, irreversible removal of the electrons from CsPbX3 NCs enabled by the photoinduced halide exchange between the NCs and the dihalomethane resulted in efficient oxidative generation of the aminium radical. In the absence of dihalomethane in solvent, the availability of both electrons and holes resulted in the production of an aminyl radical via sequential hole transfer and reductive N-H bond dissociation. The negative charge of the halide ions on the NC's lattice surface appears to facilitate the aminyl radical production, competing favorably with the reversible charge transfer reverting to the reactant.

Augmenting the Performance of Hydrogenase for Aerobic Photocatalytic Hydrogen Evolution via Solvent Tuning

This work showcases the performance of [NiFeSe] hydrogenase from Desulfomicrobium baculatum for solar-driven hydrogen generation in a variety of organic-based deep eutectic solvents. Despite its well-known sensitivity towards air and organic solvents, the hydrogenase shows remarkable performance under an aerobic atmosphere in these solvents when paired with a TiO2 photocatalyst. Tuning the water content further increases hydrogen evolution activity to a TOF of 60±3 s-1 and quantum yield to 2.3±0.4 % under aerobic conditions, compared to a TOF of 4 s-1 in a purely aqueous solvent. Contrary to common belief, this work therefore demonstrates that placing natural hydrogenases into non-natural environments can enhance their intrinsic activity beyond their natural performance, paving the way for full water splitting using hydrogenases.

Photobiocatalytic Strategies for Organic Synthesis

Biocatalysis has revolutionized chemical synthesis, providing sustainable methods for preparing various organic molecules. In enzyme-mediated organic synthesis, most reactions involve molecules operating from their ground states. Over the past 25 years, there has been an increased interest in enzymatic processes that utilize electronically excited states accessed through photoexcitation. These photobiocatalytic processes involve a diverse array of reaction mechanisms that are complementary to one another. This comprehensive review will describe the state-of-the-art strategies in photobiocatalysis for organic synthesis until December 2022. Apart from reviewing the relevant literature, a central goal of this review is to delineate the mechanistic differences between the general strategies employed in the field. We will organize this review based on the relationship between the photochemical step and the enzymatic transformations. The review will include mechanistic studies, substrate scopes, and protein optimization strategies. By clearly defining mechanistically-distinct strategies in photobiocatalytic chemistry, we hope to illuminate future synthetic opportunities in the area.

Algal cell bionics as a step towards photosynthesis-independent hydrogen production

The engineering and modulation of living micro-organisms is a key challenge in green bio-manufacturing for the development of sustainable and carbon-neutral energy technologies. Here, we develop a cellular bionic approach in which living algal cells are interfaced with an ultra-thin shell of a conductive polymer along with a calcium carbonate exoskeleton to produce a discrete cellular micro-niche capable of sustained photosynthetic and photosynthetic-independent hydrogen production. The surface-augmented algal cells induce oxygen depletion, conduct photo-induced extracellular electrons, and provide structural and chemical stability that collectively give rise to localized hypoxic conditions and concomitant hydrogenase activity under daylight in air. We show that assembly of the living cellular micro-niche opens a direct extracellular photoelectron pathway to hydrogenase resulting in photosynthesis-independent hydrogen evolution for 200 d. In addition, surface-conductive dead algal cells continue to produce hydrogen for up to 8 d due to their structural stability and retention of functional hydrogenases. Overall, the integration of artificial biological hydrogen production pathways and natural photosynthesis in surface-augmented algal cells provides a cellular bionic approach to enhanced green hydrogen production under environmentally benign conditions and could pave the way to new opportunities in sustainable energy production.

Photobio-electrocatalytic production of H2 using fluorine-doped tin oxide (FTO) electrodes covered with a NiO-In2S3 p-n junction and NiFeSe hydrogenase

Clean energy vectors are needed towards a fossil fuel-free society, diminishing both greenhouse effect and pollution. Electrochemical water splitting is a clean route to obtain green hydrogen, the cleanest fuel; although efficient electrocatalysts are required to avoid high overpotentials in this process. The combination of inorganic semiconductors with biocatalysts for photoelectrochemical H₂ production is an alternative approach to overcome this challenge. N-type semiconductors can be coupled to a co-catalyst for H₂ production in the presence of a sacrificial electron donor in solution, but the replacement of the latter with an electrode is a challenge. In this work we attach a NiFeSe-hydrogenase with high activity for H₂ production with the n-type semiconductor indium sulfide, which upon visible irradiation is able to transfer its excited electrons to the enzyme. In order to enhance the transfer of the generated holes towards the electrode for their replenishment, we have explored the inclusion of a p-type material, NiO, to induce a p-n junction for H₂ production in a photoelectrochemical biocatalytic system in absence of sacrificial reagents.

Electronic Structure and Excited State Dynamics of Cadmium Chalcogenide Nanorods

The cylindrical quasi-one-dimensional shape of colloidal semiconductor nanorods (NRs) gives them unique electronic structure and optical properties. In addition to the band gap tunability common to nanocrystals, NRs have polarized light absorption and emission and high molar absorptivities. NR-shaped heterostructures feature control of electron and hole locations as well as light emission energy and efficiency. We comprehensively review the electronic structure and optical properties of Cd-chalcogenide NRs and NR heterostructures (e.g., CdSe/CdS dot-in-rods, CdSe/ZnS rod-in-rods), which have been widely investigated over the last two decades due in part to promising optoelectronic applications. We start by describing methods for synthesizing these colloidal NRs. We then detail the electronic structure of single-component and heterostructure NRs and follow with a discussion of light absorption and emission in these materials. Next, we describe the excited state dynamics of these NRs, including carrier cooling, carrier and exciton migration, radiative and nonradiative recombination, multiexciton generation and dynamics, and processes that involve trapped carriers. Finally, we describe charge transfer from photoexcited NRs and connect the dynamics of these processes with light-driven chemistry. We end with an outlook that highlights some of the outstanding questions about the excited state properties of Cd-chalcogenide NRs.

Photodriven Chemical Synthesis by Whole-Cell-Based Biohybrid Systems: From System Construction to Mechanism Study

By simulating natural photosynthesis, the desirable high-value chemical products and clean fuels can be sustainably generated with solar energy. Whole-cell-based photosensitized biohybrid system, which innovatively couples the excellent light-harvesting capacity of semiconductor materials with the efficient catalytic ability of intracellular biocatalysts, is an appealing interdisciplinary creature to realize photodriven chemical synthesis. In this review, we summarize the constructed whole-cell-based biohybrid systems in different application fields, including carbon dioxide fixation, nitrogen fixation, hydrogen production, and other chemical synthesis. Moreover, we elaborate the charge transfer mechanism studies of representative biohybrids, which can help to deepen the current understanding of the synergistic process between photosensitizers and microorganisms, and provide schemes for building novel biohybrids with less electron transfer resistance, advanced productive efficiency, and functional diversity. Further exploration in this field has the prospect of making a breakthrough on the biotic-abiotic interface that will provide opportunities for multidisciplinary research.

Photosynthetic Polymer Dots-Bacteria Biohybrid System Based on Transmembrane Electron Transport for Fixing CO2 into Poly-3-hydroxybutyrate

Organic semiconductor-microbial photosynthetic biohybrid systems show great potential in light-driven biosynthesis. In such a system, an organic semiconductor is used to harvest solar energy and generate electrons, which can be further transported to microorganisms with a wide range of metabolic pathways for final biosynthesis. However, the lack of direct electron transport proteins in existing microorganisms hinders the hybrid system of photosynthesis. In this work, we have designed a photosynthetic biohybrid system based on transmembrane electron transport that can effectively deliver the electrons from organic semiconductor across the cell wall to the microbe. Biocompatible organic semiconductor polymer dots (Pdots) are used as photosensitizers to construct a ternary synergistic biochemical factory in collaboration with Ralstonia eutropha H16 (RH16) and electron shuttle neutral red (NR). Photogenerated electrons from Pdots promote the proportion of nicotinamide adenine dinucleotide phosphate (NADPH) through NR, driving the Calvin cycle of RH16 to convert CO₂ into poly-3-hydroxybutyrate (PHB), with a yield of 21.3 ± 3.78 mg/L, almost 3 times higher than that of original RH16. This work provides a concept of an integrated photoactive biological factory based on organic semiconductor polymer dots/bacteria for valuable chemical production only using solar energy as the energy input.

Shikimic acid biosynthesis in microorganisms: Current status and future direction

Shikimic acid (SA), a hydroaromatic natural product, is used as a chiral precursor for organic synthesis of oseltamivir (Tamiflu®, an antiviral drug). The process of microbial production of SA has recently undergone vigorous development. Particularly, the sustainable construction of recombinant Corynebacterium glutamicum (141.2 g/L) and Escherichia coli (87 g/L) laid a solid foundation for the microbial fermentation production of SA. However, its industrial application is restricted by limitations such as the lack of fermentation tests for industrial-scale and the requirement of growth-limiting factors, antibiotics, and inducers. Therefore, the development of SA biosensors and dynamic molecular switches, as well as genetic modification strategies and optimization of the fermentation process based on omics technology could improve the performance of SA-producing strains. In this review, recent advances in the development of SA-producing strains, including genetic modification strategies, metabolic pathway construction, and biosensor-assisted evolution, are discussed and critically reviewed. Finally, future challenges and perspectives for further reinforcing the development of robust SA-producing strains are predicted, providing theoretical guidance for the industrial production of SA.

Molecular-Modified Photocathodes for Applications in Artificial Photosynthesis and Solar-to-Fuel Technologies

Nature offers inspiration for developing technologies that integrate the capture, conversion, and storage of solar energy. In this review article, we highlight principles of natural photosynthesis and artificial photosynthesis, drawing comparisons between solar energy transduction in biology and emerging solar-to-fuel technologies. Key features of the biological approach include use of earth-abundant elements and molecular interfaces for driving photoinduced charge separation reactions that power chemical transformations at global scales. For the artificial systems described in this review, emphasis is placed on advancements involving hybrid photocathodes that power fuel-forming reactions using molecular catalysts interfaced with visible-light-absorbing semiconductors.

Polymer Dots as Photoactive Membrane Vesicles for [FeFe]-Hydrogenase Self-Assembly and Solar-Driven Hydrogen Evolution

A semiartificial photosynthesis approach that utilizes enzymes for solar fuel production relies on efficient photosensitizers that should match the enzyme activity and enable long-term stability. Polymer dots (Pdots) are biocompatible photosensitizers that are stable at pH 7 and have a readily modifiable surface morphology. Therefore, Pdots can be considered potential photosensitizers to drive such enzyme-based systems for solar fuel formation. This work introduces and unveils in detail the interaction within the biohybrid assembly composed of binary Pdots and the HydA1 [FeFe]-hydrogenase from Chlamydomonas reinhardtii. The direct attachment of hydrogenase on the surface of toroid-shaped Pdots was confirmed by agarose gel electrophoresis, cryogenic transmission electron microscopy (Cryo-TEM), and cryogenic electron tomography (Cryo-ET). Ultrafast transient spectroscopic techniques were used to characterize photoinduced excitation and dissociation into charges within Pdots. The study reveals that implementation of a donor-acceptor architecture for heterojunction Pdots leads to efficient subpicosecond charge separation and thus enhances hydrogen evolution (88 460 μmol_H2·g_H2ase^-1·h^-1). Adsorption of [FeFe]-hydrogenase onto Pdots resulted in a stable biohybrid assembly, where hydrogen production persisted for days, reaching a TON of 37 500 ± 1290 in the presence of a redox mediator. This work represents an example of a homogeneous biohybrid system combining polymer nanoparticles and an enzyme. Detailed spectroscopic studies provide a mechanistic understanding of light harvesting, charge separation, and transport studied, which is essential for building semiartificial photosynthetic systems with efficiencies beyond natural and artificial systems.

Bioelectronic Platform to Investigate Charge Transfer between Photoexcited Quantum Dots and Microbial Outer Membranes

Photosynthetic semiconductor biohybrids (PSBs) convert light energy to chemical energy through photo-driven charge transfer from nanocrystals to microorganisms that perform bioreactions of interest. Initial proof-of-concept PSB studies with an emphasis on enhanced CO₂ conversion have been encouraging; however, bringing the broad prospects of PSBs to fruition is contingent on establishing a firm fundamental understanding of underlying interfacial charge transfer processes. We introduce a bioelectronic platform that reduces the complexity of PSBs by focusing explicitly on interactions between colloidal quantum dots (QDs), microbial outer membranes, and native, small-molecule redox mediators. Our model platform employs a standard three-electrode electrochemical cell with supported outer membranes of Pseudomonas aeruginosa, pyocyanin redox mediators, and semiconducting CdSe QDs dispersed in an aqueous electrolyte. We present a comprehensive electrochemical analysis of this platform via electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and chronoamperometry (CA). EIS reveals the formation and electronic properties of supported outer membrane films. CV reveals the electrochemically active surface area of P. aeruginosa outer membranes and that pyocyanin is the sole species that performs redox with these outer membranes under sweeping applied potential. CA demonstrates that photoexcited charge transfer in this system is driven by the reduction of pyocyanin at the QD surface followed by diffusion of reduced pyocyanin through the outer membrane. The broad applicability of this platform across many bacterial species, QD architectures, and controlled environmental conditions affords the possibility to define design principles for future PSB systems to synergistically integrate concurrent advances in genetically engineered organisms and inorganic nanomaterials.

Self-replicating Biophotoelectrochemistry System for Sustainable CO Methanation

Efficient conversion of CO-rich gas to methane (CH₄) provides an effective energy solution by taking advantage of existing natural gas infrastructures. However, traditional chemical and biological conversions face different challenges. Herein, an innovative biophotoelectrochemistry (BPEC) system using Methanosarcina barkeri-CdS as a biohybrid catalyst was successfully employed for CO methanation. Compared with CO₂-fed BPEC, BPEC-CO significantly extended the CH₄ producing time by 1.7-fold and exhibited a higher CH₄ yield by 9.5-fold under light irradiation. This superior conversion of CO resulted from the fact that CO could serve as an effective quencher of reactive species along with the photoelectron production. In addition, CO was used as a carbon source either directly or indirectly via the produced CO₂ for M. barkeri. Such a process improved the redox activities of membrane-bound proteins for BPEC methanogenesis. These results were consistent with the transcriptomic analyses, in which the genes for the putative CO oxidation and CO₂ reduction pathways in M. barkeri were highly expressed, while the gene expression for reactive oxygen species detoxification remained relatively stable under light irradiation. This study has provided the first proof-of-concept evidence for sustainable CO methanation under a mild condition in the self-replicating BPEC system.

Solar-Driven Producing of Value-Added Chemicals with Organic Semiconductor-Bacteria Biohybrid System

Photosynthetic biohybrid systems exhibit promising performance in biosynthesis; however, these systems can only produce a single metabolite and cannot further transform carbon sources into highly valuable chemical production. Herein, a photosynthetic biohybrid system integrating biological and chemical cascade synthesis was developed for solar-driven conversion of glucose to value-added chemicals. A new ternary cooperative biohybrid system, namely bacterial factory, was constructed by self-assembling of enzyme-modified light-harvesting donor-acceptor conjugated polymer nanoparticles (D-A CPNs) and genetically engineered Escherichia coli (E. coli). The D-A CPNs coating on E. coli could effectively generate electrons under light irradiation, which were transferred into E. coli to promote the 37% increment of threonine production by increasing the ratio of nicotinamide adenine dinucleotide phosphate (NADPH). Subsequently, the metabolized threonine was catalyzed by threonine deaminase covalently linking with D-A CPNs to obtain 2-oxobutyrate, which is an important precursor of drugs and chemicals. The 2-oxobutyrate yield under light irradiation is increased by 58% in comparison to that in dark. This work provides a new organic semiconductor-microorganism photosynthetic biohybrid system for biological and chemical cascade synthesis of highly valuable chemicals by taking advantage of renewable carbon sources and solar energy.

Enzyme-photo-coupled catalysis in gas-sprayed microdroplets

Enzyme-photo-coupled catalysis produces fine chemicals by combining the high selectivity of an enzyme with the green energy input of sunlight. Operating a large-scale system, however, remains challenging because of the...

Tailoring Quantum Dot Sizes for Optimal Photoinduced Catalytic Activation of Nitrogenase

Many efforts have been directed towards elucidating the nitrogenase structure, its biocatalytic activity, and methods to artificially activate it by external stimuli. Here, we investigated how semiconductor nanoparticles (NPs) with sizes ranging between 2.3-3.5 nm form nano-biohybrids with the nitrogenase enzyme and enable its photoinduced biocatalytic activity. We examined two homogenously synthesized quantum dots (QDs), CdS, CdSe, and two nitrogenase variants, the wild-type and a cysteine-mutated. We show that the cysteine-mutated variant does not enhance the hydrogen generation amounts, as compared with the wild type. Nevertheless, we show that the 2.3 nm-sized CdSe NPs facilitate an eightfold increase compared with larger CdSe NPs. The obtained results were investigated using electrochemical techniques, transmission electron microscopy, and further confirmed by time-resolved spectroscopic measurements, which allow us to determine the electron tranfer rate constant (k_ET ) of the different configurations.

Recent Progress and Prospects in Catalytic Water Treatment

Presently, conventional technologies in water treatment are not efficient enough to completely mineralize refractory water contaminants. In this context, the implementation of catalytic processes could be an alternative. Despite the advantages provided in terms of kinetics of transformation, selectivity, and energy saving, numerous attempts have not yet led to implementation at an industrial scale. This review examines investigations at different scales for which controversies and limitations must be solved to bridge the gap between fundamentals and practical developments. Particular attention has been paid to the development of solar-driven catalytic technologies and some other emerging processes, such as microwave assisted catalysis, plasma-catalytic processes, or biocatalytic remediation, taking into account their specific advantages and the drawbacks. Challenges for which a better understanding related to the complexity of the systems and the coexistence of various solid-liquid-gas interfaces have been identified.

Abiotic-biotic hybrid for CO2 biomethanation: From electrochemical to photochemical process

Converting CO₂ into sustainable fuels (e.g., CH₄) has great significance to solve carbon emission and energy crisis. Generally, CO₂ methanation needs abundant of energy input to overcome the eight-electron-transfer barrier. Abiotic-biotic hybrid system represents one of the cutting-edge technologies that use renewable electric/solar energy to realize eight-electron-transfer CO₂ biomethanation. However, the incompatible abiotic-biotic hybrid can result in low efficiency of electron transfer and CO₂ biomethanation. Herein, we present the comprehensive review to highlight how to design abiotic-biotic hybrid for electric/solar-driven CO₂ biomethanation. We primarily introduce the CO₂ biomethanation mechanism, and further summarize state-of-the-art electrochemical and photochemical CO₂ biomethanation in hybrid systems. We also propose excellent synthetic biology strategies, which are useful to design tunable methanogenic microorganisms or enzymes when cooperating with electrode/semiconductor in hybrid systems. This review provides theoretical guidance of abiotic-biotic hybrid and also shows the bright future of sustainable fuel production in the form of CO₂ biomethanation.

Influence of QD photosensitizers in the photocatalytic production of hydrogen with biomimetic [FeFe]-hydrogenase. Comparative performance of CdSe and CdTe

Photocatalytic systems comprising a hydrogenase-type catalyst and CdX (X = S, Se, Te) chalcogenide quantum dot (QD) photosensitizers show extraordinary hydrogen production rates under visible light excitation. What remains unknown is the mechanism of energy conversion in these systems. Here, we have explored this question by comparing the performance of two QD sensitizers, CdSe and CdTe, in photocatalytic systems featuring aqueous suspensions of a [Fe₂ (μ-1,2-benzenedithiolate) CO₆] catalyst and an ascorbic acid sacrificial agent. Overall, the hydrogen production yield for CdSe-sensitized reactions QDs was found to be 13 times greater than that of CdTe counterparts. According to emission quenching experiments, an enhanced performance of CdSe sensitizers reflected a greater rate of electron transfer from the ascorbic acid (k_Asc). The observed difference in the QD-ascorbic acid charge transfer rates between the two QD materials was consistent with respective driving forces for these systems.

Biophotoelectrochemistry for renewable energy and environmental applications

Biophotoelectrochemistry (BPEC) is an interdisciplinary research field and combines bioelectrochemistry and photoelectrochemistry through the utilization of the catalytic abilities of biomachineries and light harvesters to accomplish the production of energy or chemicals driven by solar energy. The BPEC process may act as a new approach for sustainable green chemistry and waste minimization. This review provides the state-of-the-art introduction of BPEC basics and systems, with a focus on light harvesters and biocatalysts, configurations, photoelectron transfer mechanisms, and the potential applications in energy and environment. Several examples of BPEC applications are discussed including H2 production, CO2 reduction, chemical synthesis, pollution control, and biogeochemical cycle of elements. The challenges about BPEC systems are identified and potential solutions are proposed. The review aims to encourage further research of BPEC toward development of practical BPEC systems for energy and environmental applications.

Photocatalyst-enzyme hybrid systems for light-driven biotransformation

Enzymes catalyse target reactions under mild conditions with high efficiency, as well as excellent regional-, stereo-, and enantiomeric selectivity. Photocatalysis utilises sustainable and environment-friendly light power to realise efficient chemical conversion. By combining the interdisciplinary advantages of photo- and enzymatic catalysis, the photocatalyst-enzyme hybrid systems have proceeded various light-driven biotransformation with high efficiency under environmentally benign conditions, thus, attracting unparalleled focus during the last decades. It has also been regarded as a promising pathway towards green chemistry utilising ubiquitous solar energy. This systematic review gives insight into this research field by classifying the existing photocatalyst-enzyme hybrid systems into three sections based on different hybridizing modes between photo- and enzymatic catalysis. Furthermore, existing challenges and proposed strategies are discussed within this context. The first system summarised is the cofactor-mediated hybrid system, in which natural/artificial cofactors act as reducing equivalents that connect photocatalysts with enzymes for light-driven enzymatic biotransformation. Second, the direct contact-based photocatalyst-enzyme hybrid systems are described, including two different kinds of electron exchange sites on the enzyme molecules. Third, some cases where photocatalysts and enzymes are integrated into a reaction cascade with specific intermediates will be discussed in the following chapter. Finally, we provide perspective concerning the future of this field.

Stand-Alone CdS Nanocrystals for Photocatalytic CO2 Reduction with High Efficiency and Selectivity

The development of a cost-effective photocatalyst is highly anticipated to achieve efficient photocatalytic CO₂ reduction with superior selectivity, which is still facing the lack of valid settlements. Herein, 4-mercaptopyridine (PD) as the building block of a capping ligand is tightly decorated on the surface of CdS nanocrystals (CdS-PD) using a facile ligand-exchange strategy, to exploit a cost-effective photocatalyst for photocatalytic CO₂ reduction without any cocatalysts. The conjugated structure of PD can facilitate the delocalization of photogenerated electrons in CdS nanocrystals, bringing forth an improved charge separation efficiency. More importantly, N-protonated PD can enable the easy formation of a six-membered ring intermediate with CO₂ assisted by water, which can serve as the efficient active site to achieve photocatalytic CO₂ reduction. In the absence of a cocatalyst, stand-alone CdS-PD nanocrystals exhibit an excellent CO yield of 20.35 mmol g^-1 h^-1 concomitant with a high selectivity of 95.3% for the CO₂-to-CO conversion under visible light, which are remarkably superior than those of CdS nanocrystals possessing traditional alkyl-chain and other conjugated capping ligands.

Ultrafast Electron Transfer from CdSe Quantum Dots to an [FeFe]-Hydrogenase Mimic

The combination of CdSe nanoparticles as photosensitizers with [FeFe]-hydrogenase mimics is known to result in efficient systems for light-driven hydrogen generation with reported turnover numbers in the order of 10⁴-10⁶. Nevertheless, little is known about the details of the light-induced charge-transfer processes. Here, we investigate the time scale of light-induced electron transfer kinetics for a simple model system consisting of CdSe quantum dots (QDs) of 2.0 nm diameter and a simple [FeFe]-hydrogenase mimic adsorbed to the QD surface under noncatalytic conditions. Our (time-resolved) spectroscopic investigation shows that both hot electron transfer on a sub-ps time scale and band-edge electron transfer on a sub-10 ps time scale from photoexcited QDs to adsorbed [FeFe]-hydrogenase mimics occur. Fast recombination via back electron transfer is observed in the absence of a sacrificial agent or protons which, under real catalytic conditions, would quench remaining holes or could stabilize the charge separation, respectively.

Intracellular Hybrid Biosystem in a Protozoan to Trigger Visible-Light-Driven Photocatalysis

Incorporating artificial photosensitizers with microorganisms has recently been recognized as an effective way to convert light energy into chemical energy. However, the incorporated biosystem is usually constructed in an extracellular manner and is vulnerable to the external environment. Here, we develop an intracellular hybrid biosystem in a higher organism protozoa Tetrahymena pyriformis, in which the in vivo synthesized CdS nanoparticles trigger photoreduction of nitrobenzene into aniline under visible-light irradiation. Integrating a photosensitizer CdS into T. pyriformis enables the photosensitizer CdS, inherent nitroreductase, and the cytoplasmic reductive substance in T. pyriformis to synergistically engage in the photocatalysis process, generating a greatly enhanced aniline yield with a 40-fold increment. Moreover, building an intracellular hybrid biosystem in mutant T. pyriformis could even grant it new capability of reducing nitrobenzene into aniline under visible-light irradiation. Such an intracellular hybrid biosystem paves a new way to functionalize higher organisms and diversify light energy conversion.

Comprehensive insights into synthetic nitrogen fixation assisted by molecular catalysts under ambient or mild conditions

N2 is fixed as NH3 industrially by the Haber-Bosch process under harsh conditions, whereas biological nitrogen fixation is achieved under ambient conditions, which has prompted development of alternative methods to fix N2 catalyzed by transition metal molecular complexes. Since the early 21st century, catalytic conversion of N2 into NH3 under ambient conditions has been achieved by using molecular catalysts, and now H2O has been utilized as a proton source with turnover frequencies reaching the values found for biological nitrogen fixation. In this review, recent advances in the development of molecular catalysts for synthetic N2 fixation under ambient or mild conditions are summarized, and potential directions for future research are also discussed.

Bioremediation of Crude Glycerol by a Sustainable Organic-Microbe Hybrid System

Klebsiella pneumoniae with crude glycerol-utilizing and hydrogen (H₂)-producing abilities was successfully isolated from return activated sludge from Shatin Sewage Treatment Works. The H₂ production strategy used in this study was optimized with crude glycerol concentrations, and 1,020 μmol of H₂ was generated in 3 h. An organic–microbe hybrid system was constructed with metal-free hydrothermal carbonation carbon (HTCC) microspheres to enhance the H₂ production under visible light (VL) irradiation. Under optimized VL intensity and HTCC concentration, an elevation of 35.3% in H₂ production can be obtained. Electron scavenger study revealed that the photogenerated electrons (e^–) from HTCC contributed to the additional H₂ production. The variation in intercellular intermediates, enzymatic activity, and reducing equivalents also suggested that the photogenerated e^– interacted with K. pneumoniae cells to direct the metabolic flux toward H₂ production. This study demonstrated the feasibility of using an organic–microbe hybrid system as a waste-to-energy technology.

Investigating the use of conducting oligomers and redox molecules in CdS-MoFeP biohybrids

In this work we report the effect of incorporating conducting oligophenylenes and a cobaltocene-based redox mediator on photodriven electron transfer between thioglycolic acid (TGA) capped CdS nanorods (NR) and the native nitrogenase MoFe protein (MoFeP) by following the reduction of H⁺ to H₂. First, we demonstrate that the addition of benzidine-a conductive diphenylene- to TGA-CdS and MoFeP increased catalytic activity by up to 3-fold as compared to CdS-MoFeP alone. In addition, in comparing the use of oligophenylenes composed of one (p-phenylenediamine), two (benzidine) or three (4,4''-diamino-p-terphenyl)phenylene groups, the largest gain in H₂ was observed with the addition of benzidine and the lowest with phenylenediamine. As a comparison to the conductive oligophenylenes, a cobaltocene-based redox mediator was also tested with the TGA-CdS NRs and MoFeP. However, adding either cobaltocene diacid or diamine caused negligible gains in H₂ production and at higher concentrations, caused a significant decrease. Agarose gel electrophoresis revealed little to no detectable interaction between benzidine and TGA-CdS but strong binding between cobaltocene and TGA-CdS. These results suggest that the tight binding of the cobaltocene mediator to CdS may hinder electron transfer between CdS and MoFe and cause the mediator to undergo continuous reduction/oxidation events at the surface of CdS.

Photo-biocatalytic Cascades: Combining Chemical and Enzymatic Transformations Fueled by Light

In the field of green chemistry, light - an attractive natural agent - has received particular attention for driving biocatalytic reactions. Moreover, the implementation of light to drive (chemo)enzymatic cascade reactions opens up a golden window of opportunities. However, there are limitations to many current examples, mostly associated with incompatibility between the enzyme and the photocatalyst. Additionally, the formation of reactive radicals upon illumination and the loss of catalytic activities in the presence of required additives are common observations. As outlined in this review, the main question is how to overcome current challenges to the exploitation of light to drive (chemo)enzymatic transformations. First, we highlight general concepts in photo-biocatalysis, then give various examples of photo-chemoenzymatic (PCE) cascades, further summarize current synthetic examples of PCE cascades and discuss strategies to address the limitations.

Light-driven hydrogen production with CdSe quantum dots and a cobalt glutathione catalyst

A photocatalytic hydrogen (H₂) production system is reported using glutathione (GSH)-capped CdSe QDs with a cobalt precatalyst, yielding 130 000 mol H₂ per mol cobalt over 48 hours. Analysis of the reaction mixtures after catalysis indicates that the active catalyst is a labile complex of cobalt and GSH formed in situ.