To Biotic or Abiotic: Biohybrid Systems for Artificial Photosynthesis

Abstract

Biotic-abiotic interfaced configurations hold great promise for application in renewable energy and artificial photosynthesis systems. Recent advances in synthetic biology, computational, and visualization techniques, along with enhanced high-resolution characterization, have enabled a deeper fundamental understanding of the interface, which, in turn, has improved electron transfer processes and the design architecture. These developed configurations open new routes to mimic the photosynthetic apparatus or add new applications based on biotic and abiotic catalytic reactions. Aiming to surpass natural systems, researchers have examined methods to reconfigure these block sets into new designs. This review focuses on the advances in artificial photosynthesis and coupled biotic-abiotic biohybrid systems. The work presents the development of artificial photosynthesis configurations aimed at generating light-induced energy or fuels. The use of natural photosynthetic proteins, inorganic photocatalysts, and advanced biohybrid materials is presented and discussed, aiming to enable future biotic-abiotic design and the ambitious goal of developing real-world applications.

1. Introduction

The fundamental photosynthesis process that harnesses light energy evolved 3.5 billion years ago. The process altered Earth by enabling the conversion of light energy to chemical energy and the production of organic matter. The reaction also opened new routes for producing biological components that later emerged in living organisms.

Through evolution, the Z-scheme-based oxygenic process has been evolved. The improved efficiency of the Z-scheme apparatus has ushered in a new era on earth, in which oxygen serves as a key molecule, enabling the evolution of higher-level organisms. Oxygenic photosynthesis comprises the light-dependent and light-independent cycles. Those cycles harness visible light and facilitate the capture of CO₂ for the production of carbohydrates. The central photosynthesis paradigm is based on two light-activated centers that enable an extremely high-efficiency light-induced charge separation. Both photosystems, Photosystem I (PSI) and Photosystem II (PSII), hold efficiencies reaching 100%. However, the overall efficiency of photosynthesis is much lower, estimated at 1–4%. During the last century, humankind has tried to harness the Sun’s irradiation as an ultimate and unlimited energy source. Since the discovery of semiconductors and, later, the development of photovoltaics, methodologies for directly converting light energy into electrical power have been developed and implemented. Those methodologies often require smart grid installation and control and must be coupled with energy storage devices or energy management apparatus. In parallel to photovoltaics, photoelectrochemical and dye-sensitized solar cell configurations were also developed. − These cells generated electrical power or fuels, with light energy acting as the driving force.

Inspired by nature, Honda and co-workers developed the first light-induced photoelectrochemical cell, PEC, to enable full water splitting for O₂ and H₂. The cell was constructed with a TiO₂-based photoanode, which has a wide bandgap and can be activated by UV light irradiation.This first demonstration sparked a critical scientific effort to develop improved configurations, aiming to convert light energy into electrical energy or produce fuels. In parallel, a crucial understanding of the photosynthesis apparatus was achieved by Feher and co-workers. These insights into the electron and proton transfer processes have led to the development of light-induced systems inspired by the natural Reaction Center. During the 50s to the 70s, a significant scientific effort led to the Z-scheme configurations comprising two reaction centers. These findings open the way for advanced devices with greater potential for various energy applications. Furthermore, Z-scheme configurations sparked scientific efforts to develop PECs using biomimetic approaches.

Over the last few decades, tremendous efforts have been made to develop PECs. These efforts have led to an enhanced stability, light absorption, power output, and catalytic processes. The latter were mainly focused on water oxidation and oxygen reduction; nevertheless, hydrogen evolution and CO₂ and N₂ reduction processes were also pursued. A key challenge in those processes is selectivity, and cocatalysts are frequently coupled with the photoanode or photocathode to prevent side reactions. The added cocatalyst promotes improved hole or electron transfer, which, in turn, enhances photooxidation or photoreduction, respectively. While the use of photoelectrodes has great promise, light-induced catalysis can be achieved using colloidal NPs. These nanoelements exhibit unique properties that are governed by their size and morphology. The quantum size effect dictates the properties of semiconductor-based nanomaterials, which can therefore be tuned by controlling their size. By tailoring the energy band positions at the nanoelements, one can control the absorption spectrum and the catalytic properties of the generated holes and electrons at the valence and conduction bands, respectively.

Furthermore, the NPs’ surface-to-volume ratio and their outstanding solubility in various solvents enable excellent mass transfer of the NPs or substrates, promoting reactions that could not be achieved using photoelectrochemistry. Alternatively, metallic nanoelements can also be used as catalysts. However, here, the metals’ plasmonic properties dictate the energy levels and the nanomaterial absorption properties. In many cases, these nanoelements have dimensions and photophysical properties similar to those of the photosynthetic proteins; therefore, they may be used to enhance photosynthetic apparatus activity or to replace its components.

The Z-scheme light-driven configuration regenerates two critical energy carriers: ATP and NADPH. These bioenergy molecules provide cells with the energy required for CO₂ assimilation and other essential processes in organisms. In recent years, artificial photosynthesis configurations that can regenerate NADPH/NADH or ATP have been developed. These systems use chemical, electrochemical, photochemical, and photoelectrochemical apparatuses to produce energy-currency molecules. The photosynthesis process enables the critical transformation of inorganic CO₂ into hydrocarbons and energy; however, it can also be used to generate other forms of energy such as electricity and hydrogen fuels.

Furthermore, one can harness the high reduction potential of the process or its energy currency molecules to generate fuels or to develop an enzymatic cascade for fine chemical production. Such cascades open a path to the production of enantiomerically pure compounds, which are highly desired in many processes, particularly in the pharmaceutical industry. Moreover, as enzymatic cascades can be utilized for waste degradation, these processes can lead to carbon-neutral fuel generation and are, therefore, routinely pursued.

Designing biotic-abiotic configurations toward artificial photosynthesis aims to utilize the advances of each discipline. However, those are often not easy to designate and are highly dependent on the configuration. For example, using natural photosynthetic proteins as light absorbers may lead to high quantum efficiencies. However, those photosensitizers are limited to specific wavelengths and degrade because of back reactions under high light intensities. In contrast, QDs can be tuned to absorb the entire visible spectrum; however, the light-induced electron-transfer process is poorly directional and lacks specificity, thereby frequently hindering the desired reaction. While some approaches put the natural elements in the foreground (photosynthetic proteins, photosynthetic organisms, and redox enzymes), others use them as external moieties to complement robust abiotic design.

Recent efforts have aimed to couple abiotic processes with biological processes to achieve higher efficiency, robustness, and versatility. This review will focus on these directions, the different approaches, opportunities, and current limitations in applications, and on improving understanding of the biotic and abiotic interfaces. Figure exhibits the key scientific efforts toward biotic-abiotic and artificial photosynthesis systems. The blue panel exhibits photo­(bio)­electrochemical cells that convert light energy to chemical fuels. Light-driven photooxidation processes can facilitate water oxidation, mimicking the function of the photosystem II. Alternatively, the oxidation potential can be used for waste degradation. The coupled cathode enables the production of fuels or fine chemicals, facilitated by biocatalysts, such as enzymes or whole-cell bacteria. The orange panel illustrates the use of a biocatalyst to degrade polymeric waste into small molecules that, in turn, serve as sacrificial electron donors and can be further converted to value-added chemicals. Similarly, QD catalysts can be coupled with the photoinduced reactions for the production of fine chemicals while generating H₂ fuel. The green panel presents approaches for constructing biotic-abiotic biohybrids for whole-cell artificial photosynthesis configurations, where added QDs or internally grown QDs are used to enhance biochemical processes.

1.

Schematic representation of the standard biotic-abiotic and artificial photosynthesis configurations.

Herein, we wish to present the latest research in artificial photosynthesis, aiming especially at biotic-abiotic configurations. These systems aim to address obstacles to efficient and sustainable artificial photosynthesis, focusing on photochemistry, bioengineering, and photoelectrochemistry. Each of these disciplines has advantages and disadvantages; therefore, a holistic approach enables enhanced activity that can even surpass the natural one. To that end, we first present and discuss artificial photosynthesis configurations that use natural photosynthetic proteins. Methodologies for establishing electrical communication between those proteins and abiotic elements, such as quantum dots, conductive polymers, and electrodes, are presented and discussed. Architecture is a critical factor enabling remarkable efficiency in natural photosynthesis. Therefore, methodologies to design scaffolds that enable light-induced electron transfer and charge separation are presented and discussed. We then dive deeper into the construction of biotic-abiotic configurations by presenting critical published research with a focus on methodologies for constructing photosynthesis-mimicry biohybrids in vitro and in vivo. Here, the ability to couple bricks from different toolboxes provides great flexibility for designing new reactions that utilize artificial photosensitizers in conjunction with enzyme cascades or whole-cell bacteria. The next section centers on (photo)­bioelectrochemical cells, their design, critical parameters, obstacles, and suitability for real-world artificial photosynthesis applications.

Furthermore, critical parameters, such as photoinduced water oxidation and charge separation, and the challenges that arise when they are coupled to oxygen-sensitive enzymes or bacteria are discussed. The scope was then extended to present different approaches for exploiting natural or synthetic polymers to produce electrical energy carriers and value-added chemicals. In a way, we present a reversal of the natural photosynthesis process that degrades polymers into energy and critical compounds. State-of-the-art configurations comprising biohybrid PECs or photochemical ones are presented and discussed. The last part of the review presents the essential parameters for quantitative evaluation of biotic-abiotic artificial photosynthesis systems. Section finalized the review with the parameters needed to be addressed toward the real-world application based on biotic-abiotic and artificial photosynthesis configurations.

2. Photosynthesis Mimicry: Do and Undo

The engine of the photosynthetic paradigm is without doubt the photosystem II complex. The PSII protein complex comprises the Mn₄CaO₅ cluster, an inorganic complex that enables the water-oxidation reaction via five consecutive intermediate states, S0 through S4. The process facilitated by the oxygen-evolving complex, OEC, enables the oxidation of H₂O, generating an electron transfer to the Q_b site and, in parallel, releasing O₂. To achieve the strong oxidation conditions required for the process at the Mn₄CaO₅ cluster, light-absorption steps occur. By sequential excitations at the P680 reaction center, the essential energy is provided to shift the manganese ions into a higher oxidation state, enabling H₂O oxidation. Since the OEC determination, efforts to construct stable and effective OEC complex analogs have been pursued. These efforts included a similar structure or function to the natural complexes, − as well as the use of precious and novel metals or metal oxide layers that enable high turnover rates. − While the developed complexes developed reached turnover frequencies that are similar to the natural process at the range of 100–400 s^–1, the stability of those complexes at homogeneous and heterogeneous catalysis falls short. The designed complexes inspired by nature contributed tremendously to basic science understanding. Furthermore, biophysical techniques have been developed, making tremendous contributions to scientific progress. However, currently, thin-layer oxides or similar thin layers based on abundant metals such as cobalt, manganese, nickel, and iron seem more suitable for real-world applications with stable, continuous activation. Frequently, developed catalysts that present high turnover rates fall short in terms of robustness and long-term stability. In many reports, the determined parameter accounts for short measurements of seconds or minutes. Therefore, the catalyst’s aptness for applications cannot be concluded. While the catalyst’s structural and mechanistic understanding is of great importance for developing new complexes and configurations, an indication of the systems’ stability for at least 30 min should become standard practice in such scientific reports. Post analysis of the catalyst should also be included.

Developed catalysts can be activated chemically, photochemically, or photoelectrochemically. In the case of the last two mentioned, the developed water oxidation catalysts should be coupled with light-induced photosensitizers, electronic bridges, or redox mediators to enable the activation of the OEC. These systems should be designed to be bias-free and donor (or acceptor)-free, resembling the natural apparatus. In many cases, this is the hurdle that limits advancement into applications.

While some advances toward artificial photosynthesis have been made using different approaches, for example, an artificial leaf, decoupling oxygen and hydrogen production, and photochemical cells enabling full water splitting at a large scale, a commercialized system has not yet been fully introduced.

3. Photosynthetic Apparatus-Based Devices

3.1. Photosystem I-Based Configurations

3.1.1. Conjugation of Proteins with Inorganic Catalysts

PSI comprises 12 or 13 subunits that form the active protein in cyanobacteria or plants, respectively. Naturally, the highly negative potential developed at the Fb iron–sulfur cluster, which can reach −0.75 V vs Ag/AgCl at pH 7, enables the reduction of ferredoxin or flavodoxin (−0.65 V). These diffusional proteins act as redox mediators to activate the ferredoxin NADP­(+) reductase (FNR) enzyme. The latter generates an NADPH flux, which triggers the Calvin cycle. The electron transfer chain has been optimized and evolved to enable an internally efficient electron transfer process at the protein core, starting at the excited P700 and ending at the Fb iron sulfur cluster. The extremely high reducing power developed by the light reaction can be exploited to enable a variety of alternative chemical reactions or electron-transfer processes that can benefit humankind. Thus, a significant research effort has been focused on redirecting the electron flux to generate electrical or fuel energy.

As presented, the Photosystem I light-harvesting complex has attracted significant effort toward its use as a photosensitizer in biohybrid systems. In 2001, photosystem I was first coupled with a Pt cluster/NP to enable direct hydrogen generation. Millsaps and co-workers have photoirradiated isolated Photosystem I proteins in an oxygen-free environment. Alternatively, platinum salts were added in the presence of ascorbate and plastocyanin, as an electron donor and a redox mediator, respectively, to oxidized P700. The excited PSI exhibits a high reducing potential and a unique electron-transfer chain that enables the site-specific reduction of platinum salts in proximity to the Fb site. The formed PSI-Pt NP biohybrid was then irradiated in the presence of a sacrificial electron donor, enabling a similar electron-transfer process; however, the Pt catalyst facilitated hydrogen generation. This pioneering work presented the great promise of PSI-NP hybrids as a H₂ fuel generation apparatus. Since the first demonstration, different approaches have been used to enable the conjugation of Pt or gold NPs with photosystem I. Golbeck and co-workers presented a unique approach where a molecular wire interfaced gold or Pt NPs with the Fb Fe-P cluster. The interface configurations have led to hydrogen generation. The same group has reported different approaches to conjugate NPs in proximity to the Fb site, leading to hydrogen generation. Reducing a Pt cluster or interface Pt NPs at the Fb site was also demonstrated in follow-up research. Utschig and co-workers investigated the formation of PSI-Pt NP biohybrids and their photocatalytic hydrogen generation. , Bruce and co-workers have shown the use of a PSI-Pt cluster for hydrogen generation. In their work, Cyt C6 was utilized as an electron redox mediator, which in turn enabled the Pt salt reduction and the hydrogen generation as schematically presented in Figure .

2.

Schematic of the electron flow in the photosystem I catalytic nanoparticle. The monomeric form of the T. elongatus PSI is shown with chlorophyll cofactors (magnesium ligated in macrocycle shown in green van der Waals radius) and the protein colored by secondary structure (helical regions in purple, beta sheets in yellow, and unstructured regions in brown). A partially docked form of the cyt c ₆ is shown, providing the rereduction of P₇₀₀, deriving electrons from the oxidation of ascorbate (Asc_Red. → Asc_Ox. + e^–). The platinum clusters on the stromal surface of PSI are shown as gray stars catalyzing the reduction of protons to hydrogen with electrons of lower potential, using the energy of the absorbed photon. Reproduced with permission from ref . Copyright Springer 2010 Nature.

Although extensive research has been conducted, the exact positions of the Pt clusters or NPs at the PSI have not been fully determined. Recently, Gisriel and co-workers investigated a biohybrid structure to position Pt NPs at the PSI surface. The work provided significant evidence derived from cryo-EM imaging that two different sites can accommodate the formed NPs in a parallel manner. The research suggests that no perturbation to the PSI machinery occurs due to the biohybrid formation. Moreover, while two NP sites were found, one positioned in proximity to the Fd binding site, and another at an alternate position, only the larger NPs in proximity to the Fb Fe-S cluster act as catalysts for the H₂ generation. While most of the formed PSI-NP biohybrids were designed for hydrogen generation, other interesting directions were also explored. For example, the use of Au NPs or photosensitizers as a light-absorbing antenna, where the NPs’ plasmonic properties or the dye enabled light-induced energy transfer.

As mentioned, unlike PSII, PSI is highly stable and can maintain its activity for months if continually supplied with electron donors and acceptors. Therefore, a photodriven configuration for hydrogen generation using a biohybrid comprising the PSI light-harvesting protein and a metal cluster or an NP holds great promise. The use of renewable, environmentally friendly materials stands out as an alternative to chemically synthesized photosensitizers such as QDs. It should be mentioned that the high purification cost of the protein complex from plants or cyanobacteria may limit its use in real-world applications. Furthermore, in most of the research described, ascorbic acid was used as a sacrificial electron donor. While ascorbic acid is a well-known and efficient sacrificial electron donor, alternative chemicals should be explored, especially side stream chemicals that accumulate in industrial or household waste processes. Using such compounds may offer economic and environmental advantages, paving the way for real-world applications. Alternatively, Kim and colleagues have developed a biotic-abiotic configuration in which BiVO₄ particles were modified with gold clusters, which served as linkers to couple the PSI-Pt electronically. The integrated system mimicked the natural Z-scheme configuration, where BiVO₄ acts as an alternative to the natural PSII and facilitates light-induced water oxidation. At the same time, the PSI enables proton reduction to hydrogen fuel, as shown in Figure .

3.

a) Natural Z-scheme and its energy diagram; photosynthesis follows serial reactions: oxidation of water and electron excitation in PSII, electron transfer through the electron transport chain (plastoquinone (PQ), cytochrome c6 (cyt), plastocyanin (PC)), electron excitation in PSI, and reduction of NADP in ferredoxin-NADP+ reductase (FNR). b) Hybrid Z-scheme and its energy diagram (mediator is Au). Photosynthetic water splitting follows serial reactions: oxidation of water and electron excitation in BiVO₄, electron transfer through Au, electron excitation in PSI, and reduction of hydrogen catalyzed by Pt. Reproduced with permission from ref . Copyright 2015 Wiley-VCH GmbH.

3.1.2. Electrochemical Activation of Photosystem I-Based Devices

The PSI P700 acts as a photosensitizer and upon light excitation facilitates the transfer of high-energy electrons. These short-lived excited states undergo charge separation through the PSI protein’s redox-active acceptor chain. The highly efficient charge separation minimizes back reactions and the recombination process. It also develops an extraordinary potential gap between oxidized P700 and the Fb acceptor side. The developed potential reaches 1 V in the small dimensions of a 7 nm protein. , The reduced Fb site exhibits a strong reducing potential of −0.8 V vs Ag/AgCl, which, thermodynamically, can be used to reduce any natural biological process. Naturally, the electrons are transferred from the Fb Fe-S cluster to ferredoxins or flavodoxins. However, PSI can also be used for unorthodox reduction processes. Alternatively, to the natural electron-transfer process, under irradiation, the PSI can facilitate anodic or cathodic photocurrents. Generating power by activating an environmentally friendly photosynthesizer has great promise for sustainable energy-producing devices and should therefore be pursued. −

Toward this end, several prerequisites should be considered:

• (i)The PSI protein shell is an insulator. The PSI contains redox centers that enable efficient internal electron transfer. However, it allows specific routes to be externally accessed through the P700 or the F_ab sites. Therefore, an electronic bridge with a short electron transfer pathway from the protein to the electrode (or vice versa) should be developed. Such bridges can facilitate short electron-transfer processes to electron sinks or donors, respectively.
• (ii)To prevent short circuits, the protein redox centers are embedded within the protein structure with only the Fab sites and the P700 prone to electrical communication with their surroundings.
• (iii)For an efficient electron-transfer process, methods to order and orient the PSI proteins’ P700 or Fab sites relative to the electrodes or acceptors are required.
• (iv)The developed devices should provide clear paths for absorption of visible light irradiation.

Indeed, over the last two decades, extensive research has aimed to utilize PSI for electrical power generation. While PSI offers significant advantages, this research direction was first demonstrated with reaction centers that absorb light in the near-IR range. − Those demonstrations ignited extensive research to couple isolated PSI proteins with electrodes, both anodes and cathodes to unlock their potential for light-induced anodic or cathodic photocurrent generation, respectively. Carmeli and colleagues have self-assembled the isolated protein on metal electrode surfaces. The assembled device showed potential buildup due to light irradiation. Cliffel and Jennings introduced a vacuum technique to improve the PSI self-assembly on the electrode surface. By increasing the electrode surface area and improving immobilization using dialdehyde cross-linker, they managed to enhance the generated photocurrents by 8 times to reach 800 nA/cm². By applying consecutive layers of the PSI on top of the electrode surface, an increased cathodic photocurrent was reached. As mentioned in the previous section, metal clusters/NPs can be formed at the Fb site. These metal clusters enable the integration and orientation of the PSI proteins toward the electrode. , While the PSI can be used for anodic or cathodic photocurrents, the latter yield higher currents and present high quantum efficiency, QE. Badura and co-workers have shown that an Os complex-based polymer, with a potential of ca. 0.2 V vs Ag/AgCl, can act as an efficient redox mediator for the PS1 oxidized P700. In parallel with electron transfer from the electrode to P700⁺, the generated electrons can be transferred to methyl viologen, an efficient diffusional redox mediator. The viologen radical reacts efficiently with atmospheric oxygen to form high cathodic photobioelectrocatalytic currents in the range of 29 μA/cm² and Incident-Photon-to-Current Efficiency (IPCE) of 3.1%. As mentioned, the PSI redox-active centers are isolated to prevent short-circuiting and enable efficient charge separation. While charge separation limits back reactions, each electron-transfer process lowers the redox potential gradient. An elegant approach used by Terasaki involved replacing the natural VK1 quinone with a VK1 quinone analog. The analog holds a viologen end, which acts as an electron acceptor and a positive binding site for interacting with the negatively charged electrode surface, Figure .

4.

Schematic illustration of the procedures for the extraction of VK₁ used to prepare a) a quinone pocket and b) reconstitution with a molecular wire adsorbed on a gold electrode. Reproduced with permission from ref . Copyright 2009 Wiley-VCH GmbH.

This approach enabled the extraction of electrons at a higher potential, about 300 mV, compared to the naturally extracted electrons’ potential from the Fab site. The use of site-specific redox mediators has a crucial effect on protein orientation relative to the electrodes. It also controls the distance between electron donors and acceptors, which dictates the rate of electron transfer. Biotechnological tools were also used to control PSI orientation with respect to the electrode. Mershin and co-workers overexpressed ZnO metal-binding peptide in proximity to the Fab redox centers. The ZnO photoanode was then modified with the bioengineered PSI to form a fully oriented PSI monolayer. This unique configuration enables the generation of a photocurrent of 362 μA/cm² under Sun illumination and an applied bias of 0.5 V. The PSI-based photoanode was coupled with a cathode, which continuously reduced the cobalt phenanthroline complex, which acted as a redox mediator and enabled current flow initiated by the photoinduced reaction. Thus, the cell could operate continuously. The work nicely demonstrates the contribution of biotechnology, material science, and engineering toward enhanced performance.

Naturally, plastocyanin acts as an electron donor and Fd as an electron acceptor to enable the photoactivation of PSI and prevent a back-reaction. Therefore, artificial activation of the PSI should include these elements as well.

Manocchi and colleagues developed a PSI-based configuration in which the protein was self-assembled onto hydrocarbon monolayers (self-assembled monolayers, SAM) with different head groups, Figure a,b. They show that the negatively charged head enabled improved orientation on the electrode surface, thereby increasing the generated photocurrents. The work showed that both the electron donor (Os complex) and the electron acceptor (methyl viologen) are essential for the efficient electron transfer process, Figure c–f. As depicted in Figure f, it is crucial to adjust the redox mediator’s potential to the P700⁺ and the Fb Fe-S to enable correct electron flow. Furthermore, as will be discussed later, the thermodynamic parameters are essential; however, they are not consistently enough to facilitate such a reaction, and other parameters such as charge, size, affinity, and mobility should be considered.

5.

Schematic of PSI assembly on electrodes modified with alkanethiols (a) Gold electrodes are first modified with alkanethiols to form SAMs, followed by (b) PSI assembly. (c) Light-induced electron transfer associated with surface-assembled PSI on SAM-modified electrodes. Os­(bpy)₂Cl₂ and methyl viologen (MV²⁺) are used as redox mediators in solution. (d) Os­(bpy)₂Cl₂ and (e) MV²⁺ chemical structures. (f) Energy level diagram relating redox potentials of mediators and PSI. Adapted from ref . Copyright 2013 American Chemical Society.

The addition of redox-active mediators to shorten the electron-transfer distance between PSI redox centers and electrodes or oxygen indeed enhanced photocurrents and yields; however, fully integrated systems can minimize diffusion limitations, which is essential for improved bioelectrocatalytic processes. Therefore, redox-active polymers were examined, with a layer-by-layer assembly implemented. The PSI and polybenzylviologen (PBV) were deposited on an ITO electrode. The redox-active polymer enabled an improved electron transfer process from the photoexcited PSI photosynthetic proteins to the electrode while providing stabilizing structural support due to the strong electrostatic interaction between the charged electrode, the photosynthetic proteins, and the charge viologen polymer, Figure a. A layer-by-layer technique was used to integrate the PSI proteins on top of the negatively charged ITO electrode. Using a positively charged viologen-based polymer and the photosynthetic protein, a multilayer was formed. As depicted in Figure b, the ITO/PBV/PSI electrodes were further used for the photocurrent generation. Under oxygen-free conditions and with the addition of a sacrificial electron donor, anodic photocurrent correlated with PSI absorbance was measured as a function of the amount of layer deposited, as shown in Figure c. Alternatively, application of chopped light irradiation at 680 nm resulted in similar photocurrents, Figure d. Interestingly, omitting the electron donor-coupled system (DCPIP/ascorbic acid) led to the development of cathodic photobiocatalytic currents. In the presence of O₂, a single layer or double layer produced cathodic photocurrents as presented in Figure d,e, respectively. The design configuration features a fully integrated biocathode that requires only an atmospheric oxygen for its activation. Using a similar approach, integrated PSI-based photocathodes were coupled to a photoanode to develop a Z-scheme-like configuration. A Photosystem II-based photoanode, as presented in Figure , or a BiVO₄-based photoanode was coupled to the PSI-based photocathode for the construction of photobioelectrochemical cells. ,

6.

(a) Schematic assembly of the layered PBV²⁺/PSI photoactive composite on an ITO electrode. (b) Absorption spectrum corresponding to an ITO surface modified with 3 layers of the PBV²⁺/PSI composite. The inset shows the dependence of the absorption intensity, at λ = 680 nm, on the number of layers deposited. (c) Photocurrent action spectra corresponding to the layer-by-layer deposited assemblies of the PBV²⁺/PSI composites on the ITO electrode. The curves correspond to (a) Bare ITO; (b) 1; (c) 2, and (d) 3 PBV²⁺/PSI composite layers. The inset shows the dependence of the photocurrent, at λ = 680 nm, on the number of layers deposited on the electrode. (d) Photocurrent responses, at λ = 680 nm, upon the cyclic switchable illumination of the PBV²⁺/PSI-modified electrode, consisting of 3 composite layers. Switch “ON” (illumination on) - marked (a); Switch “OFF” - marked (b). Potential applied E = −0.05 V vs Ag/AgCl. The effective illumination area was 0.25 cm². All measurements were performed in an Ar-dehydrated phosphate buffer (0.1 M, pH = 7.2) that contained ascorbic acid, 40 mM, and DCPIP, 50 μM. (e) Cathodic photocurrents generated by PSI/PBV²⁺ based photocathode in the presence of O₂ and applied potential of 0 V vs Ag/AgCl. Adapted with permission from ref . Copyright 2013 Wiley-VCH GmbH.

7.

Representation of the proposed biophotovoltaic cell combining a PSII-based photoanode and a PSI-based photocathode. Upon photon absorption, water molecules split into electrons and protons. The electrons are transferred to the cathodic half-cell via the outer circuit, where PSI generates a reductive force of −580 mV vs SHE and reduces the electron acceptor (methyl viologen). The methyl viologen radical cation is reoxidized by molecular oxygen, resulting in water as the final product. Reproduced with permission from ref . Copyright 2013 Wiley-VCH GmbH.

Utilizing redox-active polymers to enable a short electron-transfer process is of great importance; however, most of these configurations can lead to nondirected electron flow, resulting in short circuits and a limited efficiency. To tackle these issues, the P700 or Fb site should be oriented toward or away from the electrode surface to generate cathodic or anodic photocurrent, respectively. A recent work by Buesen and co-workers modeled the parameters and electron transfer in biophotoelectrochemical processes. This work presents guidelines to minimize short circuits and back-reaction, which often hinder efficient photocurrent development. Schuhmann and co-workers addressed this issue by using the Langmuir–Blodgett technique to control the orientation of the PSI on the electrode surface. − It is important to note that full coverage of the electrode surface is critical for controlling the direction of electron flow and preventing short circuits. Naturally, cyanobacterial PSI forms a trimer. Therefore, in terms of geometry, casting a densely packed monolayer on the electrode surface is challenging. To tackle this issue, the electrode surfaces were first modified with trimeric PSI, followed by a second deposition of the monomeric form. The resulting PSI-based photoelectrode showed improved photocurrent generation due to greater electrode coverage, which enhanced current and blocked the unmodified electrode surface, which can contribute to undesired, uncontrolled electron-transfer processes.

The electrode and Photosystem I interfacing configurations opened the way toward energy device construction. As mentioned in the previous section, PSI can be used as a potent reducing agent and facilitate hydrogen generation when coupled with an appropriate catalyst. The main disadvantage of such configurations is the use of sacrificial electron donors. Hence, coupling such systems to the electrode surface could potentially solve these issues, as the electrode serves as the electron donor, and Pt cluster/NP or the hydrogenase , enzyme can be used as the hydrogen generation catalysts. It has been found that PSI stability is altered by peroxide species being formed by the photocatalytic process. To tackle this issue, a quinone electron acceptor was added, presenting enhanced stability.

3.2. PSII

3.2.1. Conjugation of Proteins with Inorganic Catalysts and Electrodes

The Photosystem II protein facilitates the fundamental conversion of light energy into chemical energy. In terms of energy and mechanism, water oxidation is a challenging reaction that requires both light absorption and a multielectron-catalytic process. The protein complex facilitates the light-induced water oxidation reaction by overcoming thermodynamic barriers. The evolution of oxygenic photosynthesis has tremendously changed Earth, enabling the variety that exists today. The importance of oxygen is, of course, crucial to many developed species; however, the evolved oxygen is a byproduct of the generated electrons. Naturally, the internal electron transfer and charge separation end at the Q_b site, where the electrons are transferred to PQ, a quinone that serves as a diffusional electron-transfer mediator across the membrane to reduce B6f. The PSII stability under irradiation is very limited due to back-reactions that lead to deactivation. Unlike the PSI, PSII does not develop a strong reducing power that can be utilized for essential reactions such as hydrogen generation or CO2 fixation. However, like its natural function, it can be coupled with biotic (such as phycobilisomes), inorganic photosensitizers or nanoelements to achieve the essential higher reducing power, or alternatively, an electron sink, respectively. Feng and co-workers presented an interesting configuration to facilitate the production of ATP by an artificial system. In this work, PSII and ATPase were isolated and purified. Core–shell microspheres were designed to encapsulate the PSII complex, which includes the light-harvesting complex and the OEC, Figure . To achieve active and stable PSII complexes, it has been cross-linked using glutaraldehyde and BSA as a scaffold to form a PSII-BSA-CaCO₃. The CaCO₃ was then removed, leaving a porous structure. The PSII modified protoliposomes were then modified with the ATPase. The ATPase positioned at the membrane, which by light irradiation in the presence of DCPIP as electron scavenger enables the production of ATP. The work demonstrates an artificial biotic abiotic hybrid consisting of PSII as water oxidation catalyst, which in turn, like in the natural system, produces proton gradient which activates the ATPase. The artificial scaffold allows flexible design to reach direct route to biological energy currency, ATP.

8.

Schematic representation of core–shell microspheres designed for light-driven ATP Synthesis. Adapted from ref. Copyright 2016 American Chemical Society.

This concept was further extended by expanding the light-harvesting range using both the reaction center (RC) and PSII to produce ATP independently and in parallel. The critical role of the photosynthetic apparatus is to utilize the generated energy currency (ATP and NADPH) and enable the CO₂ assimilation process. Miller and co-workers presented an advanced approach in which the photoinduced electrons generated by the thylakoids were used to activate an enzymatic cascade that enabled CO₂ capturing within water-in-oil droplets. The artificial enzymatic cascade was designed to replace the natural CO₂ assimilation process. , Toward this end, 16 enzymes of the crotonyl-CoA/ethylmalonyl-CoA/hydroxybutyryl-CoA (CETCH) cycle were incorporated in the droplets, mimicking both the light and dark cycles, aiming to artificially replicate the chloroplast’s role, as shown in Figure . As presented, NADPH could be photogenerated by the thylakoid membranes when a ferredoxin redox mediator was added, as shown in Figure a. Also, ATP was continuously produced, with light irradiation greatly enhancing its production, as shown in Figure b. As presented, the generated energy currencies, NADPH and ATP, were further coupled to enzymatic reactions for CO₂ assimilation, Figures c and d. Stability issues still hinder the adoption of more-applicable approaches. However, a clear route toward artificial photosynthesis has been established. The utilization of isolated PSII in photobiochemical configurations has been quite limited. However, its conjugation with electrodes for water oxidation and electrical energy generation has great promise and has, therefore, been a focus of artificial photosynthesis research in recent decades.

9.

(a) NADPH production is dependent on light and externally added ferredoxin. (b) ATP production is dependent on light, with some background reaction in the dark caused by membrane-bound adenylate kinase. (c) Scheme of TEM-driven carboxylation reactions of Pcc and Ccr using light-produced ATP and NADPH, respectively. (d) Reactions coupled to TEM (2.5 or 5 μg Chl and 60 μmol photons m^–2 s^–1) (N = 6). Adapted with permission from ref . Copyright 2020 AAAS.

3.2.2. PSII Electrochemical Activation

To construct PSII-based photobioelectrochemical devices, the proteins must be oriented to facilitate an efficient electron transfer to the electrode. Unlike the PSI, the protein can function only at the anode to generate photobioelectrocatalytic currents. By light irradiation, the OEC enables water oxidation, which generates electrons that can be transferred to electrodes. To enhance the generated photocurrents, the Q_b protein site should be oriented toward the electrode surface to facilitate efficient electron transfer. Alternatively, using redox mediators or redox-active polymers shortens electron-transfer pathways between the protein and the redox-active moieties, enabling an enhanced protein capacity on electrode surfaces with efficient electron transfer. PSII stability is compromised by D1 subunit degradation. In nature, D1 can be routinely replaced. However, this cannot be performed in an artificial photosynthetic configuration using an isolated PSII protein. To minimize protein degradation and, in turn, deactivation, back-reactions should be minimized. This could be achieved through tailored charge separation processes, which may be achieved through appropriate orientation and the use of redox-active charge mediators.

Miyachi and co-workers presented an interesting approach for the assembly of a PSII-nanoparticle biohybrid. The biohybrids were designed to enable improved orientation and short-range electron transfer between the PSII protein and the Pt nanoparticles, Figure a. As described, an analog to the natural PQ9 was designed. The analog consists of the PQ9 quinone head with a thiolate tail, replacing the natural hydrocarbon tail. The thiolate moiety enabled efficient integration with the novel metal nanoparticle, facilitating a short electron-transfer process. The hybrids were then deposited on electrode surfaces to enable photocurrent generation using a sacrificial electron donor. Alternatively, His-tag, , and reduced graphene oxide were used to dictate a proper orientation of the PSII toward the electrode for efficient photocurrent generation. Self-assembly monolayers have been used to improve the orientation of PSII on the electrode surface and further stabilize it. Through electrostatic interactions, the PSII Q_b site was oriented toward the negatively charged ITO layer, enabling improved photocurrent generation. Alternatively, the use of a bonded redox-active mediator was examined. By integrating a quinone into the mesoporous ITO electrodes, an efficient electron transfer process was achieved. The integration of enzymes with osmium (Os) based complexes was realized three decades ago, initially utilized for the construction of biosensing devices. It has been shown that replacing the Os complex ligands can tune the complex redox potential to the system’s requirements. Due to its high stability, pH independence, and efficient electron transfer, it can be used for a wide range of bioelectrochemical applications. Sokol and co-workers integrated Os-complex modified polymer and a phenothiazine-modified polymer with PSII on hierarchically structured inverse opal indium thin oxide (IO-ITO) electrodes. As shown in Figure b, the redox-active polymer chains can act as an “electron sink,” enabling continuous photocurrent generation. As mentioned before, quinones have a similar structure and hydrophobicity to interact with the Q_b site pocket. Therefore, quinone, as a redox mediator, may dictate proximity, leading to high turnover rates. Indeed, quinone-based polymers were synthesized to enable high photocurrent generation or a full H₂O/O₂ cycle power-generating device.

10.

(a) Schematic representation of Photosystem II (PSII)-modified gold electrodes, prepared by the deposition of PSII reconstituted with platinum nanoparticles (PtNPs) on Au electrodes. Adapted from ref . Copyright 2017 American Chemical Society. (b) Schematic representation of PSII wired via a redox polymer network to the IO-ITO electrode. (c) Energy level diagram showing electron transfer pathways between PSII and the redox polymer. Adapted from ref . Available under a CC-BY 3.0 license. Copyright 2016 The Author(s).

As in the natural process, the electrons generated by the water-oxidation process can be utilized to generate chemical energy, such as hydrogen or formic acid, by reducing H⁺ or CO₂. As in the natural photosynthesis process, PSII cannot generate enough reducing power to enable the above reactions. Thus, by utilizing Z-scheme configurations, the potential energy required for water oxidation and proton reduction, or for CO₂ reduction, can be achieved. Alternatively, the system can be biased to compensate for the missing potential. It should be noted that increased surface area or the use of redox active polymers allowing high electronically wired photosynthetic proteins are essential to reach real-world application photocurrents. Indeed, a significant effort is directed toward this end. ,,−

Riedel and co-workers have introduced an interesting approach to developing a Z-scheme biotic-abiotic configuration. In the designed configuration, PSII was coupled with QDs. An Os-based redox-active polymer was synthesized and used as a buffering layer and redox mediator between the PSII proteins and PbS QDs. The PSII photoexcited electrons were shuttled to the osmium redox-active moieties, which, in turn, acted as electron donors to the photoexcited QDs, Figure a. The osmium layer plays two important roles: (i) acting as an electron donor to prevent recombination at the QDs and (ii) acting as an electron acceptor to facilitate the transfer of electrons from the QDs to the TiO₂ to allow continuous electron flow from the PSII Q_b site to the oxidized Os polymer, which minimizes back-reactions and short circuits. The gained potential was further utilized for the activation of bilirubin oxidase, BOD, which enabled oxygen reduction in the cathode chamber, Figure b. As shown, the osmium-based redox-active polymer improves charge separation and enables an electron-accumulating layer that closes the photogenerated holes produced by excited QDs. Interestingly, this approach can be tuned to any desired absorption wavelength by controlling the NPs’ dimensions, which are governed by quantum size effects.

11.

a) Schematic of the electron transfer steps and energetic level of the components of the light-driven signal chain composed of TiO₂, PbS QDs, redox polymer (P_Os), and PSII. b) Scheme of the photobioelectrochemical cell consisting of an IO-TiO₂|PbS|P_Os|PSII anode and an IO-ATO|PC|BOD cathode. Reproduced with permission from ref . Copyright 2018 Wiley-VCH GmbH.

As presented in this section, diverse photosynthetic protein-based devices have been developed. We summarize the main performance characteristics to enable easy comparison between the systems in Table .

1. Photosynthetic Protein-Based Devices.

Photosystem unit	Photoelectrochemical/Photochemical system	Diffusional electron donor/electron acceptor	The catalytic reaction	Maximal photocurrent/photocurrent density	Stability	The applied potential	Maximum activity (in terms of product generation)	Efficiency parameters (maximal value)	Ref.
PSI purified from spinach	Purified PSI reaction centers, platinized at the reducing end and plastocyanin.	Sodium ascorbate	HER	N.A.	H2 evolution sustained during 2 h illumination cycles.	N.A.	3.93 μmol H2/h/mg Chl	N.A.	Millsaps et al. (ref )
PSI from Synechococcus sp. PCC 7002	PSI was covalently linked to Pt and Au NP surfaces by 1,6-hexanedithiol.	DCPIP, sodium ascorbate, Cyt C6	HER	N.A.	N.A.	N.A.	Hydrogen production rate of 49.3 μmol H2 mg Chl–1 h–1 was reported with Pt NPs, rebuilt PS I, 1.6-hexanedithiol, Cyt c 6 system	N.A.	Grimme at al. (ref )
PSI from Synechocystis sp. PCC 6803	PSI with site-specific attachment of a naphthoquinone molecular-wire–([NQ[CH2)15S)2]) -Pt nanoparticle at A1A/A1B sites.	DPIP, sodium ascorbate, Cyt C6	HER	N.A.	The rate of hydrogen evolution declined with a half-time of 9 h.	N.A.	The rate of hydrogen generation reached a maximum of 67.6 ± 8.2 μmol of H2 (mg of Chl)−1 h–1 after 1 h.	N.A.	Gorka et al. (ref )
PSI from Synechococcus lividus, Synechococcus leopoliensis	PSI with electrostatically associated Pt nanoparticles (PSI/Pt NP).	Ascorbate, cyt c6	HER	N.A.	Hydrogen generation stopped after 4 h due to depletion of the sacrificial electron donor.	N.A.	244 μmol H2 (mg chlorophyll)−1 h–1 or 21 034 mol H2 (mole PSI)−1 h–1	N.A.	Utschig et al. (ref )
PSI isolated from the thermophilic cyanobacterium T. Elongatus	PSI assembled into PSI-Pt ’photosynthetic NPs’	Ascorbate, cyt c6.	HER	N.A.	>85 days (hydrogen yield maintained, intermittent testing)	N.A.	5.5 μmol H2 h–1 mg–1 chlorophyll	N.A.	Iwuchukwu et al. (ref )
PSI isolated from Synechococcus lividus PCC 6717,	PSI bound to Pt NPs (PSI-Pt NPs biohybrids)	Ascorbate, cyt c6	HER	N.A.	The rate of hydrogen evolution was measured for 400 min.	N.A.	6070 mol H2 mol PSI–1 h–1 and a turnover number of 36,600 in 400 min	N.A.	Gisriel et al. (ref )
PSI purified from Thermosynechococcus vulcanus	PSI/PtNP + artificial light-harvesting dye (Lumogen Red, LR) composite. The combination of PSI with PtNP and LR enabled hydrogen production in the presence of visible light.	ascorbic acid	HER	N.A.	Hydrogen generation was measured for 12 h.	N.A.	26.3 mol H2 (mol PSI)−1 h–1	N.A.	Nagakawa et al. (ref )
PSI from Synechocystissp. PCC 6803	Cysteine mutated- PSI covalently bound to gold electrode with Pt deposited on top of each SPI	N.A.	Photovoltage generation	0.12 mA/cm–2	N.A.	N.A.	A photovoltage of 0.386 ± 1.4 was generated by the trilayer configuration	N.A.	Frolov et al. (ref )
PSI from spinach	BiVO4/Au//PSI/Pt or BiVO4/Ag//PSI/Pt. H2O was the electron donor. The Pt-PSI and the mt-BiVO4 was covalently bound.	N.A.	HER	N.A.	Hydrogen evolution was measured for 72 h.	N.A.	Hydrogen evolution of 34 and 15 nmol h–1 using Au and Ag as mediators, respectively. 0.42 ± 0.20 and 0.091 ± 0.044 μmol h–1 mg Chl–1 of H2 were produced in the Au- and Ag-mediated systems.	External quantum efficiency of 10–5	Kim et al. (ref )
RC isolated from Rhodobacter sphaeroides	Self-assembled photoelectrochemical complex, comprised of an assembly of reaction centers, membrane scaffold proteins, and phospholipids onto SWCNTs.	Ferrocyanide, ubiquinone	C	20 nA	Stability was measured for 168 h, every 32 h a regeneration cycle was operated for a duration of 8 h.	OC potential	N.A.	Photoconversion efficiency >300% over 168 h	Ham et al. (ref )
RC from Rb. sphaeroides	Monolayers of RC immobilized on gold electrodes.	2,3- dimethoxy-5-methyl-6-geranyl-1,4-benzoquinone, Cyt c	C	150 nA	Photocurrent was measured for 80 s	–0.1 V vs Ag/AgCl	Electron transfer rate of 1.16 × 102 (tilting RC orientation, with cytochrome)	N.A.	Lebedev et al. (ref )
RCs from Rhodobacter sphaeroides	RCs bound on a Ni NTA terminated self-assembled monolayers on gold surface. The RC were bound with the acceptor side looking to the solution through a poly histidine tag (His7) constructed at the C-terminal end of the RC M-subunit.	ubiquinone-10, atrazine	C	30 nA	N.A.	OC potential	N.A.	N.A.	Trammell et al. (ref )
Light-harvesting antenna core (LH1–RC) complexes isolated from Rhodoseudomonaspalustris	The LH1–RC complexes self-assembled on an alkanethiol-modified gold electrode	Methyl viologen		Maximal photocurrent density of 5.1 ± 0.5 × 107 (A cm-2 Abs–1)	N.A.	–0.2 V vs Ag/AgCl	N.A.	Maximal quantum yield of 8.7 ± 0.8 (×103%)	Kondo et al. (ref )
PSI and PSII from thermophilic cyanabacterium Mastigocladus Laminsus	Layered assemblies of PSI, or PSII, on ITO electrodes with PBV2+ used as an interprotein “glue” and electron mediator. PBV2+/PSI/PBQ/PSII photoactive composite on an ITO electrode	Poly benzyl-viologen (PVB2+), poly lysine benzoquinone (PBQ), DCPIP, ascorbate	OER, ORR	2.2 μA cm–2 for the PBV2+/PSI/PBV2+/PSI layered assembly	N.A.	–0.05 V vs Ag/AgCl	N.A.	Quantum yield 4%	Yehezkeli et al. (ref )
PSI from Synechocystis sp. PCC 6803	Cysteine mutated- PSI covalently bound to gold electrode	N.A.	Light-induced charge separation	N.A.	N.A.	N.A.	A photovoltage of 0.45 V	N.A.	Carmeli et al. (ref )
PSI from spinach	PSI monolayers assemble on a gold electrode. Several covalent attachment strategies were examined.	2,6-dichloroindophenol (DCIP), sodium ascorbate	c	100 nA/cm2	Stable photocurrent over several hrs for dense monolayers from vacuum-assisted assembly.	–0.11 V vs Ag/AgCl	N.A.	N.A.	Faulkner et al. (ref )
PSI from spinach	PSI immobilized on the surface of nanoporous gold leaf (NPGL) electrodes	DCPIP, sodium ascorbate	C	∼800 nA/cm2	N.A.	–0.1 V vs Ag/AgCl	N.A.	N.A.	Ciesielski et al. (ref )
PSI from spinach	Multilayer assemblies od PSI on gold and glass substrates	Ferricyanide, ferrocyanide	C	12 μA/cm2	N.A.	∼300 mV vs Ag/AgCl	N.A.	Internal quantum efficiency (films fabricated by three deposition steps) of 0.014% and 0.017% at of 437 and 678 nm, respectively	Ciesielski et al. (ref )
PSI from Mastigocladus laminosus	Bis-aniline-cross-linked Pt NPs/PSI composites on Au electrodes and Bis-aniline-Cross-Linked Ferredoxin/Pt NP–Pt nanocluster/PSI composite on a Au electrode.	Ascorbic acid, DCPIP	c	1000 nA	N.A.	0.3 V vs Ag/AgCl	N.A.	Quantum yields of ∼ 2.6% and ∼ 3.8% and IPCE values of ∼ 0.35% and ∼ 0.5% (λ = 420 nm) for the configuration without and with the Fd unit, respectively.	Yehezkeli et al. (ref )
PSI from Thermosynechococcus elongatus	PSI immobilized on a gold electrode surface via an Os complex containing redox polymer hydrogel	Methyl viologen	ORR	29 μA cm–2	N.A.	0 V vs Ag/AgCl	N.A.	IPCE of 3.1%	Badura et al. (ref )
PSI from Thermosynechococcus elongatus	PSI natural VK1 quinone replaced with a VK1 quinone analog, with viologen end, adsorbed on a gold electrode	Viologen, DCIP, ascorbate	c	40 nA cm–2	N.A.	0 V vs Ag/AgCl	N.A.	N.A.	Terasaki Et al. (ref )
PSI from Thermosynechococcus elongatus	PSI with fused ZnO-binding peptide tag bound to ZnO nanowire	Co(II)/Co(III) ion-containing electrolyte Z813, methyl viologen	ORR	362 μA/cm2	N.A.	0 V vs Ag/AgCl	N.A.	Incident-light to electrical external power conversion efficiency of 0.08% with UV, 0.07% without	Mershin et al. (ref )
	PSI with designer peptide A6K air-dried onto nanocrystalline TiO2 paste on FTO glass
PSI from Thermosynechoccus elongatus	PSI self- assembled onto hydrocarbon monolayers with different head groups deposited on gold electrode	Os(bpy)2Cl2 and methyl viologen	C	75 nA	>3 h of intermittent illumination.	–0.1 V vs Ag/AgCl	N.A.	N.A.	Manocchi et al. (ref )
PSI from Thermo synechococcus elongatus	PSI containing redox-hydrogel film of a pH-dependent a poly(vinyl)imidazole Os(bispyridine)2Cl	Methyl viologen	ORR	322 ± 19 μA cm–2	Depends on the irradiation intensity, half-life ranging from <30 min for the strongest irradiation up to 15 h at 1 mW cm–2	0 mV vs Ag/AgCl	Electron transfer rate of 335 ± 14 e– s–1 PS1–1	N.A.	Kothe et al. (ref )
PSI from Mastigocladus laminosus	Unbiased PEC of Polymethylene blue/PSI/polybutyl-viologen//BiVO4/CoP	Polymethylene blue, polybutyl-viologen	OER, ORR	∼200 μA/cm2	a photocurrent drop of less than 20% during 5 h of biocathode measurement.	–0.2 V vs Ag/AgCl	∼0.1 mM H2O2 generated during 2 h	IPCE of 0.12% for	Herzallh et al. (ref )
					Stable OCV for more than 15 h.
PSI and PSII from Thermosynechococcus elongatus	Biophotovoltaic cell comprised of PSII immobilized on the electrode via an imidazole-coordinated bispyridyl osmium complex-based redox hydrogel and PSI immobilized on the electrode via a pyridine-coordinated bispyridyl osmium complex-based redox hydrogel.	Methyl viologen	OER, ORR	1.5 μA cm–2 (for the two-compartment cell)	N.A.	unbiased	N.A.	conversion efficiency of 3.6 × 10–7	Kothe et al. (ref )
PSI from Thermosynechococcus elongatus	Biomimetic folded PSI monolayers on an Au electrode modified with P–Os redox polymer (poly(1-vinylimidazole-co-allylamine)-[Os(bpy)2Cl]Cl). The Langmuir–Blodgett (LB) technique was used to control PSI orientation on the electrode. Several illumination configurations were examined.	Methyl viologen	ORR	∼3 μA/cm2	N.A.	0 mV vs Ag/AgCl	N.A.	External quantum efficiency of 0.012 ± 0.003)% for the tilted configuration	Wang et al. (ref )
PSI from Thermosynechococcus elongatus and PSII	Au/P-Os(I) redox polymer/PSI/viologen modified polymer/Hydrogenase biocathode combined with a P-Os (II) redox polymer/PSII-based bioanode demonstrating a fully light-driven Z-scheme mimic biophotovoltaic cell for bias-free water splitting.	Methyl viologen, viologen modified polymer (p-vio)	OER, HER	∼−17 μA cm–2 for Au/P-Os(I)/ PSI-LB with p-vio layer (while subjected to 210 mV vs SHE)	N.A.	The anode and the cathode- 0 mV vs Ag/AgCl	electron throughput of 183 ± 25 e– PSI–1 s–1 for (Au/P-Os(I)/PSI-LB/P-vio assembly under ambient air)	N.A.	Zhao Et al. (ref )
						The full cell is unbiased
PSI from Thermosynechococcus elongatus	Au/P-vio/PSI-LB/PEGDGE/P-Os/GOx biophootoanode coupled with carbon electrode modified with 1-pyrenebutyric acid N-hydroxysuccinimide ester (PBSE), GOx and HRP biocathode	Glucose	Glucose oxidation, H2O2 reduction	The biophotoanode generated 17.6 μA cm–2 at 200 mV vs SHE	The biophotoanode stability was tested for 90 min.	The full cell is unbiased	N.A.	N.A.	Wang et al. (ref )
PSI from Thermosynechococcus elongatus	Au/Os-complex-modified redox polymer deposited with mixed PSI monolayers of PSI trimers and monomers. A top of H2ase/BPEI-[CoCp2] or H2ase/viologen modified polymer (P-vio) were examined	Methyl viologen	HER	–(9 ± 1) μA cm–2 for the Au/P-Os/mixed PSI monolayers.	N.A.	0 mV vs Ag/AgCl	N.A.	N.A.	Wang et al. (ref )
PSI from Thermosynechococcus elongatus	Au electrodes modified with PSI-Pt NP complexes integrated in a poly(vinyl)imidazole Os(bispyridine)2Cl redox hydrogel.	Methyl viologen	HER	4.8 ± 0.4 μA cm–2, under anaerobic conditions	N.A.	0 mV vs Ag/AgCl	N.A.	N.A.	Zhao et al. (ref )
PSI from Synechocystis sp. PCC 6803	PSI-H2ase fusions embedded eithin Poly(1-vinylimidazole-coallylamine)-[Os(2,2’-bipyridine)2Cl]Cl redox polymer immobilized on Au electrode	Methyl viologen	HER	∼700 nA/cm2	Hydrogen production in vivo was tested for 25 min.	0 mV vs Ag/AgCl	0.01–0.02 μmol H2/min/mg Chl (in vivo)	N.A.	Wang et al. (ref )
					Photocurrent response was measured for a few min.
PSI from Thermosynechococcus elongatus	PSI/Os-complex-modified redox polymer (Os-P) on Au electrode. Catalase and superoxide dismutase were included.	Cu-tris(2-pyridylmethyl)amine nitrate (CuTPA), methyl viologen	ORR	∼375 nA while in O2-saturated solution	87% of the initial photocurrent was maintained while in air equilibrated solution	0 mV vs Ag/AgCl/	N.A.	N.A.	Zhao et al. (ref )
PSI from Thermosynechococcus elongatus	PSI/Os-complex-modified redox polymer (Os-P) on Au electrode.	Methyl viologen, 2,3-dimethoxy-5-methyl-1,4-benzoquinone (Q0)	c	∼150 μA cm–2	Tested for 930 min, improved stability with Q0 as electron acceptor under anaerobic conditions.	0 mV or 100 mV vs Ag/AgCl for MV2+ or Q0, respectively	N.A.	N.A.	Zhao et al. (ref )
PSII from Thermosynechococcus elongatus, Phycobilisomes (PBSs) from Acaryochloris marina, Mastigocladus laminosus, and Synechocystis sp. PCC 6803	PBS–PSII supercomplexes integrated within an Os-complex-modified hydrogel (P-Os) on macro-porous indium tin oxide electrodes	N.A.	OER	>10 μA cm–2 for all PSII-containing modifications	wavelength-dependent photocurrent response was measured during 700 s	0.4 V vs Ag/AgCl	C	IPCE 10.9% at 670 nm	Hartmann et al. (ref )
PSII from Thermosynechococcus elongatus	PSII-gold NPs (GNP) conjugates	Phenyl-p-benzoquinone (PpBQ), ferricyanide	OER	N.A.	N.A.	N.A.	575 ± 80 μmol O2 (mg Chl)−1 h–1	N.A.	Noji et al. (ref )
PSII-enriched membrane particles isolated from spinach (Spinacia oleracea L.)	FoF1-ATPase proteoliposome-coated PSII-based microspheres with core@shell structures.	DCPIP, K3[Fe(CN)6]	OER, ATP synthesis	N.A.	The rate of ATP production becomes much smaller and it reaches a plateau after about 80 min.	N.A.	1100 nmol ATP (mg Chl)−1. The rate of ATP production becomes much lower, reaching a plateau after 80 min	N.A.	Feng et al. (ref )
					At least 50% functionality of photodriven ATP synthesis was retained after 3 weeks of storage at 4 °C in dark
Proteorhodopsin from Gamma proteobacterium and PSII from Spinacia oleracea	An artificial organelle combining two photoconverters -PSII proteorhodopsin and ATP synthase. The organelle is encapsulated in a giant vesicle comprising pyruvate carboxylase, acetylcoenzyme A, and bicarbonate	phenyl-p-benzoquinone	ATP synthesis, carbon fixation (oxaloacetate, C4 formation from pyruvate, C3)	∼−3 fA for the artificial organelle (described as photocurrents/chromophores)	The artificial organelle sustained ATP conversion for 3 days (half-maximum efficiency) at room temperature and 1 month at 4 °C	N.A.	∼300 (OAA chromophores) and ∼ 300 (ATP/chromophores) during 20 min	N.A.	Lee et al. (ref )
Thylakoid Membranes from the chloroplasts of Spinacia oleracea	Thylakoid membrane-based energy module coupled with two different CO2-fixing enzymes, crotonyl-coenzyme A carboxylase/reductase and propionyl-CoA carboxylase. Sixteen enzymes of the crotonyl-CoA/ethylmalonyl-CoA/hydroxybutyryl-CoA (CETCH) cycle and a glyoxylate/hydroxypyruvate reductase from E. coli coupled with the thylakoid membrane-based energy module within water-in-oil droplets	Ferredoxin	NADPH and ATP production, CO2 fixation	N.A.	The CETCH version 7.0 coupled to TEM operating inside microdroplets demostrated glycolate production during 150 min.	N.A.	The integrated system produced 47 ± 5 mM glycolate from CO2 during 90 min	Carbon-conversion efficiency ∼ 3.5% (integrated system, full CETCH cycle)	Miller et al. (ref )
PSII from Thermosynechococcus elongatus	PSII reconstituted with platinum nanoparticles deposited on gold electrodes	1,5-diphenylcarbazide	c	15 nA cm–2	N.A.	0.10 V vs Ag/AgCl	TOF of 2 e– (PSII)−1 s–1	external quantum yield of 9.2 × 10–7	Miyachi et al. (ref )
PSII from Thermosynechococcus vulcanus	PSII, tagged with six histidine residues, was immobilized on a gold surface modified with a self-assembled monolayer (SAM) of a nickel-nitrilotriacetic acid complex (Ni-NTA).	N.A.	OER	2440 nA cm–2	Anodic photocurrent was maintained for at least tens of minutes for PSII immobilized on a planar gold electrode configuration	0.2 V vs Ag/AgCl	N.A.	N.A.	Terasaki et al. (ref )
PSII from spinach	ITO functionalized with layer-by-layer assembly of PSII and polyethylenimine-reduced graphene oxide multilayered films	–	OER	37.2 nA cm–2	∼65% of the initial photocurrent remained after 20 min illumination of the PEI–rGO/PSII films	0.2 vs Ag/AgCl	N.A.	Quantum efficiency of ∼ 0.0026%	Cai et al. (ref )
PSII from Thermosynechococcus elongatus	PSII covalently bound to ITO electrode modified with a self-assembled monolayer (SAM) of phosphonic acid ITO linkers with a dangling carboxylate moiety.	2,6-dichloro-1,4-benzoquinone	OER	4.5 μA cm–2	half-life time ∼ 12 min for the C-mesoITO|SAM-C2CO2 –|PSII, MET configuration	0.3 V vs Ag/AgCl	TOF of 4.6 ± 0.6 mol O2 (mol PSII)−1 s–1 for the C-mesoITO|SAM-C2CO2 –|PSII, MET configuration	IPCE of 0.1% for the C-mesoITO|SAM-C2CO2 –|PSII, MET configuration	Kato et al. (ref )
PSII from Thermosynechococcus elongatus	PSII on a mesoporous indium–tin oxide electrode.	Potassium 1,4-naphthoquinone-2-sulfonate, 2,6-dichloro-1,4-benzoquinone	OER	1.6 ± 0.3 μA cm–2 and 22 ± 2 μA cm–2, for DET configuration and with 2,6-dichloro-1,4-benzoquinone as electron mediator, respectively	A half-life time of 4 to 5 min under continuous red light illumination	0.3 V vs Ag/AgCl	TOF of 0.18 ± 0.04 (mol O2) (mol PSII)−1 s–1 and 3.2 ± 0.4 (mol O2) (mol PSII)−1 s–1, for DET configuration and with 2,6-dichloro-1,4-benzoquinone as electron mediator, respectively	IPCE 0.3% (635 nm, + 0.5 V vs NHE, in presence of 1 mM of NQS)	Kato et al. (ref )
PSII from Thermosynechococcus elongatus	PSII, poly(1-vinylimidazole)-Os(bipy)2Cl-polymer (POs) and phenothiazine-modified polymer integrated on hierarchically structured inverse opal indium tin oxide electrodes	2,6-dichloro-1,4-benzoquinone (DCBQ)	OER	381 ± 31 μA cm–2 and 513 ± 29 μA cm–2 for IO-ITO|POs–PSII in DET configuration and with DCBQ as electron mediator, respectively	∼7% and 11% of the initial photocurrent was detected after 60 min of irradiation, for the IO-ITO|POs–PSII electrode, in DET configuration and with DCBQ as electron mediator, respectively	0.3 V vs. Ag/AgCl	Maximal TOF of 4.0 ± 0.4 s–1 and 6.7 ± 0.7 s–1 for IO-ITO|POs–PSII in DET configuration and with DCBQ as electron mediator, respectively	An external quantum efficiency of 6.9 ± 0.9% and 9.3 ± 1.2% for IO-ITO|POs–PSII in DET configuration and with DCBQ as electron mediator, respectively	Sokol et al. (ref )
PSII from Synechococcus bigranulatus	PSII deposited on poly mercapto-p-benzoquinone (polySBQ) covered gold electrodes as a sensor for herbicide detection	Duroquinone	OER	8.17 × 10–7	Tested during 10 s of illumination with a 180 s interval between each light phase. The observed signal decreased exponentially with t 1/2 within ∼ 3 h.	250 mV vs Ag/AgCl	N.A.	N.A.	Maly et al. (ref )
PSII from Mastigocladus laminosus	poly(mercapto-p-benzoquinone), pMBQ/PSII//BOD/CNTs and bis-aniline-cross-linked Au NPs/PSII//BOD/CNTs PBEC	N.A.	OER, ORR	∼400 nA for pMBQ/PSII while subjected to 0.15 V vs SCE	For pMBQ/PSII/BOD/CNTs cell, stable photocurrent during 3h and then degraded by 15% upon 10 h	0.1 V vs Ag/AgCl and >0.1 V vs Ag/AgCl for pMBQ/PSII and for bis-aniline-cross-linked Au NPs/PSII anodes, respectively.	Electron transfer turnover rate between PSII and the electrode of 518 e– s–1, for pMBQ/PSII electrode	QY of 1.0% and 0.7% for pMBQ/PSII/BOD/CNTs and for bis-aniline-cross-linked Au NPs/PSII//BOD/CNTs PBECs, respectively.	Yehezkeli et al. (ref )
PSII core particles isolated from a CP47 His-tagged mutant from T. elongatus BP-1	IO-mesoITO|PSII photoanode// IO-mesoITO|H2ase cathode	2,6- dichloro-1,4-benzoquinone	OER, HER	For IO-mesoITO|PSII photoanode: maximal 930 ± 30 μA cm–2, 40 μm film thickness at 0.5 V vs NHE.	IO-mesoITO|PSII photoanode decayed to a few percent of its initial value within 1 h while subjected to 0.5 V vs NHE.	For a full cell, potentials >0.6 V are required for photocurrent generation.	After 1 h of full cell illumination and 0.9 V applied potential, 0.52 ± 0.04 (μmol O2) cm–2, and 0.96 ± 0.08 (μmol H2) cm–2 were generated.	Faradaic yield of (104 ± 5)% and (98 ± 2)% for OER and HER, respectively (full cell activation)	Mersch et al. (ref )
				For IO-mesoITO|H2ase cathode: ∼ −1.5 mA/cm2 at −0.6 V vs NHE	IO-mesoITO|H2ase cathode: After 5 h, >80% of the initial current remained, while subjected to −0.6 V vs NHE.
				Full cell: 450 ± 10 μA cm–2, 0.9 V
PSII from Thermosynechococcus elongatus	PSII-based photoanode combining dpp (a phosphonated diketopyrrolopyrrole dye) and POs [poly(1-vinylimidazole-co-allylamine)-[Os(bipy)2Cl]Cl redox polymer] coupled with FDH adsorbed on a hierarchically structured inverse opal titanium dioxide scaffold (IO-TiO2|FDH)	N.A.	OER, CO2 reduction to formate	For IO-TiO2|FDH −240 μA cm–2 at −0.6 V vs SHE	For IO-TiO2|FDH ∼ 83% of initial activity retained after 2 h at −0.6 vs SHE	The full cell was biased with 0.3 V	0.185 ± 0.017 μmol cm–2 formate (full cell)	Faradaic efficiency of (70 ± 6)% for formate generation	Sokol et al. (ref )
					For the full cell, the photocurrent decayed from 92 to 7 μA cm–2 after 1 h with a half-life time of ∼8 min
PSII from Thermosynechococcus elongatus	PSII-loaded on three-dimensional ITO and graphene electrodes.	2,5-dichloro-1,4-benzoquinone	OER	165.17 ± 24.93 μA cm–2 for ITO-40 750 nm electrode, with electron mediator	half-life of protein films inside the 750 nm IO–ITO electrode of ∼ 4 min, while subjected to 0.5 V vs SHE, with electron mediator	0.3 V vs Ag/AgCl	TOF of 7.17 ± 0.30 s–1 for ITO-10 3 μm electrode with electron mediator	N.A.	Fang et al. (ref )
PSII from spinach and from Thermosynechococcus elongatus	PSII adsorbed on hierarchically structured ITO electrodes	2,6-dichlorobenzoquinone C60 (N,N-dimethyl pyrrolidinium) idodide	OER	∼18 μA cm–2 for PSII adsorbed on ITO in the presence of C60-DMePyl matrix while subjected to 0.7 V vs SCE	N.A.	Stepped chronoamperometry measurements in the potential range of 0.15–0.75 V vs Ag/AgCl	N.A.	N.A.	Zhang et al. (ref )
PSI from Thermosynechococcus elongatus BP-1	Biohybrid electrodes comprising electrospun (e-spun) 3D ITO, PSI and cytochrome c.	N.A.	ORR	–1.5E–5 A cm–2 for e-spun ITO/PSI/cyt c with 60 min e-spun ITO, while subjected to −0.1 V vs Ag/AgCl	Photocurrent was measured with different light intensities and bias potentials for 25 min.	–0.1/–0.2 V vs Ag/AgCl	Number of electrons processed by each protein complex per second of 6.2 ± 1.5, for electrodes with 10 min spinning time	EQE of 0.14% for electrodes with 60 min spinning time (20 mW/cm2)	Nioradze et al. (ref )
PSI from Thermosynechococcus elongatus	PSI immobilized on 3D ITO electrodes with cyt c	Cyt c, 2,3-dimetoxy-5-methyl-parabenzoquinone (Q0)	ORR	270 μA cm–2 (for 3D-ITO15×–PSI–cyt c electrode, while subjected to – 0.15 V vs. Ag/AgCl)	A photocurrent of ∼ 100 μA cm–2 is retained during 30 min with Q0 for 3D-ITO15×–PSI–cyt c electrode, while subjected to −0.15 V vs. Ag/AgCl	–0.15/–0.2 vs Ag/AgCl	turnover number of 30 e– s–1 at PSI	N.A.	Ciornii et al. (ref )
PSI from Thermosynechococcus elongatus	PSI immobilized on IO-ITO	N.A.	ORR	10.1 μA cm–2	90% of the photocurrent output was retained after repeated illuminations for 90 min.	–0.1 V vs Ag/AgCl	TOF of 6.5 e– PSI–1 s–1	IQE of 4% and 0.8% EQE	Morlock et al. (ref )
					After 3 days at room temperature and a total operation time of 3 h, 90% of the photocurrent is retained.
PSII from T. elongatus	IO-TiO2/PbS QDs/POs/PSII anode and an inverse-opal antimony tin oxide (IO-ATO)| pyrenecarboxylic acid (PC)|BOD cathode	N.A.	OER, ORR	160 ± 36 μA cm–2 (while subjected to 0 mV vs Ag/AgCl). (anode) −287 ± 18 μA cm–2, (cathode, 8 IO-ATO layers) 76 ± 7 μA cm–2 (full cell)	Full cell stability tested during 30 min with 0.4 V applied.	The anode tested while subjected to 0 V vs Ag/AgCl.	electron turnover frequency of k et = 5.2 ± 1.1 e– s–1 (anode)	EQE of 0.36 ± 0.08 for the anode	Riedel et al. (ref )
						Maximal cathodic current at 0.3 vs Ag/AgCl

^aN.A. - Data were not listed or are irrelevant for this configuration.

^bEstimated from data within the article.

^cPhotocurrent generation was used as a measure of the photoelectrochemical electron transfer, rather than the catalytic product formation.

4. Architectures for Efficient Charge Transfer Processes

The photosynthesis process comprises highly efficient light-absorbing photosensitizers with near-unity efficiencies. However, the excited electrons will undergo back reactions and recombination unless the charges are spatially separated following the initial excitation. Sophisticated biotic architecture has developed, enabling efficient intra- and interelectron transfer processes. Therefore, developing successful artificial photosynthesis configurations requires the construction of scaffolds or layers that are tuned for an optimal electron-transfer process and charge separation. Over the last several decades, the fundamental structural and functional understanding of each photosynthetic component has been investigated, shedding light on its importance. The research stemming from the basic research has led to two distinct subdirections. The first is improving the natural process, mainly by rational design and bioengineering methodologies. − The second focused on replacing natural components with external unnatural elements that can perform similarly or surpass them. , This direction can be further divided into fully abiotic components − or biotic-abiotic interfaces. − In all of the above, one should consider how to pin each element into its specific spatial position to maximize performance and prevent back reactions, inactivation processes, short circuits, and more.

Previous and current research aimed to mimic the water-oxidation machinery by constructing new inorganic complexes ,− and further conjugating them with photosensitizers. Alternatively, semiconductor-based photocatalysts were used to enable efficient, long-term performance in water oxidation reactions. ,,−

The natural model dictates precise control over the photosensitizers and the redox-active position and orientation. Hence, toward photosynthesis biomimicry, different architectures have been developed to enable controllable charge separation and electron donor distances. − This section provides a variety of scaffold materials for the design of artificial photosynthesis.

4.1. Peptides or DNA Scaffolds for Artificial Photosynthesis

The utilization of DNA for unique architecture and scaffold construction has significantly evolved during the last few decades. − The developed platform enables site-specific positioning by using sticky ends. Thus, the DNA scaffold could be used to assemble site-specific photosensitizers and charge-separation acceptors, nanomaterials, , dyes, or enzymes − with controllable distance and position. An interesting approach involves using biological components, such as peptides − or DNA strands, as building blocks for self-assembled 2D and 3D structures. ,−

Amino acids are building blocks for biological machinery. Over billions of years of evolution, nature has evolved proteins and enzymes that enable a magnificent variety of catalytic reactions to exist on our planet. Can we use these simple blocks for the design of minimal photosynthesis processes?

The use of two- or three-amino-acid peptides for self-assembled 2D and 3D structures has been demonstrated for a variety of purposes. Kim and co-workers developed a photosystem-like configuration in which the photosensitizer, tetra­(p-hydroxyphenyl) porphyrin (THPP), was coupled to diphenylalanine to form a wire-like structure, as shown in Figure . The designed system enabled light-induced generation of NADPH by activating [Cp*Rh­(bpy)­H₂O]²⁺, M, which acted as a redox catalyst. NADPH produced, in turn, was used as a cofactor to facilitate the enzymatic reaction. It should be noted that the system requires the addition of TEOA, which is a sacrificial electron donor for continuous NADPH production.

12.

Light-harvesting peptide nanotubes, synthesized by the self-assembly of diphenylalanine with THPP and platinum nanoparticles, for light-induced NADPH generation. [Cp*Rh­(bpy)­H₂O]²⁺ and triethanolamine, TEOA, were used as electron mediator (M) and electron donor, respectively. Adapted with permission from ref . Copyright 2011 Wiley-VCH GmbH.

Ma and co-workers utilized a DNA scaffold to construct a Z-scheme configuration as illustrated in Figure . As presented, the DNA scaffold provided a controlled distance between the two photocenters, the TiO₂ and the CdS nanorods, enabling water oxidation and H₂ generation, respectively. The TiO₂ acts as a “PSII-like” photocatalyst while the CdS, which holds higher reducing energy due to its conductive band edges positions, acts as a “PSI-like”. Pt clusters were grown on the CdS nanorods to improve the proton reduction process. To enable an efficient electron transfer process and improved charge separation, a redox center that can shuttle electrons and protons is required. Therefore, benzoquinone was tailored on the DNA scaffold using a single-stranded DNA tail. The quinone facilitated the electron transfer process between the two photoactive centers and enhanced H₂ generation.

13.

TiO₂ and CdS nanocrystals were organized into a Z-scheme photosynthesis system by using DNA as a structure-directing agent. Increased H₂ production from water splitting was observed compared to either the photocatalyst alone or dispersed mixtures of the two. The inclusion of the electron mediator benzoquinone equidistant between the TiO₂ and CdS through DNA assembly further increased H₂ production. Reproduced with permission from ref . Copyright 2015 Wiley-VCH GmbH.

It has been shown that the optimal configuration enables a complete water-splitting reaction, producing both hydrogen and oxygen. The designed system has several key features that are critical for future artificial apparatuses:

• a.The DNA scaffold introduced a controllable spatial separation between the photocatalysts and the redox mediator.
• b.The system absorbs a broad wavelength range, with TiO₂ absorbing at shorter wavelengths and CdS absorbing at longer ones.
• c.Most semiconductor-based hydrogen generation systems require sacrificial electron donors to operate continuously. Here, TIO₂/CdS coupling enables its operation without sacrificial electron donors.

While this model presents an interesting approach, its absorbance is still limited, and shorter bandgap semiconductors, such as CdTe, may be coupled to extend the visible-wavelength absorbance.

4.2. Molecular Bridges for Artificial Photosynthesis

The development of new chemistries and methodologies for integrating chemical moieties or molecules in a controlled manner has enabled the construction of new electronic devices. Aiming for artificial photosynthesis processes, several components should be incorporated:

• (i)Light-harvesting antenna.
• (ii)A photosensitizer.
• (iii)Charge separation processes.
• (iv)Electron acceptors/catalysts.

Gust and Moore presented the fundamental principles for developing such configurations. These include chemical bond bridges that enable precise control of the distance and orientation of each component. Indeed, over the last few decades, significant efforts have been made to develop architectures that enable artificial photosynthesis configurations. ,

Li and co-workers presented a host–guest configuration that enables the self-assembly of a photosensitizer with a water-oxidation catalyst, Figure . The Ru-based photocatalyst was conjugated to a cyclodextrin, enabling host–guest interactions with the Ru-based water oxidation catalyst. The elegantly designed system reached a quantum efficiency of 84% under photoirradiation at 450 nm with a sacrificial electron acceptor.

14.

A host–guest configuration for light-driven water oxidation, comprising a cyclodextrin-modified ruthenium as a photosensitizer and phenyl-modified ruthenium complexes as the catalysts. Adapted from ref . Copyright 2015 American Chemical Society.

As mentioned, charge separation processes are critical for improving the performance of artificial photosynthesis systems. To that end, molecular wires have been developed to enable charge transport through insulating layers, thereby enhancing activity. The conjugated molecular wire can be tuned by adding with- or donating-groups to the designed wire. Efficient hole transport through a SiO₂ membrane was achieved, enabling the activation of inorganic catalysts. , The developed methodology opens new possibilities to conjugate catalysts toward artificial photosynthesis, or to enable coupling abiotic elements with biotic ones, as will be discussed later. The use of metal–organic frameworks for artificial photosynthesis was also examined. By conjugation of water oxidation catalysts with CO₂ to formate or CO reduction processes, a standalone artificial photosynthesis configuration can be developed.

Hong and co-workers recently developed an artificial photosynthesis configuration that fully mimics the photosynthetic light cycle. The designed system comprises a water oxidation catalyst, a charge transfer unit, and an NADP reduction catalyst to NADPH. The two phasic solution configurations utilized light irradiation as the sole energy source for facilitating water oxidation to O₂ and the production of NADPH. While the system presented is not fully bridged, it clearly demonstrates that an inorganic catalyst can provide a stable water-oxidation and NADP-reduction catalyst that can be activated by light irradiation in conjugated processes.

4.3. Photosynthesis Mimicry by Catalysts or Electrode Surface Design

Utilizing electrode surfaces for photodriven electrocatalysis, and specifically for photosynthesis mimicry, is of great importance for future energy applications. Unlike homogeneous catalysis, the heterogeneous catalysis process occurs at the electrode surface, allowing operation without an installed membrane or separation barriers between the anodic and the cathodic sides. As mentioned at the beginning of this article, the first demonstration of full water splitting sparked tremendous research into artificial photosynthesis. While photodriven water splitting generates hydrogen fuel, other critical chemical fuels derived from N₂ or CO₂ can be very beneficial. The first demonstration has used a wide-band gap TiO₂ photoanode, which developed the essential potential to enable both water oxidation and reduction.

Nevertheless, the developed system was limited to UV wavelengths for activation. With advances in material fabrication and design, a photoelectrode could be fabricated with the required properties to mimic the photosynthetic apparatus. Over the last few decades, dye-sensitized solar cells have been developed, enabling electrical power generation while visible light is being used for activation. , In the developed cells, redox mediators facilitated the charge transfer between the photoanode and the cathode, enabling electrical power or H₂ fuel generation.

While wide-bandgap semiconductors can potentially meet the energy requirements to facilitate CO₂ or proton reduction without biasing or the addition of sacrificial electron donors, they have limited visible-light absorbance. Therefore, designing a Z-scheme configuration with adjustable absorbance wavelength and energy levels surpasses the capabilities of a single photosensitizer. Toward that, several key features should be considered:

• A.The semiconductor layers may be fabricated at one or two electrodes.
• B.Cocatalysts should be considered to lower the activation energy and improve the catalytic processes.
• C.The hole- and electron-transfer processes should be optimized at the cathode and anode, respectively.
• D.Stability of the electrodes through the catalytic process in aqueous conditions and under continuous light irradiation.
• E.Complementary wavelength absorbance should be used by two (or more) semiconductors to fully exploit the visible light wavelength range fully.

Over the last few decades, electrodeposition and ALD techniques have been developed and refined to enable precise control of the layer composition and thickness. These layers enabled enhanced stability, improved hole and electron transport, and cocatalyst integration for an efficient catalysis.

An interesting approach demonstrated the development of bias-free devices enabling CO₂ or proton reduction from water. Figure schematically presents the designed configuration. TiCo cocatalyst was deposited on the BiVO₄-based photoanode to facilitate an efficient and stable water-oxidation reaction. The photocathode was constructed from an organic semiconductor blend, enabling efficient light absorption and adequate energy levels to support CO₂ or proton reduction. The designed cell achieved a photovoltaic cell efficiency (PCE) of 10.7 ± 0.9%.

15.

Design of OPV-BiVO₄ artificial leaves. a) Schematic diagram of an unassisted OPV-BiVO₄ artificial leaf. The PCE10:EH-IDTBR layer is illustrated as a violet-yellow blend. b) Energy level diagram of the OPV. Vacuum energy levels and redox potentials were obtained from the reported literature. c) Cross-section SEM image of an OPV cell. Adapted from ref . Available under a CC-BY 4.0 license. Copyright 2024 The Author(s).

5. Semiartificial Cells: From Biohybrid to Functional Cells

5.1. Photosensitizers Enzyme Hybrids for Fine Chemicals or Fuel Generation

5.1.1. Developing Photocatalytic Biohybrids Performing Semiartificial Photosynthesis

Artificial photosynthesis seeks to capture sunlight and convert available feedstocks (e.g., H⁺, CO₂, N₂) into fuels and value-added chemicals. Abiotic photoelectrochemical devices typically achieve high solar-to-chemical conversion but often lack the remarkable selectivity of enzymes, which operate under mild conditions. Semiartificial photosynthetic biohybrid strategies bridge these strengths by interfacing inorganic light absorbers (e.g., semiconductors, molecular dyes) with biocatalysts (purified enzymes or whole cells).

Upon illumination, the inorganic photoabsorber generates charge-separated states (e.g., promotion of electrons to the conduction band of a semiconductor or to an excited state of a molecular dye), which are then relayed to a biological acceptor to drive reductive biocatalysis (Figure ). Electron transfer process can proceed either by direct electron transfer (DET) across a wired interface or by mediated electron transfer (MET) via a diffusible redox mediator. For MET, the mediator’s potential must lie between that of the excited photosensitizer and the acceptor to ensure thermodynamically favored electron transfer. Hole quenching is provided by a sacrificial electron donor (SED) that fills photogenerated holes (h⁺). Although water serves this role in oxygenic photosynthesis, efficient water oxidation remains challenging for most artificial absorbers. Consequently, practical systems commonly employ chemical SEDs such as triethanolamine (TEOA) or ascorbate.

16.

A comparison between native and semiartificial photosynthesis. Left – a schematic illustration of the light-harvesting process performed by photosystem I (PSI) and photosystem II (PSII) in the thylakoid membrane in photosynthetic organisms. Water is used as the sacrificial electron donor, while ferredoxin (Fd) and NADP+ reductase accept the excited electrons to facilitate the reduction of NADP+ to NADPH. Right – a schematic illustration of a typical semiartificial photosynthetic process. An inorganic photosensitizer (PS) is irradiated, resulting in the generation of excited electrons (e^–) together with a positively charged hole (h+). An external sacrificial electron donor is required to fill the generated holes while a biological acceptor (typically a redox enzyme) consumes the electrons to drive reductive biocatalysis. Figure generated with Biorender.com.

Many semiartificial photosynthetic configurations focus on wiring hydrogenases, the main catalyst for biohydrogen production across diverse biological systems, to light harvesters catalyzing proton reduction to H₂. − Owing to its high energy density and the high efficiencies achievable in conversion devices (such as fuel cells), hydrogen is viewed as one of the most promising alternative carbon-free energy sources. While encouraging advances have been achieved, biological hydrogen generation often faces challenges for practical application due to parallel photochemical reactions that are typically more robust and efficient. Alternatively, biohybrid systems have been used to drive more complex reactions for the production of fine chemicals and value-added products while using the remarkable stereo- and enantioselectivity of enzymes. When the target biocatalytic product is chiral, enzymatic reduction offers a significant advantage over traditional chemical methods, which often yield a mixture of stereoisomers. In contrast, many oxidoreductase enzymes have evolved to produce a single stereoisomer with a near-complete enantioselectivity. This property is especially critical in the pharmaceutical industry, where insufficient enantioselectivity can lead to adverse effects. Therefore, we propose that the practical implementation of semiartificial photosynthetic systems should be directed toward highly selective commodities rather than the production of relatively simple end compounds. Additionally, numerous semiartificial photosynthetic systems aim to be incorporated to address global challenges, such as CO₂ and N₂ fixation. Work in semiartificial photosynthesis is commonly grouped into two classes: nanoparticle-enzyme assemblies (also coined in vitro biohybrids) and whole cells (in vivo biohybrids).

Enzymes have evolved to enable the variety of chemical reactions facilitated on Earth. The enzymes lower the activation barriers, which are translated into the chemical reaction toolbox used in nature. Biocatalyst structures often regulate selectivity and chirality, which are crucial for many essential processes. Natural machinery, however, comprises regulations that prevent the accumulation of a single product and the accompanying high energy costs in terms of ATP or NAD­(P)­H currency. Those can limit the production of specific compounds in excess, for example, in industrial applications. Coupling photodriven reactions with an enzymatic cascade opens a route to lower energy requirements toward continuous production of chemicals or fuels. Several challenges should be addressed in order to advance this direction into applications:

• A.The photosensitizer should be stable, enabling continuous operation.
• B.The photoactivated material should be nontoxic or easily separable from the product.
• C.The electron transfer (or, in some cases, hole transfer) should be efficient, with minimal back or side reactions.
• D.Photodriven reactions can generate reactive species that may damage the biocatalyst. Preventing or hindering such reactions is critical.
• E.If a chiral product is required, the reaction should be carried out exclusively by the biocatalyst to limit interference from the photocatalyst surface or ligands, which can be nonselective.

5.1.2. NP-Enzyme Biohybrids

As mentioned, Honda and co-workers showed that illuminated aqueous suspensions of photosensitive semiconductor powders can reduce CO₂ to formic acid, formaldehyde, methanol, and methane. Bridging the gap between light-harvesting inorganic materials, often synthesized and handled in organic solvents, and aqueous biological systems, this work has laid the groundwork for research on the biotic-abiotic interface. The study of artificially activating purified redox enzymes using inorganic light-harvesting semiconductor quantum dots (QDs) was further explored in the late 2000s, with most research efforts focused on hydrogenase enzymes. A landmark study coimmobilized a [NiFeSe] hydrogenase and a ruthenium dye on TiO₂ nanoparticles, establishing visible-light H₂ evolution at room temperature and clarifying the principles of scaffold–enzyme coupling, Figure a. This system demonstrated efficient solar-driven hydrogen production, characterized by a high turnover frequency (TOF) and long-term stability. Related studies further explored coupling hydrogenase with various nanomaterials, such as CdS nanorods (NRs), CdTe quantum dots (QDs), and carbon dots to enhance hydrogen generation. ,

17.

NP-hydrogenase hybrids for solar-driven hydrogen generation. (a) The activation of [NiFeSe]-hydrogenase by a synthetic ruthenium photosensitizer coattached to colloidal TiO₂ NPs. Adapted from ref . Copyright 2009 American Chemical Society. (b) The effect of capping ligands on the electron transfer from light-irradiated carbon dots to hydrogenase and fumarate reductase. Adapted from ref. Available under a CC BY 4.0 license. Copyright 2016 The Author(s).

To improve quantum yield (i.e., the number of molecules converted relative to the number of absorbed photons), it is essential to favor electron transfer of photoexcited high-energy electrons to the enzyme over exciton recombination. To achieve this, many studies have focused on engineering the interfacial interactions between the biological and inorganic components, which critically influence electron transfer from the photoabsorber to the enzyme. These noncovalent interactions are primarily governed by the surface ligand design of the nanomaterials. A notable example was demonstrated by Reisner and colleagues, who showed that carbon dots capped with positively charged ammonium-terminated ligands could efficiently transfer photoexcited electrons to negatively charged enzymes (e.g., fumarate reductase and hydrogenase) with high efficiency and stability (Figure b).

Alternatively, the inorganic photosensitizer can be covalently linked to the biological acceptor via bioconjugation. , This covalent attachment minimizes the distance between the light-harvesting material and the enzyme, which is a key factor for enhancing electron-transfer rates. Such covalent anchoring is more common with molecular dye photosensitizers, due to their smaller size compared to nanoparticles.

Extensive characterization of nanoparticle-hydrogenase biohybrids has paved the way for investigating more complex reductive transformations, such as CO₂ reduction and nitrogen (N₂) fixation, using various redox enzymes. As previously mentioned, there has been growing interest in reducing atmospheric CO₂ due to its significant contribution to global warming. In 2019, Reisner and co-workers reported a system in which formate dehydrogenase (FDH) was coupled to TiO₂ QDs functionalized with a ruthenium dye for photocatalytic reduction of CO₂ to formate. Formate, a liquid and transportable product, serves as a valuable feedstock for fine chemical synthesis (see Figure a). In a recent study, photosensitizer-surfactant micelles that coassemble with redox enzymes (hydrogenases, FDH), formed enzyme-micelle hybrids that enable semiartificial photosynthesis in water with improved direct electron transfer operation for H₂ evolution and CO₂-to-formate conversion. Recently, MtrCAB, a multiheme transmembrane protein originating from Shewanella oneidensis MR-1, was incorporated into a transmembrane nanoreactor constructed by polar lipid extract and octyl glucoside. The micelle-based bioreactor encapsulated a hydrogenase enzyme that acted as a catalyst for H₂ production. By coupling the hydrogenase-containing bioreactor with a carbon nitride photocatalyst, light-induced hydrogen generation could be observed incorporating MV to the inner compartment improved the catalytic performance to TOF 880 ± 154 h^–1, which is doubled compared to the rates in its absence. This work introduces a new platform for semiartificial photosynthetic nanoreactors.

18.

NP-enzymes biohybrids for CO₂ and N₂ fixation using sunlight as an energy source. (a) Demonstrating CO₂ conversion with a dye–semiconductor–FDH photocatalytic system. Adapted from ref . Available under a CC BY 4.0 license. Copyright 2019 The Author(s). Licensed under (b). The artificial activation of Mo–Fe nitrogenase produces ammonia from nitrogen gas. (A) The native activity of Mo–Fe nitrogenase is equipped with an iron–sulfur cluster, a process that consumes 16 mol of ATP molecules per 1 mol of fixed nitrogen. (B) The artificial activation of the enzyme using CdS NRs as a light-harvesting unit, which transfers electrons directly into the enzyme’s active site. Adapted with permission from ref . Copyright 2016 AAAS.

One of the most energy-intensive industrial processes is the conversion of atmospheric nitrogen (N₂) into ammonia (NH₃) via the Haber–Bosch process, which accounts for over 1% of global energy consumption. A biohybrid photocatalytic approach successfully replicated this transformation using MoFe nitrogenase activated by CdS nanorods (Figure b). Naturally, the enzyme requires coupling with an iron–sulfur cluster, a process that consumes large amounts of adenosine triphosphate (ATP). Therefore, using light to power this enzyme offers a significant energy-saving alternative. This pioneering study has inspired many research groups worldwide to activate the nitrogenase enzyme using a nanobio interface artificially. Ding et al. report a CdS@ZnS-nitrogenase biohybrid that uses light to drive C–C coupling and hydrogenation of CO in water, producing hydrocarbons. It has been shown that product selectivity was tunable by illumination conditions, with higher photon flux favoring longer-chain products. Our lab has also investigated the incorporation of semiconductor QDs for the activation of MoFe nitrogenase. CdS QDs were synthesized with different capping ligands for either direct or mediated solar-driven ammonia production. In a follow-up work, the effect of the nanoparticle size on the NP-nitrogenase interface was demonstrated using CdSe QDs as photoabsorbers. It has been shown that biohybrids with small NPs resulted in higher H₂ yields, possibly due to increased proximity of the small particles to the enzyme’s active site. Together, these examples highlight the vast potential of nanoparticle-enzyme biohybrid systems for the solar-powered production of fuels and essential chemicals.

While the construction of these biohybrids enabled photodriven enzyme activation, it required sacrificial electron donors, such as TEOA, for continuous activation, which limits their real-world applications. To tackle this issue, a Z-scheme configuration has been developed, Figure . The activation of the enzymatic reaction was achieved using two different semiconductor catalysts, BiVO₄ and SrTiO₃:La,Rh, to drive water oxidation and reduction processes, respectively. To enable efficient water oxidation and minimize back-reactions, a RuO₂ cocatalyst was deposited on the BiVO₄ particles. As described above, charge separation is crucial to efficient Z-scheme configurations. Therefore, Co­(bpy)₃ was added to the reaction mixture and acted as a redox mediator between the oxidation and the reduction apparatus. While BiVO₄ can oxidize water, its conductive band falls short of reducing directly protons or CO₂. Hence, by coupling two semiconductors, we can achieve the potential required for the activation of the hydrogenase or formate dehydrogenase. This concept was further developed into biocatalytic CO₂ reduction processes in the bacterial cells.

19.

A Z-scheme configuration, which utilizes particulate SrTiO₃:La, Rh, and BiVO₄:Mo (light absorbers), hydrogenase or formate dehydrogenase (cocatalyst), and a molecular cobalt complex (a redox mediator). Adapted from ref . Available under a CC BY 4.0 license. Copyright 2024 The Author(s).

It should be noted that the semiconductors used have limited absorption in the visible range, and an improved energy-level design should extend their capacity to utilize a broader range of the visible spectrum.

As presented in the section, biotic-abiotic configurations for artificial photosynthesis can be versatile, comprising different enzymes and photosensitizers. We summarize the main findings of this part in Table . As presented, efficiency and stability parameters enable an easy comparison between the different systems. It should be mentioned that the dominant enzyme for biohybrid assemblies is hydrogenase. That is expected as green H₂ was marked by many countries and official agencies as a critical green fuel for the different industries due to its high energy capacity (>120 MJ/kg), which is approximately 3 times that of gasoline. Also, bioengineering the proteins for extended performance and discovering new hydrogenases pushes further to advanced biotic-abiotic systems.

2. Protein-Based Biohybrids.

^aN.A. - not mentioned in the text.

^bN.Q. - not quantified.

5.2. Whole Cell Biohybrids: Rewiring Nature

Compared to single-enzyme systems, whole-cell systems offer enhanced ability to catalyze complex, multistep reactions through integrated metabolic pathways and coordinated enzyme networks. ,, Moreover, whole cells exhibit intrinsic regeneration capabilities, which contribute to greater operational longevity and efficiency while eliminating the need for protein purification and extensive maintenance. Nature itself offers an example in the form of photosynthetic microorganisms such as cyanobacteria, which naturally convert solar energy into chemical compounds. Significant efforts have been made to harness synthetic biology tools to redirect these native photosynthetic systems toward the production of value-added chemicals, rather than biomass or survival. Liang et al. demonstrate a biohybrid platform in which the cyanobacterium Nostoc sphaeroides self-assembles with indium-phosphide (InP) semiconductor nanoparticles to form a semiartificial photosynthetic system that drives enhanced production of the commodity chemical ethylene. The native ethylene-forming enzyme (Efe) in N. sphaeroides was verified, and the self-assembled InP-cyanobacteria hybrid showed an elevated photosystem I quantum yield and metabolic flux through ethylene production compared to unmodified cells. While promising, such efforts face substantial challenges due to the highly regulated nature of the photosynthetic apparatus, which limits its programmability. Consequently, recent research has increasingly focused on equipping nonphotosynthetic microorganisms with light-harvesting capabilities by integrating them with inorganic nanomaterials, which are not subjected to the same level of complex regulatory constraints.

When abiotic materials are integrated into a living system, toxicity against the host organism remains a major concern. Indeed, a large fraction of photosensitizer materials is composed of toxic elements, particularly heavy metals, that might interfere with essential biological functions. Figure presents the major challenges that need to be addressed in order to design whole-cell biohybrids using internal or external stimuli. As presented, the cell membrane hinders a direct electron-transfer process and limits the activation of enzymatic cascades. In order to overcome the electrical insulation barrier, chemical or redox mediators can be used. Those could be based on natural redox-active proteins such as MtrCAB from Shewanella with OmcS from Geobacter that can be bioengineered to the desired host and enable cross-membrane electron transfer using heme moieties whose distances and orientation dictate the electron transfer rate. It should be mentioned that these natural electron transfer processes, which hold low reducing power thermodynamically, are limited to being used directly for cofactor regeneration. , At pH 7, at least −0.62 V vs Ag/AgCl is required for H₂ generation. Electrochemical measurements have shown that the CymA, the internal redox protein, is at least 200 mV more positive than the required potential. Even the more negative potential of OmcA or MtrC falls short. On the other hand, several reports presented electron transfer through those proteins to activate the hydrogenase enzyme for hydrogen generation. , These differences could be explained by a small alteration in the protein structure while bonded to the membrane, or other structural changes that affect the redox potential.

20.

Schematic illustration of the electron transfer process between the abiotic electrode or NPs and a whole cell enzymatic biocatalysis. The activation could occur by gas diffusion into the cell through the membrane, by membrane-bound redox active proteins, or by diffusional redox mediators, all of which can be activated by electrodes or extrinsic photosensitizers. Alternatively, internally grown NPs can be used for photoactivation of overexpressed enzymes. By bioengineering, plasmids can be designed for different protein expression levels. The total scheme presents the leading routes toward interfacing abiotic with biotic and artificial photosynthesis configurations.

The phenomena can be found in other redox enzymes, for example, the activation by the nitrogenase enzyme by its electron donor, the iron protein. The interesting mechanism of the Omc and Mtr redox proteins should be fully elucidated, as it has great implications for whole-cell biocatalysis and future applications. Alternatively, photochemically or (photo)­electrochemically generated hydrogen can act as a chemical to electron carrier, enabling diffusion through the membrane. The hydrogenases can convert the generated H₂ in the cell to energy currencies such as NADH. Alternatively, non-natural redox-active species, such as cobaltocene or viologens, can be used to enable transmembrane electron transfer. It should be noted that the periplasmic space can introduce changes in terms of trapped reducing equivalents or biocatalysts, which hinder desired processes. To establish efficient photoinduced electron transfer processes, the photocatalyst can be internally grown. This can be achieved by introducing salts into the host growth medium and allowing their absorption. Those methodologies can introduce toxicity, which can lead to a destructive outcome (see separate section on stability), or can lead to nonhomogeneous NP growth that limits their performance as photocatalysts. For further reading on this critical subject, a comprehensive study focusing on integrating an electrochemical biotic-abiotic biohybrid system was recently introduced. ,

For example, chalcogenide-based semiconductors (e.g., CdS, CdSe, CdTe) are widely used in semiartificial photosynthetic systems, yet the presence of cadmium poses a significant biological risk. It should be noted that heavy metal toxicity is largely attributed to their ionic form (e.g., Cd²⁺, In³⁺) rather than their neutral form as nanomaterials. Once released, these cations interact with various cellular components, including enzymes and nucleic acids, causing an oxidative stress. Subsequently, many essential cellular processes could be damaged, including DNA replication, ATP synthesis, and membrane integrity. As mentioned, many organisms use biomineralization as a detoxification strategy to avoid the presence of such heavy metal ions. Hence, using directed and controlled biosynthesis of heavy-metal-containing nanomaterials can significantly reduce their toxicity. ,

Photosensitizer cytotoxicity is not limited to heavy metals, as other organic light-harvesting compounds can inhibit cell viability. For instance, the widely used organic dye proflavine, an acridine derivative, exhibits strong antimicrobial activity through its reactivity with DNA and various essential cellular proteins. Therefore, employing heavy-metal-free and nontoxic photosensitizers (e.g., carbon dots) in semiartificial photosynthetic configurations holds great promise. By combining efficient light-harvesting capabilities with low cytotoxicity, such photosensitizers allow for improved biocompatibility, enhanced scalability, and low environmental impact.

A notable example of whole-cell semiartificial photosynthesis was reported in 2016 by Sakimoto and co-workers, who utilized the acetogenic bacterium Moorella thermoacetica, known for its ability to precipitate CdS particles on its surface. Remarkably, this process enables the bacterium to acquire photocatalytic activity. Upon light irradiation, these hybrid cells significantly enhanced the conversion of CO₂ into acetic acid without the need for external electron mediators. This biomineralization occurs naturally as a detoxification response, wherein M. thermoacetica forms ∼ 10 nm CdS QDs on its surface when exposed to elevated Cd²⁺ concentrations. In their study, cysteine was introduced as both the sulfur source for QD formation and a sacrificial electron donor. When irradiated, the photoexcited QDs donated electrons, either directly or indirectly, to the Wood–Ljungdahl pathway, a native metabolic route responsible for CO₂ fixation into acetic acid. Impressively, approximately 90% of the light energy harvested was directed toward acetic acid production, representing one of the first demonstrations of in vivo semiartificial photosynthesis.

Expanding on this concept, a similar system was later developed using externally synthesized gold nanoclusters (AuNCs) as the light-harvesting component. Owing to their semiconductor-like properties, small size (1–3 nm), and lower toxicity compared to cadmium-based materials, AuNCs have gained attention as a more biocompatible alternative. These nanoclusters exhibited improved cytoplasmic penetration, leading to enhanced acetic acid production within host cells.

The integration of genetic engineering with whole-cell biohybrid systems holds great potential to diversify the metabolites that can be produced in vivo. Genetically tunable organisms such as E. coli and S. cerevisiae offer ideal platforms for implementing electron-driven biocatalytic processes. For instance, Ishihara et al. (2016) reported the coupling of commercially available anatase-phase TiO₂ nanoparticles with recombinant E. coli expressing [Fe–Fe]-hydrogenase genes for light-driven hydrogen production (Figure a). Methyl viologen (MV) served as an electron mediator, exploiting its membrane-permeable nature and redox activity. This study marked the first successful example of using a recombinant E. coli and inorganic NP hybrid for photocatalytic hydrogen generation.

21.

Enhanced hydrogen evolution in recombinant E. coli coupled with inorganic semiconductors. (a) TiO₂ anatase semiconductor enabled photocatalytic production of hydrogen, using [Fe–Fe]-hydrogenase and methylviologen (MV) as an electron transfer mediator. Adapted with permission from ref . Copyright 2016 Wiley-VCH GmbH (b) Precipitation of CdS QDs on the cell due to the expression of a sulfur-reducing enzyme, followed by enhanced hydrogen production by endogenous [Ni–Fe]-hydrogenase. Adapted with permission from ref . Copyright 2017 Wiley-VCH GmbH.

In a complementary study published in 2017, Wang and co-workers engineered E. coli to biosynthesize CdS QDs on their surface for hydrogen production (Figure b). This was achieved by expressing cysteine desulfhydrase, an enzyme that converts cysteine into hydrogen sulfide (H₂S), a necessary precursor for CdS precipitation. Under anaerobic and illuminated conditions, the activity of native [Ni–Fe]-hydrogenase in E. coli was significantly enhanced, resulting in an additional 400 μmol of hydrogen over 3 h compared to unmodified controls.

Metabolomic profiling further revealed substantial shifts in key intracellular metabolites (e.g., NADH, pyruvate, lactate, and formate), indicating the significant metabolic impact of the light-driven biohybrid activity.

As the field of light-powered whole-cell biohybrids continues to expand, most studies have focused on hydrogen production, either via native or recombinant hydrogenases. However, a significant challenge remains in distinguishing hydrogen generated directly by the semiconductor from that produced via biohybrid electron transfer pathways. Electron transfer (ET) from inorganic photosensitizers to target redox enzymes within cells is often hampered by two primary limitations: (i) the complex intracellular environment, which can lead to off-target electron transfer, and (ii) spatial separation between key functional modules, which reduces ET efficiency and overall product yields.

To overcome these challenges, various strategies have been developed to improve the extracellular electron transfer (EET) in whole-cell biohybrids. One prominent approach leverages the Mtr complex, a native multiheme electron conduit in the electroactive bacterium Shewanella oneidensis MR-1. , This system enables the exchange of electrons between the cell and its external environment. The Mtr pathway has also been heterologously expressed in E. coli, significantly enhancing EET capabilities. Inspired by this mechanism, Li and colleagues engineered a modular system wherein the outer membrane protein OmpA was fused with a TiO₂-binding peptide to facilitate close coupling between TiO₂ nanoparticles and intracellular redox machinery (Figure a). This design led to an 81-fold increase in hydrogen production mediated by the hydrogenase enzymes. Kalathil and colleagues have offered an elegant strategy to overcome EET limitations. In their study, a synthetic light harvester (carbon nitride) is interfaced with a two-microbe coculture. First, the inorganic light-harvesting module is interfaced with the electrically conductive bacterium Geobacter sulfurreducens (rich in multiheme c-type cytochromes), thereby enabling efficient EET. Then, the excited electrons are transferred to the methanogenic archaeon Methanosarcina barkeri via a conductive protein filament, allowing for solar-driven CH₄ formation.

22.

Establishing extracellular electron transfer (EET) in photosynthetic whole-cell biohybrids. (a) Using a membrane anchoring domain to establish a new NP-acceptor interface for enhanced EET. The anchoring protein OmpA binds both light-harvesting TiO₂ NPs and the hydrogenase enzyme, generating a directed electron transfer pathway for improved light-driven hydrogen production. Adapted from ref . Copyright 2024 American Chemical Society. (b) and (c) Employing a liquid–liquid phase separation (LLPS) strategy to overcome limited EET in whole-cell biohybrid systems using gold nanoclusters and CdS QDs as light-harvesters, respectively. Adapted with permission from ref . Copyright 2024 Elsevier, Adapted from ref . Available under a CC BY 4.0 license, Copyright 2024 The Author(s), respectively.

A more straightforward and widely used approach involves the application of membrane-permeable redox mediators to shuttle electrons from external light-activated materials to intracellular targets. While effective, this strategy has notable drawbacks. Many redox mediators (such as MV) have limited biocompatibility and are known toxins (some were initially developed as herbicides). Their membrane permeability and intracellular distribution are also often unclear, and the reliance on exogenous chemicals limits system sustainability and scalability. Alternatively, by bioengineering, one can overexpress and tune natural redox mediators such as phenazines or derivatives to enable MET without toxicity.

Another innovative strategy to overcome spatial compartmentalization involves the engineering of membraneless organelles within cells. In one such study, Li et al. colocalized AuNCs and hydrogenase enzymes inside E. coli using liquid–liquid phase separation (LLPS). A recombinant fusion protein containing hydrogenase and an intrinsically disordered region was used to facilitate phase-separated compartment formation (Figure b). Additionally, histidine-rich tags were introduced into hydrogenase to enhance its interaction with the AuNCs, resulting in a 40-fold enhancement in hydrogen production compared to that of control systems. In follow-up work, the same team demonstrated in situ synthesis of CdS QDs using hydrogenase as a capping agent (Figure c), creating defined intracellular nanoenvironments with improved photostability and catalytic performance. Luo et al. constructed a whole-cell biohybrid system by incorporating CuInS₂/ZnS quantum dots into the periplasm of the electroactive bacterium Shewanella oneidensis, thereby enabling light-driven hydrogen production. Importantly, by colocalizing the nanomaterials and the acceptor hydrogenase enzyme within the periplasmic space, the authors overcome electron transfer limitations. Thus, the periplasmic strategy offers a promising route to improve robustness and modularity of semiartificial photosynthetic systems.

An increasingly attractive alternative is to enable whole cells to biosynthesize inorganic nanomaterials from metal salt precursors. This bottom-up approach eliminates the need for presynthesized nanomaterials, supports self-sustaining systems, and allows for continuous regeneration and turnover of active components. Furthermore, such biosynthesis can occur intracellularly, potentially bypassing EET limitations caused by compartmentalization. Kim and co-workers report an elegant whole-cell biohybrid approach in which the diazotroph Azotobacter vinelandii biosynthesizes cadmium-sulfide (CdS) semiconductor nanoparticles intracellularly by supplying CdCl₂ and leveraging endogenous cysteine desulfurase activity to generate the sulfide precursor. Under illumination, the excited electrons in the in vivo-grown CdS QDs are transferred to the native nitrogenase enzyme, boosting light-driven ammonia production.

Bachar and co-workers presented a novel strategy for constructing whole-cell biohybrids based on the SP1 self-assembled protein scaffold to guide the biosynthesis of size-constrained inorganic nanomaterials within living cells (please see section ). Stable protein 1 (SP1) is a homooligomeric protein comprised of 12 identical subunits that assemble into a ring-shaped structure with an inner diameter of ∼ 3 nm. This engineered nanocage functions as a template for nanomaterial production in the cell, endowing the host organism with entirely new capabilities. We showed that integrating semiconductor QDs biosynthesis could facilitate intracellular electron transfer to drive desirable redox transformations.

While most self-synthesized nanoparticle systems lack precise control over particle characteristics, protein-mediated biosynthesis offers a more tailored approach. For example, Wei and colleagues used a surface-display strategy to express the metal-binding protein PbrR on E. coli, facilitating the controlled formation of CdS QDs directly on the cell surface. This resulted in smaller, more uniform QDs with improved photocatalytic performance and reproducibility. Wang and co-workers developed a biotic-abiotic configuration that fully converts CO₂ and H₂O into chemical fuels and oxygen using solely light irradiation as an energy source. Here, the Z-scheme photocatalysts Cr₂O₃/Ru-SrTiO₃:La,Rh|ITO|RuO₂-BiVO₄:Mo facilitated water splitting to oxygen and H₂. The produced hydrogen was continuously uptaken into S. ovata bacteria, enhancing the production of acetic acid, Figure . The system presented a decline in activity after 15 h of operation. The authors attribute the activity loss to the Cr₂O₃ cocatalyst degradation. Indeed, the activity was restored by redepositing the Cr₂O₃ for at least 45 h with 82% activity as compared to the initial run. The generated acetic acid was further utilized for a “dark cycle” in which G. sulfurreducens, a commonly used bacterium in microbial fuel cells, consumed the acetic acid while generating electrical current. The work demonstrates the advantages of using biotic-abiotic interfaced systems, for example reloading cocatalyst similarly to the D1 replacement that occurred in PSII. The work presents a Z-scheme light cycle coupled with a dark cycle toward chemical production or electrical energy generation.

23.

Schematic representation of a biotic-abiotic interface configuration that converts CO₂ and H₂O into chemical fuels and oxygen using solely light irradiation as an energy source. The system comprises Cr₂O₃/Ru-SrTiO₃:La, Rh|ITO|RuO₂-BiVO₄:Mo that facilitates the water oxidation reaction to yield oxygen and hydrogen. The latter was uptaken into S. ovata bacteria, enhancing the production of acetic acid. Adapted with permission from ref . Copyright 2022 SNCSC.

As shown, whole-cell biohybrid configurations hold great potential for artificial photosynthetic systems. Transforming nonphotosynthetic bacterial cells into light-induced machineries by introducing abiotic photosensitizers may lead to real-world applications. Using whole-cell bacteria should allow a straightforward transformation into large-scale production. Table summarizes the leading biotic-abiotic whole-cell configurations presented in this section.

3. Whole-Cell Biohybrid.

The microorganism	The abiotic catalyst	The catalytic reaction	Applied electric potential (V) \ vs Ag/AgCl	Apparent quantum yield/efficiency	Faradaic efficiency	Stability (h)	Added value products	Power output	Product selectivity	Ref.
E. coli, recombinant	[Ru(bpy)3]2+, Eosin Y, methyl viologen	HER	N.A.	1.50%	N.A.	120	H2 (138 nmol mL–1 h–1)	N.A.	N.A.
E. coli, recombinant	Eosin Y photosensitizer	HER	N.A.	1.1% (over 5 h)	N.A.	24	H2 (1.6 μmol mL–1 OD600–1)	N.A.	N.A.
E. coli, recombinant	CdS NPs	HER	N.A.	≤5.94%	N.A.	12	H2 (13.4 mmol over 12 h)	N.A.	N.A.
E. coli, recombinant	TiO2, methyl viologen	HER	N.A.	1.57%	N.A.	15	H2 (216 μmol over 15 h)	N.A.	N.A.
E. coli	CdS NPs	HER	N.A.	7.93%-9.59%	N.A.	3	H2 (>1.8 mmol μmol over 3 h)	N.A.	N.A.
E. coli, recombinant	Graphite felt electrode	EET	0.2	N.A.	N.A.	96	N.A.	N.A.	N.A.
E. coli, recombinant	TiO2 photosensitizer, methyl viologen	HER	N.A.	5.68%, 4.06% at t = 12 h	N.A.	12	H2 (3.6 μmol over 12 h)	N.A.	N.A.
E. coli strain 1/HM, recombinant	AuNPs, methyl viologen	HER	N.A.	20.30%	N.A.	168	H2 (>80 μmol over 24 h)	N.A.	N.A.
E. coli, recombinant	CdSeS NPs	HER	N.A.	11.63%	N.A.	27 (using 3h “on” and “off” cycles)	H2 (650.5 μmol over 15 h)	N.A.	N.A.
Shewanella oneidensis MR-1	CuInS2/ZnS	HER	N.A.	15.02%	N.A.	48	H2 (≤1.6 mmol over 48 h)	N.A.	N.A.
E. coli, recombinant	CdS NPs	HER	N.A.	N.A.	N.A.	96	H2 (81.8 μmol over 24 h)	N.A.	N.A.
X. autotrophicus 7C	CoPi, Co–P alloy cathode	HER, OOR, CO2 fixation, acetylene reduction	3	N.A.	4.50%	120	Ammonia (rate of 1.9 × 104 molecules cell–1), ethylene (127 μM h–1 OD600–1)	N.A.	N.A.
Shewanella oneidensis MR-1	Eosin Y, proflavine, Ru(bpy)3 2+, fluorescein, methyl viologen	HER, CO2 reduction, pyruvate reduction, fumarate reduction	N.A.	0.5%-2.1%	N.A.	96	H2 (1 μmol over 24 h)	N.A.	93% lactate (from pyruvate), 70% succinate and 30% malate (from fumarate)
Shewanella oneidensis MR-1	CuO/ZnO/CuO photoanode	HER	–0.8 ∼ −0.85	N.A.	20%-90%	48	H2 (13.6 mmol m–2 day–1)	N.A.	N.A.
Azotobacter vinelandii	CdS QDs	nitrogen reduction	N.A.	N.A.	N.A.	8	ammonia (3.56 mg g–1 cells over 8 h)	N.A.	N.A.
Geobacter sulfurreducens KN400, Methanosarcina barkeri	Carbon nitride photosensitizer	CO2 reduction, Interspecies electron transfer	N.A.	N.A.	N.A.	120 per cycle (three cycles recorded)	CH4 (110 μmol over 120 h)	N.A.	Up to 100% methane
Chlorella minutissima	ITO-WO3–PDA electrode	CO2 reduction	0.5	4.20%	86%	1	formate (4.2 μM after 1 h), 24 μA cm–2	N.A.	N.A.
Methanobacterium sp., Hydrogenophilaceae	TiO2/CdS, carbon paper electrode	CO2 reduction	bias free	N.A.	94.40%	144	CH4 (15L m–2 day–1)	N.A.	N.A.
Microbe-containing wastewater	Ni single-atom catalysts, Si wires	CO2 reduction, hydrogen production	0.294	N.A.	>96.6% (CO H2 mixture)	45	Gas mixture (45 μmol H2 and 30 μmol CO after 45 h), 1.0–1.1 mA cm–2	N.A.	60% H2,40% CO (after 45h)
Synechocystis sp. PCC 6803	Dichloro-benzoquinone, ITO, Pt	CO2 fixation, EET	–0.497 to −0.697	29%	N.A.	N.A.	236 μA cm–2	N.A.	N.A.
Moorella thermoacetica	CdS NPs	CO2 fixation	N.A.	2.50%	N.A.	120	acetic acid (1.25 mM over 60 h)	N.A.	N.A.
M. thermoacetica	Au nanoclusters	CO2 fixation	N.A.	Up to 3%	N.A.	96	acetic acid (4.5 mM over 4 days, per 1 g cells)	N.A.	N.A.
Sporomusa ovata	SrTiO3:La,Rh, BiVO4:Mo, ITO	CO2 fixation, OER	N.A.	21.30%	88.20%	15 per cycle (three cycles recorded)	acetate (9 mM over 15 h)	N.A.	N.A.
R. palustris	Zinc–nickel based catalyst (ZGGO:Ni)	CO2 fixation, lycopene biosynthesis	N.A.	N.A.	N.A.	480 (CO2 fixation)	Lycopene (8.80 mg L–1)	N.A.	N.A.
R. palustris, Synechocystis PCC 6803	Zinc based catalyst (ZGGO), Zinc–nickel based catalyst (ZGGO:Ni)	CO2 fixation, HER, NADPH regeneration, lycopene biosynthesis	N.A.	N.A.	N.A.	96	H2 (1.38 mmol h–1 g–1), lycopene (8.80 mg L–1)	N.A.	N.A.
Clostridium thermocellum	BiVO4/CoP photoanode	Cellulose degradation, glucose\cellobiose oxidation	0	60% (before cocatalyst deposition)	40.60%	76	H2 (33 μmol over 2h), >400 μA cm–2 (over 72 h)	N.A.	N.A.
E. coli, recombinant (SP-1 cadmium binding peptide)	CdS QDs, methyl viologen	NADPH regeneration, Stereospecific imine reduction	N.A.	N.A.	N.A.	9	(R)-MPN (∼8 mM over 9 h)	N.A.	99.2% (R)-MPN
E. coli, recombinant	CdS/NiO	NADPH regeneration, Stereospecific imine reduction	bias free	N.A.	85%, 51% (bias-free setup)	14	(R)-MPN (≤10 mM after 14 h)	N.A.	99.5% (R)-MPN
E. coli, recombinant	Cd QDs, Rhodium dye	NADH regeneration, CO2 fixation	N.A.	N.A.	N.A.	36	formate (up to 14.2 mM)	N.A.	N.A.
Saccharomyces cerevisiae	InP (polyphenol functionalized)	NADH regeneration, shikimic acid production	N.A.	1.58%	N.A.	72	shikimic acid (∼45 mg L–1)	N.A.	N.A.
Ralstonia eutropha	Eosin Y	NADH regeneration, CO2 fixation, acetoin production	N.A.	2.60%	N.A.	50	acetoin (1.41 mM over 25 h)	N.A.	N.A.
Chlorella vulgaris	Osmium redox polymer	Power generation, ORR	0.35	7.4 × 10–4	N.A.	N.A.	N.A.	1.5 μW cm–2	N.A.
E. coli, recombinant	Bioproduced phenazine, Carbon paper electrode	power generation, EET	–0.1 to 0.7 (SWV)	N.A.	N.A.	300	phenazines (>200 μM after 30 h), 200 μA cm–2	80.6 μW cm–2	N.A.
Nostoc sphaeroides	Indium phosphide NPs	Ethylene production	N.A.	40%	N.A.	120	ethylene (4.4 μmol per gr cells per day)	N.A.	N.A.

6. (Photo)bioelectrochemical Cells for Fine Chemical or Fuel Generation

6.1. Photodriven Water Oxidation Configurations

As discussed, PSII has a critical role as the engine of life. The light-induced water oxidation reaction provides the essential step to convert inorganic materials into chemical energy and biomass. Emulating this process using photobioelectrochemistry can be achieved by advanced material design and a well-interfaced biotic-abiotic configuration. During the past decades, two different approaches were mainly developed to allow water oxidation and fuel generation (H₂ generation, CO₂ reduction to fuels):

• A.Using bioanodes, mainly PSII isolated proteins, thylakoid membranes, leaves, or cyanobacteria cells as photocatalysts on an electrode support.
• B.Inorganic catalysts (semiconductors, homogeneous catalysts) were used to drive the water oxidation reaction.

The utilization of isolated PSII proteins for water oxidation has been discussed in Section .

As shown, the photocurrents and stability of such devices are limited and lag far behind those of alternatives. Recently, it has been demonstrated that the cyanobacterium Synechocystis sp. PCC 6803 can be coupled to ITO electrodes to enable photocurrents of 245 μA/cm² with 29% efficiency. These outstanding high photocurrents were achieved through state-of-the-art electrode surface design, in which micropillar ITO structures were printed onto an FTO-coated glass. While this is a significant advance and presents top performance, the designed configuration requires diffusional redox mediators and an applied bias of 0.5 V for its activation. By excluding the redox mediators, the system can still operate, however, with significantly lower photocurrents. Alternatively, semiconductor-based photoanodes, e.g., BiVO₄/MO _x , can achieve such performance using a 1 V lower applied potential!

Therefore, with the current advances, utilization of the biotic process at the anode is less favorable. On the other hand, CO₂, N₂, H⁺, and O₂ reduction processes may be facilitated at the cathode, activated by the photoanode’s abiotic photoinduced reaction. In the next section, which focuses on waste degradation for energy generation, a holistic approach that uses both biotic and abiotic catalysts at the anode is presented and discussed.

A decade ago, Choi and co-workers presented an elegant, straightforward methodology for electrodepositing porous BiVO₄ layers on conductive surfaces. , The outstanding stability and efficiency of these fabricated photoanodes paved the way for their use in water oxidation devices, mainly PECs. As part of these scientific efforts, it was shown that these photoanodes can be coupled with biological processes. BiVO₄ photoanodes can reach up to 90% incident photon-to-current efficiency, and the developed electrodeposition protocol can be easily scaled up. ,

Toward realizing a biotic-abiotic photobioelectrochemical cell, Mukha and co-workers presented a design comprising a BiVO₄ photoanode coupled with a biocathode. The BiVO₄ photoanode was coupled with a biocathode based on bilirubin oxidase (BOD) as an oxygen reduction biocatalyst, Figure . The BiVO₄ photoanode was used for water oxidation to generate the oxygen. To improve photoanode performance, water oxidation was further enhanced by electrodeposition of a nickel–iron (FeOOH/NiOOH) or cobalt phosphate (CoP) layer as a cocatalyst. Although the nickel–iron layer was more robust at high potentials, the CoP layer produced higher photocurrents at lower voltages, down to 0 V (vs Ag/AgCl). The mesoporous structure of the BiVO₄-CoP results in a high surface area, leading to high photocurrent generation.

24.

Highlights of a BOD-based PBEC. (a) Water oxidation by the BiVO₄–CoP photoanode under irradiation (in yellow). In parallel, the BOD-based biocathode reduces molecular oxygen into water. The BOD biocathode utilized either direct or mediated ET (gray and brown-gray cathode, respectively). The power curves for direct and mediated ET PBEC are shown (b+c, respectively). Adapted with permission from ref . Copyright 2020 Wiley-VCH GmbH.

Under light irradiation and applied potential above −0.5 V vs Ag/AgCl, anodic photocurrent could be obtained. Therefore, to construct a bias-free photobioelectrochemical cell, a cathodic photocurrent should be reached while an applied potential is positive than −0.5 V. In the presented work, two different biocathodes were developed, based on either mediated or direct electron transfer process (MET and DET, respectively). Although both configurations generated bioelectrocatalytic currents of up to 3 mA/cm² under oxygen-saturated conditions, the mediated approach using MvBOD and the ABTS redox mediator entrapped in a polydopamine layer displayed superior stability and lower onset potentials than DET configurations. Entrapping redox-active molecules or binding them to polymeric chains via enzymes has been proven successful in many bioelectrochemical systems, e.g., for sensing , or hydrogen generation. − By coupling the DET and MET biocathodes with the BiVO₄ photoanode, power outputs of ∼ 0.8 and 0.7 mW/cm² were observed under oxygen saturation, Figure b, c, respectively. These high-power outputs were dictated by the high photocurrents, bioelectrocatalytic currents, and open-circuit voltages (OCVs) of 1 V. Overall, the developed biotic-abiotic configuration demonstrates a bias-free O₂/H₂O recycling system that produces high-power outputs at low fabrication and materials cost.

Converting light energy into electrical power in a bias-free configuration is of great importance. Nevertheless, it requires an electrical grid or energy-storing devices for practical use in the future. Alternatively, water-oxidation reactions can be coupled to biocathodes to generate fuels. BiVO₄-based photoanodes fall short in terms of developed potential to generate H₂ without biasing the electrodes. Therefore, an additional photocatalyst is required for such a task. As learned from nature, the energy levels of the photosensitizers should be aligned to enable at least −0.41 and 0.82 V vs NHE at pH 7 to catalyze H₂ generation or water oxidation, respectively. The above potential usually requires overpotentials for an effective catalytic process. Therefore, a few hundred mV should be added to the thermodynamic potential, depending on the catalysts and the system configuration. During the last decades, highly efficient photocathodes have been developed. However, often, their stability in aqueous solutions was limited. Moore and co-workers have developed a tandem system that couples the perovskite’s photo absorber with the BiVO₄-based photooxidation catalyst. The biotic-abiotic configuration comprises a charge-separation layer that limits back-reactions and recombination processes, Figure . While perovskites have outstanding light-absorption efficiencies, they degrade rapidly at high-humidity conditions or in aqueous solutions. Therefore, protective layers were added to limit any contact between the perovskite layer and the solution. To enable efficient hydrogen generation, a hydrogenase originating from Desulfovibrio vulgaris Hildenborough was deposited as the biocatalyst. The Desulfovibrio vulgaris Hildenborough was selected due to its improved stability and activity in the presence of oxygen. The Z-scheme was designed to generate the required potential to enable bias-free activation of the biotic-abiotic photobioelectrochemical cell, with solar-to-hydrogen efficiency (STH) of 1.1%. Due to hydrogenase’s stability under oxygen, the system did not require a separation membrane for operation.

25.

(a) Schematic representation of the tandem PEC cell consisting of a FM-encapsulated perovskite photocathode with H₂ase integrated into an IO-TiO₂ layer and a BiVO₄ photoanode. TiCo refers to the water oxidation layer precursor: [Ti₄O­(OEt)₁₅(CoCl)]. PCBM: [6,6]-phenyl-C₆₁-butyric acid methyl ester. PEIE: polyethylenimine. (b) Representative LSV of PVK|TiO₂|H₂ase (blue) and BiVO₄ (green) electrodes with chopped illumination, forward scan, 10 mV s^–1 scan rate, showing the absolute current densities. Adapted from ref . Copyright 2019 American Chemical Society.

Furthermore, the system enables activation via single-directional light irradiation, in which BiVO₄ absorbs at shorter wavelengths (below 500 nm) and the cathode is harnessed above 500 nm. It should be noted that covering the entire visible light spectrum surpasses the natural photosynthetic systems. While the designed system demonstrated significant advances toward an abiotic water-splitting device, its stability was limited. Furthermore, to scale up the system, a separation between oxygen and hydrogen should be implemented. Cheng and co-workers developed a similar approach. BiVO₄-based photoanode was utilized to facilitate photodriven water oxidation, while a PB6 dye was used at the cathode to supply the required potential for bias-free water splitting. As presented in Figure , to enable efficient activation and stabilization of the [FeFe]-H2ase from Chlamydomonas reinhardtii, a 2,2′-viologen-based redox polymer was mixed with the enzymes. Under light irradiation, the developed biotic-abiotic configuration reached a faradaic efficiency of 81% and STH of 0.124%. These results gained by biotic-abiotic water splitting devices are aligned with or even surpass similar abiotic configurations. The water-splitting process indeed follows the principles of natural photosynthesis, converting water and sunlight into fuels (H₂).

26.

Performance of the overall water splitting system. A scheme of the device for overall water spitting. b LSV scans of NiO|PB6|PolyV|H₂ase (photocathode) and modified BiVO₄ (photoanode) under illumination, obtained from three-electrode measurements. The photocurrent density of the photocathode was inverted for comparison. Adapted from ref . Available under a CC BY 4.0 license. Copyright 2024 The Author(s).

Nevertheless, coupling the CO₂ reduction process with water oxidation has several advantages:

• a.It captures CO₂ and helps to limit elevated atmospheric concentrations.
• b.Liquid fuels, e.g., formic acid and methanol, produced from the CO₂ reduction processes, can be stored and transported directly. Liquid fuels can also be used in industrial processes and in standard fuel engines.
• c.The resulting CO₂ reduction products can undergo further reduction steps as part of an enzymatic cascade.

Photoelectrochemical, electrochemical, and bioelectrochemical configurations for CO₂ reduction have been developed over the last few decades. Holistic approaches were also developed. A flow electrolysis and bioreactor for microbial fermentation were coupled to enable CO₂ conversion into long-chain fatty acids production. The developed system provides high currents and high conversion efficiency of CO₂ into CO (100%), which is further converted into acetic acid that could be used for fermentation. However, those systems were commonly dependent on sacrificial electron donors, high temperatures, or applied bias for activation. Keun Kuk et al. presented a photobioelectrochemical cell that enables the conversion of CO₂ and water into O₂ and methanol fuel. The developed system coupled Co-Pi/α-Fe₂O₃|BiFeO₃ with CpRh­(bpy)­(H₂O)]²⁺ to enable the regeneration of NAD into NADH. The cofactor regeneration was, in turn, utilized for the activation of a sequential biocatalytic reduction process, enabling the conversion of CO₂ into formic acid, formaldehyde, and methanol, a 6-electron reduction process. The configuration could operate without external bias, reaching 3% spontaneous regeneration of NADH. The NADH regeneration was further enhanced using a photobioelectrochemical cell constructed with BiVO₄/NiOOH photoanode and perovskite cathode, enabling a dual absorbance tandem cell. The cell was conjugated to CpRh­(bpy)­(H₂O)]²⁺ NADH regeneration system, which facilitated the bioelectrocatalytic conversion of ketoglutamate to l-glutamate.

Just recently, See Yeung and co-workers presented a holistic configuration for the photoelectrochemical conversion of H₂O + CO₂ into formic acid fuel, Figure . Toward that, the BiVO₄ photoanode was coupled with a PEDOT/PSS organic photosensitizer-based photocathode. The PEDOT/PSS photocathode was constructed with added charge-separation layers and a TiO₂ high-surface-area mesoporous layer. The developed layered configuration was integrated with the formate dehydrogenase enzyme, which served as the biocatalyst for converting CO₂ to formate. Naturally, the cyanobacterial photosynthetic process utilizes carboxysomes, microcompartments, to concentrate CO₂ levels and limit the adverse effects of oxygen on CO₂ assimilation. To mimic this process and increase the availability of CO₂/bicarbonate in proximity to the FDH enzyme, carbonic anhydrase was codeposited on the cathode, next to the FDH. The developed cell demonstrates an architecture that enables water oxidation, a charge separation system, and CO₂ reduction into liquid fuel in a bias-free biotic-abiotic configuration. Formic acid could be used as a H₂ carrier or directly as a fuel.

27.

Biohybrid device incorporating organic semiconductors and enzymes for the photoelectrochemical conversion of H₂O + CO₂ into formic acid fuel. Reproduced from ref . Available under a CC BY 4.0 license. Copyright 2025 The Author(s).

Nevertheless, using it to produce pure chiral precursors for pharmaceuticals and other essential industries has greater economic and sustainability value. Therefore, the work demonstrated a proof-of-concept integration of the photobioelectrochemical cell with an asymmetric catalyst. As shown in Figure , the produced formate was used as a reductant to convert acetophenone into the chiral aromatic alcohol (R)-1-phenylethanol, with 78% yield and 94% enantiomeric selectivity.

28.

A light focusing PBEC for CO₂ reduction. (a) Schematic representation of the system. The FeOOH/BiVO₄ photoanode (yellow) and the CIGS solar cell (green) both absorb solar energy to generate an electric current from water oxidation. Then, the current is utilized by the CN/rGO cathode to reduce oxygen to hydrogen peroxide. The AaeUPO enzyme further reduces the peroxide while oxidizing ethylbenzene in a chiral manner. (b) Overpotential reduction by the CIGS solar absorber. Photocurrent is shown before (black) and after (red) addition of the CIGS solar cell. (c) Ethylbenzene conversion over time with (black) and without (red) the CIGS solar cell. Adapted from ref . Copyright 2019 American Chemical Society.

Harnessing solar radiation to drive chemical reactions is advantageous from an economic and environmental standpoint. In a paper by Choi et al., light was utilized to synthesize (R)-1-phenylethanol from ethylbenzene using a PBEC at ambient conditions and without applied bias. The PBEC’s anodic component was assembled from a FeOOH/BiVO₄ photoanode and a copper solar absorber (CIGS). While the FeOOH/BiVO₄ photoanode absorbed light for water oxidation, the CIGS absorbed the remained irradiation. Absorbing the unabsorbed irradiation increased the thermodynamic driving force and reduced overpotential by 0.57 V. However, CIGS contains expensive elements such as indium and gallium, which limit its commercial potential. Still, alternative solar absorbers could offer greater economic feasibility for commercialization. , To complement the BiVO–CIGS photoanode, the abiotic cathode contained a layer of carbon nitride and reduced graphene oxide (CN/rGO) on FTO for oxygen reduction. Additionally, the CN/rGO ratio was optimized to allow maximal faradaic efficiency of 92%. On the cathode’s surface, CN/rGO reduced O₂ to H₂O₂, which in turn activates the AaeUPO peroxygenase enzyme. AaeUPO catalyzes stereospecific hydroxylation to produce ethylbenzene to (R)-1-phenylethanol. Furthermore, several structural analogs of ethylbenzene were also shown to undergo hydroxylation with either lower turnover or lower stereospecificity. (R)-1-phenylethanol is a key component for polishes, washing agents, and air fresheners, as well as a building block for pharmaceutical drugs. Using the full system, ethylbenzene was converted to (R)-1-phenylethanol with a total conversion rate of ∼ 2.5% over 3 h and a turnover rate of 12.9 s^–1. The system operated at neutral pH, ambient temperature, and atmospheric conditions. The ethylbenzene conversion rate of ∼ 0.9 mM h^–1 correlated with a previously reported system. Taken together, this work demonstrates the potential of combining specific enzymatic reactions with well-known electrochemical processes.

6.2. Enzymes or Bacterial Fuel Cells toward Electrical Energy, Fuels, or Ammonia Generation

One of the most critical chemical processes today is the Haber-Bosch reaction for ammonia generation. This abiotic reaction requires extreme conditions and an extensive energy input. Therefore, alternative methods to enable ammonia generation under more ambient conditions should be pursued. Coupling the natural nitrogen fixation process with electrodes or semiconductors has been at the center of extensive research over the past decade. ,,,,− Milton and co-workers have presented an elegant approach to construct a Haber–Bosch-like system. The work-coupled H₂ oxidation anode was facilitated by a hydrogenase enzyme and a nitrogenase cascade-based cathode for the generation of ammonia. In both the anode and cathode, methyl viologen was used as a redox mediator to enable electron transfer. The system provides a route for utilizing enzymatic reactions to convert H₂ and N₂ into ammonia under ambient conditions, similar to the Haber-Bosch process. However, this process depends on H₂ for operation; hence, it requires an additional activation step.

A recent work by Meirovich and co-workers has suggested a different path for activation. Nitrogen was chemically reduced into ammonia using a biotic-abiotic PEC-based configuration, Figure a. To mitigate the energy consumption used in the industrial nitrogen reduction process, the nitrogenase (MoFe-P) enzyme was used. MoFe-P, the Fe-P auxiliary protein, the (SPr)₂V redox mediator, and carbon electrodes were all coupled to form a biocathode. To artificially activate MoFe-P and enable nitrogen reduction, semiconductor photoanodes of two types were examined: (i) BiVO₄/CoP and (ii) CdS/NiO. Both photoanodes absorb in the visible range and can suppress the water oxidation reaction. The suppression effect negated side reactions and enabled the specific oxidation of glucose and ascorbic acid, respectively. BiVO₄ can oxidize water into oxygen, which can inactivate the nitrogenase enzyme. Thus, suppressing the water oxidation is required. However, alternatively, oxidizing glucose is critical for a membrane-less configuration. Both photoanode and biocathode were then integrated to form a PBEC for ammonia production under minimal to no bias at ambient aqueous conditions. Although the BiVO₄ photoanode generated a higher anodic current, ammonia production was higher using the CdS/NiO photoanode: ca. 55 μmol compared to 100 μmol, respectively, Figure b and Figure c. As CdS/NiO’s conductive band is higher than that of BiVO₄, it enabled higher photocurrent at lower potentials, leading to higher ammonia generation. Although the system’s stability is limited by several components (ATP and enzymes), it succeeded in maintaining ambient conditions to produce ammonia without the need for expensive catalysts. The amounts produced are, of course, not yet practical for real-world applications. However, further development may lead to better yields and improved design.

29.

A nitrogen-reducing, light-assisted PBEC. (a) Schematic representation of the system, where the semiconductor photoanode is coupled with a graphite rod. The rod reduces the soluble (SPr)₂V redox mediator to enable electron transfer. The electrons are first transferred to the auxiliary protein Fe-P, then to MoFe-P for nitrogen reduction. Using ATP, Fe-P both changes the conformation of MoFe-P and supplies electrons for nitrogen reduction. Ammonia generation without applied bias can be seen for two types of photoanodes – BiVO₄-CoP (b) and CdS-NiOx (c). Adapted with permission from ref . Copyright 2024 Elsevier.

While photoelectrochemistry can facilitate the synthesis of valuable chemicals, it can also break down materials. This property is advantageous for the treatment of the chemical pollutants. These remains a consistent issue to this day, with materials such as pesticides, oil, organic solvents, cyanide, strong acids and bases, various sludges, and so on. A recent paper by Ding and co-workers described the breakdown of phenols in wastewater using a PBEC, Figure . The system comprised a biotic-abiotic configuration consisting of a biophotoanode and a cobalt catalyst buckypaper or an HRP cathode. The biophotoanode was fabricated using TiO₂ NPs, and a dye photosensitizer, SA-TCP.P A NAD-dependent glucose dehydrogenase (NAD-GDH), was used to regenerate NADH. In the presence of NADH and visible light, a photocurrent was observed where NADH acted as the electron donor. To ensure continuous regeneration of the NADH cofactor, the NAD-GDH enzyme oxidizes soluble glucose and reduces NAD⁺ to NADH. The induced photocurrent was utilized by the cobalt catalyst (CoNOC) to drive the two-electron oxygen reduction reaction (ORR) and generate hydrogen peroxide. The observed OOR remained stable for 5h while generating up to 1 mA/cm² under oxygen-saturated conditions. The generated H₂O₂ was used to activate the HRP enzyme, which, in turn, facilitated the degradation of phenols. The presented systems show a holistic approach for wastewater treatment and electrical energy generation. Coupling energy generation to an adjunct beneficial process is a key direction for developing new energy processes with economic viability.

30.

PBEC for wastewater treatment employing either HRP or a cobalt oxide nanozyme. (a) Schematic representation of the system. (b) Power curve of the PBEC in real wastewater. The voltage to current and power to current curves are presented (blue and orange, respectively). (c) Cathodic phenol degradation without a catalyst, with HRP, and with the cobalt oxide nanozyme (black, blue, and red curves, respectively). The blue area represents a time phase without illumination. Adapted with permission from ref . Copyright 2024 Elsevier.

One of the growing fields in energy storage is renewable fuel. As fossil fuels are finite and polluting, research has focused on developing alternatives, such as renewable fuels. For example, dozens of billions of liters of bioethanol are produced each year. However, ethanol’s energy density is relatively low, making it insufficient for some applications. Therefore, energy-rich renewable fuels are required to supplement or even replace fossil fuels. In an article by Harris et al., an abiotic PEC was integrated with enzymes to generate renewable fuel. The PEC was composed of abiotic electrodes and a solution containing two biocatalysts - alcohol oxidase (AOx) and beta-alanine. The BiVO₄ photoanode oxidizes water, generating electrons. The PEC generated power by coupling a BiVO₄ photoanode to a platinum cathode, Figure a. The generated oxygen served as an electron acceptor for AOx, which oxidized butanol to butanal. Then, pairs of butanal molecules underwent aldol condensation to form 2-ethylhexanal (2-EH). The organocatalyst beta-alanine catalyzed the condensation to form the nonvolatile ethylhexanal. While the observed butanol to butanal total conversion rate was 2.6%, the butanal to 2-EH conversion rate was significantly higher. Furthermore, removing the light from the system significantly reduced both butanal and 2-EH amounts. The authors suggested that AOx may have an unknown inhibition mechanism, even when beta-alanine pushes the reaction forward.

31.

A PBEC that generates jet fuel from simple alcohols. (a) Schematic representation of the system. The BiVO₄ photoanode oxidizes water, enriching the solution with oxygen and protons. Alcohol oxidase (AOx) reduces oxygen to hydrogen peroxide while oxidizing butanol to butanal (BA). Then, BA undergoes aldol condensation to form 2-EH. Simultaneously, the Pt cathode reduces the excess protons to hydrogen gas. (b) Yields of butanal and 2-EH with or without light at specific time points. (c) The effect of AOx presence on water oxidation at 0 V vs Ag/AgCl. According to the Le Chatelier principle, AOx reduces oxygen and pushes the water oxidation reaction forward. The improved water oxidation rate translates to increased current in AOx-containing cells (blue) vs the control cells (black) upon irradiation. Adapted from ref . Copyright 2017 American Chemical Society.

7. Electrochemical Photoreforming of Waste into Electrical Energy, Fuels, or Value-Added Chemicals

7.1. (Photo)­bioelectrochemical Cells for the Conversion of Plastic or Cellulose into Electrical Energy, Fuels, or Fine Chemicals

As agriculture expands and develops, various types of carbon-rich waste tend to accumulate (e.g., weeds, cutoff branches, peels, etc.). Biomass waste contains energy-rich polysaccharides, such as cellulose, the most abundant polymer on earth. Valorization of cellulose or lignin, a highly abundant material, could prove to be beneficial from environmental and economic perspectives.

Naturally, biomass undergoes enzymatic degradation of its biopolymers. Cellulose-degrading bacteria secrete an enzymatic complex, the cellulosome, which degrades the β-d-glucose polymer into cellobiose, a soluble sugar molecule that most organisms cannot use as a carbon or energy source. The bacterium breaks the cellobiose into glucose monomers using an internally expressed β-deglycosylase after its intake. This strategy limits the bacteria’s competition for critical resources. Developing methodologies to utilize cellulose waste directly could be highly beneficial and should therefore be pursued.

Fossil fuels are a great energy source that is easy to use and transport. Nevertheless, millions of years are required for the conversion of biomass into fossil fuels through natural processes. Over the last few centuries, humankind has burned biomass to generate heat, chemical energy, and electricity. However, these methodologies are harmful to the environment and have a limited efficiency.

Utilizing photochemical, , (photo)­electrochemical, , biochemical, − and (photo)­bioelectrochemical configurations, − for direct conversion of biomass into electricity or chemical fuel could be beneficial. − However, several obstacles need to be addressed toward realization:

• a.Polysaccharide polymers have poor solubility in aqueous solutions.
• b.Cellulose is highly crystalline and densely packed, hindering its degradation by using (bio)­catalysts.
• c.Harsh conditions are required for its chemical degradation.

Therefore, to exploit polysaccharides for the generation of fuels or electrical energy using (photo)­electrochemical tools, one should design a multidisciplinary approach that enables degradation and efficient oxidation. − Such processes, in a way, will rewind the classical photosynthetic process and use biomass in reverse to generate energy. Ideally, to overcome activation barriers, these processes should occur spontaneously or be derived from a sustainable source, such as sunlight. A TiO₂-based PEC was introduced for the degradation of cellulose. The cellulose degradation is facilitated by OH radicals formed by the photooxidation of hydroxy ions. While the system indeed facilitated the photooxidation of the deposited polymer, developed 1.1 V of photovoltage and a quantum efficiency up to 52%, strong basic pH and limited visible light absorbance hindered the presented configuration.

Furthermore, the system photo-oxidized only the deposited cellulose matrix and could not reach the nonsoluble material in the electrochemical cell. An improved PEC configuration was developed, in which the TiO₂ layer was modified with CuBi₂O₄. The latter significantly suppressed the water oxidation reaction and improved the Faradaic efficiency under basic conditions. Suppressing water oxidation during organic matter oxidation is a key feature that enables an improved efficiency and reduced side reactions.

Alternatively, Shemesh and co-workers presented a biotic-abiotic PEC that utilizes cellulose waste to generate electrical power. Here, the cellulose waste was enzymatically digested by a cellulase complex added to the cell. The complex consists of 3 hydrolytic enzymes that enable the degradation of the β-d-glycosidic bonds into small glucose repeats at neutral pH. These were subsequently photooxidized by the N-type BiVO₄/CoP photoanode, Figure a. The addition of a thick cobalt phosphate layer dictated a valence band position shift, leading to a full suppression of the water oxidation reaction, Figure b. Interestingly, a thin CoP layer commonly dictates an enhanced water oxidation reaction; however, tuning the CoP layer’s thickness and morphology enables suppression. Hence, the BiVO₄ photoanode favors oxidation of both glucose and cellobiose and fully suppresses the water oxidation. This phenomenon was attributed to band bending, which led to a higher valence-band position at the CoP interface. The shift dictates a new energy level above the one that thermodynamically enables water oxidation. The oxidation products were detected by NMR and identified as small organic molecules useful for other processes, e.g., acetic acid and formic acid.

32.

PBEC for the valorization of cellulose-rich waste. (a) a schematic of the cellulose-degrading PBEC. Cellulose waste is placed in the cell, where a cellulase enzyme complex degrades it into glucose and cellobiose. The BiVO₄-CoP photoanode further oxidizes the degradation products while suppressing water oxidation. Then, the BOD-based biocathode utilizes the current to reduce oxygen, thus generating power. (b) Effect of the CoP water oxidation suppressing layer. The photocurrent of pristine BiVO₄ before and after illumination is shown (gray and light blue curves, respectively). Additionally, the photocurrents of BiVO₄ with thin and thick CoP layers are shown (Purple and pink curves, respectively). The colored areas around the curve denote the error for each measurement type. (c) Power curve of the full PBEC with the addition of cellulose. The gray curve describes potential to current, while the pink curve describes power to current. Adapted with permission from ref . Copyright 2023 Elsevier.

Furthermore, the remarkable stability of BiVO₄/CoP enabled cellulose-derived photocurrents of ca. 0.6 mA/cm² for at least 5 days. The BiVO₄/CoP photoanode was then coupled with a bilirubin oxidase (BOD) biocathode to form a bias-free two-electrode photobioelectrochemical cell. The BOD biocathode was coupled with the BiVO₄ photoanode to produce electrical power by facilitating the bioelectrocatalytic oxygen reduction, Figure c. By coupling the photoanode with the BOD-based cathode, an OCV of 1 V was reached, which in turn yielded up to ∼ 1 mW/cm² power output (at O₂ saturation). The wide OCV stems in part from the redox potential of BOD (0.6 V vs Ag/AgCl), which is 200 mV higher than that of the expensive Pt cathodes at pH 7. The PBEC displayed a faradaic efficiency of 48.7%, which is lower than the BiVO₄/CoP photoanodes designed for water splitting. −

Poly terephthalate (PET) is a widely used plastic, commonly used in packaging, textiles, and films. However, spontaneous PET degradation occurs very slowly, which is insufficient, as 350 tons of waste are generated each year. Moreover, active abiotic PET degradation generally requires harsh conditions, which are therefore less attractive. , Taking the environmental issue into account, biodegradable polymers such as polyethylene furan dicarboxylate (PEF) have been garnering increasing interest. To synthesize mass quantities of PEF, large amounts of the monomer 2,5-furandicarboxylic acid (FDCA) must be obtained. Therefore, a robust and sustainable method for FDCA synthesis is crucial for PEF production. In a recent work by Zhang and co-workers, FDCA was synthesized from 5-hydroxymethylfurfural using a NiFe/BiVO₄ photoanode and a BOD-based biocathode, Figure . To improve both photocurrents and onset potential, pristine BiVO₄ was modified with NiFe using a hydrothermal methodology. The NiFe modification lowered the onset potential and increased photocurrent output by improving the photoanode’s charge separation. On the photoanode’s surface, the NiFe cocatalyst oxidizes TEMPO, which oxidizes HMF in turn. TEMPO is regenerated by the constant photocurrent, enabling high conversion rates. The biocathode was formed on an ITO substrate coated with porphyrin-modified (PTCC) buckypaper. The ITO/BP/PTCC cathode was further modified with BOD and coated with a Nafion layer to enhance stability. The buckypaper enabled increased conductive surface area, while PTCC mediated facile electron transfer. The ITO/BP/PTCC/BOD biocathode achieved currents of over 2 mA/cm² under oxygen saturation in phosphate buffer, pH 7. Since a nafion membrane separated the anodic and cathodic compartments, optimizing the pH in the cathodic compartment could have improved current density even further. According to HPLC measurements, HMF underwent oxidation to 2,5-diformyl furan or 5-formyl-2-furan carboxylic acid. Both these intermediate compounds could undergo further oxidation to FDCA, the PEF precursor. In a typical experiment, 3 mM were partially converted using pristine BiVO₄ and TEMPO, while NiFe/BiVO₄ fully converted HMF to FDCA over 9h of continuous activity, Figure c. As a result, the FE for pristine BiVO₄ reached 79.4% while NiFe/BiVO₄ reached 100% FE. In addition, the system demonstrated impressive stability, with up to 30h of continuous activity. While BiVO₄ is known for its stability, , enzyme-based electrodes may suffer from stability issues. In summary, this work demonstrates the combination of photoelectrochemistry with catalysis to synthesize valuable chemicals.

33.

PBEC for the synthesis of FDCA, a precursor for biodegradable plastics. (a) Schematic representation of the system. While illuminated, the NiFe/BiVO₄ photoanode oxidizes TEMPO, which in turn oxidizes HMF to FDCA. Simultaneously, the ITO/BP/PTCC/BOD biocathode uses the photocurrent to reduce oxygen to water and generate power. (b) Power curve (red) and current–voltage curve (black) of the illuminated PBEC under oxygen saturation. (c) Conversion rates of HMF after 9 h of continuous activity. While pristine and modified BiVO₄ catalyzed HMF oxidation, only NiFe/BiVO₄ achieved complete conversion of HMF to FDCA. Adapted from ref . Copyright 2025 American Chemical Society.

Bhattacharjee and co-workers have developed a bias-free photodriven biohybrid system that facilitates the conversion of PET into glycolate and formic acid, Figure . The designed photobioelectrochemical cell facilitated the degradation of PET fibers under strongly basic conditions.

34.

Schematic representation of the two-compartment Cu₂₇Pd₇₃||PVK|CO₂R_cat PEC system with FDH. Adapted with permission from ref . Copyright 2023 SNCSC.

The anode comprising a Cu₂₅Pd₇₃ catalyst enables the reforming of the PET into glycolic acid. The photocathode developed the required potential to activate the oxidation of PET derivatives and the reduction of CO₂ to formic acid using the FDH enzyme as the biocatalyst. The system was constructed with a bipolar membrane to enable the use of an appropriate pH value for the raw PET degradation and CO₂ reduction by the enzymatic process. Furthermore, the designed configuration showed good stability for at least 10 h and continuous generation of formate and glycolate as the sole products with high faradaic efficiencies, Figure . This work presented for the first time an unbiased PEC biohybrid that enables the reforming of PET into fuel or value-added chemicals. While this process clearly points toward real-world application, it also holds some drawbacks: (i) it requires the utilization of strong pH values at the anode. (ii) It uses novel metal ions as catalysts. (iii) The photocathode fabrication is not straightforward.

In recent years, biotic PET degradation has been suggested. Since its initial discovery, intensive work has aimed to improve PETase enzyme activity to achieve reasonable rates suitable for industrial applications. Unlike chemical degradation, it can operate under ambient conditions and could be coupled with a PEC for further reforming. Recently, Murphy and co-workers introduced a batch-fed system that enables one-pot extrusion and enzymatic hydrolysis of PET with high efficiency and at a comparable cost to an industrial process. Such a system should be fully integrated with biotic-abiotic photoelectrochemical cells to enable light-induced reactions toward fuel or fine chemicals without any applied bias. A great demonstration toward this goal was recently presented by Kim and co-workers, Figure . In this research work, a Fe₂O₃-based photoanode was used to enable the photoconversion of degraded PET polymers into small hydrocarbons such as formate and acetate. The photoanode hole enabled the oxidation of the hydrolyzed PET under alkaline conditions, while the photogenerated electrons were further used for bioconversion reactions. As presented, the carbon-based cathode was coupled with 3 different routes that can enable biocatalysis. H₂O₂ or NADH was generated electrochemically using anthraquinone-2-carboxylic acid or [Cp*Rh­(bpy)­H₂O]²⁺ electrocatalyst, respectively. The authors demonstrated key processes such as oxyfunctionalization, reductive animation, and asymmetric hydrogenation. While the developed photobioelectrochemical cell requires applied bias and alkaline conditions for its operation, it clearly puts the foundation toward efficient light-induced waste conversion into beneficial commodities such as formic and acetic acids and further enables fine chemical production using biocatalysis, where the cofactors are recycled electrochemically.

35.

CFP-based cathodes reduce O₂ to H₂O₂ for biocatalytic oxyfunctionalization and NAD⁺ to NADH for enzymatic amination and asymmetric hydrogenation. For the biocatalytic synthesis, a Zr:α-Fe₂O₃ photoanode extracts electrons from hydrolyzed PET solutions obtained from postconsumer PET waste. Adapted with permission from ref . Copyright 2022 Springer Nature.

7.2. Coupling PEC with Bacteria Cells toward Electrical Energy, Fuel, or Fine Chemical Production

Microbial fuel cells for the generation of electrical current were introduced over 100 years ago, and the power outputs and device constructions have since significantly evolved. Nevertheless, these cells require a feedstock, such as glucose, to generate currents and are mainly limited to electrical energy. Bacterial cells contain sophisticated machinery that can produce high-value chemicals or, alternatively, facilitate essential degradation processes.

As presented, photodriven CO₂ reduction via enzymatic processes has great promise. Indeed, small biotic catalysts can assist in CO₂ reduction, but they have a limited lifetime, high production costs, and cannot regenerate. Those limitations led researchers to utilize the living organisms directly as catalysts. Several families and genera of microorganisms have been known to convert gases such as carbon dioxide and nitrogen into valuable chemicals, e.g., ammonia. , Interestingly, these micro-organisms’ activity can be enhanced by external stimuli such as electrical current applied via electrodes. , Therefore, an external electron flow could drive microorganisms to enhance catalytic conversion toward fuel production.

In a recent study by Xiao et al., a hybrid microbial photoelectrochemical system for CO₂ reduction was presented. The system was composed of a TiO₂/CdS photoanode and a carbon-cloth biocathode with adsorbed bacteria toward the conversion of CO₂ into methane, a useful gas fuel for industrial processes, Figure a. The TiO₂/CdS photoanode absorbed visible light and generated photocurrent. TiO₂ is a wide-band gap semiconductor with limited absorbance in the visible range. Therefore, it was coupled with a CdS layer, which has improved visible-range absorption. Under light irradiation, an excited electron was transferred to the biocathode, while the sacrificial electron donor S^2– was used to close the holes formed and prevent self-oxidation of the CdS layer. The photocurrent was transferred to the bacteria via the chitosan-coated electrode. Chitosan was covalently bound to the cathode after acid treatment and NHC-EDS cross-linking. The electrostatic interaction between chitosan’s positive charge and the bacterial membrane’s negative charge enabled ET through the cathode. The photoanode and the biocathode operated at different pH levels. Therefore, a cation exchange membrane was required. To examine the biocathode’s CO₂ reduction capabilities, cyclic voltammetry was used. bioelectrocatalytic currents could be observed at an onset of −0.45 V vs Ag/AgCl. Control experiments lacking the methanobacteria did not present any currents under the same conditions.

36.

PBEC for the synthesis of FDCA, a precursor for biodegradable plastics. (a) Schematic representation of the system. The left cell contains the TiO₂/CdS photoanode, where photocurrent is generated in response to visible light. Using sulfur as an electron source, the generated photocurrent flows toward the cathode. There, electrons are shuttled inside the methanogen bacteria to allow CO₂ reduction. (b) PBEC photocurrent stability over time. The arrow represents an additional influx of high-purity CO₂ gas. (c) Methane production over time in the presence of visible light. Adapted with permission from ref . Copyright 2020 Elsevier.

The onset potential was very close to the thermodynamic redox potential of CO₂/CH₄ (ca. −0.44 V vs Ag/AgCl, pH 7.0), with <50 mV overpotential for CO₂ reduction. The authors suggest a direct electron-transfer mechanism, which is consistent with the observed low overpotential, Figure b. The cathodic current slightly decreased over time due to CO₂ depletion, which was restored upon reintroducing higher CO₂ levels. In addition, FE reached 92.7%, Figure c, indicating that most photocurrent was effectively channeled toward CO₂ reduction. Overall, despite the system’s dependence on anaerobic conditions, electron donors, and CO₂ influx, it displayed high selectivity and activity.

The system produced 15.0 L m^–2 of methane gas per day, where the final product was a mixture of methane and CO₂. Such outstanding selectivity minimizes downstream processing costs. Overall, the presented system provided a photocurrent to CO₂-reducing bacteria. The bacteria were adsorbed to the cathode via electrostatic interactions with chitosan. The system’s impressive operational stability could be further enhanced by using other photoanodes. A different photoanode with a similar bandgap could eliminate the need for a cation exchange membrane. Furthermore, an improved understanding of the electron transfer process could provide essential knowledge toward the development of more efficient PECs.

Alternatively, a photocathode can be coupled to a microbial bioanode toward bias-free syngas generation. Lu and co-workers presented a biofuel cell coupled with a PEC. The developed bioanode uses wastewater streams for its activation. The mixed microbial community was integrated into anodes wired to two different circuits, Figure . The first bioanode was coupled to an oxygen-reducing cathode. The power generated was added to a second photoactivated PEC via a flyback circuit. The developed potential enabled the photoreduction of CO₂ to CO at a silicon-based photocathode. The presented work provides an unbiased CO₂ reduction system that operates without any external energy input other than light irradiation. The use of wastewater as an electron donor for the bioanode activation provides both a cheap carbon source and, in parallel, can be useful for water treatment. Unlike inorganic catalysts or enzyme-based fuel cells, bacterial cells can be regenerated, thereby providing extended lifespan operation.

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Proposed MPEC system for spontaneous solar-driven CO₂RR. The voltage (V_in) generated by coupling waste organics (C_XH_YO_Z) oxidation on one electroactive bioanode with oxygen reduction on an air cathode (Reaction II) was first boosted to a larger output voltage (V_out) by an energy-harvesting circuit (EHC). The electrical energy produced was temporarily stored as electromagnetic energy. Further, a flyback circuit (FBC) was able to isolate V_out from Reaction II to power the solar-driven CO₂RR on a photocathode, which is coupled with another electroactive bioanode (Reaction I). The FBC decouples Reactions I and II and combines them in one solution environment without inducing a chemical short-circuit. The waste organics (C_XH_YO_Z) in brewery wastewater (BWW) served as electron donors. Adapted with permission from ref . Copyright 2020 Elsevier.

A different approach integrated a CuO/ZnO/CuO-based photoanode with S. oneidensis MR-1 bacteria to assemble a biotic-abiotic photobioelectrochemical cell for hydrogen generation, Figure . Under light irradiation, the photoanode generated a photocurrent that was utilized at the cathode for hydrogen generation. The holes accumulated at the photoanode were closed by the bacteria, which acted as electron donors and catalysts for lactate oxidation. Although the generated photocurrents were low, the cell showed impressive long-term stability, losing only 25% of its activity over 47 h of operation.

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Schematic image of a proposed new M-PEC cell for H₂ production with a biophotoanode consisting of CuO/ZnO/CuO and electroactive S. oneidensis MR-1. Adapted with permission from ref . Copyright 2024 Elsevier.

As presented, using waste as an electron source could be greatly beneficial in such biotic-abiotic cells. Furthermore, constructing unbiased systems could enable their use in real-world applications, even in areas not connected to the grid. Cellulose is the most abundant polymer on Earth. Therefore, utilizing raw waste materials comprising high cellulose content for energy generation could be very beneficial. Most known bacterial cells that facilitate catalytic cellulose degradation processes are susceptible to oxygen. Therefore, integrating biotic cellulose degradation with a water-oxidation process is hindered by the oxygen released. Moreover, water concentration in aqueous solutions reaches 55 M, which limits the oxidation of other small molecules. Another critical issue is solubility. Cellulose and other high-cellulose raw materials have low solubility. Therefore, direct photooxidation of these materials by photoanodes cannot be facilitated. Shemesh and co-workers presented a holistic approach to tackle those challenges. A photobioelectrochemical system was developed, in which Clostridium thermocellum was used as a biocatalyst for the degradation of raw waste materials. Clostridium bacteria are extremely oxygen-sensitive bacteria that secrete the cellulosome complex into the medium, Figure a.

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Hydrogen is generated in the MPEC comprising BiVO₄/CoP photoanode, Clostridium thermocellum, and cellulose as a carbon source. (a) Schematic illustration of the constructed cell (left). An image of the constructed cell (right). For the cell construction, a specially designed sealed electrochemical cell was used. The cell consisted of BiVO₄/CoP photoanode as the working electrode, Ag/AgCl as a reference electrode, and carbon as a counter. The outer cell solution was PB, pH 7.3, 0.2 M. The cellulose and bacteria were placed in a screw cap and were separated from the outer cell solution using a poly­(ether sulfone) membrane. A water jacket maintained the temperature in the cell solution. (b) CV measurements were performed before and after a 5 h preincubation step. A scan rate of 5 mV/s was used. (c) Chronoamperometry (CA) test performed while the photoanode was subjected to 0 V vs Ag/AgCl. Samples were taken from the headspace of the cell during the CA test. The sampling times are marked with arrows. (d) The H₂ generated during the CA run was monitored using gas chromatography equipped with a thermal conductivity detector (GC-TCD). The cell solution was maintained at 60 °C under anaerobic conditions (O₂ < 0.5 ppm). The photoanode was back-illuminated with white LED light during the CA measurement. Adapted from ref . Available under a CC BY-NC-ND license. Copyright 2025 The Author(s).

The complex consists of exo and endo β-1,4 glycosidic bond hydrolases. These enzymes enable the degradation of cellulose into cellobiose, a diglucose molecule. To utilize directly the energy stored in these sugar molecules, a photo bioanode was coupled to the bioreactor, Figure a. The designed BiVO₄-based photoanode fully suppressed the water oxidation reaction while enabling the photooxidation of cellobiose, Figure b. As presented, photooxidation can generate high photocurrents with an onset potential of −0.45 V vs Ag/AgCl (pH 7.3). At this pH, the generated potential falls short by ∼ 200 mV to enable bias-free hydrogen generation. Therefore, a 0 V vs Ag/AgCl reference electrode was applied to drive the reaction toward hydrogen generation. As presented in Figure c, d, H₂ fuel can be generated by light irradiation and an applied potential. It should be noted that the cathode in this work was not optimized to produce hydrogen, and therefore, the applied potential could be minimized by lowering the required overpotential. The designed photobioelectrochemical cell consisted of an internal bioreactor chamber that separates the insoluble raw material and the enzymes from the electrochemical cell. The poly­(ether sulfone) membrane allows degraded cellulose materials, mainly cellobiose, to diffuse to the photoanode, generating photocurrents. The electrochemical cell design, therefore, prevents fouling on the electrode surface and limits the adverse effects of shear forces on the electrode caused by the floating cellulosic material. Furthermore, an unseparated cell design presented a colored solution attributed to the oxidized quinone derivatives, which limit light penetration to the photoanode. Overall, the presented biotic-abiotic cell enabled the generation of electrical power or hydrogen (biased) while photo-oxidizing raw cellulose-containing materials such as leaves and paper sheets.

Photoinduced chemical reactions open new possibilities for novel chemistries or for eliminating the need for external energy for activation. The significant advances in biotechnology and synthetic biology provide us with state-of-the-art tools to redesign and reconfigure bacterial cells for the production of essential chemical products, sometimes even noncanonical ones. , Furthermore, most chemical reactions facilitated by natural oxidoreductases are chiral, which in many cases is critical to activate specific processes in the cells, or could lead to adverse activity if the wrong enantiomer is used. The pharmaceutical industry aims to develop processes that lead to enantioselective manufacturing. Taking this approach, only the active molecules are produced, which, in turn, minimizes costs, increases the drug safety, and eliminates the need for expensive postmanufacturing processes for separation, such as HPLC. By coupling photoinduced reactions, for example, by combining PECs with enzymatic cascades or bioengineered bacteria, energy costs can be reduced and efficient precursor production improved. Bachar, Meirovich, and co-workers introduced a biotic-abiotic system that promoted the production of a chiral amine in a light-induced photobioelectrochemical cell. Imine reductase, IRED, is an essential NADPH-dependent enzyme that facilitates the production of chiral cyclic imines that are being used as precursors in the pharmaceutical industry. For its continuous operation, NADPH is required. As mentioned in the Introduction section, the photosynthesis process utilizes PSII and sequential charge separation to activate the FNR enzyme, which regenerates NADP and is further used in the dark cycle. Here, the photoanode oxidizes a sacrificial electron donor to generate a high reducing potential, which is then used to reduce methyl viologen, a redox-active mediator. The developed photobioelectrochemical cell can activate an enzymatic cascade to generate NADPH, which in turn is used for facilitating the (R)-2-methylpyrrolidine enantiomer, as shown in Figure a. The enzymatic cascade operated without any externally applied bias with the only input being visible-light irradiation. This concept was extended to whole-cell bacterial catalysis. E. coli was bioengineered to express both the FNR and IRED. NADPH is an essential cofactor that is naturally expressed in the cell. Therefore, IRED can be directly activated during a naturally occurring dark process in the cell. However, by coupling the PEC with the cathodic bioprocess, the NADPH levels in the cell were increased, thereby improving (R)-2-methylpyrrolidine enantiomer production by 5-fold, Figure b,c. The MV concentration was kept low at 0.3 mM to prevent toxicity to the bacterial cell; and enabled a continuous electron supply to FNR, thereby activating the enzymatic cascade. The developed in vitro system reached 99.5% substrate-to-product conversion, whereas the unbiased microbial one reached 69%. As recently suggested, the developed methodology could be easily adapted to any NADPH-dependent enzymatic cascade, both in vivo and in vitro.

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Bias-free PEC for microbial chiral amine production. (a) Schematic of the full microbial photoelectrochemical cell (MPEC) constructed by coupling a CdS/NiO photoanode to a biocathode comprising bacteria expressing the ferredoxin NADP⁺ reductase (FNR) and imine reductase (IRED) for light-induced enantioselective chiral amine generation. Protein structures were visualized by ChimeraX, Protein Data Bank (PDB) 2B5O, and 4D3S for FNR and IRED, respectively. (b) (R)-2-methylpyrrolidine [(R)-2MP] concentration in the cathodic cell chamber after 20 h of cell activation. All measurements were performed in PB 0.2 M pH 7.3 in an anaerobic environment (Ar-filled glovebox, O₂ < 0.2 ppm), while methyl viologen (MV) (0.3 mM) and 2-MPN (10 mM) were dissolved in the cell solution (final OD_600 nm: 0.7). Cell activation was performed in the presence of BL21 Escherichia coli expressing FNR and/or IRED. Carbon rods were used as working and counter electrodes, while Ag/AgCl served as a reference electrode. A bias potential of −0.75 V versus Ag/AgCl was applied during cell activation. Additional measurement without any external bias application was performed as a control. (c) (R)-2MP concentration quantification after 20 h of unbiased MPEC under white LED illumination (5 W, LEDSupply) compared with the same configuration under dark conditions. Here, 50 mM of ascorbic acid (AA) was dissolved in the anodic cell solution. The cathodic cell solution contained 0.3 mM MV, 10 mM 2-MPN, and BL21 E. coli expressing FNR and IRED in a final optical density (OD)_600 nm of 0.7. All measurements were performed in phosphate buffer (PB) 0.2 M, pH 7.3, in an anaerobic environment (Ar-filled glovebox, O₂ < 0.2 ppm). Reproduced with permission from ref . Copyright 2025 Elsevier.

While the system offers many advantages and can be adapted to activate any NADPH- or NADH-dependent enzyme, it still requires an unnatural redox mediator for activation. Furthermore, although the system allows bias-free, light-induced activation, it still depends on a sacrificial electron donor. Future configurations should aim to use natural redox mediators and waste materials as sacrificial electron donors. Alternatively, Z-scheme configurations that could develop ∼ −0.7 V vs Ag/AgCl while oxidizing water could lead to outstanding performance.

8. Photodriven Bioconversion of Waste into Fuels or Value-Added Chemicals

8.1. Biomass Photobioconversion to Fuel and Value-Added Chemicals

As presented in previous sections, colloidal nanomaterials, such as quantum dots and nanorods, can be used for fuel generation. As presented, upon light irradiation, electrons gain the energy to overcome the bandgap, switching from the valence band to the conduction band. If the excited electrons’ energy level is high enough, it can thermodynamically allow H₂ or CO₂ reduction processes. The energy levels of nanomaterials can be tuned by controlling their dimensions. Therefore, it can be adapted to various chemical reactions. Furthermore, QDs exhibit extremely high excitation coefficients, which make them ideal for artificial photosynthesis systems. Indeed, tremendous efforts toward the utilization of NPs or nanorods for H₂, N₂, or CO₂ reduction have been developed. CdS NPs have a band gap of 2.4–2.7 eV, which allows absorption in the visible spectrum, and their CB edges are positioned at ∼ −1 V vs Ag/AgCl. This strong reducing power can readily reduce protons to hydrogen (−0.62 V). However, kinetic barriers often limit production rates. The use of core–shell structures improved charge separation, extending the excited-electron lifetime and limiting recombination. Alivisatos’ lab and others , introduced methodologies for synthesizing such nanoparticles or nanorods with an addon, a metal cluster or tip catalyst grown on top of the nanoelements for improved catalytic performance.

Alternatively, the Dukovic group coupled biocatalysts to the semiconductor nanomaterials to enable efficient hydrogen generation. , The hydrogenase enzyme was integrated into the CdS nanorods, which injected the excited electrons to facilitate the water reduction process. These biohybrids have great promise toward a more sustainable and efficient H₂ generation platform. Nevertheless, those biohybrids require sacrificial electron donors for continuous activation. The excitation process leaves holes in the valence band, which have a strong oxidation potential that can be further utilized for oxidative photocatalytic processes. CdS NP, a common photocatalyst, develops a potential of ∼ −1.4 V vs Ag/AgCl when excited. This potential could, in principle, facilitate the water oxidation reaction (+0.81 V). However, these photogenerated holes promote rapid self-oxidation of sulfide ions, which causes a change in the lattice structure and subsequently leads to a loss of photocatalytic activity. , Replacing the essential sacrificial electron donor with waste material could be greatly beneficial, as both waste degradation and fuel generation can be achieved in one photocatalytic process. Moreover, H₂ generation from renewable sources is not economical. Valorization of waste by the photocatalytic process, while coupling it with H₂ generation or CO₂ reduction processes, can make it cost-effective for real-world applications.

Wakerley and co-workers have presented a waste-to-hydrogen photocatalytic process. CdS NPs were placed in a basic pH to form a cadmium oxide layer. By irradiation, wood or leaves were photooxidized to generate hydrogen fuel. Unlike many photodegradation configurations, which produced OH^– or O₂ ^– radicals, here the use of a strong base partially degrades the cellulose, hemicellulose, or lignin, which in turn, photooxidizes to yield formic acid or is released as CO₂, Figure . Nevertheless, the extreme basic condition used in this work prevents coupling it with the biodegradation process. Also, in terms of sustainability, ambient conditions should be sought in order to minimize the need for further waste treatment.

41.

Lignocellulose exists as microfibrils in plant cell walls and is comprised of cellulose surrounded by the less crystalline polymers hemicellulose and lignin. These components can be photoreformed into H₂ using semiconducting CdS coated with CdOx (the CdOx surface is believed to contain some −OH functionality, but H atoms have been removed in the illustration for clarity). Light absorption by CdS generates electrons and holes, which travel to the CdOx surface and undertake proton reduction and lignocellulose oxidation, respectively. This combination creates a highly robust photocatalyst that can generate H₂ from crude sources of lignocellulose when suspended in alkaline solution and irradiated with sunlight. Adapted with permission from ref . Copyright Springer Nature 2017.

It has been shown that glucose can be used as a sacrificial electron donor. Ideally, hydrogen production by the photochemical process should be coupled to an enzymatic process that facilitates biomass or plastic waste degradation into small soluble molecules. The latter, in turn, may be further exploited as a sacrificial electron donor. Furthermore, an optimized system may produce value-added chemicals that can be further used in other industrial processes. As presented, raw biomass materials contain a high content of cellulose, which can be degraded by enzymatic reactions into sugars (glucose and cellobiose). Photooxidation of these materials into gluconic acid, a common food additive, or formic acid, which can be used as a fuel while producing H₂, could provide a sustainable system with clear economic advantages compared to systems based on the commonly used ascorbic acid as a sacrificial electron donor.

Recently, Liu and co-workers presented a holistic configuration for the photochemical generation of H₂. The designed configuration utilized a cellulose enzymatic complex to facilitate the degradation of wood chips that have a high cellulose content into glucose, Figure . The MoS₂/ZnIn₂S₄ macrostructures facilitated the photogeneration of H₂ fuel while photooxidizing the released glucose molecules. Rhamnose, glucuronic acid, glucose, xylose, and cellobiose were analyzed as products of the photocatalytic processes. These molecules may be further utilized as a side stream process. However, a single product is preferred to minimize postprocess separations. Similar approaches were recently reviewed − or presented, focusing on the photocatalytic mechanism and substrate treatment.

42.

Schematic illustration for photocatalytic H₂ production over the MoS₂/ZnIn₂S₄ photocatalyst with the assistance of cellulose. Reproduced with permission from ref . Copyright 2022 Wiley-VCH GmbH.

While inorganic semiconductors are mainly used to promote photocatalytic reactions, Cd-based photocatalysts are toxic, and the use of cadmium-free materials is beneficial. Carbon nitride QDs are a great substitute. These materials are cheap to produce and absorb in the visible wavelength. Utilizing carbon nitride QDs as a photocatalyst for H₂ fuel generation has high promise for future applications. Indeed, the use of carbon nitride for the photoreforming of lignocellulose has recently been demonstrated. For a broader scope, please refer to several key review articles recently published. ,−

8.2. Plastic Bioconversion

As mentioned, cellulose is the most abundant polymer on earth. Nevertheless, manufactured polymers are accumulating on earth and pose a threat to human health. While minimizing plastic use could reduce threats, reuse, recycling, or efficient degradation and valorization methodologies could significantly mitigate their adverse effects. Common polymers produced industrially on large scales include polyesters. Those include polyterephthalic acid, PET, and polylactic acid. The polymer chains can be degraded to their monomer form by heating under strong basic conditions. However, alternatives that use ambient conditions are more preferable. In the past decade, it has been shown that the PETase enzyme can degrade PET into bis-hydroxyethyl terephthalate (BHET) and monohydroxyethyl terephthalate (MHET), which can lead to full degradation to ethylene glycol and terephthalic acid. The enzyme degradation rate was very slow. Thus, for future applications, their rates and stability should be improved. Bioengineering techniques were used to enhance the catalytic activity of the enzyme (and its complementary enzyme, MHETase). − In parallel, it has been found that proteinase K and other hydrolytic enzymes, such as cutinase, can facilitate the degradation of poly lactic acid into lactate. −

Coupling chemical or biological degradation processes with fuel production is highly desirable as both recycling and fuel production are gained in the process. Moreover, processes that yield value-added chemicals and do not require energy inputs or expensive separations may lead to real-world applications. ,

Uekert and co-workers have presented a single-chamber reactor for the direct conversion of PET or PLA into H₂ fuel and valuable organic chemicals. In their work, a cadmium-free photocatalyst, CN _x |Ni₂P, was utilized for H₂ generation. The authors used pretreatment under basic conditions to allow polyester degradation. Subsequently, visible light was applied to the CN _x |Ni₂P photocatalyst to enable the generation of H₂. As shown in Figure a,b, polyester microfiber and PET can be degraded and used as sacrificial electron sources for continuous hydrogen generation for at least 5 days. The authors also examined the activity of a scaled-up cell of 120 mL, as presented in Figure c. This work clearly presents the great potential of photocatalytic reforming as a tool for both hydrogen generation and parallel production of value-added chemicals. As mentioned above, coupling such processes may yield economically viable applications.

43.

Photoreforming of nonrecyclable plastic waste. (a) Long-term photoreforming of polyester microfibers, a PET bottle, and an oil-coated PET bottle. (b) Upscaled photoreforming of polyester microfibers; the sample was purged every 24 h. Conditions: CN _x |Ni₂P (1.6 mg mL^–1), 1 M KOH (2 mL for part a and 120 mL for part b), pretreated microfibers (5 mg mL^–1) or PET bottle (25 mg mL^–1) without or with soybean oil (5 mg mL^–1), simulated solar light (AM 1.5G, 100 mW cm^–2). (c) Photograph of the batch reactor in use. Reproduced from ref . Available under a CC BY 4.0 license. Copyright 2019 The Author(s). As presented, a high concentration of base was required for the degradation of the polyester. These conditions are less favorable, and alternative directions should include biotic-abiotic configurations mitigated by enzymatic or bacterial cell biodegradation. In this case, the degraded products can be used as electron donors to neutralize the holes generated by light irradiation.

Indeed, Bhattacharjee and co-workers coupled photocatalysis with enzymatic biodegradation. Two different PETase enzymes were used to facilitate PET or polycaprolactone, PCL, degradation. A Pt-loaded TiO₂ (TiO₂|Pt) or Ni₂P-loaded carbon-nitride (CNx|Ni₂P) photocatalyst with a cocatalyst was utilized for photoreforming and H₂ generation, Figure . In the optimized process, AM 1.5G light irradiation and ambient temperature and pH conditions were used. It was found that TiO₂-Pt presented much higher photoreforming performance. However, it has limited absorption in the visible wavelength range.

44.

Schematic illustration of the photoreforming process with enzyme pretreatment. (a, b) Illustration of PET and PCL plastics undergoing (a) enzymatic pretreatment in solution followed by (b) photoreforming to yield valuable products. Adapted from ref . Available under a CC BY 4.0 license. Copyright 2023 The Author(s).

Recently, CdS@NiS nanocomposites were constructed and used for light-induced conversion of PLA into H₂ fuel and pyruvate, a critical metabolite for the biotechnology fermentation industry, as shown in Figure a. Yelin and co-workers coupled the biodegradation process of PLA films facilitated by proteinase K with the photocatalytic process. Proteinase K has a wide range of hydrolytic capabilities. It can cleave amide bonds and degrade peptides or, alternatively, degrade polyesters such as PLA. The enzyme was used to degrade the PLA films into lactic acid monomers that were then used as sacrificial electron donors. The photocatalytic hydrogen generation process was enhanced by the deposition of NiS on the 9 nm CdS NPs. It was found that a very low concentration of NiS clusters on the NPs’ surface dramatically enhanced the hydrogen generation rates. Interestingly, the lactic acid photooxidation process has led to a single product, pyruvate, as shown in Figure b. The highly selective process offers significant advantages for future industrial applications as it minimizes the need for additional separation steps. The biotic-abiotic configuration used ambient conditions for its activation, which is critical for sustainability. Photoinduced H₂ generation has been pursued for many years. However, the use of sacrificial electron donors and novel metals makes the process noneconomic. Here, no novel metals are used and waste plastic is used to produce sacrificial electron donors.

45.

a. Schematic representation of the biotic-abiotic configuration. (i) biodegradation, (ii) photocatalytic reaction. Inset: High-resolution TEM image of the CdS@NiS nanoparticles. b. ¹H NMR spectrum (A) only PB 0.2 M pH = 7.3 (B) illuminated NPs with no LA (C) NPs with LA not illuminated (D) illuminated NPs with LA. Adapted from ref . Available under a CC BY-NC 4.0 license. Copyright 2026 The Author(s).

Furthermore, the photooxidation process and the generation of pyruvate add to the economic gain. Ironically, the process has higher financial contribution potential than that achieved by the H₂ fuel production. Thus, coupling these two processes is economically viable. Such production-coupling processes may incentivize the industry to adopt the biotic-abiotic approach. Just recently, an interesting bacterial consortium and an abiotic photocatalyst biohybrid were introduced. The microbial consortia were coupled with a TiO₂/Cu₂O photocatalyst to enable both the photodegradation of poly­(vinyl alcohol) and its further assimilation by the bacteria to produce enhanced cofactor regeneration and amino acid production.

9. NAD(P)H Regeneration, Using a Photocatalytic Process

Biocatalytic systems enable the sustainable synthesis of complex molecules by minimizing the use of harsh reagents and reducing byproduct formation. However, many enzymatic processes depend on redox cofactors, which serve as essential electron or energy carriers. In photosynthetic organisms, NADPH drives carbon fixation through the Calvin–Benson–Bassham (CBB) cycle, converting CO₂ into carbohydrates. Beyond photosynthesis, NADPH serves as a universal hydride donor for a broad spectrum of oxidoreductases, facilitating key redox transformations, such as fatty acid and nucleotide biosynthesis. Its regeneration and selective use are essential for enabling continuous biotransformations in both natural and engineered systems. Since the stoichiometric use of NADPH and other cofactors (e.g., ATP, FAD, and FMN) is often economically unfeasible at scale, efficient cofactor regeneration strategies are crucial for maintaining enzyme activity while minimizing costs.

NAD­(P)H regeneration can be achieved through various approaches, including chemical reduction, photochemical methods, electrochemical methods, or coupling complementary enzymatic reactions. , Among these, photochemical and (photo)­electrochemical methodologies have emerged as particularly attractive due to their ability to harness light energy for cofactor recycling. In the context of NAD­(P)H regeneration, the stereoselective reduction of NAD­(P)⁺ to its active form, 1,4-NAD­(P)­H, is a key consideration. The development of a homogeneous catalyst for electro-regeneration of NAD or NADP to the 1,4 active form opened new possibilities to interface electrodes with enzymatic activation. − The CpRh­(bpy)­H₂O catalyst was also conjugated to a photoinduced reaction to enable the generation of NAD­(P)­H. The conjugated process ties light-induced reactions with NAD­(P) regeneration and acts as an alternative to the photosynthesis process. − It is important to note that direct photochemical reduction of NAD­(P) could be achieved using the NP or the photosensitizer as a catalyst; however, this leads to lower stereoselectivity. Limited NADPH regeneration selectivity forms dimers or the 1,2 or 1,6 isoforms, resulting in a nonviable process. In these configurations, sacrificial electron donors are required for continuous NADPH photorecycling. Alternatively, the photocatalytic reaction for NADPH generation can be coupled with a water oxidation reaction and a tailored charge separation and electron transfer chain to achieve an artificial Z-scheme configuration.

In nature, only the 1,4-isomer is utilized by NADPH-dependent oxidoreductases. At the same time, nonselective photochemical reduction often leads to enzymatically inactive byproducts such as 1,2- and 1,6-NADPH isomers or dimers, which can inhibit enzyme activity. To address this, nature employs ferredoxin-NADP⁺ reductase (FNR), an enzyme that catalyzes the stereoselective formation of 1,4-NADPH. By coupling inorganic photocatalysts (including semiconductors, metal complexes, or carbon-based materials) with FNR, highly selective and efficient NADPH regeneration can be achieved under mild, light-driven conditions. Once a robust NADPH regeneration platform is established, it can be linked to downstream enzymatic reactions that require this cofactor. In such integrated systems, only catalytic quantities of NADP⁺ are needed as continuous regeneration sustains the biocatalytic cycle.

Solar-driven NADPH regeneration broadly divides into two main categories: photochemical and (photo)­electrochemical. Photochemical schemes use dissolved photosensitizers to harvest light and deliver reducing equivalents in homogeneous solutions, whereas (photo)­electrochemical strategies employ illuminated or biased electrodes as the driving force for selective cofactor recycling.

9.1. Photochemical NAD­(P)H Regeneration

The native solar-driven regeneration of NADPH in the photosynthetic electron transport chain has been demonstrated in vitro using purified PSI as the photosensitizer and FNR as the acceptor. The activation of various NADPH-dependent enzymes demonstrated the versatility of the system. However, it requires the labor-intensive isolation of PSI from cyanobacteria as well as the addition of DCPIP and ferredoxin as electron mediators. King and co-workers developed biohybrid systems comprising CdSe QDs coupled with FNR for the light-driven regeneration of NADPH. This regenerated NADPH was then used by alcohol dehydrogenase to reduce aldehydes to alcohols, simultaneously recycling NADP⁺ to close the catalytic cycle. This concept has been further developed by Bachar and co-workers in a study that demonstrated protein-mediated synthesis of photocatalytically active CdS QDs (Figure a). These biosynthesized QDs facilitated solar-driven NADPH regeneration that, in turn, was used for the activation of the imine reductase (IRED) enzyme. The latter catalyzes the asymmetric reduction of cyclic imines to yield chiral amines, which are highly valuable in pharmaceutical and agrochemical syntheses. For 1,4-NADH regeneration, the diaphorase enzyme has been employed for the direct reduction of NAD⁺ into enzymatically active NADH without oxidizing any other product. Yuan and co-workers reported high 1,4-NADH selectivity using immobilized diaphorase in a redox polymer modified with a cobaltocene mediator. Another interesting study has tested the incorporation of CdTe QDs with thylakoid extracts in artificial photosynthetic cells. The authors used inorganic CdTe QDs to enhance the flux of photogenerated electrons for the regeneration of various cofactors, including NADH and ATP. The developed platform demonstrated its promise in activating various cofactor-consuming oxidoreductases, including formate dehydrogenase and nitrogenase.

46.

Using engineered protein cages in photosynthetic whole-cell biohybrids for directed bottom-up biosynthesis of inorganic nanomaterials to drive intracellular NADPH regeneration. (a) A demonstration of an in vitro cascade for CdS QDs biosynthesis and their use in FNR-facilitated NADPH regeneration. In turn, the reduced cofactor activates NADPH-dependent enzymatic activity for chiral product generation. Adapted with permission from ref . Copyright 2022 Wiley-VCH GmbH. (b) the in vivo implementation of the system presented in (a) inside a living E. coli for whole-cell biomanufacturing. Adapted from ref . Available under a CC BY-NC 4.0 license. Copyright 2023 The Author(s). (c) Designing protein cages for directed inorganic nanomaterials synthesis and colocalization of NADPH-consuming enzyme. This methodology is incorporated inside a living host for enhanced ET between the generated nanoparticles and the target reductive cascade. Adapted with permission from ref . Copyright 2025 Wiley-VCH GmbH.

Alternatively to FNR and diaphorase, selective 1,4-NADPH regeneration is commonly achieved using Rh-based electron mediators such as [Cp*Rh­(bpy)­(H₂O)]. Since first introduced as a selective catalyst for NAD­(P) regeneration, its use was widely extended to be activated electrochemically, photoelectrochemically, and photochemically. In a recent work, the latter was incorporated with InP QDs to enable solar-driven NAD­(P)­H regeneration for the activation of alcohol dehydrogenase. Additionally, Macchioni and co-workers have shown the incorporation of iridium-based complexes, enabling highly selective NAD⁺ reduction. Xing and co-workers incorporated a photocatalytically active ZnIn₂S₄ semiconductor with a Rh-based catalyst to enable selective NADH regeneration. Interestingly, in parallel with NAD⁺ reduction, the polarized semiconductor enables selective alcohol oxidation, which is utilized as a sustainable sacrificial electron donor platform in the system. However, these chemical catalysts are often required in relatively high concentrations, which might be incompatible with biological systems. Moreover, the use of enzymes to enable 1,4-NAD­(P)H selectivity is preferable in whole-cell biohybrid systems since it could be intrinsically expressed by the host bacterium, minimizing the external addition of chemicals to the system.

9.2. (Photo)­electrochemical NAD­(P)H Regeneration

Although colloidal inorganic nanomaterials are attractive light harvesters, practical deployment is hampered by reliance on toxic heavy-metal precursors and colloidal instability, which together undermine biocompatibility and scalability. Alternatively, NADPH regeneration can be embedded within a photoelectrochemical architecture using photoelectrodes to deliver electrons to a biocathode. This approach decouples the light absorber from the biological entity, offering a more controllable, scalable route to real-world implementation. Hambourger and co-workers have presented a dye-sensitized photobioelectrochemical cell which enables the oxidation of NAD­(P)­H, and the generation of hydrogen fuel. The designed system comprises the hydrogenase enzyme as a catalyst for hydrogen generation at the cathode, as shown in Figure . By excitation, high-energy electrons are shuttled from the anode to an external circuit. In parallel, the strong oxidative potential developed at the photoanode surface enables the oxidation of NADH to NAD⁺. While in most cases the NAD­(P)⁺ reduction process is preferred, as will be presented next, in some processes, regeneration of oxidation states of oxyreductases, such as alcohols to aldehydes, can be beneficial.

47.

Schematic drawing of the dye-sensitized photobioelectrochemical cell comprising a NADH oxidation process by the photoanode and H₂ generation by the hydrogenase enzyme at the cathode. Adapted from ref . Copyright 2008 American Chemical Society.

Minteer and co-workers reported a bioelectrocatalytic method for regenerating NADH via both enzymatic and electrocatalytic routes. In this work, an electrode has been modified to facilitate NADH regeneration and the reduction of acetoacetyl-CoA, which, in turn, was applied to the synthesis of polyhydroxybutyrate. Plumeré and colleagues demonstrated FNR-mediated solar-driven NADPH regeneration using a redox-active hydrogel to activate enzymatic reductive carboxylation. Our recent work has integrated an NADPH-regeneration module into the cathodic compartment of a PEC to achieve continuous, light-driven synthesis of chiral amines. In a representative setup, an illuminated CdS photoanode delivers electrons to a biocathode that regenerates NADPH to power an imine-reductase cascade; the device operates bias-free as a stand-alone system. The concept has been demonstrated both in vitro (with FNR and IRED) and in vivo (with whole-cell catalysts expressing the same enzymatic pathway).

9.3. Employing NAD­(P)H Regeneration Systems in Whole-Cell Biohybrids

In whole-cell photocatalytic biohybrids, light-driven NADPH regeneration is leveraged not merely to feed a single enzyme but to affect the cell’s intracellular redox state toward target reductive pathways. As previously mentioned, this methodology could be practically implemented after establishing an interface between an external light harvester and a living organism. Guo and co-workers engineered yeast–InP nanoparticle biohybrids in which light-excited, cadmium-free nanoparticles inject electrons into the cytosol, raising the free NADPH/NADP⁺ ratio and thereby driving shikimic-acid production. This study established a viable whole-cell route to in vivo, light-powered NADPH regeneration for fine-chemical synthesis, with maintained cell viability and a modular interface that is broadly adaptable to other NADPH-dependent pathways. Yuan and co-workers coated Ni-doped semiconductor ZnGa₂O₄ with polydopamine, claiming improved electron transfer between the inorganic photocatalyst and intracellular components, including NADPH. Moreover, the heterologous expression of rhodopsin allowed for solar-driven ATP regeneration, thereby boosting CO₂ fixation and lycopene biosynthesis. In a recent article, a similar approach for dual intracellular NAD­(P)H and ATP enhancement was enabled by eosin Y and rhodopsin, respectively, in Gram-negative Cupriavidus necator. These energy-rich cofactors were used to enhance CO₂ fixation to acetoin. Liu and colleagues investigated photocatalytic material-microbial interfaces using proteomics and metabolomics and reported that illuminated biohybrids maintain redox and energy homeostasis comparable to hydrogen-fed controls. This work supports the assumption that a steady photoelectron supply can sustain intracellular reducing power (including NADPH) in whole-cell systems without disrupting cell metabolism. Recently, photosynthetic microbes were combined with inorganic photoabsorbers, allowing the organisms to store chemical energy that can be released in the dark to enable “all-weather” biomanufacturing. It has been shown that steady external electron transfer provided by the inorganic light-harvesters allows for enhancing intracellular NADPH pools that, in turn, expand the operating window for light-driven reductive biocatalysis. Spatial organization further improves efficacy. Engineered condensates or protein cages can colocalize photosensitizers with NADPH-consuming enzymes, shorten electron-transfer distances, and insulate productive chemistry from competing sinks. Liu et al. designed a modular self-assembled protein-cage strategy in living bacteria that spatially colocalizes photoactive components with redox enzymes to achieve selective intracellular NADH regeneration, which in turn boosts CO₂-fixation activity (Figure c). The desired effect is ultimately measured by sustained elevation of the NADPH/NADP⁺ ratio, representing the enhancement of reducing power to drive desired reductive biotransformations. It is noteworthy to mention that this NADPH/NADP⁺ ratio is commonly measured using commercial colorimetric kits based on simple redox processes. While these methods are valid for ordinary in vivo redox balance measurements, one should take into consideration the effect of the artificial inorganic redox moieties on the assay readout. Therefore, we believe that validating such data with other methodologies (e.g., LC-MS) is highly important. Remaining challenges include mediator biocompatibility, competition with native metabolism, matching NADPH supply with ATP demand, and adoption of heavy-metal-free light harvesters for scalable operation. Overall, the integration of inorganic photocatalysis and whole-cell biocatalysis offers a powerful and sustainable route for cofactor recycling and light-driven enzymatic synthesis, with significant implications for green chemistry and synthetic biology.

The biotic-abiotic configurations presented in this work use redox proteins, enzymes, or bacterial cells, in conjunction with redox polymers and abiotic elements. Each of these elements has a specific redox potential for activation. In order to design new artificial photosynthesis and biotic-abiotic configurations, one should take into consideration the energy levels and the redox potential of each component. While most of the redox proteins have a clear redox potential, in some, like the iron-protein, Fe-P of the nitrogenase, their potentials are shifted as a result of ATP binding. Also, both biotic and abiotic elements can undergo photoexcitation and have excited electrons with higher energy levels. The band gap and energy levels of semiconductor materials at the nanoscale can be tuned by controlling their dimensions, a phenomenon known as the quantum size effect. Toward new configuration design, we summarized the majority of the elements used in the review in comprehensive energy diagrams, as shown in Figure . For convenience, we have separated the energy levels and redox potentials into three panels: (i) band gap and energy levels of semiconductors. (ii) Redox mediators and photosensitizers. (iii) Photosynthetic proteins, redox proteins, and enzymes.

48.

Energy diagrams of commonly used semiconductors, redox mediators and biotic elements.

10. Qualitative and Quantitative Methodologies to Follow Waste Conversion Using Biohybrid Configurations

The photosynthesis process converts light energy into chemical energy through a series of light- and dark-reaction steps. Over the last couple of decades, various configurations have been developed to enable the capture and conversion of CO₂ into fuels. These methodologies comprise a chemical catalyst, a biocatalyst, or a photocatalyst. Most designed systems do not include both light-induced water oxidation and CO₂ assimilation apparatuses. The 2e^– and 2H⁺ reduction processes could generate formic acid, which can be used directly as a fuel or as a hydrogen carrier. Many photo­(bio)­catalytic and photo­(bio)­electrocatalytic configurations have been developed to facilitate CO₂ reduction processes to formic acid. These configurations omit the water oxidation process for electron generation and instead use sacrificial electron donors. One such experiment should consider several issues:

Most of the experiments were performed in aqueous solutions. Therefore, the dominant form is that of formate. In its charged form, it cannot be measured accurately by a gas chromatograph.

Formic acid can be extracted after acidification and distillation and separation. However, one should be cautious as such a process can yield false-positive results. Photoinduced reactions oxidize sacrificial electron donors, such as ascorbic acid, TEOA, DTT, and EDTA. These molecules undergo oxidation and further degradation. The process may lead to the degradation of the oxidized sacrificial electron donor backbone, yielding formic acid, which is mistakenly thought to originate from the CO₂ reduction process. To minimize false-positive results, ¹³CO₂ should be used as the CO₂ precursor. H NMR measurements of ¹³C formic acid show a distinguishable signal, which confirms that the formate origin is the marked CO₂. Furthermore, an internal standard, e.g., DSS, should be added.

While ammonia production is not directly connected to photosynthesis, it is of great importance to many industrial processes and is therefore indirectly related. Ammonia is commonly detected by fluorescence or absorption probes. The most frequently used probe is phthalaldehyde, which reacts with ammonia in the presence of thiols to exhibit a strong fluorescence at 475 nm. One should take into account that the obtained fluorescence is greatly affected by the pH, the presence of proteins, and other amines. Therefore, to limit false positives, H NMR measurements should also be included. Ammonium ions give a distinguishable triplet split at 6.42, 7.07, and 7.72 ppm, with a spacing between the triplet peaks of 52 Hz. The origin of the ammonium can be further verified using 15N-labeled substrate material. The ammonium exhibits doublet peaks at 6.57 and 7.48, separated by 72.8 Hz. additional spiking tests with ammonia should be performed to confirm that the results align with the desired products.

While GC can easily detect methane, H₂, and small MW alcohols, equipped with TCD and FID detectors and a molecular sieve column, fine chemicals may require several steps of derivatization and separation. Moreover, fine chemicals are often chiral and should therefore be detected and identified using a chiral column. In many cases, a derivatization process is required for improved separation into the organic phase.

The efficiency of waste conversion using biohybrid configurations can be assessed using various parameters that reflect the utilization of light energy, charge, and chemical energy.

To enable an accurate assessment and valid comparison between the different system configurations, standardized experimental protocols should be established. Several parameters have been defined to characterize various system configurations including electrochemical, photoelectrochemical, and photochemical configurations.

The efficiency of electrochemical reactions is commonly evaluated using Faradaic efficiency. Faradaic efficiency (FE) reflects the selectivity of an electrochemical reaction toward the generation of a particular product. , It is defined as the ratio between the charge utilized for the generation of the specific product to the total charge passed through the electrochemical system, according to eq :

$$FE(\%)=\frac{Q_{\mathrm{product}}}{Q_{\mathrm{total}}}\times 100$$ 1

For products present in the liquid phase, the FE is calculated using eq :

$$F{E}_{\mathrm{liquid}}(\%)=\frac{N\times n\times F}{Q_{\mathrm{total}}}\times 100$$ 2

Where N is the number of moles of the generated product, n is the number of electron moles required for the generation of 1 mol of product, and F is the Faraday constant (96485 C·mol^–1).

Several parameters have been defined to characterize the efficiency of photoelectrochemical systems. Incident photon to current efficiency (IPCE) is a widely used measure for photoactive devices that describes how photon-induced electron–hole pairs are converted into useful electrical energy. IPCE is also referred to as the External Quantum Efficiency (EQE). The IPCE is a function of photon wavelength (λ), as it reflects the photocurrent response of the system under illumination at each specific wavelength point. The IPEC measure is described according to the eq :

$$IPC{E}_{(\lambda )}\%=\frac{I_{\mathrm{el}}}{I_{\mathrm{ph}}}\times 100$$ 3

Where I _el represents the flux of electrons flowing through the external circuit (mol·m^–2·sec^–1) and I _ph denotes the incident photon flux (mol·m^–2·sec^–1). The electron flux is derived from the ratio of the photocurrent density (J, μA·cm^–2) to the Faraday constant (96485 C·mol^–1). The photon flux is calculated from the power density of the incident light (W, W·m^–2) and the wavelength (λ, nm) divided by the Avogadro number (N _A = 6.022 × 10²³ mol^–1), Planck’s constant (h = 6.626 × 10^–34 J·sec) and the speed of light (c = 2.998 × 10⁸ m·sec^–1), according to eq : ,

$$IPC{E}_{(\lambda )}\%=\frac{\frac{J\times {10}^{-2}}{F}}{\frac{W\times \lambda \times {10}^{-9}}{N_{\mathrm{A}}\times h\times c}}\times 100\cong \frac{1240\times J}{\lambda \times W}$$ 4

Applied bias Photon-to-Current Efficiency (ABPE) is an additional performance measure that describes the efficiency of a photoelectrochemical system to convert incident photons into electrical current under an externally applied bias. It is calculated according to eq :

$$ABPE\%=\frac{J\times (1.23-V)}{P}\times 100$$ 5

where J is the photocurrent density generated under an applied voltage of V vs RHE. P stands for the light intensity.

Quantum yield (QY) or quantum efficiency is commonly used to evaluate the activity of photocatalysts. The quantum yield of a photochemical reaction is defined as the ratio between the number of reacted electrons and the number of photons, of a specific wavelength, absorbed by the photocatalyst, according to eq : ,

$$QY(\%)=\frac{numberofreactedelectrons}{numberofabsorbedphotons}\times 100\%$$ 6

The absorbed photon flux is affected by factors such as light intensity, the optical properties of the photocatalysts, and the substrate. , Measuring the absorbed photon flux is straightforward in homogeneous systems but challenging for heterogeneous reactions. In these systems, light scattering and reflection reduce the fraction of incident light that is absorbed. The losses can amount to 13–76% of the total incident photon flux. , The apparent quantum yield (AQY) is widely used to evaluate the photocatalytic performance, as it considers the total number of incident photons rather than only those absorbed by the photocatalyst. By replacing the number of absorbed photons with the number of incident photons in the quantum yield calculation, the resulting value corresponds to the AQY.

The number of incident photons can be determined according to the ratio between the total energy of the monochromatic light and the energy of one photon, according to eq :

$$Numberofincidentphotons=\frac{E_{total}}{E_{photon}}=\frac{P\times S\times t_{irr}\times \lambda }{h\times c}$$ 7

Where P is the incident monochromatic light power density (W·m^–2), S is the irradiated area (m²), t _irr is irradiation time (sec), λ is the wavelength (m), h is Planck’s constant (6.626 × 10^–34 J·sec), and c is the speed of light (2.998 × 10⁸ m·sec^–1).

The number of reacted electrons can be evaluated according to the number of target product molecules generated and the number of electrons required for their generation. The AQY is therefore calculated according to eq :

$$\mathit{A}\mathit{Q}\mathit{Y}(\%)=\frac{number\mathrm{of}\mathrm{reacted}\mathrm{electrons}}{\mathrm{number}\mathrm{of}\mathrm{incident}\mathrm{photons}}=\frac{n\times N_{\mathrm{A}}\times N}{\frac{P\times S\times t_{\mathrm{irr}}\times \lambda }{h\times c}}=\frac{N\times N_{\mathrm{A}}\times n_{\mathrm{product}}\times h\times c}{P\times S\times t_{\mathrm{irr}}\times \lambda }$$ 8

Where N is the number of electrons required for the generation of one molecule of the target product, NA is the Avogadro number (6.022 × 10²³ mol^–1), and n _product is the amount of the target product generated (mol).

10.1. Electron Transfer Process in Biotic-Abiotic Systems

As presented, the different biotic-abiotic artificial photosynthesis configurations require efficient electron transfer processes. These processes include electrical communication between enzymes and electrodes, enzymes and nanomaterials, the biotic internal electron transfer process to enzymes’ active sites, and more. As can be learned from the photosynthesis process, efficient electron transfer processes can be achieved by compatible redox centers with appropriate redox potentials and tailored electron transfer distances between those centers. When these parameters are optimized, efficient electron transfer rates can be gained. Furthermore, it is critical to design configurations that minimize back reactions, which hinder efficient electron transfer. These principles are essential in any electron transfer process, as proved in the Marcus theory. eq presents the Marcus equation, which correlates the electron transfer rate with the distance between the electron donor and the electron acceptor. In many of the designed configurations presented, enzymes or photosynthetic proteins are electronically coupled with electrodes or nanomaterials. In most cases, protein scaffolds act as insulators that prevent and direct electron transfer processes as a result of short distances between the proteins’ active sites and the electrode. Exceptionals can be found in the case of hydrogenases, laccase, bilirubin oxidase, and FNRs. In order to overcome those limitations, MET processes can be used where redox mediators or redox polymers can act as relays to enable the electrical wiring of biotic elements. Advances in bioengineering, computational structure prediction, and the use of noncanonical amino acids open new ways to conjugate and orient proteins’ active sites or internal redox mediators with surfaces. ,− Dictating the conjugation position prevents stochastic orientation, which can ultimately lead to short and efficient electron transfer processes.

$$k_{et}\propto e^{[-\beta (\mathrm{d}-{\mathrm{d}}_0)]}·e^{[\frac{-{(\mathrm{\Delta }G+\lambda )}^2}{4RT\lambda }]}$$ 9

• β = electron - coupling constant

• ΔG = free energy change

• λ = reorganization energy

• d = distance separating the electron and donor.

• d ₀ = van der Waals distance.

eq . The electron transfer Marcus theory equation. Where d is the distance between the electron donor and the electron acceptor, d₀ is the van der Waals distance. ΔG is the free energy change, and λ is the reorganization energy associated with the electron transfer process.

While most of the configurations presented in this work are facilitated by electron transfer processes to isolated proteins, in some biohybrid systems, for example, in living cells or thylakoid membranes, different approaches are required. Membranes are insulators with a thickness above 2 nm that prevent any direct electron transfer process through hopping or tunneling mechanisms. Gram-negative bacteria often have a membrane reaching 4–7 nm; therefore, the DET process should not occur. To overcome these issues, redox relay structures and proteins have evolved to enable electrical communication of bacterial cells with their surroundings. , Multiheme proteins such as MtrA and MtrC enable short-distance electron transfer processes on a microsecond time scale. , This can be achieved by a hopping mechanism occurring between neighboring hemes with distances shorter than 2 nm. Using these internal electron transfer processes, a long-chain electron transfer process can be achieved. The heme orientation plays a critical role in the electron transfer process. More details of this fascinating electron transfer process through membranes can be found in recent reviews and research articles. −

11. Conclusion and Outlook

11.1. The Boundaries between the Biotic and the Abiotic Have Faded

Over the last few decades, blocks from different sets have been integrated to overcome the known limitations of materials science, biotechnology, (photo)­electrochemistry, and more. Once the methodologies for connecting the different blocks and characterizing them were developed and implemented, new possibilities emerged for developing applications using both the new and old blocks. The quest for renewable energy and artificial photosynthesis processes has been reignited in the past decade. Coupling light absorbers with extinction coefficients of a magnitude higher than those of the natural apparatus with living cells can improve the natural photoinduced reactions. A well-designed whole-cell interfaced configuration could potentially morph nonphotosynthetic bacteria into photosynthetic ones.

Clarivate citation report of the terms “biohybrid” or “biotic abiotic” exhibits exponential growth both in published papers and citations. An “old-school” organic chemist or biotechnology engineer can leverage their strong skill sets and knowledge to achieve further goals. Why limit the cofactor set to NAD­(P)­H? One can now design analogs − that can shuttle through organic solvents , or activate noncanonical enzymes. , With advances in synthetic biology, these analogs can be activated in vivo to drive a specific enzymatic cascade without interfering with natural biochemical pathways. Proteins can be used as a “metal organic framework-like” system, or DNA can be used as a scaffold for photosensitizers, charge separation, , or as an electronic chip.

As presented, the development of biotic-abiotic configurations opens many new directions and will soon be available for real-world industrial applications. Critical development focuses on the photosynthetic apparatus mimicry, and indeed, significant advances have been made. Toward the full realization of biotic-abiotic configurations, several points should be addressed:

11.2. Sacrificial Electron Donor

While many creative and novel biohybrid configurations have been presented in the previous sections, these systems require sacrificial electron donors, such as TEOA, DTT, or ascorbic acid, for continuous activation. Derived from sustainability and economic reasons, these molecules cannot be used in practical applications. As shown, the generation of formic acid or H₂ by biocatalytic, electrocatalytic, or photocatalytic reactions cannot be justified while consuming equal or greater chemical energy or incurring higher production costs. Another drawback is the release of CO₂. To solve these issues, two approaches may be used:

• Incorporate water oxidation (bio)­catalysts.

• Utilized waste has a sacrificial electron donor.

On the one hand, following nature by incorporating a water-oxidation catalyst offers significant advantages; on the other hand, many (bio)­catalysts for H₂ generation or CO₂ reduction are adversely affected in the presence of oxygen. This issue can be addressed by smart design that limits oxygen diffusion to the photoreduction process, or by biotechnological engineering to develop a (bio)­catalyst selective for the desired process. The incorporation of incorporating oxygen-protecting layers could be a valid solution. , Alternatively, exploiting waste material as a sacrificial electron donor offers significant advantages, as it does not lead to evolved oxygen (if water-oxidation competing reaction is suppressed), , and it degrades undesired materials that could have value for recycling or valorization. Such processes can be coupled with oxygen-sensitive catalysts that facilitate the reduction of CO₂ or H₂O. Another critical aspect is technoeconomic validity. Green hydrogen is currently too expensive to compete with conventional hydrogen production methodologies. Its price ranges between 3 and 8$ per kg, while the aim is to reach 1$. By coupling a hydrogen generation system with the valorization of waste into fine chemicals or commodities, e.g., vanillin or pyruvate, the overall gain makes it economical. Pyruvate is currently in high demand for fermentation processes as the cultured meat industry grows, and its price range per kg is 1–11$.

Hence, producing pyruvate from PLA waste while generating H₂ fuel could make it economical! Those directions should be the primary focus of researchers advancing this field. With that in mind, current maturation can be found mainly in the production of commodities. For example, New Iridium (Kemvera), offers light-activated organic photocatalysts for commodity production. The company offers commodities like acetic acid and ethyl acetate using biobased and CO₂ feedstocks. Fronting fossil fuels commodities production is a challenging task due to the low price of the produced materials. On the other hand, producing pharmaceuticals may lead better cost-effectiveness margin due to the high price of such materials. A critical challenge toward that realization is device engineering. How can light-induced reactions fit industrial applications? Should it use LED light or sun irradiation? Can we combine both to limit dark cycles? Can we scale up current photoelectrochemical and photochemical configurations into industrial size reactors? This gap is not yet filled.

Another interesting direction to pursue is the activation of whole-cell-based biohybrids that can operate without the addition of an external electron donor. This can be achieved by coupling the photochemical reaction to internally produced donors. These could be generated by knockouts or added enzymatic processes. For example, by the addition of the thiosulfate reductase gene, sulfides can be accumulated internally and further used as an electron donor or for NP formation. In a different path, lactate production or H₂O₂ production by oxidases may lead to an appropriate electron donor. These directions should be further developed and coupled with whole-cell biohybrid systems.

11.3. Stability

A significant obstacle in catalysis is stability. Unlike what is commonly thought, this obstacle is shared across the different disciplines, including both biological and inorganic origin catalysts. In many cases, transient photocurrents or short-term fuel production are reported in leading journals, suggesting a bright future for renewable energy and artificial photosynthesis. Fundamental research is critical to generating new knowledge. Nonetheless, in H₂ production or CO₂ reduction, the research has matured to the point that it is mandatory to demonstrate stability for long-term activation, ranging from a few hours to days or more. If photoelectrochemical or electrochemical cells are utilized, one should clarify the activation barriers in terms of applied bias or the gained photocurrent in a bias-free configuration. In many cases, the applied potential is so high that electrical currents can be facilitated by the electrode surface toward oxidizing (or reducing) electron donor/acceptor molecules, which makes the light irradiation redundant!

Biohybrid configurations’ stability is limited both by the biological components and entities, and the abiotic elements. Often, the biohybrid photosensitizer, its cocatalyst, or the conjugated electrode is constructed using metal ions such as Cd, Cu, Ni, Co, Bi, Mo, Ti, Bi, and more. While some of these metal ions are available in living systems, their concentrations are extremely low, which limits toxicity. However, at higher concentrations required for internal or external NPs growth, or in conjugation with photo or electroactive electrodes, these ions can leach and act as enzyme inhibitors. Eventually, these ions can accumulate and fully inhibit the biological cascade in artificial configurations or bacteria growth and proper functioning. To limit their adverse effect, one should aim for alternating their oxidation state or accumulating in an insensitive compartment. These methodologies also occur naturally in bacterial species; − however, they can be enhanced and improved by bioengineering and synthetic biology. , In some cases, Metallothioneins (MTs), protein-rich in strong intercalation moieties such as cysteines, are used to remove heavy metals from the intercellular fluids and limit their negative effect. As presented, these challenges were mainly addressed by maintaining the environment with relatively low concentrations of toxic metal ions (>0.3 mM). For enhanced tolerance against those toxic elements, activated elements that can specifically react with the ions by reduction or complexation should be added or internally grown. Such elements (proteins, peptides, or externally added metal–organic complexes) that mimic the natural processes at metal ion toxic-tolerant bacteria should improve above the state of the art. − As mentioned, the abiotic elements also suffer from instability and degradation in conjugation to biological components, aqueous solutions, natural and non-natural reducing agents, and photoirradiation. The addition of protecting layers, such as ligands (NPs) or thin films (electrodes), could greatly improve stability. The addition of sacrificial electron donors and limiting photoirradiation flux is crucial to limit back reactions, self-oxidation, and degradation of photosensitizers. Designing configurations that can internally generate electron donors could greatly improve any nanobio hybrid design. In general, such systems have great promise; however, maturation is required for real-world applications that may enable mass production of commodities and fuels.

With all achieved advances, without worldwide national policies supporting the shift to renewable energy, and specifically to artificial photosynthesis systems, most of these technologies will not advance beyond the pilot stage. The dependence on fossil fuels was challenged recently during the shortage of gas supply from Russia to Germany. This induced the use of alternative methods, including connecting the renewable energy resources with the smart grid. , Currently, 23% of Germany’s energy can be attributed to renewable resources, although artificial photosynthesis technologies are not yet adapted. Governments and policymakers should not be led by energy crises or political hurdles to push toward renewable energy resources. Alternatively, we should continuously seek to reduce the dependency on fossil fuels, bearing in mind that the sun’s irradiation can support all the required energy on earth, as has been demonstrated for billions of years of photosynthesis. Utilizing artificial photosynthesis processes may add the missing block to enable sustainability while meeting the needs of modern life. , An interesting demonstration of bench-to-application technology is H2Pro, a recently established startup company utilizing electrochemical cells to generate green H₂ fuel. The company uses a technology developed at the Technion to decouple the generation of H₂ and O₂ in the electrolysis process. This important feature allows the direct generation and collection of green H₂ without the use of an expensive membrane or separation techniques, which simplifies the required engineering of large-scale production and reduces costs. Coupling the technology directly to PV enables the generation of a photoinduced H₂ fuel. Another interesting direction uses core–shell QDs with a cocatalyst for the generation of H₂. Here, the technology developed at QD-SOL uses a sacrificial electron donor and light irradiation to produce fuel with a high efficiency. The maturation of the interfaced biotic-abiotic configuration is still hindered, and these processes are currently ambitious in terms of industry. Adapting the conditions that should fit the biological element, the abiotic element, and the industrial limitations and constraints is challenging. Efficient collection of the sun’s irradiation is another great challenge. Nevertheless, some scaled-up PEC devices have already been introduced. With the continuous development in the field, it is reasonable to suggest that higher TRLs should be reached in the near future.

In summary, the review summarizes the development of biotic-abiotic systems, the different approaches, and critical obstacles that need to be addressed in order to realize artificial photosynthesis, photoreforming, and light-induced energy generation systems.

Glossary

Abbreviations

ABPE

applied bias photon to current efficiency

ABTS

2,2′ azino bis­(3 ethylbenzothiazoline 6 sulfonic acid)

ALD

atomic layer deposition

AQY

apparent quantum yield

ATP

adenosine triphosphate

BFC

biofuel cell

BOD

bilirubin oxidase

BHET

bis­(2 hydroxyethyl) terephthalate

C _x Ny

carbon nitride

COF

covalent organic framework

DCPIP

2,6 dichlorophenolindophenol

DET

direct electron transfer

DTT

dithiothreitol

EET

extracellular electron transfer

EQE

external quantum efficiency

FDCA

2,5 furandicarboxylic acid

FDH

formate dehydrogenase

FAD

flavin adenine dinucleotide

FMN

flavin mononucleotide

FNR

ferredoxin–NADP⁺ reductase

FTO

fluorine doped tin oxide

HER

hydrogen evolution reaction

HMF

Five hydroxymethylfurfural

HRP

horseradish peroxidase

IPCE

incident photon to current efficiency

IQE

internal quantum efficiency

ITO

indium tin oxide

kET

electron transfer rate constant

MET

mediated electron transfer

MWCNT

multi walled carbon nanotube

MV

methyl viologen

N.A.

not available

N.Q.

not quantified

NAD⁺

nicotinamide adenine dinucleotide

NADH

reduced nicotinamide adenine dinucleotide

NADP⁺

nicotinamide adenine dinucleotide phosphate

NADPH

reduced nicotinamide adenine dinucleotide phosphate

NAD­(P)­H

reduced nicotinamide adenine dinucleotide (phosphate)

NHE

normal hydrogen electrode

NP(s)

nanoparticle(s)

NR(s)

nanorod(s)

OCV

open circuit voltage

OER

oxygen evolution reaction

ORR

oxygen reduction reaction

PBEC

photo bioelectrochemical cell

PBV

polybenzyl viologen

PEC

photoelectrochemical cell

PET

polyethylene terephthalate

PCE

power conversion efficiency

PLA

polylactic acid

PS

photosensitizer

PSI

photosystem I

PSII

photosystem II

QD(s)

quantum dot(s)

QE

quantum efficiency

QY

quantum yield

RHE

reversible hydrogen electrode

SAM

self-assembled monolayer

SED

sacrificial electron donor

SHE

standard hydrogen electrode

SP1

stable protein 1

STH

solar to hydrogen efficiency

TEOA

triethanolamine

TEMPO

2,2,6,6 tetramethylpiperidine 1 oxyl

TOF

turnover frequency

TRL

technology readiness level

Biographies

Dr. Yifat Cohen received her PhD in Biotechnology and Food Engineering from the Technion – Israel Institute of Technology (2018). In the same year, she joined the Yehezkeli research group, where she serves as a researcher and laboratory manager. Her current research focuses on the development of methods for valorization of industrial residual streams.

Dr. Oren Bachar received his PhD (2025) in Biotechnology and Food Engineering from the Technion, Israel Institute of Technology. Currently, he is a postdoctoral researcher at the Max Planck Institute of Terrestrial Microbiology, in the Department of Biochemistry and Synthetic Metabolism. His research interests span from synthetic biology, material science, photodriven cofactor regeneration, and developing new-to-nature enzymatic cascades for CO₂ fixation.

Dr. Roy Cohen received his PhD (2023) from the Technion – Israel Institute of Technology, where he studied and developed amperometric biosensors for biomarker monitoring and disease detection. During his PhD, he was awarded the Jacobs scholarship for academic excellence. Later on, he developed topical patches that enable continuous biomarker monitoring through the Shulamit Aloni fellowship. He is currently a postdoctoral fellow at the Technion, developing valorization methods for carbon-rich waste.

Dr. Matan M. Meirovich received his BSc and PhD (2025) in Biotechnology & Food Engineering from the Technion – Israel Institute of Technology. Under the supervision of Prof. Omer Yehezkeli, his research focused on nitrogenase-based nanobiohybrid systems for photobio­(electro)­catalytic applications. Matan aims to integrate photocatalysis and electrocatalysis with biological elements, specifically enzymatic and microbial systems, to develop next-generation applied biotechnological solutions.

Prof. Omer Yehezkeli received his Bachelor’s (2006) in Chemistry from the Hebrew University of Jerusalem. He conducted his MSc (2008) and PhD (2014) research at the Hebrew University of Jerusalem with Prof. Itamar Willner. He later joined the University of Colorado, Boulder, as a postdoctoral research associate with Prof. Jennifer N. Cha (2017). He is currently serving as Prof. of Biotechnology and Food Engineering at the Technion, Israel Institute of Technology. Prof. Yehezkeli’s research focuses on the interface between biotic and abiotic elements with implications in health, (photo)­bioelectrochemistry, artificial photosynthesis, and waste valorization.

The manuscript was written through the contributions of all authors. All authors have approved the final version of the manuscript.

The authors declare no competing financial interest.

Published as part of Chemical Reviews special issue “Semi-artificial Photosynthesis”.