Rational design of artificial redox-mediating systems toward upgrading photobioelectrocatalysis

Photobioelectrocatalysis has recently attracted particular research interest owing to the possibility to achieve sunlight-driven biosynthesis, biosensing, power generation, and other niche applications. However, physiological incompatibilities between biohybrid components lead to poor electrical contact at the biotic-biotic and biotic-abiotic interfaces. Establishing an electrochemical communication between these different interfaces, particularly the biocatalyst-electrode interface, is critical for the performance of the photobioelectrocatalytic system. While different artificial redox mediating approaches spanning across interdisciplinary research fields have been developed in order to electrically wire biohybrid components during bioelectrocatalysis, a systematic understanding on physicochemical modulation of artificial redox mediators is further required. Herein, we review and discuss the use of diffusible redox mediators and redox polymer-based approaches in artificial redox-mediating systems, with a focus on photobioelectrocatalysis. The future possibilities of artificial redox mediator system designs are also discussed within the purview of present needs and existing research breadth.

Electrospinning for building 3D structured photoactive biohybrid electrodes

We describe the development of biohybrid electrodes constructed via combination of electrospun (e-spun) 3D indium tin oxide (ITO) with the trimeric supercomplex photosystem I and the small electrochemically active protein cytochrome c (cyt c). The developed 3D surface of ITO has been created by electrospinning of a mixture of polyelthylene oxide (PEO) and ITO nanoparticles onto ITO glass slides followed by a subsequent elimination of PEO by sintering the composite. Whereas the photosystem I alone shows only small photocurrents at these 3D electrodes, the co-immobilization of cyt c to the e-spun 3D ITO results in well-defined photoelectrochemical signals. The scaling of thickness of the 3D ITO layers by controlling the time (10 min and 60 min) of electrospinning results in enhancement of the photocurrent. Several performance parameters of the electrode have been analyzed for different illumination intensities.

Solar Energy Conversion through Thylakoid Membranes Wired by Osmium Redox Polymer and Indium Tin Oxide Nanoparticles

For several decades, much attention has been paid to thylakoid membranes (TMs) as photocatalysts for converting solar light to electricity. Despite extensive research, current technology provides only limited photocurrents. Here, a novel method based on TM-composite material was developed for achieving high photocurrent. When a thin film composed of TMs, osmium redox polymer (Os-RP), and indium tin oxide nanoparticles (ITOnp) was formed on a porous graphite surface, appreciable photocurrent as high as 0.5 mA cm^-2 was achieved at 0.4 V vs. Ag/AgCl. Each component plays its own role in transferring electrons from TMs to the anode, resulting in sharp drop in photocurrent with missing any component. Optimization between these three components showed 1 : 0.5 : 30 (TM/Os-RP/ITOnp) was the best ratio. Action spectra confirmed that TMs was the origin of photocurrent. It was inferred from blocking experiments using 3-(3,4-dichlorophenyl)-1,1-dimethylurea as an inhibitor that about 41 % of photocurrent was transferred from Q_A in photosystem II to the electrode via Os-RP and ITOnp. Quantum efficiencies at 430 and 660 nm were 12.2 and 18.5 %, respectively. Turnover frequency for water oxidation depended upon the amount of the composite. A complete cell with Pt/C cathode produced P_max of 122 μW cm^-2 at 758 μA cm^-2 under one sun illumination, which is the highest power density to our knowledge. This study opened a possibility of using TMs as photocatalysts for solar energy conversion.

Chlorophyll-Based Organic Heterojunction on Ti3 C2 Tx MXene Nanosheets for Efficient Hydrogen Production

The Z-scheme process is a photoinduced electron-transfer pathway in natural oxygenic photosynthesis involving electron transport from photosystem II (PSII) to photosystem I (PSI). Inspired by the interesting Z-scheme process, herein a photocatalytic hydrogen evolution reaction (HER) employing chlorophyll (Chl) derivatives, Chl-1 and Chl-2, on the surface of Ti₃ C₂ T_x MXene with two-dimensional accordion-like morphology, forming Chl-1@Chl-2@Ti₃ C₂ T_x composite, is demonstrated. Due to the frontier molecular orbital energy alignments of Chl-1 and Chl-2, sublayer Chl-1 is a simulation of PSI, whereas upper layer Chl-2 is equivalent to PSII, and the resultant electron transport can take place from Chl-2 to Chl-1. Under the illumination of visible light (>420 nm), the HER performance of Chl-1@Chl-2@Ti₃ C₂ T_x photocatalyst was found to be as high as 143 μmol h^-1  g_cat ^-1 , which was substantially higher than that of photocatalysts of either Chl-1@Ti₃ C₂ T_x (20 μmol h^-1  g^-1 ) or Chl-2@Ti₃ C₂ T_x (15 μmol h^-1  g^-1 ).

Membrane Protein Modified Electrodes in Bioelectrocatalysis

Transmembrane proteins involved in metabolic redox reactions and photosynthesis catalyse a plethora of key energy-conversion processes and are thus of great interest for bioelectrocatalysis-based applications. The development of membrane protein modified electrodes has made it possible to efficiently exchange electrons between proteins and electrodes, allowing mechanistic studies and potentially applications in biofuels generation and energy conversion. Here, we summarise the most common electrode modification and their characterisation techniques for membrane proteins involved in biofuels conversion and semi-artificial photosynthesis. We discuss the challenges of applications of membrane protein modified electrodes for bioelectrocatalysis and comment on emerging methods and future directions, including recent advances in membrane protein reconstitution strategies and the development of microbial electrosynthesis and whole-cell semi-artificial photosynthesis.

Photocurrent generation by a photosystem I-NiO photocathode for a p-type biophotovoltaic tandem cell

Photosynthesis is a process used by algae and plants to convert light energy into chemical energy. Due to their uniquely natural and environmentally friendly nature, photosynthetic proteins have attracted attention for use in a variety of artificial applications. Among the various types, biophotovoltaics based on dye-sensitized solar cells have been demonstrated in many studies. Although most related works have used n-type semiconductors, a p-type semiconductor is also a significant potential component for tandem cells. In this work, we used mesoporous NiO as a p-type semiconductor substrate for Photosystem I (PSI) and demonstrated a p-type PSI-biophotovoltaic and tandem cell based on dye-sensitized solar cells. Under visible light illumination, the PSI-adsorbed NiO electrode generated a cathodic photocurrent. The p-type biophotovoltaic cell using the PSI-adsorbed NiO electrode generated electricity, and the IPCE spectrum was consistent with the absorption spectrum of PSI. These results indicate that the PSI-adsorbed NiO electrode acts as a photocathode. Moreover, a tandem cell consisting of the PSI-NiO photocathode and a PSI-TiO2 photoanode showed a high open-circuit voltage of over 0.7 V under illumination to the TiO2 side. Thus, the tandem strategy can be utilized for biophotovoltaics, and the use of other biomaterials that match the solar spectrum will lead to further progress in photovoltaic performance.

The emerging science of electrosynbionics

Dramatic changes in electricity generation, use and storage are needed to keep pace with increasing demand while reducing carbon dioxide emissions. There is great potential for application of bioengineering in this area. We have the tools to re-engineer biological molecules and systems, and a significant amount of research and development is being carried out on technologies such as biophotovoltaics, biocapacitors, biofuel cells and biobatteries. However, there does not seem to be a satisfactory overarching term to describe this area, and I propose a new word-'electrosynbionics'. This is to be defined as: the creation of engineered devices that use components derived from or inspired by biology to perform a useful electrical function. Here, the phrase 'electrical function' is taken to mean the generation, use and storage of electricity, where the primary charge carriers may be either electrons or ions. 'Electrosynbionics' is distinct from 'bioelectronics', which normally relates to applications in sensing, computing or electroceuticals. Electrosynbionic devices have the potential to solve challenges in electricity generation, use and storage by exploiting or mimicking some of the desirable attributes of biological systems, including high efficiency, benign operating conditions and intricate molecular structures.

Investigation of photocurrents resulting from a living unicellular algae suspension with quinones over time

Plants, algae, and some bacteria convert solar energy into chemical energy by using photosynthesis. In light of the current energy environment, many research strategies try to benefit from photosynthesis in order to generate usable photobioelectricity. Among all the strategies developed for transferring electrons from the photosynthetic chain to an outer collecting electrode, we recently implemented a method on a preparative scale (high surface electrode) based on a Chlamydomonas reinhardtii green algae suspension in the presence of exogenous quinones as redox mediators. While giving rise to an interesting performance (10-60 μA cm-2) in the course of one hour, this device appears to cause a slow decrease of the recorded photocurrent. In this paper, we wish to analyze and understand this gradual fall in performance in order to limit this issue in future applications. We thus first show that this kind of degradation could be related to over-irradiation conditions or side-effects of quinones depending on experimental conditions. We therefore built an empirical model involving a kinetic quenching induced by incubation with quinones, which is globally consistent with the experimental data provided by fluorescence measurements achieved after dark incubation of algae in the presence of quinones.

Photobiocatalysis: Activating Redox Enzymes by Direct or Indirect Transfer of Photoinduced Electrons

Biocatalytic transformation has received increasing attention in the green synthesis of chemicals because of the diversity of enzymes, their high catalytic activities and specificities, and mild reaction conditions. The idea of solar energy utilization in chemical synthesis through the combination of photocatalysis and biocatalysis provides an opportunity to make the "green" process greener. Oxidoreductases catalyze redox transformation of substrates by exchanging electrons at the enzyme's active site, often with the aid of electron mediator(s) as a counterpart. Recent progress indicates that photoinduced electron transfer using organic (or inorganic) photosensitizers can activate a wide spectrum of redox enzymes to catalyze fuel-forming reactions (e.g., H₂ evolution, CO₂ reduction) and synthetically useful reductions (e.g., asymmetric reduction, oxygenation, hydroxylation, epoxidation, Baeyer-Villiger oxidation). This Review provides an overview of recent advances in light-driven activation of redox enzymes through direct or indirect transfer of photoinduced electrons.

Highly stable protein immobilizationviamaleimido-thiol chemistry to monitor enzymatic activity

Combining a novel protein immobilisation method with multivariate curve resolution enables the direct observation of biocatalysis by ATR-FTIR spectroscopy.

Material science lesson from the biological photosystem

Inspired by photosynthesis, artificial systems for a sustainable energy supply are being designed. Each sequential energy conversion process from light to biomass in natural photosynthesis is a valuable model for an energy collection, transport and conversion system. Notwithstanding the numerous lessons of nature that provide inspiration for new developments, the features of natural photosynthesis need to be reengineered to meet man's demands. This review describes recent strategies toward adapting key lessons from natural photosynthesis to artificial systems. We focus on the underlying material science in photosynthesis that combines photosystems as pivotal functional materials and a range of materials into an integrated system. Finally, a perspective on the future development of photosynthesis mimetic energy systems is proposed.

Structure and Modification of Electrode Materials for Protein Electrochemistry

The interactions between proteins and electrode surfaces are of fundamental importance in bioelectrochemistry, including photobioelectrochemistry. In order to optimise the interaction between electrode and redox protein, either the electrode or the protein can be engineered, with the former being the most adopted approach. This tutorial review provides a basic description of the most commonly used electrode materials in bioelectrochemistry and discusses approaches to modify these surfaces. Carbon, gold and transparent electrodes (e.g. indium tin oxide) are covered, while approaches to form meso- and macroporous structured electrodes are also described. Electrode modifications include the chemical modification with (self-assembled) monolayers and the use of conducting polymers in which the protein is imbedded. The proteins themselves can either be in solution, electrostatically adsorbed on the surface or covalently bound to the electrode. Drawbacks and benefits of each material and its modifications are discussed. Where examples exist of applications in photobioelectrochemistry, these are highlighted.

Engineering of supramolecular photoactive protein architectures: the defined co-assembly of photosystem I and cytochrome c using a nanoscaled DNA-matrix

The engineering of renewable and sustainable protein-based light-to-energy converting systems is an emerging field of research. Here, we report on the development of supramolecular light-harvesting electrodes, consisting of the redox protein cytochrome c working as a molecular scaffold as well as a conductive wiring network and photosystem I as a photo-functional matrix element. Both proteins form complexes in solution, which in turn can be adsorbed on thiol-modified gold electrodes through a self-assembly mechanism. To overcome the limited stability of self-grown assemblies, DNA, a natural polyelectrolyte, is used as a further building block for the construction of a photo-active 3D architecture. DNA acts as a structural matrix element holding larger protein amounts and thus remarkably improving the maximum photocurrent and electrode stability. On investigating the photophysical properties, this system demonstrates that effective electron pathways have been created.

Light Induced H2 Evolution from a Biophotocathode Based on Photosystem 1--Pt Nanoparticles Complexes Integrated in Solvated Redox Polymers Films

We report on a biophotocathode based on photosystem 1 (PS1)-Pt nanoparticle complexes integrated in a redox hydrogel for photoelectrocatalytic H2 evolution at low overpotential. A poly(vinyl)imidazole Os(bispyridine)2Cl polymer serves as conducting matrix to shuttle the electrons from the electrode to the PS1-Pt complexes embedded within the hydrogel. Light induced charge separation at the PS1-Pt complexes results in the generation of photocurrents (4.8 ± 0.4 μA cm(-2)) when the biophotocathodes are exposed to anaerobic buffer solutions. Under these conditions, the protons are the sole possible electron acceptors, suggesting that the photocurrent generation is associated with H2 evolution. Direct evidence for the latter process is provided by monitoring the H2 production with a Pt microelectrode in scanning electrochemical microscopy configuration over the redox hydrogel film containing the PS1-Pt complexes under illumination.

A Decaheme Cytochrome as a Molecular Electron Conduit in Dye-Sensitized Photoanodes

In nature, charge recombination in light-harvesting reaction centers is minimized by efficient charge separation. Here, it is aimed to mimic this by coupling dye-sensitized TiO₂ nanocrystals to a decaheme protein, MtrC from Shewanella oneidensis MR-1, where the 10 hemes of MtrC form a ≈7-nm-long molecular wire between the TiO₂ and the underlying electrode. The system is assembled by forming a densely packed MtrC film on an ultra-flat gold electrode, followed by the adsorption of approximately 7 nm TiO₂ nanocrystals that are modified with a phosphonated bipyridine Ru(II) dye (RuP). The step-by-step construction of the MtrC/TiO₂ system is monitored with (photo)electrochemistry, quartz-crystal microbalance with dissipation (QCM-D), and atomic force microscopy (AFM). Photocurrents are dependent on the redox state of the MtrC, confirming that electrons are transferred from the TiO₂ nanocrystals to the surface via the MtrC conduit. In other words, in these TiO₂/MtrC hybrid photodiodes, MtrC traps the conduction-band electrons from TiO₂ before transferring them to the electrode, creating a photobioelectrochemical system in which a redox protein is used to mimic the efficient charge separation found in biological photosystems.

Simultaneous measurements of photocurrents and H2O2 evolution from solvent exposed photosystem 2 complexes

In plants, algae, and cyanobacteria, photosystem 2 (PS2) catalyzes the light driven oxidation of water. The main products of this reaction are protons and molecular oxygen. In vitro, however, it was demonstrated that reactive oxygen species like hydrogen peroxide are obtained as partially reduced side products. The transition from oxygen to hydrogen peroxide evolution might be induced by light triggered degradation of PS2's active center. Herein, the authors propose an analytical approach to investigate light induced bioelectrocatalytic processes such as PS2 catalyzed water splitting. By combining chronoamperometry and fluorescence microscopy, the authors can simultaneously monitor the photocurrent and the hydrogen peroxide evolution of light activated, solvent exposed PS2 complexes, which have been immobilized on a functionalized gold electrode. The authors show that under limited electron mediation PS2 displays a lower photostability that correlates with an enhanced H2O2 generation as a side product of the light induced water oxidation.

Photoelectrochemistry of photosystem I bound in nafion

Developing a solid state Photosystem I (PSI) modified electrode is attractive for photoelectrochemical applications because of the quantum yield of PSI, which approaches unity in the visible spectrum. Electrodes are constructed using a Nafion film to encapsulate PSI as well as the hole-scavenging redox mediator Os(bpy)2Cl2. The photoactive electrodes generate photocurrents of 4 μA/cm(2) when illuminated with 1.4 mW/cm(2) of 676 nm band-pass filtered light. Methyl viologen (MV(2+)) is present in the electrolyte to scavenge photoelectrons from PSI in the Nafion film and transport charges to the counter electrode. Because MV(2+) is positively charged in both reduced and oxidized states, it is able to diffuse through the cation permeable channels of Nafion. Photocurrent is produced when the working electrode is set to voltages negative of the Os(3+)/Os(2+) redox potential. Charge transfer through the Nafion film and photohole scavenging at the PSI luminal surface by Os(bpy)2Cl2 depends on the reduction of Os redox centers to Os(2+) via hole scavenging from PSI. The optimal film densities of Nafion (10 μg/cm(2) Nafion) and PSI (100 μg/cm(2) PSI) are determined to provide the highest photocurrents. These optimal film densities force films to be thin to allow the majority of PSI to have productive electrical contact with the backing electrode.

Photosynthesis at the forefront of a sustainable life

The development of a sustainable bio-based economy has drawn much attention in recent years, and research to find smart solutions to the many inherent challenges has intensified. In nature, perhaps the best example of an authentic sustainable system is oxygenic photosynthesis. The biochemistry of this intricate process is empowered by solar radiation influx and performed by hierarchically organized complexes composed by photoreceptors, inorganic catalysts, and enzymes which define specific niches for optimizing light-to-energy conversion. The success of this process relies on its capability to exploit the almost inexhaustible reservoirs of sunlight, water, and carbon dioxide to transform photonic energy into chemical energy such as stored in adenosine triphosphate. Oxygenic photosynthesis is responsible for most of the oxygen, fossil fuels, and biomass on our planet. So, even after a few billion years of evolution, this process unceasingly supports life on earth, and probably soon also in outer-space, and inspires the development of enabling technologies for a sustainable global economy and ecosystem. The following review covers some of the major milestones reached in photosynthesis research, each reflecting lasting routes of innovation in agriculture, environmental protection, and clean energy production.