Solid-state biophotovoltaic cells containing photosystem I

Photosystem I complexes form remarkably stable self-assembled tunneling junctions

This paper describes large-area molecular tunneling junctions comprising self-assembled monolayers (SAMs) of light-harvesting protein complexes using eutectic Ga-In (EGaIn) as a top contact. The complexes, which are readily isolable in large quantities from spinach leaves, self-assemble on top of SAMs of [6,6]-phenyl-C61-butyric acid (PCBA) on gold (Au) supported by mica substrates (AuMica), which induces them to adopt a preferred orientation with respect to the electron transport chain that runs across the short axis of each complex, leading to temperature-independent rectification. We compared trimeric protein complexes isolated from thermophilic cyanobacteria to monomeric complexes extracted from spinach leaves by measuring charge-transport at variable temperatures and over the course of at least three months. Transport is independent of temperature in the range of 130 to 310 K for both protein complexes, affirming that the likely mechanism is non-resonant tunneling. The junctions rectified current and were stable for at least three months when stored at room temperature in ambient conditions, with the yield of working junctions falling from 100% to 97% over that time. These results demonstrate a straightforward strategy for forming remarkably robust molecular junctions, avoiding the fragility that is common in molecular electronics.

Molecular interactions of photosystem I and ZIF-8 in bio-nanohybrid materials

Bio-nanohybrid devices featuring natural photocatalysts bound to a nanostructure hold great promise in the search for sustainable energy conversion. One of the major challenges of integrating biological systems is protecting them against harsh environmental conditions while retaining, or ideally enhancing their photophysical properties. In this mainly computational work we investigate an assembly of cyanobacterial photosystem I (PS I) embedded in a metal-organic framework (MOF), namely the zeolitic imidazolate framework ZIF-8. This complex has been reported experimentally [Bennett et al., Nanoscale Adv., 2019, 1, 94] but so far the molecular interactions between PS I and the MOF remained elusive. We show via absorption spectroscopy that PS I remains intact throughout the encapsulation-release cycle. Molecular dynamics (MD) simulations further confirm that the encapsulation has no noticeable structural impact on the photosystem. However, the MOF building blocks frequently coordinate to the Mg2+ ions of chlorophylls in the periphery of the antenna complex. High-level quantum mechanical calculations reveal charge-transfer interactions, which affect the excitonic network and thereby may reversibly change the fluorescence properties of PS I. Nevertheless, our results highlight the stability of PS I in the MOF, as the reaction center remains unimpeded by the heterogeneous environment, paving the way for applications in the foreseeable future.

Dependence of Protein Immobilization and Photocurrent Generation in PSI-FTO Electrodes on the Electrodeposition Parameters

This study investigates the immobilization of cyanobacterial photosystem I (PSI) from Synechocystis sp. PCC 6803 onto fluorine-doped tin oxide (FTO) conducting glass plates to create photoelectrodes for biohybrid solar cells. The fabrication of these PSI-FTO photoelectrodes is based on two immobilization processes: rapid electrodeposition driven by an external electric field and slower adsorption during solvent evaporation, both influenced by gravitational sedimentation. Deposition and performance of photoelectrodes was investigated by UV-Vis absorption spectroscopy and photocurrent measurements. We investigated the efficiency of PSI immobilization under varying conditions, including solution pH, applied electric field intensity and duration, and electrode polarization, with the goals to control (1) the direction of migration and (2) the orientation of the PSI particles on the substrate surface. Variation in the pH value of the PSI solution alters the surface charge distribution, affecting the net charge and the electric dipole moment of these proteins. Results showed PSI migration to the positively charged electrode at pH 6, 7, and 8, and to the negatively charged electrode at pH 4.4 and 5, suggesting an isoelectric point of PSI between 5 and 6. At acidic pH, the electrophoretic migration was largely hindered by protein aggregation. Notably, photocurrent generation was consistently cathodic and correlated with PSI layer thickness, and no conclusions can be drawn on the orientation of the immobilized proteins. Overall, these findings suggest mediated electron transfer from FTO to PSI by the used electrolyte containing 10 mM sodium ascorbate and 200 μM dichlorophenolindophenol.

Photoactive and conductive biohybrid films by polymerization of pyrrole through voids in photosystem I multilayer films

The combination of conducting polymers with electro- and photoactive proteins into thin films holds promise for advanced energy conversion materials and devices. The emerging field of protein electronics requires conductive soft materials in a composite with electrically insulating proteins. The electropolymerization of pyrrole through voids in a drop-casted photosystem I (PSI) multilayer film enables the straightforward fabrication of photoactive and conductive biohybrid films. The rate of polypyrrole (PPy) growth is reduced by the presence of the PSI film but is insensitive to its thickness, suggesting that rapid diffusion of pyrrole through the voids within the PSI film enables initiation at vacant areas on the gold surface. The base thickness of the composite tends to increase with time, as PPy chains propagate through and beyond the PSI film, coalescing to exhibit a tubule-like morphology as observed by scanning electron microscopy. Increasing amounts of PPy greatly increase the capacitance of the composite films in a manner almost identical to that of pure PPy films grown from unmodified gold, consistent with a high polymer/aqueous interfacial area and a conductive composite film. While PPy is not photoactive here, all composite films, including those with large amounts of PPy, exhibit photocurrents when irradiated by white light in the presence of redox mediator species. Optimization of the Py electropolymerization time is necessary, as increasing amounts of PPy lead to decreased photocurrent density due to a combination of light absorbance by the polymer and reduced accessibility of redox species to active PSI sites.

Protein Orientation Defines Rectification of Electronic Current via Solid-State Junction of Entire Photosystem-1 Complex

We demonstrate that the direction of current rectification via one of nature's most efficient light-harvesting systems, the photosystem 1 complex (PS1), can be controlled by its orientation on Au substrates. Molecular self-assembly of the PS1 complex using four different linkers with distinct functional head groups that interact by electrostatic and hydrogen bonds with different surface parts of the entire protein PS1 complex was used to tailor the PS1 orientation. We observe an orientation-dependent rectification in the current-voltage characteristics for linker/PS1 molecule junctions. Results of an earlier study using a surface two-site PS1 mutant complex having its orientation set by covalent binding to the Au substrate supports our conclusion. Current-voltage-temperature measurements on the linker/PS1 complex indicate off-resonant tunneling as the main electron transport mechanism. Our ultraviolet photoemission spectroscopy results highlight the importance of the protein orientation for the energy level alignment and provide insight into the charge transport mechanism via the PS1 transport chain.

Observation of anomalous carotenoid and blind chlorophyll activations in photosystem I under synthetic membrane confinements

The role of natural thylakoid membrane confinements in architecting the robust structural and electrochemical properties of PSI is not fully understood. Most PSI studies till date extract the proteins from their natural confinements that can lead to non-native conformations. Recently our group had successfully reconstituted PSI in synthetic lipid membranes using detergent-mediated liposome solubilizations. In this study, we investigate the alterations in chlorophylls and carotenoids interactions and reorganization in PSI based on spectral property changes induced by its confinement in anionic DPhPG and zwitterionic DPhPC phospholipid membranes. To this end, we employ a combination of absorption, fluorescence, and circular dichroism (CD) spectroscopic measurements. Our results indicate unique activation and alteration of photoresponses from the PSI carotenoid (Car) bands in PSI-DPhPG proteoliposomes that can tune the Excitation Energy Transfer (EET), otherwise absent in PSI at non-native environments. Specifically, we observe broadband light harvesting via enhanced absorption in the otherwise non-absorptive green region (500-580 nm) of the Chlorophylls (Chl) along with ~64% increase in the full-width half maximum of the Qy band (650-720 nm). The CD results indicate enhanced Chl-Chl and Chl-Car interactions along with conformational changes in protein secondary structures. Such distinct changes in the Car and Chl bands are not observed in PSI confined in DPhPC. The fundamental insights into membrane microenvironments tailoring PSI subunits reorganization and interactions provide novel strategies for tuning photoexcitation processes and rational designing of biotic-abiotic interfaces in PSI-based photoelectrochemical energy conversion systems.

Printable logic circuits comprising self-assembled protein complexes

This paper describes the fabrication of digital logic circuits comprising resistors and diodes made from protein complexes and wired together using printed liquid metal electrodes. These resistors and diodes exhibit temperature-independent charge-transport over a distance of approximately 10 nm and require no encapsulation or special handling. The function of the protein complexes is determined entirely by self-assembly. When induced to self-assembly into anisotropic monolayers, the collective action of the aligned dipole moments increases the electrical conductivity of the ensemble in one direction and decreases it in the other. When induced to self-assemble into isotropic monolayers, the dipole moments are randomized and the electrical conductivity is approximately equal in both directions. We demonstrate the robustness and utility of these all-protein logic circuits by constructing pulse modulators based on AND and OR logic gates that function nearly identically to simulated circuits. These results show that digital circuits with useful functionality can be derived from readily obtainable biomolecules using simple, straightforward fabrication techniques that exploit molecular self-assembly, realizing one of the primary goals of molecular electronics.

Proteins are promising molecular materials for next-generation electronic devices. Here, the authors fabricated printable digital logic circuits comprising resistors and diodes from self-assembled photosystem I complexes that enable pulse modulation.

Electron Transfer in a Bio-Photoelectrode Based on Photosystem I Multilayer Immobilized on the Conducting Glass

A film of ~40 layers of partially oriented photosystem I (PSI) complexes isolated from the red alga Cyanidioschyzon merolae formed on the conducting glass through electrodeposition was investigated by time-resolved absorption spectroscopy and chronoamperometry. The experiments were performed at a range of electric potentials applied to the film and at different compositions of electrolyte solution being in contact with the film. The amount of immobilized proteins supporting light-induced charge separation (active PSI) ranged from ~10%, in the absence of any reducing agents (redox compounds or low potential), to ~20% when ascorbate and 2,6-dichlorophenolindophenol were added, and to ~35% when the high negative potential was additionally applied. The origin of the large fraction of permanently inactive PSI (65–90%) was unclear. Both reducing agents increased the subpopulation of active PSI complexes, with the neutral P700 primary electron donor, by reducing significant fractions of the photo-oxidized P700 species. The efficiencies of light-induced charge separation in the PSI film (10–35%) did not translate into an equally effective generation of photocurrent, whose internal quantum efficiency reached the maximal value of 0.47% at the lowest potentials. This mismatch indicates that the vast majority of the charge-separated states in multilayered PSI complexes underwent charge recombination.

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

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

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

Cryo-EM structure of a tetrameric photosystem I from Chroococcidiopsis TS-821, a thermophilic, unicellular, non-heterocyst-forming cyanobacterium

Photosystem I (PSI) is one of two photosystems involved in oxygenic photosynthesis. PSI of cyanobacteria exists in monomeric, trimeric, and tetrameric forms, in contrast to the strictly monomeric form of PSI in plants and algae. The tetrameric organization raises questions about its structural, physiological, and evolutionary significance. Here we report the ∼3.72 Å resolution cryo-electron microscopy structure of tetrameric PSI from the thermophilic, unicellular cyanobacterium Chroococcidiopsis sp. TS-821. The structure resolves 44 subunits and 448 cofactor molecules. We conclude that the tetramer is arranged via two different interfaces resulting from a dimer-of-dimers organization. The localization of chlorophyll molecules permits an excitation energy pathway within and between adjacent monomers. Bioinformatics analysis reveals conserved regions in the PsaL subunit that correlate with the oligomeric state. Tetrameric PSI may function as a key evolutionary step between the trimeric and monomeric forms of PSI organization in photosynthetic organisms.

Photosynthetic Nanomaterial Hybrids for Bioelectricity and Renewable Energy Systems

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

Fullerenes Enhance Self-Assembly and Electron Injection of Photosystem I in Biophotovoltaic Devices

This paper describes the fabrication of microfluidic devices with a focus on controlling the orientation of photosystem I (PSI) complexes, which directly affects the performance of biophotovoltaic devices by maximizing the efficiency of the extraction of electron/hole pairs from the complexes. The surface chemistry of the electrode on which the complexes assemble plays a critical role in their orientation. We compared the degree of orientation on self-assembled monolayers of phenyl-C61-butyric acid and a custom peptide on nanostructured gold electrodes. Biophotovoltaic devices fabricated with the C61 fulleroid exhibit significantly improved performance and reproducibility compared to those utilizing the peptide, yielding a 1.6-fold increase in efficiency. In addition, the C61-based devices were more stable under continuous illumination. Our findings show that fulleroids, which are well-known acceptor materials in organic photovoltaic devices, facilitate the extraction of electrons from PSI complexes without sacrificing control over the orientation of the complexes, highlighting this combination of traditional organic semiconductors with biomolecules as a viable approach to coopting natural photosynthetic systems for use in solar cells.

Probing size variations of molecular aggregates inside chlorosomes using single-object spectroscopy

We theoretically investigate the possibility to use single-object spectroscopy to probe size variations of the bacteriochlorophyll aggregates inside chlorosomes. Chlorosomes are the light-harvesting organelles of green sulfur and non-sulfur bacteria. They are known to be the most efficient light-harvesting systems in nature. Key to this efficiency is the organization of bacteriochlorophyll molecules in large self-assembled aggregates that define the secondary structure inside the chlorosomes. Many studies have been reported to elucidate the morphology of these aggregates and the molecular packing inside them. It is widely believed that tubular aggregates play an important role. Because the size (radius and length) of these aggregates affects the optical and excitation energy transport properties, it is of interest to be able to probe these quantities inside chlorosomes. We show that a combination of single-chlorosome linear polarization resolved spectroscopy and single-chlorosome circular dichroism spectroscopy may be used to access the typical size of the tubular aggregates within a chlorosome and, thus, probe possible variations between individual chlorosomes that may result, for instance, from different stages in growth or different growth conditions.

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.

Enhancement of the Photocurrent of a Single Photosystem I Complex by the Localized Plasmon of a Gold Nanorod

A combination of conductive atomic force microscopy (AFM) and confocal fluorescence microscopy was used to measure photocurrents passing through single trimeric photosytem I (PSI) complexes located in the vicinity of single gold nanorods (AuNRs). Simultaneous excitation of PSI and of the AuNR longitudinal plasmon mode and detection of photocurrents from individual PSI in relation to the position of single AuNRs enable insight into plasmon-induced phenomena that are otherwise inaccessible in ensemble experiments. We have observed photocurrent enhancement by the localized plasmons by a factor of 2.9 on average, with maximum enhancement values of up to 8. Selective excitation of the longitudinal plasmon modes by the polarization of the excitation laser enables controllable switch-on of the photocurrent enhancement. The dependence of the extent of enhancement on the distance between PSI and AuNRs indicates that, apart from the enhancement of absorption, there is an additional enhancement mechanism affecting directly the electron transport process. The present study provides deeper insight into the molecular mechanisms of plasmon-enhanced photocurrents, not only in PSI but also potentially in other systems as well.

Aqueous-soluble bipyridine cobalt(ii/iii) complexes act as direct redox mediators in photosystem I-based biophotovoltaic devices

Sustainable energy production is critical for meeting growing worldwide energy demands. Due to its stability and reduction potential, photosystem I (PSI) is attractive as the photosensitizer in biophotovoltaic devices. Herein, we characterize aqueous and organic solvent soluble synthetic bipyridine-based cobalt complexes as redox mediators for PSI-based biophotovoltaics applications. Cobalt-based complexes are not destructive to protein and have appropriate midpoint potentials for electron donation to PSI. We report on PSI stability in organic solvents commonly used in biophotovoltaics. We also show the effects of a mixed organic solvent phase on PSI reduction kinetics, slowing reduction rates approximately 8–38 fold as compared to fully aqueous systems, with implications for dye regeneration rates in PSI-based biophotovoltaics. Further, we show evidence of direct electron transfer from cobalt complexes to PSI. Finally, we report on photocurrent generation from Co mediator-PSI biophotovoltaic devices. Taken together, we discuss the development of novel Co complexes and our ability to fine-tune their characteristics via functional groups and counteranion choice to drive interaction with a biological electron acceptor on multiple levels from redox midpoints, spectral overlap, and solvent requirements, among others. This work suggests that fine-tuning of redox active species for interaction with a biological partner is possible for the creation and improvement of low cost, carbon-neutral energy production in the future.

Sustainable energy production is critical for meeting growing worldwide energy demands.

A new platform for development of photosystem I based thin films with superior photocurrent: TCNQ charge transfer salts derived from ZIF-8

The transmembrane photosynthetic protein complex Photosystem I (PSI) is highly sought after for incorporation into biohybrid photovoltaic devices due to its remarkable photoactive electrochemical properties, chiefly driving charge separation with ∼1 V potential and ∼100% quantum efficiency. In pursuit of these integrated technologies, three factors must be simultaneously tuned, namely, direct redox transfer steps, three-dimensional coordination and stabilization of PSI aggregates, and interfacial connectivity with conductive pathways. Building on our recent successful encapsulation of PSI in the metal-organic framework ZIF-8, herein we use the zinc and imidazole cations from this precursor to form charge transfer complexes with an extremely strong organic electron acceptor, TCNQ. Specifically, the PSI-Zn-H2mim-TCNQ charge transfer salt complex was drop cast on ITO to form dense films. Subsequent voltammetric cycling induced cation exchange and electrochemical annealing of the film was used to enhance electron conductivity giving rise to a photocurrent in the order of 15 μA cm-2. This study paves the way for a myriad of future opportunities for successful integration of this unique class of charge transfer salt complexes with biological catalysts and light harvesters.

Photosystem I Enhances the Efficiency of a Natural, Gel-Based Dye-Sensitized Solar Cell

The photosystem I (PSI) protein complex is known to enhance bioelectrode performance for many liquid-based photoelectrochemical cells. A hydrogel as electrolyte media allows for simpler fabrication of more robust and practical solar cells in comparison to liquid-based devices. This paper reports a natural, gel-based dye-sensitized solar cell that integrates PSI to improve device efficiency. TiO₂-coated FTO slides, dyed by blackberry anthocyanin, act as a photoanode, while a film of PSI deposited onto copper comprises the photocathode. Ascorbic acid (AscH) and 2,6-dichlorophenolindophenol (DCPIP) are the redox mediator couple inside an agarose hydrogel, enabling PSI to produce excess oxidized species near the cathode to improve device performance. A comparison of performance at low pH and neutral pH was performed to test the pH-dependent properties of the AscH/DCPIP couple. Devices at neutral pH performed better than those at lower pH. The PSI film enhanced photovoltage by 75 mV to a total photovoltage of 0.45 V per device and provided a mediator concentration-dependent photocurrent enhancement over non-PSI devices, reaching an instantaneous power conversion efficiency of 0.30% compared to 0.18% without PSI, a 1.67-fold increase. At steady state, power conversion efficiencies for devices with and without PSI were 0.042 and 0.028%, respectively.

Excitation dynamics in Photosystem I trapped in TiO2 mesopores

Excitation decay in closed Photosystem I (PSI) isolated from cyanobacterium Synechocystis sp. PCC 6803 and dissolved in a buffer solution occurs predominantly with a ~ 24-ps lifetime, as measured both by time-resolved fluorescence and transient absorption. The same PSI particles deposited in mesoporous matrix made of TiO₂ nanoparticles exhibit significantly accelerated excitation decay dominated by a ~ 6-ps component. Target analysis indicates that this acceleration is caused by ~ 50% increase of the rate constant of bulk Chls excitation quenching. As an effect of this increase, as much as ~ 70% of bulk Chls excitation is quenched before the establishment of equilibrium with the red Chls. Accelerated quenching may be caused by increased excitation trapping by the reaction center and/or quenching properties of the TiO₂ surface directly interacting with PSI Chls. Also properties of the PSI red Chls are affected by the deposition in the TiO₂ matrix: they become deeper traps due to an increase of their number and their oscillator strength is significantly reduced. These effects should be taken into account when constructing solar cells' photoelectrodes composed of PSI and artificial matrices.

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.

Photovoltaic activity of electrodes based on intact photosystem I electrodeposited on bare conducting glass

We demonstrate photovoltaic activity of electrodes composed of fluorine-doped tin oxide (FTO) conducting glass and a multilayer of trimeric photosystem I (PSI) from cyanobacterium Synechocystis sp. PCC 6803 yielding, at open circuit potential (OCP) of + 100 mV (vs. SHE), internal quantum efficiency of (0.37 ± 0.11)% and photocurrent density of up to (0.5 ± 0.1) µA/cm². The photocurrent measured for OCP is of cathodic nature meaning that preferentially the electrons are injected from the conducting layer of the FTO glass to the photooxidized PSI primary electron donor, P700⁺, and further transferred from the photoreduced final electron acceptor of PSI, F_b^-, via ascorbate electrolyte to the counter electrode. This observation is consistent with preferential donor-side orientation of PSI on FTO imposed by applied electrodeposition. However, by applying high-positive bias (+ 620 mV) to the PSI-FTO electrode, exceeding redox midpoint potential of P700 (+ 450 mV), the photocurrent reverses its orientation and becomes anodic. This is explained by "switching off" the natural photoactivity of PSI particles (by the electrochemical oxidation of P700 to P700⁺) and "switching on" the anodic photocurrent from PSI antenna Chls prone to photooxidation at high potentials. The efficient control of the P700 redox state (P700 or P700⁺) by external bias applied to the PSI-FTO electrodes was evidenced by ultrafast transient absorption spectroscopy. The advantage of the presented system is its structural simplicity together with in situ-proven high intactness of the PSI particles.

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

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

Jolly green MOF: confinement and photoactivation of photosystem I in a metal-organic framework

Photosystem I (PSI) is a ∼1000 kDa transmembrane protein that enables photoactivated charge separation with ∼1 V driving potential and ∼100% quantum efficiency during the photosynthetic process. Although such properties make PSI a potential candidate for integration into bio-hybrid solar energy harvesting devices, the grand challenge in orchestrating such integration rests on rationally designed 3D architectures that can organize and stabilize PSI in the myriad of harsh conditions in which it needs to function. The current study investigates the optical response and photoactive properties of PSI encapsulated in a highly stable nanoporous metal-organic framework (ZIF-8), denoted here as PSI@ZIF-8. The ZIF-8 framework provides a unique scaffold with a robust confining environment for PSI while protecting its precisely coordinated chlorophyll networks from denaturing agents. Significant blue shifts in the fluorescence emissions from UV-vis measurements reveal the successful confinement of PSI in ZIF-8. Pump-probe spectroscopy confirms the photoactivity of the PSI@ZIF-8 composites by revealing the successful internal charge separation and external charge transfer of P₇₀₀ ⁺ and F_B ^- even after exposure to denaturing agents and organic solvents. This work provides greater fundamental understanding of confinement effects on pigment networks, while significantly broadening the potential working environments for PSI-integrated bio-hybrid materials.

Polyviologen as Electron Transport Material in Photosystem I-Based Biophotovoltaic Cells

The photosynthetic protein complex, photosystem I (PSI), can be photoexcited with a quantum efficiency approaching unity and can be integrated into solar energy conversion devices as the photoactive electrode. The incorporation of PSI into conducting polymer frameworks allows for improved conductivity and orientational control in the photoactive layer. Polyviologens are a unique class of organic polycationic polymers that can rapidly accept electrons from a primary donor such as photoexcited PSI and subsequently can donate them to a secondary acceptor. Monomeric viologens, such as methyl viologen, have been widely used as diffusible mediators in wet PSI-based photoelectrochemical cells on the basis of their suitable redox potentials for accepting electrons. Polyviologens possess similar electronic properties to their monomers with the added advantage that they can shuttle electrons in the solid state. Depositing polyviologen directly onto a film of PSI protein results in significant photocurrent enhancement, which confirms its role as an electron-transport material. The polymer film not only improves the photocurrent by aiding the electron transfer but also helps preserve the protein film underneath. The composite polymer-PSI assembly enhances the charge-shuttling processes from individual protein molecules within the PSI multilayer, greatly reducing charge-transfer resistances. The resulting PSI-based solid-state platform demonstrates a much higher photocurrent than the corresponding photoelectrochemical cell built using a similar architecture.

Natural Dyes and Their Derivatives Integrated into Organic Solar Cells

Natural photosynthetic systems contain several dyes such as carotenoids or chlorophylls which are adequately arranged to produce efficient photoinduced charge separation and electron transfer. Several research groups have attempted integrating these natural dyes and photosynthetic systems into functional organic solar cells (OSCs) producing power conversion efficiencies (PCEs) up to 0.99%. The studies presented in this short review emphasize that functionalization of natural dyes can considerably improve their PCEs. For instance, chlorophyll derivatives can yield PCEs up to 2.1%, and copolymers produced with isoindigo as an electron-deficient unit generate high PCEs up to 8%, respectively, when combined with fullerene C70 based electron acceptors in the OSC active layers. An alternative approach for natural dye integration into OSC architectures is to place these light-harvesting antennas at the interface between the active layer and the charge collection layers in these low-cost photovoltaic devices. This strategy produces large PCE increases up to 35% with respect to OSCs prepared without the interlayer. When light-harvesting systems are combined with silver nanoprisms as interlayers, additional localized surface plasmon resonance effects result in high-performance OSCs that integrate natural photosynthetic systems and demonstrate a PCE over the milestone value of 10%.

Identification of Short- and Long-Wavelength Emitting Chlorophylls in Cyanobacterial Photosystem I by Plasmon-Enhanced Single-Particle Spectroscopy at Room Temperature

A peculiarity of cyanobacterial Photosystem I (PSI) is the presence of so-called red chlorophylls absorbing at wavelengths longer than the reaction center P700. The origin and function of these chlorophylls have been debated in literature, but so far no consensus has been reached on either question. Here, we use plasmon-enhanced single-particle fluorescence spectroscopy to elucidate the origin of both short- and long-wavelength emitting species in monomeric PSI from Thermosynechococcus elongatus at room temperature. Polarized fluorescence spectra of single PSI complexes reveal a phase shift in the modulation of the short-wavelength (687 nm) and long-wavelength (717 nm) peaks. Numerical simulations show that this phase shift reflects a spatial angle of 15° between the transition dipole moments of the two forms. Quantum chemical calculations, together with reported X-ray structural and spectroscopic data, were used to assign the chlorophyll a monomer A3 as a candidate for the short-wavelength emitter and the B31-B32 chlorophyll dimer as a candidate for the long-wavelength emitter.

Self-Regenerating Soft Biophotovoltaic Devices

This paper describes the fabrication of soft, stretchable biophotovoltaic devices that generate photocurrent from photosystem I (PSI) complexes that are self-assembled onto Au electrodes with a preferred orientation. Charge is collected by the direct injection of electrons into the Au electrode and the transport of holes through a redox couple to liquid eutectic gallium-indium (EGaIn) electrodes that are confined to microfluidic pseudochannels by arrays of posts. The pseudochannels are defined in a single fabrication step that leverages the non-Newtonian rheology of EGaIn. This strategy is extended to the fabrication of reticulated electrodes that are inherently stretchable. A simple shadow evaporation technique is used to increase the surface area of the Au electrodes by a factor of approximately 10⁶ compared to planar electrodes. The power conversion efficiency of the biophotovoltaic devices decreases over time, presumably as the PSI complexes denature and/or detach from the Au electrodes. However, by circulating a solution of active PSI complexes the devices self-regenerate by mass action/self-assembly. These devices leverage simple fabrication techniques to produce complex function and prove that photovoltaic devices comprising PSI can retain the ability to regenerate, one of the most important functions of photosynthetic organisms.

A Biogenic Photovoltaic Material

A proof-of-concept for the fabrication of genetically customizable biogenic materials for photovoltaic applications is presented. E. coli is first genetically engineered to heterologously express the carotenoid biosynthetic pathway from plants. This modification yields a strain that overproduces the photoactive pigment lycopene. The pigment-producing cells are then coated with TiO₂ nanoparticles via a tryptophan-mediated supramolecular interface, and subsequent incorporation of the resulting biogenic material (cells@TiO₂ ) as an anode in an I^- /I₃^- -based dye-sensitized solar cell yields an excellent photovoltaic (PV) response. This work lays strong foundations for the development of bio-PV materials and next-generation organic optoelectronics that are green, inexpensive, and easy to manufacture.

Photoswitchable Sn-Cyt c Solid-State Devices

Electron transfer across proteins plays an important role in many biological processes, including those relevant for the conversion of solar photons to chemical energy. Previous studies demonstrated the generation of photocurrents upon light irradiation in a number of photoactive proteins, such as photosystem I or bacteriorhodopsin. Here, it is shown that Sn-cytochrome c layers act as reversible and efficient photoelectrochemical switches upon integration into large-area solid-state junctions. Photocurrents are observed both in the Soret band (λ = 405 nm) and in the Q band (λ = 535 nm), with current on/off ratios reaching values of up to 25. The underlying modulation in charge-transfer rate is attributed to a hole-transport channel created by the photoexcitation of the Sn-porphyrin.

Liquefaction of Biopolymers: Solvent-free Liquids and Liquid Crystals from Nucleic Acids and Proteins

Biomacromolecules, such as nucleic acids, proteins, and virus particles, are persistent molecular entities with dimensions that exceed the range of their intermolecular forces hence undergoing degradation by thermally induced bond-scission upon heating. Consequently, for this type of molecule, the absence of a liquid phase can be regarded as a general phenomenon. However, certain advantageous properties usually associated with the liquid state of matter, such as processability, flowability, or molecular mobility, are highly sought-after features for biomacromolecules in a solvent-free environment. Here, we provide an overview over the design principles and synthetic pathways to obtain solvent-free liquids of biomacromolecular architectures approaching the topic from our own perspective of research. We will highlight the milestones in synthesis, including a recently developed general surfactant complexation method applicable to a large variety of biomacromolecules as well as other synthetic principles granting access to electrostatically complexed proteins and DNA.

These synthetic pathways retain the function and structure of the biomacromolecules even under extreme, nonphysiological conditions at high temperatures in water-free melts challenging the existing paradigm on the role of hydration in structural biology. Under these conditions, the resulting complexes reveal their true potential for previously unthinkable applications. Moreover, these protocols open a pathway toward the assembly of anisotropic architectures, enabling the formation of solvent-free biomacromolecular thermotropic liquid crystals. These ordered biomaterials exhibit vastly different mechanical properties when compared to the individual building blocks. Beyond the preparative aspects, we will shine light on the unique potential applications and technologies resulting from solvent-free biomacromolecular fluids: From charge transport in dehydrated liquids to DNA electrochromism to biocatalysis in the absence of a protein hydration shell. Moreover, solvent-free biological liquids containing viruses can be used as novel storage and process media serving as a formulation technology for the delivery of highly concentrated bioactive compounds. We are confident that this new class of hybrid biomaterials will fuel further studies and applications of biomacromolecules beyond water and other solvents and in a much broader context than just the traditional physiological conditions.

Orientation and Incorporation of Photosystem I in Bioelectronics Devices Enabled by Phage Display

Interfacing proteins with electrode surfaces is important for the field of bioelectronics. Here, a general concept based on phage display is presented to evolve small peptide binders for immobilizing and orienting large protein complexes on semiconducting substrates. Employing this method, photosystem I is incorporated into solid-state biophotovoltaic cells.

Biofunctionalisation of p-doped silicon with cytochrome c553minimises charge recombination and enhances photovoltaic performance of the all-solid-state photosystem I-based biophotoelectrode

Passivation of p-doped silicon substrate was achieved by its biofunctionalisation with hexahistidine-tagged cytochrome c₅₅₃, a soluble electroactive photosynthetic protein responsible for electron donation to photooxidised photosystem I.

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.

Dipole-induced asymmetric conduction in tunneling junctions comprising self-assembled monolayers

The collective action of embedded dipoles causes asymmetric tunneling charge-transport through self-assembled monolayers.

Construction of a Semiconductor-Biological Interface for Solar Energy Conversion: p-Doped Silicon/Photosystem I/Zinc Oxide

The interface between photoactive biological materials with two distinct semiconducting electrodes is challenging both to develop and analyze. Building off of our previous work using films of photosystem I (PSI) on p-doped silicon, we have deposited a crystalline zinc oxide (ZnO) anode using confined-plume chemical deposition (CPCD). We demonstrate the ability of CPCD to deposit crystalline ZnO without damage to the PSI biomaterial. Using electrochemical techniques, we were able to probe this complex semiconductor-biological interface. Finally, as a proof of concept, a solid-state photovoltaic device consisting of p-doped silicon, PSI, ZnO, and ITO was constructed and evaluated.

Improved Device Performance of Polymer Solar Cells by Using a Thin Light-harvesting-Complex Modified ZnO Film as the Cathode Interlayer

In this study, a high-performance inverted polymer solar cell (PSC) has been fabricated by incorporating a zinc oxide (ZnO)/light-harvesting complex II (LHCII) stacked structure as the cathode interlayer. The LHCII not only smoothens the film surface of ZnO, improves the contact between ZnO and the photoactive layer, but also suppresses the charge carrier recombination at the interface, hence all the device parameters of PTB7-based solar cells are simultaneously improved, yielding higher power conversion efficiency (PCE) up to 9.01% compared with the control one (PCE 8.01%). And the thin LHCII modification layer also presents similar positive effects in the PTB7-Th:PC71BM system (PCE from 8.31% to 9.60%). These results put forward a facile approach to the interfacial modification in high-performance PSCs and provide new insight into developing and utilizing inexpensive and environmentally friendly materials from the fields of biological photosynthesis.

Mechanism of Orientation-Dependent Asymmetric Charge Transport in Tunneling Junctions Comprising Photosystem I

Recently, photoactive proteins have gained a lot of attention due to their incorporation into bioinspired (photo)electrochemical and solar cells. This paper describes the measurement of the asymmetry of current transport of self-assembled monolayers (SAMs) of the entire photosystem I (PSI) protein complex (not the isolated reaction center, RCI), on two different "director SAMs" supported by ultraflat Au substrates. The director SAMs induce the preferential orientation of PSI, which manifest as asymmetry in tunneling charge-transport. We measured the oriented SAMs of PSI using eutectic Ga-In (EGaIn), a large-area technique, and conducting probe atomic force microscopy (CP-AFM), a single-complex technique, and determined that the transport properties are comparable. By varying the temperatures at which the measurements were performed, we found that there is no measurable dependence of the current on temperature from ±0.1 to ±1.0 V bias, and thus, we suggest tunneling as the mechanism for transport; there are no thermally activated (e.g., hopping) processes. Therefore, it is likely that relaxation in the electron transport chain is not responsible for the asymmetry in the conductance of SAMs of PSI complexes in these junctions, which we ascribe instead to the presence of a large, net dipole moment present in PSI.

Electrochemical preparation of Photosystem I-polyaniline composite films for biohybrid solar energy conversion

In this work, we report for the first time the entrapment of the biomolecular supercomplex Photosystem I (PSI) within a conductive polymer network of polyaniline via electrochemical copolymerization. Composite polymer-protein films were prepared on gold electrodes through potentiostatic electropolymerization from a single aqueous solution containing both aniline and PSI. This study demonstrates the controllable integration of large membrane proteins into rapidly prepared composite films, the entrapment of such proteins was observed through photoelectrochemical analysis. PSI's unique function as a highly efficient biomolecular photodiode generated a significant enhancement in photocurrent generation for the PSI-loaded polyaniline films, compared to pristine polyaniline films, and dropcast PSI films. A comprehensive study was then performed to separately evaluate film thickness and PSI concentration in the initial polymerization solution and their effects on the net photocurrent of this novel material. The best performing composite films were prepared with 0.1 μM PSI in the polymerization solution and deposited to a film thickness of 185 nm, resulting in an average photocurrent density of 5.7 μA cm(-2) with an efficiency of 0.005%. This photocurrent output represents an enhancement greater than 2-fold over bare polyaniline films and 200-fold over a traditional PSI multilayer film of comparable thickness.

Large photocurrent response and external quantum efficiency in biophotoelectrochemical cells incorporating reaction center plus light harvesting complexes

Bacterial photosynthetic reaction centers (RCs) are promising materials for solar energy harvesting, due to their high ratio of photogenerated electrons to absorbed photons and long recombination time of generated charges. In this work, photoactive electrodes were prepared from a bacterial RC-light-harvesting 1 (LH1) core complex, where the RC is encircled by the LH1 antenna, to increase light capture. A simple immobilization method was used to prepare RC-LH1 photoactive layer. Herein, we demonstrate that the combination of pretreatment of the RC-LH1 protein complexes with quinone and the immobilization method results in biophotoelectrochemical cells with a large peak transient photocurrent density and photocurrent response of 7.1 and 3.5 μA cm(-2), respectively. The current study with monochromatic excitation showed maximum external quantum efficiency (EQE) and photocurrent density of 0.21% and 2 μA cm(-2), respectively, with illumination power of ∼6 mW cm(-2) at ∼875 nm, under ambient conditions. This work provides new directions to higher performance biophotoelectrochemical cells as well as possibly other applications of this broadly functional photoactive material.

Enhanced photocurrent production by bio-dyes of photosynthetic macromolecules on designed TiO2 film

The macromolecular pigment-protein complex has the merit of high efficiency for light-energy capture and transfer after long-term photosynthetic evolution. Here bio-dyes of A. platensis photosystem I (PSI) and spinach light-harvesting complex II (LHCII) are spontaneously sensitized on three types of designed TiO2 films, to assess the effects of pigment-protein complex on the performance of bio-dye sensitized solar cells (SSC). Adsorption models of bio-dyes are proposed based on the 3D structures of PSI and LHCII, and the size of particles and inner pores in the TiO2 film. PSI shows its merit of high efficiency for captured energy transfer, charge separation and transfer in the electron transfer chain (ETC), and electron injection from FB to the TiO2 conducting band. After optimization, the best short current (JSC) and photoelectric conversion efficiency (η) of PSI-SSC and LHCII-SSC are 1.31 mA cm(-2) and 0.47%, and 1.51 mA cm(-2) and 0.52%, respectively. The potential for further improvement of this PSI based SSC is significant and could lead to better utilization of solar energy.