A Photovoltaic Device Using an Electrolyte Containing Photosynthetic Reaction Centers

Preparation of Photo-Bioelectrochemical Cells With the RC-LH Complex From Roseiflexus castenholzii

Roseiflexus castenholzii is an ancient green non-sulfur bacteria that absorbs the solar energy through bacteriochlorophylls (BChls) bound in the only light harvesting (LH) complex, and transfers to the reaction center (RC), wherein primary charge separation occurs and transforms the energy into electrochemical potentials. In contrast to purple bacteria, R. castenholzii RC-LH (rcRC-LH) does not contain an H subunit. Instead, a tightly bound tetraheme cytochrome c subunit is exposed on the P-side of the RC, which contains three BChls, three bacteriopheophytins (BPheos), two menaquinones, and one iron for electron transfer. These novel structural features of the rcRC-LH are advantageous for enhancing the electron transfer efficiency and subsequent photo-oxidation of the c-type hemes. However, the photochemical properties of rcRC-LH and its applications in developing the photo-bioelectrochemical cells (PBECs) have not been characterized. Here, we prepared a PBEC using overlapped fluorine-doped tin oxide (FTO) glass and Pt-coated glass as electrodes, and rcRC-LH mixed with varying mediators as the electrolyte. Absence of the H subunit allows rcRC-LH to be selectively adhered onto the hydrophilic surface of the front electrode with its Q-side. Upon illumination, the photogenerated electrons directly enter the front electrode and transfer to the counter electrode, wherein the accepted electrons pass through the exposed c-type hemes to reduce the excited P⁺, generating a steady-state current of up to 320 nA/cm² when using 1-Methoxy-5-methylphenazinium methyl sulfate (PMS) as mediator. This study demonstrated the novel photoelectric properties of rcRC-LH and its advantages in preparing effective PBECs, showcasing a potential of this complex in developing new type PBECs.

Photocurrent Generation by Photosynthetic Purple Bacterial Reaction Centers Interfaced with a Porous Antimony-Doped Tin Oxide (ATO) Electrode

The ability to exchange energy and information between biological and electronic materials is critical in the development of hybrid electronic systems in biomedicine, environmental sensing, and energy applications. While sensor technology has been extensively developed to collect detailed molecular information, less work has been done on systems that can specifically modulate the chemistry of the environment with temporal and spatial control. The bacterial photosynthetic reaction center represents an ideal photonic component of such a system in that it is capable of modifying local chemistry via light-driven redox reactions with quantitative control over reaction rates and has inherent spectroscopic probes for monitoring function. Here a well-characterized model system is presented, consisting of a transparent, porous electrode (antimony-doped tin oxide) which is electrochemically coupled to the reaction center via a cytochrome c molecule. Upon illumination, the reaction center performs the 2-step, 2-electron reduction of a ubiquinone derivative which exchanges with oxidized quinone in solution. Electrons from the electrode then move through the cytochrome to reoxidize the reaction center electron donor. The result is a facile platform for performing redox chemistry that can be optically and electronically controlled in time and space.

Supramolecular nanostructures based on bacterial reaction center proteins and quantum dots

Design of the nanostructures based on membrane proteins (the key functional elements of biomembranes) and colloid nanoparticles is a fascinating field at the interface of biochemistry and colloids, nanotechnology and biomedicine. The review discusses the main achievements in the field of ultrathin films prepared from bacterial reaction center proteins and light-harvesting complexes, as well as these complexes tagged with quantum dots. The principles of preparation of these thin films and their structure and properties at different interfaces are described; as well as their characteristics estimated using a combination of the modern interfacial techniques (absorption and fluorescence spectroscopy, atomic force and Brewster angle microscopy, etc.) are discussed. Further approaches to develop the nanostructures based on the membrane proteins and quantum dots are suggested. These supramolecular nanostructures are promising prototypes of the materials for photovoltaic, optoelectronic and biosensing applications.

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.

Application of Wide Band Gap Semiconductors to Increase Photocurrent in a Protein Based Photovoltaic Device

Reaction centers (RCs) from natural photosynthetic cells are photoactive proteins, which generate electron-hole pairs in presence of light. In a new approach presented in this work, a solution of suspended RCs with mediators has been applied as the electrolyte to build electrochemical based photovoltaic (PV) devices. In this approach, the mediators transfer charges from the RCs to the electrodes (indirect charge transfer). Various metallic and wide bandgap semiconducting materials, including Carbon, Au, Indium Tin Oxide (ITO), SnO2, WO3, have been tested as the electrodes. Among all WO3, which is a semiconductor, have shown the largest photocurrent density with an amount of ∼5.1 μA/cm2. The results show that the material of the electrode can affect the rates of the reactions in the cell. Choosing an appropriate material for the electrode, the charge transfer from the mediators to the electrode would be rectified to achieve a large photocurrent.

Free-floating Reaction Centers (RCs) versus Attached Monolayer of RCs in Bio-photoelectrochemical Cells

The high quantum efficiency (~100%) in the bacterial photosynthetic reaction center (RC) has inspired research on the application of RCs to build protein based solar cells. Conventionally, applying RCs as the photosensitive layer on the surface of a carbon electrode has shown poor photocurrents in the cells. The low photocurrent is partly due to the weak absorption of light in the monolayer of RCs. Also, an Atomic Force Microscopy image of the electrode shows lots of defects on the immobilized RCs at the electrode surface. In this work, we have built a bio-photoelectrochemical cell in which the RCs are floating in the electrolyte instead of being attached to the surface of an electrode. Despite the simple structure of the cell, the photocurrent is significantly higher in the new cell compared to when RCs are attached to an electrode. The amplitude of current reached to ~40 nA for free floating RCs, about five times larger than that in the cell with attached RCs. The aging effect was studied in both cells in a course of a week. The lifetime of attached RCs on electrode surface was slightly better than solubilized RCs in the electrolyte. Also, it is found that the mechanism which governs the charge transfer from RCs to the electrodes is the same in both bio-photoelectrochemical cells.