Near-IR Absorbing Solar Cell Sensitized With Bacterial Photosynthetic Membranes

Generation of Electric Potential Difference by Chromatophores from Photosynthetic Bacteria in the Presence of Trehalose under Continuous Illumination

Measurement of electrical potential difference (Δψ) in membrane vesicles (chromatophores) from the purple bacterium Rhodobacter sphaeroides associated with the surface of a nitrocellulose membrane filter (MF) impregnated with a phospholipid solution in decane or immersed into it in the presence of exogenous mediators and disaccharide trehalose demonstrated an increase in the amplitude and stabilization of the signal under continuous illumination. The mediators were the ascorbate/N,N,N'N'-tetramethyl-p-phenylenediamine pair and ubiquinone-0 (electron donor and acceptor, respectively). Although stabilization of photoelectric responses upon long-term continuous illumination was observed for both variants of chromatophore immobilization, only the samples immersed into the MF retained the functional activity of reaction centers (RCs) for a month when stored in the dark at room temperature, which might be due to the preservation of integrity of chromatophore proteins inside the MF pores. The stabilizing effect of the bioprotector trehalose could be related to its effect on both the RC proteins and the phospholipid bilayer membrane. The results obtained will expand current ideas on the use of semi-synthetic structures based on various intact photosynthetic systems capable of converting solar energy into its electrochemical form.

Conversion of light into electricity in a semi-synthetic system based on photosynthetic bacterial chromatophores

Chromatophores (Chr) from photosynthetic nonsulfur purple bacterium Rhodobacter sphaeroides immobilized onto a Millipore membrane filter (MF) and sandwiched between two semiconductor indium tin oxide (ITO) electrodes (termed ITO|Chr - MF|ITO) have been used to measure voltage (ΔV) induced by continuous illumination. The maximum ΔV was detected in the presence of ascorbate / N,N,N'N'-tetramethyl-p-phenylenediamine couple, coenzyme UQ₀, disaccaride trehalose and antimycin A, an inhibitor of cytochrome bc₁ complex. In doing so, the light-induced electron transfer in the reaction centers was the major source of photovoltages. The stability of the voltage signal upon prolonged irradiation (>1 h) may be due to the maintenance of a conformation that is optimal for the functioning of integral protein complexes and stabilization of lipid bilayer membranes in the presence of trehalose. Retaining ∼70 % of the original photovoltage performance on the 30th day of storage at 23 °C in the dark under air was achieved after re-injection of fresh buffer (∼40 μL) containing redox mediators into the ITO|Chr - MF|ITO system. The approach we use is easy and can be extended to other biological intact systems (cells, thylakoid membranes) capable of converting energy of light.

Alternative sources of natural pigments for dye-sensitized solar cells: Algae, cyanobacteria, bacteria, archaea and fungi

Dye-sensitized solar cells have been of great interest in photovoltaic technology due to their capacity to convert energy at a low cost. The use of natural pigments means replacing expensive chemical synthesis processes by easily extractable pigments that are non-toxic and environmentally friendly. Although most of the pigments used for this purpose are obtained from higher plants, there are potential alternative sources that have been underexploited and have shown encouraging results, since pigments can also be obtained from organisms like bacteria, cyanobacteria, microalgae, yeast, and molds, which have the potential of being cultivated in bioreactors or optimized by biotechnological processes. The aforementioned organisms are sources of diverse sensitizers like photosynthetic pigments, accessory pigments, and secondary metabolites such as chlorophylls, bacteriochlorophylls, carotenoids, and phycobiliproteins. Moreover, retinal proteins, photosystems, and reaction centers from these organisms can also act as sensitizers. In this review, the use of natural sensitizers extracted from algae, cyanobacteria, bacteria, archaea, and fungi is assessed. The reported photoconversion efficiencies vary from 0.001 % to 4.6 % for sensitizers extracted from algae and microalgae, 0.004 to 1.67 % for bacterial sensitizers, 0.07-0.23 % for cyanobacteria, 0.09 to 0.049 % for archaea and 0.26-2.3 % for pigments from fungi.

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.

Modelling of the cathodic and anodic photocurrents from Rhodobacter sphaeroides reaction centres immobilized on titanium dioxide

As one of a number of new technologies for the harnessing of solar energy, there is interest in the development of photoelectrochemical cells based on reaction centres (RCs) from photosynthetic organisms such as the bacterium Rhodobacter (Rba.) sphaeroides. The cell architecture explored in this report is similar to that of a dye-sensitized solar cell but with delivery of electrons to a mesoporous layer of TiO₂ by natural pigment-protein complexes rather than an artificial dye. Rba. sphaeroides RCs were bound to the deposited TiO₂ via an engineered extramembrane peptide tag. Using TMPD (N,N,N',N'-tetramethyl-p-phenylenediamine) as an electrolyte, these biohybrid photoactive electrodes produced an output that was the net product of cathodic and anodic photocurrents. To explain the observed photocurrents, a kinetic model is proposed that includes (1) an anodic current attributed to injection of electrons from the triplet state of the RC primary electron donor (P^T) to the TiO₂ conduction band, (2) a cathodic current attributed to reduction of the photooxidized RC primary electron donor (P⁺) by surface states of the TiO₂ and (3) transient cathodic and anodic current spikes due to oxidation/reduction of TMPD/TMPD⁺ at the conductive glass (FTO) substrate. This model explains the origin of the photocurrent spikes that appear in this system after turning illumination on or off, the reason for the appearance of net positive or negative stable photocurrents depending on experimental conditions, and the overall efficiency of the constructed cell. The model may be a used as a guide for improvement of the photocurrent efficiency of the presented system as well as, after appropriate adjustments, other biohybrid photoelectrodes.

Fluorescence property of photosystem II protein complexes bound to a gold nanoparticle

Development of an efficient photo-anode system for water oxidation is key to the success of artificial photosynthesis. We previously assembled photosystem II (PSII) proteins, which are an efficient natural photocatalyst for water oxidation, on a gold nanoparticle (GNP) to prepare a PSII-GNP conjugate as an anode system in a light-driven water-splitting nano-device (Noji et al., J. Phys. Chem. Lett., 2011, 2, 2448-2452). In the current study, we characterized the fluorescence property of the PSII-GNP conjugate by static and time-resolved fluorescence measurements, and compared with that of free PSII proteins. It was shown that in a static fluorescence spectrum measured at 77 K, the amplitude of a major peak at 683 nm was significantly reduced and a red shoulder at 693 nm disappeared in PSII-GNP. Time-resolved fluorescence measurements showed that picosecond components at 683 nm decayed faster by factors of 1.4-2.1 in PSII-GNP than in free PSII, explaining the observed quenching of the major fluorescence peak. In addition, a nanosecond-decay component arising from a 'red chlorophyll' at 693 nm was lost in time-resolved fluorescence of PSII-GNP, probably due to a structural perturbation of this chlorophyll by interaction with GNP. Consistently with these fluorescence properties, degradation of PSII during strong-light illumination was two times slower in PSII-GNP than in free PSII. The enhanced durability of PSII is an advantageous property of the PSII-GNP conjugate in the development of an artificial photosynthesis device.