Generation of Alternating Current in Response to Discontinuous Illumination by Photoelectrochemical Cells Based on Photosynthetic Proteins

Abstract

There is developing interest in the device properties of natures chlorophyll-containing photochemical reaction centers (RCs) driven by the need to establish a range of technologies for harvesting solar energy and the development of nanoscale electronic components and sensing materials. [1] These RCs operate as photodiodes, converting light energy into a linear or cyclic flow of electrons, [2] and at the heart of this biological function lies a light-induced separation of electrical charge. A much commented feature of natural RCs is that they operate with a quantum yield close to unity. [2] In addition to uses in photovoltaic devices [3] and light-powered fuel cells, [4] RCs have potential applications as photosensors, [5] phototransistors, [6] biosensors for detection of environmental pollutants. [7] A variety of photoelectrochemical cells based on RCs have been constructed, [3] mostly comprising two or three electrodes with RCs adhered to a working electrode that is submerged in a buffer solution containing one or more redox mediators. [8][9][10][11] A number of linkers have been employed to achieve oriented adhesion of RCs to the electrode surface, and in many cases construction of the working electrode has required a complex, multistep fabrication procedure. Photocurrent densities between one and several hundred nA cm À2 have typically been obtained in response to red light of an intensity of a few tens of mW cm À2 , [8,9] and a small number of studies have reported higher photocurrents in the mA or even mA range. [9][10][11][12] Herein, protein photoelectrochemical cells were constructed using either RCs or RC-LH1 complexes from Rhodobacter (Rba.) sphaeroides. In the RC-LH1 complex a central RC is surrounded by a light harvesting 1 (LH1) pigment protein (the structures and mechanism of these complexes are described in Figures S1 and S2 in the Support-ing Information). A very simple fabrication procedure was used in which a mixture of protein and the redox mediator N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) was injected into an approximately 10 mL cavity formed between a fluorine-doped tin oxide (FTO) glass front electrode and a Pt-coated rear electrode (see the Supporting Information). The cells had an area of around 1 cm 2 , and the absorbance spectra of the proteins within the cells were consistent with their spectra in detergent solution ( Figure S3 in the Supporting Information). Cells were illuminated with white light passed through a 570 nm long-pass filter.Illumination of a cell that was constructed using a mixture of RC-LH1 complexes and TMPD produced a short-circuit current density (J SC ) that stabilized at a value of 0.15 mA cm À2 after approximately 20 s (Figure 1 a); the direction of the current indicates electron flow into the cell cavity from the FTO-glass front electrode. The initial spike of current flow was similar to a feature that has been seen in photoelectric measurements on RCs in planar lipid bilayers [13] and Langmuir Blodgett films. [14] The initial spike was consistently se...

Enhanced Output from Biohybrid Photoelectrochemical Transparent Tandem Cells Integrating Photosynthetic Proteins Genetically Modified for Expanded Solar Energy Harvesting

“…[7,11,12] Previous studies utilizing these proteins have been aimed at enhancing photocurrent generation by improving the effectiveness of protein-electrode electron transfer processes in threeelectrode cells [13] or, to a lesser extent, in two-electrode cells. [14,15] Manipulating protein orientation and the nature of chemical or biochemical linkers for immobilizing proteins on electrodes good electronic conductivity achievable on chemical treatment and also due to its low cost. [20] To our knowledge, PEDOT:PSS has not so far been evaluated as an electrode material in a protein based photo-bioelectrochemical cell.…”

“…Solutions of 50 × 10 −6 m protein injected into the cavity of each subcell included 10 × 10 −3 m ubiquinone-0 (Q 0 ) and 1 × 10 −3 m N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) as electrolytes. [14] Assembled subcells and tandem cells were visibly transparent (Figure 3), with transmittances at 550 nm of 82% for the RC-LH1 red subcell ( Figure 3b) and 87% for the RC-LH1 green subcell (Figure 3a). The lower transmittance of the former was consistent with the higher absorbance of spheroidene at this wavelength compared to neurosporene (Figure 1d).…”

On the mechanism of ubiquinone mediated photocurrent generation by a reaction center based photocathode

“…This provides sufficient time for diffusional processes to remove electrons from the "negative terminal" of the RC and donate electrons to the "positive terminal", resetting the RC for the next turnover. Developments in our understanding of RC mechanism have influenced the design of new synthetic materials for solar energy conversion, and there is interest in the hybridization of both natural and engineered RC proteins with man-made materials for applications in photovoltaics [4][5][6][7][8][9], biosensing [10,11] and molecular electronics [12,13].…”

“…Particular attention has been paid to the Photosystem I complex from oxygenic phototrophs [15][16][17] and both the RC and RC/light harvesting complexes from anoxygenic purple photosynthetic bacteria such as Rhodobacter (Rba.) sphaeroides [4][5][6][7][8]10,[18][19][20][21][22][23]. Major preoccupations of this work have been the development of strategies for effective interfacing of the positive and negative terminals of the photovoltaic protein to the working and counter electrode, either through direct binding or the use of diffusional mediators [4][5][6][7][8][16][17][18][19][20][21][22][23][24][25][26][27].…”

“…sphaeroides [4][5][6][7][8]10,[18][19][20][21][22][23]. Major preoccupations of this work have been the development of strategies for effective interfacing of the positive and negative terminals of the photovoltaic protein to the working and counter electrode, either through direct binding or the use of diffusional mediators [4][5][6][7][8][16][17][18][19][20][21][22][23][24][25][26][27]. In the case of RC's from purple bacteria a particularly effective strategy has been to use analogues of the natural electron donor and electron acceptor, cytochrome (cyt) c2 and ubiquinone-10 (UQ10), to connect the photovoltaic protein to the two electrodes [18,20,22,[27][28][29].…”