The Role of Gold-Adsorbed Photosynthetic Reaction Centers and Redox Mediators in the Charge Transfer and Photocurrent Generation in a Bio-Photoelectrochemical Cell

Abstract

Bacterial photosynthetic reaction centers (RCs) are promising materials for solar energy harvesting, due to their high quantum efficiency. A simple approach for making a photovoltaic device is to apply solubilized RCs and charge carrier mediators to the electrolyte of an electrochemical cell. However, the adsorption of analytes on the electrodes can affect the charge transfer from RCs to the electrodes. In this work, photovoltaic devices were fabricated incorporating RCs from purple bacteria, ubiquinone-10 (Q2), and cytochrome c (Cyt c) (the latter two species acting as redox mediators). The adsorption of each of these three species on the gold working electrode was investigated, and the roles of adsorbed species in the photocurrent generation and the cycle of charge transfer were studied by a series of photochronoamperometric, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and cyclic voltammetry (CV) tests. It was shown that both redox mediators were required for photocurrent generation; hence, the RC itself is likely unable to inject electrons into the gold electrode directly. The reverse redox reactions of mediators at the electrodes generates electrical current. Cyclic voltammograms for the RC-exposed gold electrode revealed a redox couple due to the adsorbed RC at ∼ +0.5 V (vs NHE), which confirmed that the RC was still redox active, upon adsorption to the gold. Photochronoamperometric studies also indicated that RCs adsorb, and are strongly bound to the surface of the gold, retaining functionality and contributing significantly to the process of photocurrent generation. Similar experiments showed the adsorption of Q2 and Cyt c on unmodified gold surfaces. It was indicated by the photochronoamperometric tests that the photocurrent derives from Q2-mediated charge transfer between the RCs and the gold electrode, while solubilized Cyt c mediates charge transfer between the P-side of adsorbed RC and the Pt counter electrode. Also, the stability of the adsorbed RCs and mediators was evaluated by measuring the photocurrent response over a period of 1 week. It is found that ∼46% of the adsorbed RCs remain active after a week under aerobic conditions. A significantly extended lifetime is expected by removing oxygen from the electrolyte and sealing the device.

Plasmon‐Enhanced Photocurrent of Photosynthetic Pigment Proteins on Nanoporous Silver

“…This established that the RC-LH1 complex was the source of the photocurrent and that it had maintained its functional integrity despite the lack of a self-assembled monolayer (SAM) that is generally thought necessary to maintain protein integrity on bare metals. [30][31][32] The peak EQE of 1.3% at 875 nm (5 µW cm -2 irradiation) for the RS|RCLH1 electrode ( Figure 2b) was ~6-fold greater than the largest reported to date of 0.21% under 6 mW cm -2 irradiation at 875nm using a gold surface. [33] The decreased EQE at approximately 375 nm on RS ( .…”

Progress and perspectives in exploiting photosynthetic biomolecules for solar energy harnessing

“…13(b)). 116 It was found that about 78% of the RCs bind preferentially to the gold 40 electrode while the rest bind to the counter electrode or float freely in the electrolyte negligibly affecting the photocurrent generation. Though the RCs were in direct contact with the gold electrode, the photocurrent was close to zero in the absence of the two electron transfer mediators, suggesting the least possibility of 45 a direct electron transfer and the importance of the mediators in achieving the electron transfer.…”

“…It is also interesting to note that two different approaches may coexist as in ref. 116 where the RCs are added to the electrolyte while some extent of oriented attachment to the electrode was also attempted by using mutant RCs with an 45 external Cys-group that tends to preferentially attach to the electrode. 116 Unlike other strategies, using very high light intensity may not be very useful as solar cells ultimately are to use the sunlight, which is discussed in the next section of this review.…”

“…116 where the RCs are added to the electrolyte while some extent of oriented attachment to the electrode was also attempted by using mutant RCs with an 45 external Cys-group that tends to preferentially attach to the electrode. 116 Unlike other strategies, using very high light intensity may not be very useful as solar cells ultimately are to use the sunlight, which is discussed in the next section of this review. As also notable in the roadmap, some aspects like the 50 counter electrode material and the protein stability in the solar cell have rarely been explored and are worthy to be our future research avenues in this field.…”

“…42 The stability has also been found to increase by removing oxygen from the environment which has been realized by implanting RCs in anaerobic condition and by perfectly sealing the device. 116,22 One major approach to improve 15 the stability of RCs in the solar cell is to achieve an RCcompatible environment which can be best achieved by mimicking the native membrane of RCs. In contrast to the isolated RCs, the photosynthetic membranes used as the photovoltaic component in the solar cell exhibited more 20 resistance to stress factors and the functionality of RC was maintained for three days when continuously illuminated.…”