Abstract
Biohybrid photoelectrochemical systems in photovoltaic or biosensor applications have gained considerable attention in recent years. While the photoactive proteins engaged in such systems usually maintain an internal charge separation quantum yield of nearly 100%, the subsequent steps of electron and hole transfer beyond the protein often limit the overall system efficiency and their kinetics remain largely uncharacterized. To reveal the dynamics of one of such charge-transfer reactions, we report on the reduction of Rhodobacter sphaeroides reaction centers (RCs) by Os-complex-modified redox polymers (P-Os) characterized using transient absorption spectroscopy. RCs and P-Os were mixed in buffered solution in different molar ratios in the presence of a water-soluble quinone as an electron acceptor. Electron transfer from P-Os to the photoexcited RCs could be described by a three-exponential function, the fastest lifetime of which was on the order of a few microseconds, which is a few orders of magnitude faster than the internal charge recombination of RCs with fully separated charge. This was similar to the lifetime for the reduction of RCs by their natural electron donor, cytochrome c2. The rate of electron donation increased with increasing ratio of polymer to protein concentrations. It is proposed that P-Os and RCs engage in electrostatic interactions to form complexes, the sizes of which depend on the polymer-to-protein ratio. Our findings throw light on the processes within hydrogel-based biophotovoltaic devices and will inform the future design of materials optimally suited for this application.
Highlights
Research into biohybrid solar energy conversion devices has expanded significantly over the past couple of decades, with a wide variety of device designs reported.[1−6] The biological component has usually been a photosynthetic protein such as photosystem I, photosystem II, or the reaction center (RC) from a purple photosynthetic bacterium; recently, there has been an increase in the usage of whole living organisms such as cyanobacteria immobilized directly on electrodes.[5]
The benefit of high yield of charge separation is sustained by the rapid reduction of the redox center carrying the electron hole by an external donor, preventing wasteful charge recombination within the RC.[9]
A photoprotein that has been used extensively for the fabrication of biohybrid electrodes and devices is the RC from the purple bacterium Rhodobacter sphaeroides
Summary
Research into biohybrid solar energy conversion devices has expanded significantly over the past couple of decades, with a wide variety of device designs reported.[1−6] The biological component has usually been a photosynthetic protein such as photosystem I, photosystem II, or the reaction center (RC) from a purple photosynthetic bacterium; recently, there has been an increase in the usage of whole living organisms such as cyanobacteria immobilized directly on electrodes.[5]. This relatively simple RC conducts charge separation with a quantum yield of nearly 100%.10 This functionality is provided by a set of cofactors buried inside an amino acid scaffold, namely, four bacteriochlorophylls (BChl; two of them are coupled in a socalled special pair and form the primary electron donor (P) and two are accessory BChls, BA and BB), two bacteriopheophytins (BPhe; HA and HB), two quinones (QA and QB), and one carotenoid (Car; see Figure 1A).[11] The absorption spectrum of the RC is characterized by bands that are attributed to different chromophores (Figure 1B). The excitation energy within the RCs is typically transferred to P, Received: September 24, 2020 Revised: November 10, 2020 Published: November 25, 2020
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