Functionalized gold-nanoparticles enhance photosystem II driven photocurrent in a hybrid nano-bio-photoelectrochemical cell

Photosystem II (PSII) is a multi-subunit pigment-protein complex found in thylakoid membranes of oxygenic photosynthetic organisms, including cyanobacteria, algae, and plants. PSII is an example of a well-known redox enzyme that is responsible for initiating oxygenic photosynthesis, the process by which these organisms convert solar to chemical energy. Considerable interest has recently been directed to the utilization of PSII charge separation with applied goals in solar energy applications such as hydrogen production and generation of photocurrent in photovoltaic devices. With the advances in nano-chemistry and nanotechnology, mimicking the activity of PSII to create high-performance synthetic systems like polymers and electrodes hold potential application in different fields like energy production. Recently, the design of nanoparticles by conjugating redox enzymes, specifically photosynthetic reaction centers, have been identified as a promising approach for enhancing their stability and activity for broadening their application in solar energy production.

Extensive research on the potential development of hybrid bio-photoelectrochemical (PEC) cells composed of synthetic nanoparticles and PSII has been reported. Moreover, the use of silver and plasmonic gold nanoparticles has also drawn significant research attention owing to their unique catalytic and optical properties. For instance, plasmonic nanostructures generate intense near-fields and strong optical resonances that can enhance light-induced photophysical and photochemical processes. Nevertheless, despite the significant research progress, the application of PSII in hybrid bio-PEC cells for efficient conversion of solar energy to electrical current remains a big challenge due to numerous limitations. The narrow absorption cross-section of PSII combined with the relatively poor electrical connection between the anode materials and isolated complex interfere with the conversion process.

To address these design challenges, Ms. Hagit Shoyhet, Dr.. Nicholas Pavlopoulos, led by Prof.. Lilac Amirav and Professor Noam Adir from Technion – Israel Institute of Technology proposed the successful design and construction of a functionalized gold-nanoparticle (AuNP) enhanced photosystem II (otherwise referred to us PSII-AuNP conjugate) to increase the photocurrent generation capability of a hybrid nano-bio-PEC cell. The PSII-AuNP conjugate consisted of 2,6-dichloro-1,4-benzoquinone (DCBQ) modified cysteamine protected gold nanoparticles and PSII complexes isolated from market-grade spinach. The functional association of the hybrid particles and the role of the quinone-modified AuNPs was investigated in detail. The main function of the PSII electron acceptor (DCBQ) was to facilitate electronic communication across the PSII and AuNP interface. Their research work is currently published in the Journal of Material Chemistry A.

The research team showed that the resulting PSII-AuNP conjugate system achieved a higher photocurrent of 35mA cm^-2 mg^-1 chlorophyll when used in a PEC cell, which was about six times higher than that of unconjugated PSII. The enhanced photocurrent was mainly attributed to the dual role of the AuNPs in simultaneously improving the harvesting and transfer of light energy to PSII and improving direct electron injection into the graphite anode. This combination was facilitated by the cysteamine-DCBQ ligand shell, and it produced the highest anodic photocurrent ever to be reported for isolated PSII. Furthermore, spectroscopic studies confirmed the strong functional relationship of the hybrid particles.

In summary, Israeli scientists at Technion developed a feasible approach for electronically linking plasmonic AuNPs and PSII to significantly increase anodic photocurrent when utilized in a hybrid nano-bio-PEC cell. Importantly the conjugation process exhibited no detrimental impact on the original oxygen evolution properties of PSII, paving the way for the construction of more bio-hybrid complex systems for improved current production. The impressive findings reported in the study is one of the major enhancements of photocurrent activity for isolated PSII ever to be reported to date. In a statement to Advances in Engineering, Professor Noam Adir, The Bertha Hertz-Axel Chair in Chemistry and the corresponding author explained that their findings will contribute to the coupling of PSII to hydrogen evolution catalysis and improve hydrogen energy generation.