Precursor-Based 3D Indium Tin Oxide Coated with Photosystem I and Cytochrome C for High Performing Photobioelectrodes

Abstract

The attraction of bioelectrochemistry has risen constantly in recent times. Here, nature is imitated in biohybrid systems. One interesting example are photobioelectrodes where artificial materials are linked to biological components which are responsible for photosynthesis in plants or cyanobacteria. To date, biophotovoltaics using bacterial reaction centers, photosystem I or II (PSI or PSII), thylakoid membranes or whole cells have been set up successfully. [1] In this way, not only the generation of photocurrent is possible. Additionally, photosystem-based biosensors as well as devices that convert light to valuable chemicals have been constructed. [2, 3]The performance of biophotovoltaics was improved significantly in the last years. One important step in the process was the setup of 3D electrode structures as the accessible surface for the interaction with biomolecules is enhanced multiple times. However, key prerequisite is that the material is semitransparent to allow the transmission of light to the photosystem. One example, which was used successfully on different occasions, is indium tin oxide (ITO). [2, 4, 5]The work described here aims to upgrade ITO-based photobioelectrodes. Key thereto is the use of precursors for the fabrication of the electrode as it improves the transparency of the structure significantly compared to nanoparticle-approaches as confirmed by UV/VIS spectroscopy. [6] The liquid precursors were mixed with latex beads (LB) in isopropyl alcohol and dropped onto a rotating FTO electrode. After sintering at 535 °C for 2 hours the received structure was first incubated with PSI and subsequently with the redoxmediator cytochrome c.The diameter of the LB was varied from 0.1 to 3.0 µm resulting in an optimum for 0.46 µm. Up to 15 spin-coating (ca. 17 µm) steps were applied without any obtained limitations in active surface, protein coverage, and photocurrent. Thus, cathodic currents up to 270 µA/cm² were achieved at an applied potential of -200 mV vs Ag/AgCl and an illumination with 100 mW/cm². Stability of the photobioelectrode was evaluated for half an hour and drastically improved by the addition of 2 mM 2,3-dimethoxy-5-methyl-1,4-benzoquinone.