Development and Characterization of Titanium Dioxide Gel with Encapsulated Bacteriorhodopsin for Hydrogen Production

We study bacteriorhodopsin (BR) in its native purple membrane encapsulated within amorphous titanium dioxide, or titania, gels and in the presence of titania sol particles to explore this system for hydrogen production. Förster resonance energy transfer between BR and titanium dioxide sol particles was used to conclude that there is nanometer-scale proximity of bacteriorhodopsin to the titanium dioxide. The detection of BR-titania sol aggregates by fluorescence anisotropy and particle sizing indicated the affinity amorphous titania has for BR without the use of additional cross-linkers. UV-vis spectroscopy of BR-titania gels shows that methanol addition did not denature BR at a 25 mM concentration presence as a sacrificial electron donor. Additionally, confinement of BR in the gels significantly limited protein denaturation at higher concentration of added methanol or ethanol. Subsequently, titania gels fabricated through the sol-gel process using a titanium ethoxide precursor, water, and the addition of 25 mM methanol were used to encapsulate BR and a platinum reduction catalyst for the production of hydrogen gas under white light irradiation. The inclusion of 5 μM bacteriorhodopsin resulted in a hydrogen production rate of about 3.8 μmol hydrogen mL^-1 h^-1, an increase of 52% compared to gels containing no protein. Electron transfer and proton pumping by BR in close proximity to the titania gel surface are feasible explanations for the enhanced production of hydrogen without the need to cross-link BR to the titania gel. This work sets the stage for further developments of amorphous, rather than crystalline, titania-encapsulated bacteriorhodopsin for solar-driven hydrogen production through water splitting.