Biomembrane-Compatible Sol–Gel-Derived Photocatalytic Titanium Dioxide

Titanium dioxide gel monoliths were synthesized using an organic precursor and 0–30 vol % ethanol in water. The visible-light-activated proton pump, bacteriorhodopsin, in its native purple membrane form, was successfully encapsulated within the titanium dioxide gels. Absorption spectra showed that the folded functional state of the protein remained intact within gels made with 0 and 15 vol % ethanol and retained the ability to make reversible conformational changes associated with the photocycle within the gel made with 0 vol % ethanol. The photocatalytic activity of gels made with no ethanol was significantly detectable and gels made with 0–30 vol % ethanol were comparable to commercial crystalline nanoparticles in similar solution conditions when irradiated with UV light. Our results show that sol–gel-derived photocatalytic titanium dioxide can be made biocompatible for a membrane-associated protein by minimizing the amount of ethanol and maximizing the amount of water in the synthesis procedure. The entrapment of the membrane protein, bacteriorhodopsin, in sol–gel-derived titanium dioxide provides the first step in future explorations of this bionanocomposite for visible light photocatalysis, including hydrogen production.