Biocatalytically templated synthesis of titanium dioxide

Silicatein, an enzymatic biocatalyst purified from the glassy skeletal elements of a marine sponge, and previously shown capable of catalyzing and structurally directing the hydrolysis and polycondensation of silicon alkoxides to yield silica and silsesquioxanes at low temperature and pressure and neutral pH, is shown to be capable of catalyzing and templating the hydrolysis and subsequent polycondensation of a water-stable alkoxide-like conjugate of titanium to form titanium dioxide. The structure and behavior of the TiO2 formed through this biocatalytic route, including thermally induced crystal grain growth and phase transformation from anatase to rutile, differ from those of TiO2 formed from the same precursor via alkali catalysis or thermal pyrolysis. This enzymatic route affords a path to templated synthesis that avoids the high temperatures and extremes of pH typically required for synthesis of metallo-oxanes from the corresponding alkoxide-like precursors, and thus provides access to a new and potentially useful parameter space of structures and properties. The proteins may also be nanoscopically structure-directing, as evidenced by the formation of nanocrystallites of anatase, a polymorph usually formed at much higher temperatures. The summation of weak interactions between the protein and mineral may induce this stabilization and thus may afford a new level of nanostructural control, with associated enhancement of selected performance properties.