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 TiO 2 formed through this biocatalytic route, including thermally induced crystal grain growth and phase transformation from anatase to rutile, differ from those of TiO 2 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.
The molecular mechanisms underlying the biological synthesis of nanostructured mineral/organic composites have long been recognized to offer exciting prospects for materials science.[1±8] In addition to their benign conditions for synthesis (including neutral pH, low temperature, low pressure, and the absence of caustic chemicals), these mechanisms often reveal a precision of nanostructural control not yet achievable in anthropogenic syntheses. Investigations into such mechanisms have shown that protein filaments occluded within the silica skeletal elements of a marine sponge consist of structure-directing enzymes capable of catalyzing, in vitro, the hydrolysis and polycondensation of molecular precursors of silica, silsesquioxanes, [5,9±11] and titania. [12] We show here that these protein filaments are not only capable of the hydrolysis and polycondensation of a gallium oxide molecular precursor to yield (depending on the reaction conditions) either gallium oxo-hydroxide (GaOOH) or spinel gallium oxide (c-Ga 2 O 3 , a gassensing semiconductor) at room temperature, but also to direct their resulting structures. This control is seen in the defined orientation of nanocrystals with respect to the surface of the protein, suggesting that structural determinants on the surface of the protein catalyze the formation of the c-Ga 2 O 3 polymorph at low temperatures and may direct its crystallographic orientation. These results demonstrate the feasibility of a low-temperature catalytic route to the synthesis and COMMUNICATIONS 314
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