Nanostructured silicon-titanium oxide materials possess novel electronic and catalytic properties that may be tuned by manipulating the nanostructure, elemental composition, or dimensions of the composite nanophases. These materials can potentially be used as photocatalysts or in dye sensitized solar cells. The silicon-titanium oxide structures are fabricated by harnessing the biomineralization capacity of single-celled marine diatoms, which naturally form highly ordered silica microshell skeletons called frustules. Diatoms produce sillafin proteins which are contained in subcellular membrane bound compartments called Silicon Deposition Vesicles (SDVs). Within the SDV nanobioreactor the sillafin proteins catalyze the conversion of soluble silicon tetrahydroxide and titanium tetrahydroxide into insoluble silicon-titanium oxides that are structured at both the submicron and nanoscales. While traditional approaches to forming structured oxide nanocomposites are energy intensive and often require large and sophisticated equipment the self-assembly of these materials using diatom cell culture can be done at ambient temperatures and pressures and without the use of harmful or exotic materials. Additionally, producing large quantities of composite materials that are simultaneously structured at both the micro and nanoscales has proven elusive while diatoms can produce billions of composite structures per liter of cell culture.

Titanium was metabolically incorporated into the diatom frustule by a two-stage bioreactor cultivation process. In Stage I of the cultivation a diatom cell suspension was grown in artificial seawater medium with silicon as the limiting nutrient. After depletion of the soluble silicon the diatoms ceased to grow the culture was kept at cell number stationary phase for one photoperiod. In Stage II of the cultivation titanium tetrahydroxide in HCl and soluble silicon in NaOH were slowly dosed into the photobioreactor by perfusion via a syringe pump. Silicon and titanium were added to the cell culture stoichiometrically to facilitate one cell division. The diatoms took the soluble silicon and titanium from the cell culture medium into the cytoplasm via an active transport process. The soluble substrates were transported into the SDV and were catalyzed by sillafin proteins into insoluble oxides. Each diatom then underwent cell division and bore a new microshell containing silicon and titanium oxides.

Whole diatom cells were treated with methanol, the surfactant SDS, and the chelator EDTA to remove organic material and residual salts to yield organic free metal oxide microshells. The inorganic structures were analyzed by inductively coupled plasma emission spectroscopy (ICP-ES) and thermogravimetric analysis (TGA) to determine elemental and residual sillafin and organic content. The microshells were analyzed by SEM, TEM, and STEM-EDX to determine micro and nanostructure as well as localized elemental content. It was determined by both ICP-ES and STEM-EDX that titanium is alloyed into the structured silica. To the best of our knowledge, this is the first report of using “whole cell biosynthesis” to direct the synthesis and self-assembly of titanium metal oxide nanocomposite materials into defined submicron structures.

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