Metabolic Insertion of Nanostructured TiO2 into the Patterned Biosilica of the Diatom Pinnularia sp by a Two-Stage Bioreactor Cultivation Process

Diatoms are single-celled algae that make silica shells or frustules with intricate nanoscale features imbedded within periodic two-dimensional pore arrays. A two-stage photobioreactor cultivation process was used to metabolically insert titanium into the patterned biosilica of the diatom Pinnularia sp. In Stage I, diatom cells were grown up on dissolved silicon until silicon starvation was achieved. In Stage II, soluble titanium and silicon were continuously fed to the silicon-starved cell suspension (similar to 4 x 10(5) cells/mL) for 10 h. The feeding rate of titanium (0.85-7.3 mu mol Ti L-1 h(-1)) was designed to circumvent the precipitation of titanate in the liquid medium, and feeding rate of silicon (48 mu mol Si L-1 h(-1)) was designed to sustain one cell division. The addition of titanium to the culture had no detrimental effects on cell growth and preserved the frustule morphology. Cofeeding of Ti and Si was required for complete intracellular uptake of Ti. The maximum bulk composition of titanium in the frustule biosilica was 2.3 g of Ti/100 g of SiO2. Intact biosilica frustules were isolated by treatment of diatom cells with SDS/EDTA and then analyzed by TEM and STEM-EDS. Titanium was preferentially deposited as a nanophase lining the base of each frustule pore, with estimated local TiO2 content of nearly 80 wt %. Thermal annealing in air at 720 degrees C converted the biogenic titanate to anatase TiO2 with an average crystal size of 32 nm. This is the first reported study of using a living organism to controllably fabricate semiconductor TiO2 nanostructures by a bottom-up self-assembly process.