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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 (approximately 4 x 10(5) cells/mL) for 10 h. The feeding rate of titanium (0.85-7.3 micromol Ti L(-1) h(-1)) was designed to circumvent the precipitation of titanate in the liquid medium, and feeding rate of silicon (48 micromol 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 SiO(2). 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 TiO(2) content of nearly 80 wt %. Thermal annealing in air at 720 degrees C converted the biogenic titanate to anatase TiO(2) with an average crystal size of 32 nm. This is the first reported study of using a living organism to controllably fabricate semiconductor TiO(2) nanostructures by a bottom-up self-assembly process.

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Chitosan, a linear biopolymer of glucosamine residues, selectively adsorbs transition-metal ions such as cadmium from dilute solution. In order to process chitosan into a more durable form, a 5 wt % chitosan solution was cast into spherical gel beads of 3 mm diameter and then reacted with glutaraldehyde at free amine sites to form imine cross-links between linear chitosan chains. The rate processes of the heterogeneous cross-linking reaction and the effect of cross-linking on the cadmium ion adsorption capacity were determined. The cross-linking reaction was complete within 48 h at 27 °C, and the final extent of cross-linking ranged from 0.07 to 2.40 mol of glutaraldehyde consumed/mol of amine. Heterogeneous cross-linking was modeled as a shrinking core process where the molecular diffusion of glutaraldehyde through the cross-linked shell of the gel bead limited the overall rate of glutaraldehyde consumption. The effective diffusion coefficient of glutaraldehyde through the cross-linked layer was 4.7 × 10-8 cm2/s. The saturation adsorption capacity of cadmium ions on the cross-linked gel beads exponentially decreased from 250 to 100 mg of Cd/g of chitosan as the extent of cross-linking increased from 0 to 1.3 mol of glutaraldehyde consumed/mol of amine. At higher extents of cross-linking, the saturation adsorption capacity remained at 100 mg of Cd/g of chitosan. Highly porous chitosan beads formed by freeze-drying of cross-linked gel beads had the same cadmium ion adsorption capacity as the cross-linked gel beads over the same extents of cross-linking.