(521d) Two-Stage Diatom Cell Culture for the Supramolecular Assembly of Silicon-Germanium Oxides Ordered at the Submicron and Nanoscales

Nanostructured metal oxide semiconductor materials possess novel optical and electronic properties that can be tuned by manipulating the dimensions of the composite nanophases in the 1-100 nm size range. These materials have a host of potential applications, particularly for the fabrication of optoelectronic devices. However, it is extremely difficult to fabricate nanostructured semiconductor materials and at the same time integrate these materials into patterned device features that possess the nanoscale and microscale elements needed for the next generation of optoelectronic devices. Current technologies for fabrication of nano-patterned semiconductor materials are ?top down?, meaning that large and sophisticated equipment must be used to fabricate microscale and nanoscale features into the device structure.

We are using bioprocess technology to harness the biomineralization capacity of single-celled algae called diatoms (the Bacillariophyceae) for the controlled biological fabrication of nanostructured materials composed of silicon and germanium oxides. The nanostructured metal oxide materials are self-assembled by a ?bottom up? approach into submicron scale (100-500 nm) structures, such as periodic aperture arrays reminiscent of photonic crystals. These biologic materials also possess nano-enabled optoelectronic properties, including strong blue photoluminescence and electroluminescence. Germanium was metabolically incorporated into the silica microshell of the diatom by a two-stage bioreactor cultivation process. In Stage I of the cultivation, the diatom cell suspension was grown on nutrient medium containing soluble silicon as the growth-limiting substrate to the point of silicon starvation. In Stage II of the cultivation, a mixture of soluble silicon and germanium dissolved in the liquid feed medium was added to silicon-starved cell culture. The diatom cells co-assimilated soluble silicon and germanium by a surge uptake mechanism, and then underwent one cell division or more cell divisions, depending upon the amount of silicon added. After one cell division, cells bearing a new microshell containing both silicon and germanium oxides were formed.

The diatom cells were treated with aqueous hydrogen peroxide to oxidize organic materials and isolate the inorganic silica microshell, which ranged from 2 to 50 microns in nominal size, depending upon the species. The resulting amorphous metal oxide microshells were imaged by SEM, TEM, and STEM-EDS and characterized for optoelectronic properties by photoluminesence and electroluminescence. Metabolic insertion of germanium into the diatom silica during Stage II of the bioreactor cultivation experiment changed the diatom microshell morphology at the submicron and nanoscales. The geometry of these morphological changes could be controlled, e.g. by changing 200 nm pore aperture arrays into 50-100 nm slit arrays. The final nano- and microstructure of the diatom microshell bearing the silicon and germanium oxides depended upon the amount of germanium and silicon added to the Stage 2 of the cultivation process. Furthermore, metabolic insertion of germanium into the diatom microshell during Stage II of the bioreactor cultivation by the single-pulse Si+Ge co-addition strategy resulted in uniform distribution of germanium and silicon within the microshell.

To the best of our knowledge, this is the first report of using "whole cell biosynthesis" to direct the synthesis and supramolecular assembly of metal oxide nanocomposite materials into defined submicron structures that possess optoelectronic properties.