Beyond silicon biotechnology: new routes to structure-directed nanofabrication

With a precision of nanostructural control that exceeds present human capabilities, marine biological systems fabricate 3-dimensionally organized multifunctional high-performance silicon-based materials at low temperatures and near-neutral pH. Four years ago, we proposed that "silicon biotechnology" could augment traditional routes of high-performance materials synthesis by adapting the proteins, genes and molecular mechanisms governing the biological formation of silicon-based materials in marine organisms. That vision is now becoming a reality. The fundamental molecular biology of silica production in living organisms is now being elucidated, and aspects of these processes are being harnessed for industrially and technologically relevant processes. Working with the silica needles produced by marine sponges, our laboratory discovered that the proteins we named "silicateins" catalyze and structurally direct the polymerization of silica from silicon alkoxides at neutral pH and low temperature. The silicateins are true enzymes, closely related to a well-known family of hydrolases. Site-directed mutagenesis (genetic engineering) of the cloned recombinant DNAs coding for the silicateins has allowed us to confirm the mechanism of catalysis we postulated for the enzymatic polycondensation of siloxanes from the alkoxide substrates, and has been used to increase the rate of catalysis as well. These studies enabled the synthesis of self-assembling "biomimetic" catalysts that incorporate the functionalities we identified as essential for catalysis in the natural enzymes, yielding new structure-directing catalysts of polymerization. The silicateins and their biomimetic counterparts catalyze the structure-directing synthesis from a wide range of precursors, yielding not only silica, but organometallic silsesquioxanes (and other complex organosilicon materials) as well. Most recently, we discovered that the silicateins also catalyze and structurally direct the condensation of alkoxide-like conjugates of metals such as titanium at neutral pH and low temperature. Perhaps most remarkably, molecular recognition results in preferential alignment of the resulting nanocrystallites of the mineral. The result is the first enzyme-catalyzed, nanostructure-directed polymerization of - titanium dioxide. Successful extension to other semiconductors including gallium oxide and zinc oxide recently has been accomplished. Potential uses for electronic applications, sensors, energy transducers, cosmetics and pharmaceuticals are indicated. Encouraged by these results, a major manufacturer and a leading biotechnology firm recently announced a multimillion dollar strategic alliance aimed at the commercialization of silicon biotechnology. Other firms are following.