The carbothermal reduction of silica into silicon requires the use of temperatures well above the silicon melting point (> or =2,000 degrees C). Solid silicon has recently been generated directly from silica at much lower temperatures (< or =850 degrees C) via electrochemical reduction in molten salts. However, the silicon products of such electrochemical reduction did not retain the microscale morphology of the starting silica reactants. Here we demonstrate a low-temperature (650 degrees C) magnesiothermic reduction process for converting three-dimensional nanostructured silica micro-assemblies into microporous nanocrystalline silicon replicas. The intricate nanostructured silica microshells (frustules) of diatoms (unicellular algae) were converted into co-continuous, nanocrystalline mixtures of silicon and magnesia by reaction with magnesium gas. Selective magnesia dissolution then yielded an interconnected network of silicon nanocrystals that retained the starting three-dimensional frustule morphology. The silicon replicas possessed a high specific surface area (>500 m(2) g(-1)), and contained a significant population of micropores (< or =20 A). The silicon replicas were photoluminescent, and exhibited rapid changes in impedance upon exposure to gaseous nitric oxide (suggesting a possible application in microscale gas sensing). This process enables the syntheses of microporous nanocrystalline silicon micro-assemblies with multifarious three-dimensional shapes inherited from biological or synthetic silica templates for sensor, electronic, optical or biomedical applications.

A low-temperature (r7001C) magnesiothermic reaction has been used for the first time to convert three-dimensional (3-D) silica-based diatom microshells into nanocrystalline magnesiabased replicas. Exposure of diatom microshells to Mg(g) at only 6501-7001C resulted in the initial (direct) formation of MgO and Si nanocrystals (r5 nm), that is, no intermediate magnesium silicate compounds were detected. Further reaction of Si with Mg(g) led to the formation of Mg 2 Si, and then a Mg-Si liquid that sweated away to yield free-standing, nanocrystalline MgO-based replicas. Such direct low-temperature magnesiothermic conversion of diatom microshells enables the synthesis of large numbers of 3-D nanocrystal assemblies with well-controlled morphologies for catalytic/chemical, biological, optical, and other applications. 2005 S. Bhandarkar-contributing editor Supported by the Air Force Office of Scientific Research (Hugh De Long, Program Manager).

A sol-gel process was used, for the first time, to apply a multi-component, nanocrystalline, functional ceramic compound (BaTiO3) to a three-dimensional, self-replicating scaffold derived from a single-celled micro-organism (a diatom).

Appreciable global efforts are underway to develop processes for fabricating three‐dimensional (3‐D) nanostructured assemblies for advanced devices. Widespread commercialization of such devices will require: (i) precise 3‐D fabrication of chemically tailored structures on a fine scale and (ii) mass production of such structures on a large scale. These often‐conflicting demands can be addressed with a revolutionary new paradigm that couples biological self‐assembly with synthetic chemistry: Bioclastic and Shape‐preserving Inorganic Conversion (BaSIC). Nature provides numerous examples of microorganisms that assemble biominerals into intricate 3‐D structures. Among the most spectacular of these microorganisms are diatoms (unicellular algae). Each of the tens of thousands of diatom species assembles silica nanoparticles into a microshell with a distinct 3‐D shape and pattern of fine (nanoscale) features. The repeated doubling associated with biological reproduction enables enormous numbers of such 3‐D microshells to be generated (e.g., only 40 reproduction cycles can yield >1 trillion 3‐D replicas!). Such genetic precision and massive parallelism are highly attractive for device manufacturing. However, the natural chemistries assembled by diatoms (and other microorganisms) are rather limited. With BaSIC processes, biogenic assemblies can be converted into a wide variety of new functional chemistries, while preserving the 3‐D morphologies. Ongoing advances in genetic engineering promise to yield microorganisms tailored to assemble nanoparticle structures with device‐specific shapes. Large‐scale culturing of such genetically tailored microorganisms, coupled with shape‐preserving chemical conversion (via BaSIC processes), would then provide low‐cost 3‐D Genetically Engineered Micro/nano‐devices (3‐D GEMs).