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.

Significance By determining the structure of a pantothenate energy-coupling factor (ECF) transporter, Lb ECF-PanT, we revealed the structural basis of how one EcfAA'T module can interact with different S subunits among group II ECF transporters. We also identified the residues that mediate the intermolecular conformational transmission and/or affect the transporter complex stability, and thus are essential for transporter activity. In addition, we identified the pantothenate-binding pocket and the residues constituting the pocket. Last but not least, we found that the structure of EcfT is dynamic and undergoes dramatic changes in the three different transporter complexes, which confer scaffold-mediating complex formations of the ECF module with various EcfS proteins. These findings are incorporated into an updated working model of the ECF transporter.

Porous nanostructured assemblies of noble metals can be attractive materials for use in a number of catalytic, [1] gas sensing, [2] biochemical, [3] electronic, [4] thermal, [5] and other [6] applications. Approaches used to synthesize such porous noble-metal nanostructures include selective etching (''dealloying'') of noble-metal-bearing alloys, [1a,2b,3a,4c,6b,7] combustion synthesis, [8] and deposition onto porous organic or inorganic templates via physical vapor, chemical vapor, or wet chemical routes. [1c,2a,c,3c,4a,c,9] Relatively recent work involved the deposition of metals onto porous biomineral templates, such as the calcium carbonate skeletal plates of sea urchins (with pore diameters of 10-15 mm [10a] ) and the silica microshells (frustules) of diatoms (with pore diameters of tens to hundreds of nanometers [10b-d] ). Such biomineral templates provide unique and attractive structural characteristics. For example, diatoms (unicellular algae) form porous silica frustules with intricate, hierarchically-patterned (micro-to-nanoscale) 3D structures. [11] As diatom-frustule morphologies are species specific, a spectacular variety of frustule morphologies may be found among the thousands of extant diatom species.[11] Furthermore, the sustained reproduction of a given diatom species may be used to generate enormous numbers of frustules with the same 3D morphology.[12] Such direct, genetically-precise, and massively parallel assembly of intricate, porous 3D templates in a wide range of morphologies under ambient conditions has no analog in synthetic processing. The deposition of gold or silver coatings onto diatom frustules via thermal evaporation has been reported recently.[10b,c] While 2D metallic structures that preserved the frustule nanotopography were generated, the line-of-sight nature of such physical vapor deposition inhibited complete replication of the 3D frustule morphology.[10b,c] The DNA-mediated binding of gold nanoparticles onto diatom frustules has also been reported.[10d] While gold nanoparticles were successfully bound to the 3D frustule surfaces, this process was not used to generate free-standing (silica-free) porous gold replicas of the frustules (as selective dissolution of the underlying silica from the bound gold nanoparticles would have resulted in the release of the nanoparticles). The objective of the present work is to demonstrate how self-supporting (silica-free) porous metalnanoparticle assemblies that retain the 3D morphologies of diatom frustules can be synthesized via a scalable combination of gas/solid reaction and wet chemical processes.Electroless deposition was used to generate porous 3D replicas of diatom frustules comprised of noble-metal (Ag, Au, Pd) nanoparticles. The direct electroless deposition of noble-metal coatings onto 3D silica diatom frustules is inhibited by the insulating character of silica. However, electroless deposition has been successfully used to apply platinum, gold, copper, and nickel coatings to porous silicon.[13] Hence, a two-step pr...