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.

The attractive optical, chemical, biochemical, and mechanical properties of the rutile polymorph of titanium dioxide (titania) have led to its use in powder or film form in paints, plastics, cosmetics, sunscreens, interference coatings, separation membranes, gas sensors, and as a food additive. [1][2][3][4][5][6][7] Threedimensional (3D) porous networks of rutile titania are also of considerable interest for separation/sorption, optical, and biomedical applications. [8][9][10][11] The assembly of 3D porous networks of rutile with well-controlled solid and pore morphologies is an active area of research. A number of groups have synthesized titania structures that have wellorganized 3D arrays of macropores/voids by applying coatings of the anatase polymorph of titania on organic templates and then removing the templates. [9][10][11][12][13][14] However, attempts to completely convert organized 3D anatase/pore structures into rutile titania replicas, by heat treatment at ! 800 8C, have resulted in appreciable grain growth and distortion of the solid/pore structures. [11][12][13][14] Herein, we demonstrate for the first time how an intricate, 3D, nanocrystalline rutile structure may be generated with the morphology and nanoscale features inherited from a bioorganic, chitin-based template. Chitin is a natural polysaccharide that is formed into well-organized structures by a variety of organisms (for example, fungi, yeast, arthropods, cephalopods, mollusks, insects). [15][16][17][18][19] The chitin-based templates in this work are the scales present on the wings of a Morpho butterfly. As revealed by the secondary electron (SE) image in Figure 1 a-1, these overlapping scales possess an overall rectangular shape with pointed tips. Figure 1 a-2 reveals the porous architecture of one such scale. Each scale is comprised of parallel ridges, spaced several microns apart, aligned along the scale length. The ridges are decorated with nanoscale ribs (ca. 50 nm in width) that are spaced approximately 150 nm apart.Thin, conformal, and continuous oxide coatings were deposited on these chitin scales through a computer-controlled surface sol-gel process.[20] Hydroxy groups needed for initiation of the surface sol-gel coating process [21,22] were provided by the native chitin on the individual wing scales. A 1.5 1.5 cm portion of a Morpho butterfly wing was clipped to a glass slide, positioned at a 608 angle (with respect to Figure 1. SE images of chitin-based scales obtained from a Morpho butterfly (a-1, a-2) that were then exposed to 20 surface sol-gel deposition cycles using titanium(IV) isopropoxide (0.010 m solution in 2-propanol) followed by firing at 450 8C for 4 h (b-1, b-2) or 900 8C for 1 h (c-1, c-2).

Appreciable effort is underway to develop robust protocols for synthesizing functional nanostructured assemblies. Desired characteristics for such protocols include precise control of structure (down to the nanoscale), versatile control of chemistry (for tailored functionality), and massively parallel assembly (for large-scale manufacturing). The precise, versatile, and scalable fabrication of functional nanostructured assemblies, particularly those with intricate 3D morphologies, remains a significant challenge for nanotechnology.Nature provides spectacular examples of precise and highly replicable 3D self-assembly of inorganic materials on the micrometer to nanometer scales.[1] Diatoms, for example, generate nanostructured silica microshells (frustules) with thousands of species-specific morphologies.[1d] Sustained reproduction of a particular diatom species can yield enormous numbers of frustules with similar 3D morphologies.[1e]Such intricate, genetically precise, and massively parallel 3D self-assembly under ambient conditions lies well beyond the current capabilities of synthetic micro-and nanofabrication. However, to utilize the precision and massive parallelism of such biological assembly for a variety of nanostructured devices, the silica-frustule chemistry needs to be altered for desired electronic, optical, chemical, or other properties.While gas-solid reactions [2a-c] and gas-phase deposition [2d,e] have been used to alter the chemistries of bioclastic structures, these approaches require volatile reactants orprecursors. An alternate range of chemistries may be accessed with liquid-phase deposition methods. For example, Kunitake, et al. [3] have used a layer-by-layer surface sol-gel process to deposit conformal oxide-bearing coatings on hydroxy-or carboxy-bearing surfaces (for example, latex spheres [3b,c] or cellulose [3d,e] ). However, this surface sol-gel process has not been used to generate thin, conformal, and continuous functional oxide coatings on intricate 3D biomineral structures.Herein, we demonstrate, for the first time, how the intricate nanostructured silica valves of diatom frustules may be coated with a thin (50 nm), conformal, and continuous layer of a functional oxide (SnO 2 ) through dendritic amplification of hydroxy groups on the silica surfaces and then use of an automated surface sol-gel process. A device built from such SnO 2 -coated diatom frustule valves acts as a sensitive detector for NO gas.Initial experiments produced patchy, discontinuous oxide coatings, which we attribute to a low density of surface hydroxy groups available to initiate the sol-gel reaction on the silica valves. Therefore, it was necessary to increase the number of accessible hydroxy functionalities through a series of chemical reactions (Scheme 1). The diatom silica was first subjected to an oxidizing RCA-1 cleaning solution and then treated with (3-aminopropyl)triethoxysilane to functionalize the surfaces with amine groups. The surfaces were then exposed, in alternating fashion, to solutions of po...

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).

Here, a straightforward and general method for the rapid dendritic amplification of accessible surface functional groups on hydroxylated surfaces is described, with focus on its application to 3D biomineral surfaces. Reaction of hydroxyl‐bearing silica surfaces with an aminosilane, followed by alternating exposure to a dipentaerythritol‐derived polyacrylate solution and a polyamine solution, allows the rapid, layer‐by‐layer (LBL) build‐up of hyperbranched polyamine/polyacrylate thin films. Characterization of such LBL‐grown thin films by AFM, ellipsometry, XPS, and contact angle analyses reveals a stepwise and spatially homogeneous increase in film thickness with the number of applied layers. UV–Vis absorption analyses after fluorescein isothiocyanate labeling indicate that significant amine amplification is achieved after the deposition of only 2 layers with saturation achieved after 3–5 layers. Use of this thin‐film surface amplification technique for hydroxyl‐enrichment of biosilica templates facilitates the conformal surface sol–gel deposition of iron oxide that, upon controlled thermal treatment, is converted into a nanocrystalline (∼9.5 nm) magnetite (Fe3O4) coating. The specific adsorption of arsenic onto such magnetite‐coated frustules from flowing, arsenic‐bearing aqueous solutions is significantly higher than for commercial magnetite nanoparticles (≤50 nm in diameter).