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...

A 12-mer peptide, identified through phage display biopanning, has been used for the first time to induce the rapid formation of ferroelectric (tetragonal) nanocrystalline BaTiO3 at room temperature from an aqueous salt precursor solution at near neutral pH. BaTiO3 is widely used in capacitors, thermistors, displays, and sensors owing to its attractive dielectric, ferroelectric, pyroelectric, optical, and electrochemical properties. Two 12-mer peptides (BT1 and BT2) were selected from a phage-displayed peptide library via binding to tetragonal BaTiO3 powder. While these peptides possessed various types of amino acids, 8 of the 12 amino acids were common to both peptides. Each of these peptides induced the formation of faceted nanoparticles (50-100 nm diameter) from an aqueous precursor solution. X-ray diffraction and selected area electron diffraction patterns obtained from these faceted nanoparticles were consistent with the BaTiO3 compound. Rietveld analyses of the X-ray diffraction patterns yielded good fits to tetragonal crystal structures, with the BaTiO3 formed in the presence of the BT2 peptide exhibiting the most tetragonal character. A coating of the latter BaTiO3 nanoparticles exhibited polarization hysteresis (a well-known characteristic of ferroelectric materials) at room temperature and a relative permittivity of 2200. Such rapid, peptide-induced precipitation at room temperature provides new opportunities for direct BaTiO3 formation on low-melting or reactive materials (e.g., plastics, cloths, bio-organics) and the low temperature integration of BaTiO3 into electronic devices (e.g., on silicon or flexible polymer substrates).