The fabrication of novel bioinorganic nanostructures by using biological templates with capsid-like architectures has received significant attention. [1][2][3][4][5][6][7] An important archetype of this approach involves the synthesis of polypeptide-encapsulated inorganic nanoparticles within the spheroidal 8-nm-diameter cage of the iron-storage protein, apoferritin. [1,2] Native ferritin consists of 24 polypeptide subunits arranged in 432 symmetry to produce a spherical hollow shell which is permeated by eight hydrophilic and six hydrophobic channels with pore sizes between 0.3-0.5 nm and which encloses a mineral core of hydrated iron oxide. [8] The channels facilitate the influx and efflux of metal ions and small molecules, such that the native iron oxide core can be removed from the protein cage by reductive dissolution and the resulting apoferritin molecules can be reconstituted synthetically with various types of inorganic nanoparticles. Inorganic nucleation occurs specifically within the polypeptide cavity due to metal-ion binding to anionic amino acids located on the internal surface of the cavity, which, in the presence of an appropriate counterion, increase the local supersaturation. In most cases, this is faciliated by intracavity oxidation of the bound metal ions, which lowers the solubility of the product and increases the rate of hydrolysis to produce nanoparticles such as Fe 3 O 4 , [9] MnOOH, [1,10] CoOOH, [11] or Co 3 O 4 . [12] Similarly, reduction of metal ions, anionic complexes, or both within the apoferritin cage can be used to prepare biomimetic ferritins with entrapped Ag, [13] Pd, [14] or CoPt [15] nanoparticles, respectively. Alternatively, ferritins containing uranyl, [1] indium, [16] nickel, [17] or chromium [17] hydrated oxides and hydroxides have been synthesized by using nonredox hydrolysis reactions and slow coprecipitation routes developed for the preparation of ferritin-encapsulated CdS, [18] CdSe, [19] or ZnSe [20] nanoparticles. Some of these biomimetic ferritins have been shown to have functional properties, for example, in magnetic storage [15,21] and imaging [22] and in size-selective catalytic hydrogenation. [14] In addition, by embedding the inorganic nanoparticles within the polypeptide cage, the hybrid nanostructures can be reversibly assembled into higher order superstructures by attach-[*] Dr.

Synthesis of calcium carbonate in water-in-oil microemulsions results in the spontaneous formation of stacked superstructures of 20 nm-thick pseudo-hexagonal calcite plates in crystallographic register.

Nanoscale objects with advanced structure and function are of considerable interest in areas such as sensing, drug delivery and bioelectronics, [1,2] and have important implications for biotoxicity [3,4] and the emergence of life. [5] In many cases, the synthesis and structuration of hybrid nano-objects is achieved under equilibrium or non-equilibrium conditions through a range of strategies involving integrative, higher-order, or transformative self-assembly. [6,7] Often these approaches involve the confinement and templating of reactions on or within supramolecular assemblies such as dendrimers, [8] organogel nanofilaments, [9] peptide fibers, [10] helical micelles, [11] virus capsids, [12] and protein cages. [13] Recently, cross-linked lysozyme crystals, approximately 200 mm in size, have been used to prepare nanoplasmonic arrays by intracrystalline metallization, [14] suggesting that the high mesoporosity of protein crystals might be exploited in general for the template-directed assembly of organized inorganic nanostructures across a range of length scales. Whilst many common proteins readily form crystals with macroscopic dimensions, it is generally difficult to produce nanoscale counterparts that would be effective as templates for the preparation of discrete hybrid nanomaterials. In this regard, the iron storage protein, ferritin, which consists of a 12 nm diameter spherical polypeptide shell enclosing a 5-6 nm sized iron oxide core [15] is known to readily form two-dimensional (2D) superlattices on various substrates [16] and can be clustered into aggregates in solution using biotin-streptavidin linkages or inorganic nanoparticles.[17] It should therefore be possible to control the self-assembly of discrete nanometersized ferritin crystals, and as a consequence use these nanocrystals as porous templates for the fabrication of hybrid nanoparticles with ordered mesostructured interiors.Here, we use water-in-oil microemulsion droplets as a medium for controlling the aggregation of entrapped ferritin molecules to produce discrete protein nanocrystals that can be stabilized by in situ silicification of the intracrystalline voids to produce mesostructured silica-ferritin hybrids. Microemulsions are versatile reaction media for the confinement and synthesis of inorganic nanoparticles, [18] nanowires, [19] nanoparticle superlattices, [20] and complex hierarchical architectures.[21] In addition, microemulsion droplets have been used for the encapsulation of drugs, [22] exploration of organic chemical reactions, [23] entrapment of functional enzymes, [24] and for the separation of protein mixtures.[25]Although droplet instability can often be a problem in these applications, herein we demonstrate that protein-mediated aggregation of the water pools can be exploited to produce discrete ferritin nanocrystals and silicified counterparts with well-ordered close packed structures. As silicification of the interstitial pores occurs with high precision and without degradation of the protein, it should be possible to ex...

Nanoscale objects with advanced structure and function are of considerable interest in areas such as sensing, drug delivery and bioelectronics, [1,2] and have important implications for biotoxicity [3,4] and the emergence of life. [5] In many cases, the synthesis and structuration of hybrid nano-objects is achieved under equilibrium or non-equilibrium conditions through a range of strategies involving integrative, higher-order, or transformative self-assembly. [6,7] Often these approaches involve the confinement and templating of reactions on or within supramolecular assemblies such as dendrimers, [8] organogel nanofilaments, [9] peptide fibers, [10] helical micelles, [11] virus capsids, [12] and protein cages. [13] Recently, cross-linked lysozyme crystals, approximately 200 mm in size, have been used to prepare nanoplasmonic arrays by intracrystalline metallization, [14] suggesting that the high mesoporosity of protein crystals might be exploited in general for the template-directed assembly of organized inorganic nanostructures across a range of length scales. Whilst many common proteins readily form crystals with macroscopic dimensions, it is generally difficult to produce nanoscale counterparts that would be effective as templates for the preparation of discrete hybrid nanomaterials. In this regard, the iron storage protein, ferritin, which consists of a 12 nm diameter spherical polypeptide shell enclosing a 5-6 nm sized iron oxide core [15] is known to readily form two-dimensional (2D) superlattices on various substrates [16] and can be clustered into aggregates in solution using biotin-streptavidin linkages or inorganic nanoparticles. [17] It should therefore be possible to control the self-assembly of discrete nanometersized ferritin crystals, and as a consequence use these nanocrystals as porous templates for the fabrication of hybrid nanoparticles with ordered mesostructured interiors.Here, we use water-in-oil microemulsion droplets as a medium for controlling the aggregation of entrapped ferritin molecules to produce discrete protein nanocrystals that can be stabilized by in situ silicification of the intracrystalline voids to produce mesostructured silica-ferritin hybrids. Microemulsions are versatile reaction media for the confine-ment and synthesis of inorganic nanoparticles, [18] nanowires, [19] nanoparticle superlattices, [20] and complex hierarchical architectures. [21] In addition, microemulsion droplets have been used for the encapsulation of drugs, [22] exploration of organic chemical reactions, [23] entrapment of functional enzymes, [24] and for the separation of protein mixtures. [25] Although droplet instability can often be a problem in these applications, herein we demonstrate that protein-mediated aggregation of the water pools can be exploited to produce discrete ferritin nanocrystals and silicified counterparts with well-ordered close packed structures. As silicification of the interstitial pores occurs with high precision and without degradation of the protein, it should be possible ...