Bioinspired synthesis of mesoporous silicas

Bioinspired synthesis of mesoporous silicas

Current Opinion in Solid State and Materials Science 8 (2004) 111–120 Bioinspired synthesis of mesoporous silicas Qianyao Sun a a,b , Engel G. Vrie...

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Current Opinion in Solid State and Materials Science 8 (2004) 111–120

Bioinspired synthesis of mesoporous silicas Qianyao Sun a

a,b

, Engel G. Vrieling c,1, Rutger A. van Santen b, Nico A.J.M. Sommerdijk a,*

Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands b Schuit Institute of Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands c Department of Marine Biology, Center for Ecological and Evolutionary Studies, University of Groningen, P.O. Box 14, 9750 AA Haren, The Netherlands Received 23 June 2003; accepted 30 January 2004

Abstract Recent years have witnessed rapid growth in the number of new investigations at the interface of materials chemistry and biology. This review highlights the recent developments in the studies of protein-mediated silica biomineralization in diatoms and the ‘‘downscaling’’ and ‘‘upscaling’’ models derived thereof, as well as the recent progress in the fabrication of artificial silicas with novel pore structures and morphologies at different length scales based on these new insights into biosilica formation. Ó 2004 Elsevier Ltd. All rights reserved.

1. Introduction Silica and silica-based materials have found widespread application for industrial, technological and domestic purposes. The demand for improved silica types with specific properties such as mechanical strength, pore volume, pore-size distribution, specific surface area or surface reactivity [1–3] is fueling the search for innovative production of such materials [4,5]. Indeed, extensive work has been undertaken to prepare mesoporous silica and to understand the complicated physical chemistry accompanying the relatively simple condensation reaction in which silica monomers are used to build the large polymeric structures typical for such porous silicas. In contrast to the chemically produced silicas that generally are prepared under harsh conditions, e.g. at elevated temperatures, high pressures and/or strongly acidic or alkaline media, in Nature silica architectures

* Corresponding author. Tel.: +31-40-2475870; fax: +31-402451036. E-mail address: [email protected] (N.A.J.M. Sommerdijk). 1 Current address: Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands.

1359-0286/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.cossms.2004.01.005

with delicate morphologies are generated in aqueous medium under ambient conditions [*6]. Unicellular organisms, such as diatoms use structuring and templating biomolecules to produce silica shells that not only contain hierarchically ordered pore structures with dimensions ranging from the nanometer to the micrometer domain, but also possess remarkable mechanical and structural [*7,8,9] properties. It is these intriguing architectures that have inspired many scientists not only to investigate the processes underlying their formation, but also to mimic these processes in order to obtain better control over the structure and morphology of chemically produced silica [10–12,*13,*14]. The introduction in 1992 of MCM-type materials [15], using surfactants as structure-directing agents, brought the development of methods to prepare mesoporous silicas forward with a great leap. In this approach different lyotropic phases of a large variety of surfactants and amphiphilic polymers have been used to structure the developing silica phase that grows around these self-assembled organic templates, resulting in numerous new mesophases [**16,*17]. Although the pore sizes of as-made porous silica range from a few to tens of nanometers using this method, the large welldefined pore dimensions observed in diatoms are still out of reach. For this reason natural silica production (i.e. silica biomineralization) receives increasing attention, since it holds the key to the formation of silica

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morphologies with a dedicated organization of hierarchically structured elements and the ability to synthesize such silicas under ambient conditions. This review gives a brief overview of recent developments in the field of silica biomineralization in diatoms, and the progress in the biomimetic fabrication of silica with hierarchical structure and morphology at different length scales.

2. Protein-mediated silica formation In unicellular photosynthesizing diatoms, biopolymer-mediated silica formation proceeds via a different mechanism. As was shown by Kr€ oger et al. [*18,19], small proteins and polyamines could be liberated from diatomaceous silica by hydrofluoric acid (HF)-assisted dissolution. The proteins best studied from this series are the Silaffin-1A isoforms, which are small (2.4–3.1 kDa) polycationic proteins with highly modified amino acid residues in their native state. Particularly interesting is the fact that the lysine groups of this protein are modified with oligo-N-methylpropyleneamines, as it was demonstrated that only silaffins bearing these side chains are able to precipitate silica [*18]. This is consistent with the observation that polyamines induce fast silica precipitation and flocculation to larger aggregates instead of gel formation [**21]. Recently, Noll et al. [*22] demonstrated that the use of tripropylenetetramine, as an analog of the native oligo-N-methylpropyleneamines present in silaffin-1A, resulted in the formation of smooth silica spheres. These spheres exhibit a surface structure that reasonably agrees with the surface of the exoskeleton of diatoms from the genus Coscinodiscus,

although the size of the formed particles was appreciably larger than of those observed in other diatoms [*23]. More recently it was found that, apart from the oligoN-methylpropyleneamine modifications of the lysine residues, the native silaffin-1A is also characterized by multiple phosphorylated serine residues. Kr€ oger et al. [**20] developed a mild method for the dissolution of biosilica that allowed the isolation of the associated silaffins in their native state. This method employed a solution of ammonium fluoride of pH 5 rather than the normally used hydrofluoric acid. Based on 31 P NMR and mass spectroscopy it was concluded that silaffin-1A from the diatom Cylindotheca fusiformis contained seven phosphorylated serine residues and one phosphorylated trimethylhydroxy lysine moiety. This eightfold phosphorylated protein showed even higher ability to induce the precipitation of silica than the previously reported silaffin-1A, although the dimensions of the resulting spheres suggested that these do not represent biologically relevant silica structures. Importantly, however, it was realized that the zwitter ionic structure of the native silaffins leads to the self-assembly of these molecules and may explain their extremely efficient induction of silica precipitation [**20]. Following up on this model Sumper proposed, on the basis of simulations, a new model that consists of multiple steps in which phase separation processes occur [**24]. At the initial stage phase separation enables the formation of the large, for diatoms characteristic, honeycomb structures, followed by several intermitted steps of silica formation––each mediated by phase separation processes––to create smaller structures (Fig. 1). In this ‘‘downscaling’’ model, the largest

Fig. 1. Schematic drawing of the templating mechanism by a downscaling phase separation model [(A)–(D)] in comparison with scanning electron micrographs of the valves from the diatom Coscinodiscus wailesii [(E)–(H)]. (A) The monolayer of polyamine-containing droplets in close-packed arrangement guides silica deposition. (B and C) Consecutive segregations of smaller (about 300 nm) droplets open new routes for silica precipitation. (D) Dispersion of 300-nm droplets into 50-nm droplets guides the final stage of silica deposition. Silica precipitation occurs only within the water phase (white areas). The repeated phase separations produce a hierarchy of self-similar patterns. (E–H) SEM images of valves at the corresponding stages of development. Modified from Ref. [**24] with kind permission of Prof. Dr. M. Sumper and the American Association for the Advancement of Science.

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structures are formed at the start (Fig. 1A) followed by the formation of the smaller ones (Fig. 1B and C) and finally the more delicate details (Fig. 1D). In contrast, Vrieling et al. [**21] derived an ‘‘upscaling’’ model based on experimental data obtained from in situ timeresolved ultrasmall angle X-ray scattering analysis of silica transformations mediated by synthetic polymers (polyethyleneglycol, polyethyleneimidine) and proteins (myoglobin, horseradish peroxidase). In the course of the polymerization and transformation reaction––the latter induced by aging––phase separation occurred and led to discrimination of silica-rich and templaterich phases, which continuously interact during the transformation of silica from smaller aggregates to the larger structures (Fig. 2). Ultimately removal of the template-rich phase would account for the creation of porous network.

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3. Bioinspired synthesis of silica 3.1. Silica structure control by (bio)polymers As mentioned above, studies on biomineralization have highlighted the role of proteins in mediating the formation of biogenic silica. Inspired by these findings important progress has been made in the biomimetic preparation of a variety of silica morphologies. Naik et al. [*25] examined the role in silicification of a 19 amino acid peptide derived from silaffin-1A and showed that through careful manipulation of the environment and the use of mechanical force, the formation of several silica nano-morphologies could be achieved, ranging from common spheres to organized and complex fibrils. It was found that silica spheres, the lowest free energy structures, are formed in a static environment, whereas

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c 10 nm Densification

Aggregation

2 nm

10 nm

Aggregation

Precursor Input

Phase separation

e

50 nm

d > 100nm

2-D expansion

Removal of proteinrich mesophase

3-D Growth

g

f

Casing formation

> 5µm

Fig. 2. Schematic presentation of the described upscaling model. At the onset of valve formation (a) silica precursors and peptides are imported into the silica deposition vesicle (SDV), where precipitation of silica is induced by small organic molecules (e.g., silaffins and/or polyamines indicated by the red curved structures to form silica sols shown as grey spheres (b). These sols further densify and grow to larger silica particles (bluish spheres), while larger peptides (green ellipsoids) start to interact with the silica (c). When aggregation continues (d), silica forms larger particles (up to 50 nm). At this stage silica and protein aggregates become transferred to silica- and protein-rich mesophases (blue–green ovals) by phase separation (e). This process proceeds until the SDV has reached it final two-dimensional and three-dimensional size (f) and the protective casing has been formed (g). In order to leave pores, the protein-rich mesophase is somehow removed prior to assembly of a protective casing before the wall leaves the cell. Reproduced from Vrieling et al. [**21] with permission of Wiley–VCH. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)

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elongated strands occur when the polymerization front is in motion. Patwardhan et al. [26] showed that using poly-L -lysine as the template, fiber-like and laddershaped silica morphologies with periodic voids could be observed by flowing the reaction mixture through a tube or by stirring during the reaction, respectively. The observations of Naik et al. and Patwardhan et al. demonstrate that in vitro biocatalysis may be employed in order to form tailor-made silica structures. In addition Livage and coworkers studied the influence of amine-containing peptides (such as polylysine, polyarginine [*27] and arginine-based surfactants [28]) on silica condensation. Their results indicated that these (bio)polymers act as gelating agents of silica oligomers in the case of silicic acid, and as flocculation agents in silica sols. The generation of porous materials with ordered nanostructure is inevitably coupled to the detailed characterization of those materials following transformation of intermediates during the reaction, as well as to the analysis of the product with various techniques. The use of any single analytical technique cannot adequately reveal the detailed pore structure of such materials [**29,30]. With in situ (ultra) small-angle X-ray scattering, scanning electron microscopy and nitrogen physisorption Sun et al. [*31] investigated the silica aggregation behavior and pore formation controlled by polyethylene glycol (PEG). This model system allows the in situ monitoring of the silicification process as PEG induces a slower precipitation than is observed in the case of for example polyethylene imine. It was shown that PEG plays three different roles in silica aggregation: (1) it serves as a flocculation agent in silica sols, (2) the hydrophobic silica–PEG interactions steer the silica polymerization, and (3) it induces phase separation, where a phase rich in both PEG and silica is formed in

Fig. 3. A general overview of PEG–silicas as a function of PEG/silica ratio and size of PEG polymers. At a PEG/silica ratio of 0.5 (upper row), the increase of the length of the PEG chains leads to a replacement of the fractal silicas (dimension D ¼ 2:5) composed of fractal particles ðD ¼ 2:3Þ found for PEG 600 by smooth silica spheres (PEG2000), and by silica spheres covered by fractal particles (D ¼ 2:7, PEG20000). At a high PEG/silica ratio of 2.0 (lower row), if hydrophobicity is simultaneously low, the homogeneous porous silicas are composed of fractal particles, whereas they are covered with particles (when hydrophobicity is high). Reproduced from Sun et al. [*31] with permission of American Chemical Society 2002.

the formerly homogeneous aqueous solution. It was demonstrated that PEG chains of different lengths, but also different PEG/silica ratios can be used to create by choice silicas with a variety of pore dimensions of approximately 2–20 nm (Fig. 3). In the biomimetic preparation of silica structures, PEG may therefore well be a promising, cheap and versatile substitute for the biomolecules involved in silica biomineralization. 3.2. Silica morphology control Inorganic materials, although diverse in composition, lack the structural and morphological variety that is one of the characteristics of polymeric, supramolecular and self-assembled structures [*32,*33,*34,*35,*36]. A great deal of research effort has been devoted towards the development of efficient and innovative fabrication methods to obtain inorganic materials with well-defined morphologies, because of their potential applications in e.g. catalysis, chromatography, absorbance, drug-delivery, etc. To develop novel inorganic materials closely resembling these organic architectures, supramolecular structures such as organogels based on low molecular mass amines, and amphiphilic mesophases have been transcribed into inorganic materials [37,38]. 3.3. Fibrillar silica prepared from amine-containing organogels Increasing attention has been focused on the selfassembly of low molecular mass gelators into fiber-like structures that can efficiently gelate organic solvents via physical interactions, including hydrogen-bonding, p–p-stacking and solvophobic effects [*32,*33]. Cholesterol- and cyclohexane-based organogel templates have enabled the formation of hollow silica fibers with linear, helical, and multilayered morphologies through transcription mediated by electrostatic or hydrogen-bonding interactions. Recently silica fibrils with a double stranded helical structure were prepared through mineralization of twisted bilayer ribbons formed from a cationic gemini surfactant with chiral tartrate counter ions [*39], whereas hollow silica fibers with a monodisperse inner diameter of 4–5 nm were obtained from a sugar-appended porphyrin gelator [*40]. Silica nanotubes with adjustable meso- or macroscale inner diameters were obtained using a sugar-appended azonaphthol gelator [41]. It has been shown that the presence of positive charges or hydrogen-bonding sites (e.g. amine groups) is indispensable for a successful transcription of the organogel template into the silica structure. In the case of the sugar-appended azonaphthol gelator, an azohydrazone tautomeric form provides a secondary amino group that is thought to play a major role in the successful sol–gel transcription, mimicking the silica biomineralization process. In addition, the first example of

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Fig. 4. Schematic representation of the creation of the helically structured silica from the organogel based on azobenzene-appended cholesterol derivative 1: (a) gelator, (b) incipient organogel fiber, (c) silica adsorption (lower) and aggregation of organogel fiber (upper), and (d) the inner helical structure of the silica formed after calcinations. Reproduced from Jung et al. [**42] with permission of American Chemical Society 2003.

silica nanotubes with a helical inner channel with an internal diameter of 7.5 nm was reported [**42], using an organogel from a dimeric azobenzene-appended cholesterol derivative (Fig. 4). Such chiral silica nanotubes have potential for applications in chiral separation and catalysis. In addition to the well-accepted mechanism for the catalyst mediated formation of silica in solution, a surface mechanism was also proposed by van Bommel [43]. This new mechanism allows the transcription process to take place exclusively on the surface of the template. Further growth of inorganic materials takes place solely on this surface, resulting in an inorganic product containing only templated silica. Interestingly this process resembles the one observed for silicification in marine sponges. It has been established that inside siliceous spines of these sponges a welldefined filamentous proteinaceous structure is present, consisting of oligomers of silicatein [44–46]. Silicatein resembles the structure of cathepsin and has been demonstrated to be active in the hydrolysis of silicon

alkoxides such as TEOS, thereby inducing silica polymerization at the surface of the protein [46]. These intriguing results indicate that the use of sol-gel transformation mediated by organogelators may well lead to synthesis of novel materials that possess elaborate complex architectures. 3.4. Nanotubular silica in a condensed state––mesoporous silica It was already mentioned that since the discovery of surfactant-mediated syntheses of mesoporous silica nanostructures, several mesophases e.g. hexagonal, cubic, lamellar have been defined [**16]. These materials, synthesized by a cooperative assembly of surfactants and associated inorganic species, unlike microporous zeolitic materials, that usually show well-defined crystal morphology corresponding to ordering at the atomic level, possess significant ordering on the mesoscopic scale. The short-range ordering at the nanoscale level,

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Fig. 5. TEM images, electron diffraction patterns and the resulting structural model of mesoporous benzene–silica. (A) Image and pattern taken with [0 0 1] incidence, parallel to the channels. Uniform mesopores with a diameter of 3.8 nm are arranged in a hexagonal manner. (B) Image and pattern taken with [1 0 0] incidence, perpendicular to the channels. Many lattice fringes with a spacing of 0.76 nm are observed in the pore walls. The electron diffraction pattern also shows diffused spots due to the 0.76 nm periodicity (large arrow) in the perpendicular direction to the spots due to channel arrangement with d ¼ 4:55 nm (small arrow). (C) Schematic model of as-made benzene–silica. Reproduced from Inagaki et al. [**48] with permission of Nature 2002.

however, is generally poor, which dramatically limits their practical utility [*47]. Although efforts to prepare highly crystalline materials continue, some major progress has been made recently. Inagaki et al. [**48] presented a benzene–silica hybrid mesoporous material, comprising a hexagonal array of mesopores, with crystal-like pore walls that exhibit surface structural periodicity along the channel direction (Fig. 5). Further characterization revealed that the pore walls were made of a covalently bonded network composed of O1:5 Si– C6 H4 –SiO1:5 units; the sharp signals observed in the 29 Si NMR spectra indicated a unique and uniform environment surrounding the Si atoms in the walls. Furthermore, Yu et al. [*49] reported the synthesis of mesoporous silica single crystals with a body-centered cubic space group, making use of a nonionic block copolymer EO132 –PO50 –EO132 as a template. Inorganic salts such as K2 SO4 were employed to increase the interaction of the silicate species with hydrophilic head groups of the nonionic block copolymers, resulting in long-range ordered domains of the silica-surfactant mesostructures and eventually in the formation of mesoporous single crystals. The preparation of ordered mesoporous carbon materials has been achieved, which mirror the pore structure of periodic silica templates after removing the silica matrix from silica–carbon composites [*47,50]. Such synthesis routes rely on the fact that ordered mesoporous silica can be filled with a carbon precursor

that is subsequently pyrolyzed, after which the silica is removed by HF or alkaline leaching. For a successful transformation from mesoporous silica templates to mesoporous carbon, the thermal and hydrothermal stability of mesoporous silica templates has to be high enough to withstand the harsh high-temperature treatment, and the yield of silica templates with desired morphology should be high. Yu et al. [52] addressed this matter by an inorganic salts synthesis approach. Highly ordered and condensed mesoporous SBA-15 rods were obtained using the triblock copolymer EO20 –PO70 –EO20 as the template, in the presence of large amounts of the inorganic salts. From the resulting silica scaffold, hexagonally ordered mesoporous carbon rods could then be prepared, which were characterized by a very large surface area (up to 1823 m2 /g) and a pore volume up to 2.23 cm3 /g. This technique, however, implies the possibility of silica leaching/dissolution that also affects the material that is filled into the silica pore system or replaces silicon. Lu et al. [*53] were able to circumvent the problem of silica leaching effects by going one step further and using mesoporous ordered carbons as a matrix for the production of an ordered mesoporous silica, from which carbon then could be removed easily by combustion or possibly by high-temperature hydrogenation techniques for materials not stable enough against oxidation. This innovative approach may well be extended to other inorganic framework compositions, if only they can be

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Fig. 6. Scanning electron microscopy images of the same exoskeletons of the diatom species Aulacoseira. (a) Before and (b) after reaction with MgðgÞ for 4 h at 900 °C. Ten specific protuberances can be seen to be retained after reaction, along with the larger hole (H) and finer pores and ridges on the surfaces of the frustules. The solidified Mg–Si is a product of the reaction of reduced silicon with MgðgÞ . Reproduced from Sandhage et al. [51] with permission of Wiley–VCH 2002.

introduced in the carbon matrix and retain the integrity during carbon removal. Sandhage et al. [51] demonstrated that silica-based diatom frustules could be converted into new metal oxide compositions using the technique of shape-preserving displacement. In this process fluid/solid displacement (oxidation ± reduction) reactions have been employed for the conversion of diatomaceous silica into magnesium oxide while retaining the morphology and architecture of the exoskeleton (Fig. 6). With this approach they show that the displacement procedure has a potential for the development of hybrid, bioprocessing routes i.e. genetic engineering of cell wall shapes, of the biological silica assembly, and of chemical conversion. It can be easily envisaged how the feature of biological replication of complex-shaped biosilicas with welldefined meso/nanoscale features can be coupled to processing of fluid/solid displacement reactions to yield

large numbers of chemically tailored, complex-shaped, 3-D meso- or nanodevices. 3.5. Hierarchically structured silicas Various structuring agents such as block copolypeptides, nonionic block copolymers, diamines, and gemini surfactants, have been used to mediate the specific shapes of the mesoporous silica. Mesoporous silica with hollow morphologies has attracted a lot of attention because of its low-density, as well as of its thermal and mechanical stability that allows possible application as e.g. insulators, catalysts, sorbents, and containers for chemically active or vulnerable agents (e.g. in controlled drug-delivery). Previous preparation methods have involved various templating approaches using lyotropic liquids [15,54,*55], block copolymer mesophases [56,57], bicontinuous (micro)emulsions [*58], emulsion foams

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[59], colloidal arrays [60], bacterial superstructures [*61], and even polystyrene latex spheres [62]. Most of the mesostructures reported possess non-uniform shapes, undesirable shell thickness, or a lack of structural stability and moreover often require high temperature routes. Recently some major progress has been reported in this field. Sun et al. [**63] reported a facile and efficient single-step synthesis of hollow silica spheres with a well-defined multilamellar structure and with a high size-uniformity, using an EO76 –PO29 –EO76 triblock copolymer-based emulsion in combination with the inexpensive sodium silicate solution (Fig. 7). The hybrid materials could be obtained via a rapid (4 h) synthesis procedure at either a reaction temperature of 80 °C or

room temperature. The first route is suitable to produce thermally stable materials with a highly defined wall structure for applications, whereas the second route (at RT) provides a promising tool to enclose temperaturesensitive or volatile agents e.g. for drug-delivery application. In a different approach, Yu et al. [*64] reported the synthesis of hollow spheres with ultra large mesoporous wall structures from EO39 –BO47 –EO39 -based reverse emulsions. The silica wall of the product consists of uniform and hexagonally ordered ultra large mesopores with pore sizes up to 50 nm, which may be suitable for the storage, release and transport of biomolecules. In addition, Prouzet et al. [*65] showed that hollow nanoscaled mesoporous spherical silicas could be obtained by applying sonication to the reaction mixture of PEOpolymer Tergitol C15 (EO)12 and silicon alkoxide in order to form a stable emulsion. It was described that the cavitation bubbles created by ultrasonication become trapped in the solution, and are frozen by the subsequent hydrolysis of silicon alkoxide and condensation of silica. The striking feature of the formed silica is the large hysteresis loop in nitrogen physisorption, which corresponds to the retention of condensed nitrogen within the voids, suggesting that these nano-scaled hollow silica spheres may be applied in storing or stabilizing volatile compounds. Finally, Wong et al. [**66] reported a wet chemical synthesis procedure in which silica (10–12 nm) and gold nanoparticles (10–12 nm) were cooperatively assembled at room temperature with lysine–cysteine diblock polypeptides into hollow spheres. In this system the L -lysine block of the copolypeptide provided preferential attachment sites for silica, while the L -cysteine block was used for binding to gold nanoparticles due to the strong affinity of gold surfaces for sulfide and disulfide moieties resulting in the formation of hollow spheres with amorphous walls composed of two distinct layers of silica–gold nanoparticles. Their results suggest that block copolypeptides designed with specific recognition sites for nanoparticles of various compositions may lead to development of a general approach for the hierarchical organization of nanoparticles into multidimensional composite arrays.

4. Conclusion and perspective

Fig. 7. Electron microscopy of silica hollow spheres with a welldefined multilamellar structure. (a) A SEM overview showing the size-uniformity of the synthesized particles prepared from PEO–PPO– PEO-based emulsions (bar represents 10 lm). In the inset a detailed view clearly indicate the hollowness of spheres (bar represents 1 lm); (b) high resolution TEM micrograph demonstrating the multilamellar shell (arrows) of the hollow silica spheres (bar represents 33 nm). The images were reproduced from Sun et al. [**63] with permission of Wiley–VCH 2003.

The biomimetic approaches for silica formation as summarized above are very encouraging, as they strongly suggest that the use of principles learned from Nature and in particular from diatom biomineralization will enable the elaboration of novel silica-based materials such as silica fibrils with inner chiral nanochannels and well-defined multilamellar hollow spheres, presenting hierarchical structures and morphologies. The integration of biomineralization principles such as supramolecular template synthesis, phase separation and

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self-assembly into the strategy of materials fabrication opens up the great potential for the tailored structuring of materials from nanometer to micrometer scale. The application of these biological principles would allow the transfer of the superior structures, morphologies and properties of biogenic materials towards large scale synthetic materials. Meanwhile, the understanding of the bio-inspired novel materials could, vice versa, shed new light onto the understanding of the mechanisms of formation of complex architectures that are generated in Nature. As bioinspired silica synthesis is an intriguing field of materials chemistry; the quick expansion of the further understanding of principles and the future application of these systems may be foreseen.

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Acknowledgements QS and EGV were supported by the Technology Foundation STW (grant GFc4983), the applied science division of NWO and the technology program of the Ministry of Economic Affairs. Dr. W.W.C. Gieskes (Department of Marine Biology, University of Groningen, NL) is thanked for contributive discussions.

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