Biomineral formation is widespread in Nature and occurs in bacteria, single-celled protists, plants, invertebrates and vertebrates. Minerals formed in the biological environment often show unusual physical properties (e.g., strength, degree of hydration) and often have structures that exhibit order on many length scales. Biosilica, found in single cell organisms through to higher plants and primitive animals (sponges) is formed from an environment that is undersaturated with respect to silicon and under conditions of around neutral pH and low temperature ca. 4–40 °C. Formation of the mineral may occur intra- or extra-cellularly and specific biochemical locations for mineral deposition that include lipids, proteins and carbohydrates are known. In most cases the formation of the mineral phase is linked to cellular processes, understanding of which could lead to the design of new materials for biomedical, optical and other applications. In this contribution we describe the aqueous chemistry of silica, from uncondensed monomer through to colloidal particles and three dimensional structures, relevant to the environment from which the biomineral forms. We then describe the chemistry of silica formation from alkoxides such as tetraethoxysilane as this and other silanes have been used to study the chemistry of silica formation using silicatein and such precursors are often used in the preparation of silicas for technological applications. The focus of this article is on the methods, experimental and computational by which the process of silica formation can be studied with emphasis on speciation.

In order to understand the role that proteins play in the generation of well regulated biosilica structures we need to understand the contribution of the components, singly and in combination. To this end we have performed a systematic study of the effect of amino acids and small peptide oligomers on silica formation from aqueous solution. Silicas produced from a potassium silicon catecholate salt at ca. pH 7 in the presence of the amino acids (Gly, Arg, Asn, Gln, Glx, Ser, Thr, Tyr, Pro, Ala, Lys) at a 2 Si : 1 amino acid molar ratio have shown that these amino acids affect the kinetics of small oligomer formation, the growth of aggregate structures and the morphology and surface properties of the silicas produced. The effects seen during the early stages of oligomer formation carry through to the properties of the particles and aggregates produced after extended periods of reaction. The behaviour of the amino acids relates to the pI and hydrophobicity of the individual amino acids. The presence of the nitrogen containing amino acids generates larger particles and the presence of amino acids containing hydroxyl and hydrophobic groups generates silicas with smaller particles than are produced for silicas produced in the absence of amino acids. An extensive study of the effect of the number of lysine and glycine units per peptide was also performed ( for lysine, 1 - 5 and ca. 150 and for glycine, 1,4,5). Increasing the number of glycine units per additive molecule had little effect on the kinetics, aggregation, sample morphology, surface area and porosity of the silicas produced. A distinct relationship between the number of lysine units per additive molecule and an increased rate of oligomer formation, aggregate growth and a reduction in silica surface area and broadening of pore sizes was observed. A distinct change over in behaviour, particularly in regard to the porosity characteristics of the silica produced was noted for between (lys)(3) and (lys)(4) as well as this being the smallest size of peptide that was incorporated into the siliceous material formed. Aggregation was observed to accelerate exponentially over the full range of lysine oligomers used. Consecutive sequences of the same amino acid residues were shown to produce effects much larger than the sum of the effects of the individual residues, and at extremes mediate macroscopic morphological changes. The consequences of these findings for biosilicification are discussed. It is clear that all amino acid functional groups in proteins that are accessible to silica during the stages of formation from orthosilicic acid through to the final material have a role to play in determining the physical nature and structure of the material that forms

Considerable research has been directed toward identifying the mechanisms involved in biosilicification to understand and possibly mimic the process for the production of superior silica-based materials while simultaneously minimizing pollution and energy costs. Molecules isolated from diatoms and, most recently sponges, thought to be key to this process contain polyamines with a propylamine backbone and variable levels of methylation. In a chemical approach to understanding the role of amine (especially propylamine) structures in silicification we have explored three key structural features: (i) the degree of polymerization, (ii) the level of amine methylation, and (iii) the size of the amine chain spacers. In this article, we show that there are two factors critical to their function: the ability of the amines to produce microemulsions and the presence of charged and uncharged amine groups within a molecule, with the latter feature helping to catalyze silicic acid condensation by a proton donor/acceptor mechanism. The understanding of amine-silicate interactions obtained from this study has enabled the controlled preparation of hollow and nonporous siliceous materials under mild conditions (circumneutral pH, room temperature, and in all aqueous systems) possibly compatible with the conditions used by biosystems. The ''rules'' identified from our study were further used predictively to modulate the activity of a given amine. We believe that the outcomes of the present contribution will form the basis for an approach to controlling the growth of inorganic materials by using tailor-made organic molecules.bioinspired materials ͉ biosilica ͉ hollow particles T he commercialization of siliceous materials has generated a multibillion pound industry with the value of precipitated silica products alone estimated to have reached 1.4 billion pounds by the year 2006 (1). This value largely depends on the ability to produce silica for specific applications through control of surface chemistry and morphology during the condensation and aggregation process. Industrial processes involved in the production of high-value siliceous materials typically involve harsh conditions of high temperature and acidity or basicity, environmentally damaging waste streams, and often toxic silicate precursors. These processes also generally exert poor chemical and morphological control over the materials produced. In comparison, natural silica production in organisms such as sponges, diatoms, and radiolaria occurs under biologically benign conditions and produces silica of exquisite form and function, all from a monosilicic acid source of only a few parts per million in concentration (2). Considerable research has been directed toward identifying the mechanisms involved in the biosilicification process to understand and possibly mimic the process to allow us to produce superior materials without the attendant pollution and high-energy usage (3, 4).Studies of the processes associated with the formation of siliceous diatom shells (thecae) have i...

Biosilicifying organisms such as diatoms, sponges and higher plants deposit ornate "glassy" siliceous materials with well defined properties such as particle size and porosity at precisely controlled growth rates. Here we present the in vitro synthesis and characterisation of "glassy" silica with tailored properties by using naturally occurring amines - spermidine and spermine - and their analogues. These additives were found to regulate the growth rates, particle sizes, maturation, surface areas, porosities and morphologies of the siliceous materials prepared. In particular, the combination of unique catalytic effects and aggregation behaviours that are dependent on or related to chain length, intramolecular N-N spacing and C : N ratio of the additives was found to be responsible for controlling materials properties. Mechanisms regulating the generation of silicas showing a range of material characteristics are proposed

Silica skeletal architectures in diatoms are characterized by remarkable morphological and nanostructural details. Silk proteins from spiders and silkworms form strong and intricate self-assembling fibrous biomaterials in nature. We combined the features of silk with biosilica through the design, synthesis, and characterization of a novel family of chimeric proteins for subsequent use in model materials forming reactions. The domains from the major ampullate spidroin 1 (MaSp1) protein of Nephila clavipes spider dragline silk provide control over structural and morphological details because it can be self-assembled through diverse processing methods including film casting and fiber electrospinning. Biosilica nanostructures in diatoms are formed in aqueous ambient conditions at neutral pH and low temperatures. The R5 peptide derived from the silaffin protein of Cylindrotheca fusiformis induces and regulates silica precipitation in the chimeric protein designs under similar ambient conditions. Whereas mineralization reactions performed in the presence of R5 peptide alone form silica particles with a size distribution of 0.5-10 m in diameter, reactions performed in the presence of the new fusion proteins generate nanocomposite materials containing silica particles with a narrower size distribution of 0.5-2 m in diameter. Furthermore, we demonstrate that composite morphology and structure could be regulated by controlling processing conditions to produce films and fibers. These results suggest that the chimeric protein provides new options for processing and control over silica particle sizes, important benefits for biomedical and specialty materials, particularly in light of the all aqueous processing and the nanocomposite features of these new materials.biomaterials ͉ nanostructures ͉ silaffin ͉ biomineralization ͉ ceramics C omplex mineralized composite systems in nature provide rich ground for insight into mechanisms of biomineralization and novel materials designs (1-4). Some of the more common sources of inspiration include seashells, insect exoskeletons, extracellular matrices involved in bone and other hard tissues, and biosilica skeletons. The formation of natural inorganic-organic composites is a multistep process, including the assembly of the extracellular matrix, the selective transportation of inorganic ions to discrete organized compartments with subsequent mineral nucleation, and growth delineated by preorganized cellular compartments. Silica skeletons found in nature are based on nanoscale composites wherein the organic components, usually proteins, are functional parts of the skeletal structures while also serving as silica-forming components (5, 6). As a result, materials' toughness is improved, strength is retained, and fine morphological control is achieved, all hallmark attributes of biological composites.Silica is widespread in biological systems and serves different functions, including support and protection in single-celled organisms, such as diatoms through to higher plants and animals (7,8...

Osteoinductive and biodegradable composite biomaterials for bone regeneration were prepared by combining silk fibroin with silica particles. The influence of these composite systems on osteogenesis was evaluated with human mesenchymal stem cells (hMSCs) subjected to osteogenic differentiation. hMSCs adhered, proliferated, and differentiated towards osteogenic lineages on silk/silica films. The addition of the silica to the silk films influenced gene expression leading to upregulation of bone sialoprotein (BSP) and collagen type 1 (Col 1) osteogenic markers. Evidence for early bone formation in the form of collagen fibers and apatite nodules was obtained on the silk/silica films. Collagen fibers were closely associated with apatite deposits and overall collagen content was higher for the silica containing samples. Also, smaller sized silica particles (24 nm -2 μm) with large surface area facilitated silica biodegradation in vitro through particle dissolution, leading to ~5 fold decrease in silica content over 10 weeks. These results indicate suitability of silk/silica composite system towards bone regeneration, where degradation/remodeling rates of the organic and inorganic components can be controlled.

A systematic model study on the role(s) of putrescine homologues on silicification is presented and it is proposed that electrostatic forces between additive and silicic acid, and the hydrophobic behaviour of the additives are both important in silicification.

With both mild synthesis conditions, a high level of organisation and functionality, biosilicas constitute a source of wonder and inspiration for both materials scientists and biologists. In order to understand how such biomaterials are formed and to apply this knowledge to the generation of novel bioinspired materials, a detailed study of the materials, as formed under biologically relevant conditions, is required. In this contribution, data from a detailed study of silica speciation and condensation using a model bio-inspired silica precursor (silicon catechol complex, SCC) is presented. The silicon complex quickly and controllably dissociates under neutral pH conditions to well-defined, metastable solutions of orthosilicic acid. The formation of silicomolybdous (blue) complexes was used to monitor and study different stages of silicic acid condensation. In parallel, the rates of silicomolybdic (yellow) complex formation, with mathematical modelling of the species present was used to follow the solution speciation of polysilicic acids. The results obtained from the two assays correlate well and monomeric silicic acid, trimeric silicic acids and different classes of oligomeric polysilicic acids and silica nuclei can be identified and their periods of stability during the early stages of silica condensation measured. For experiments performed at a range of temperatures (273–323K) an activation energy of 77kJ·mol−1 was obtained for the formation of trimers. The activation energies for the forward and reverse condensation reactions for addition of monomers to polysilicic acids (273–293 ± 1K) were 55.0 and 58.6kJ·mol−1 respectively. For temperatures above 293K, these energies were reduced to 6.1 and 7.3 kJ·mol−1 indicating a probable change in the prevailing condensation mechanism. The impact of pH on the rates of condensation were measured. There was a direct correlation between the apparent third order rate constant for trimer formation and pH (4.7–6.9 ± 0.1) while values for the reversible first order rates reached a plateau at circumneutral pH. These different behaviours are discussed with reference to the generally accepted mechanism for silica condensation in which anionic silicate solution species are central to the condensation process. The results presented in this paper support the use of precursors such as silicon catecholate complexes in the study of biosilicification in vitro. Further detailed experimentation is needed to increase our understanding of specific biomolecule silica interactions that ultimately generate the complex, finely detailed siliceous structures we observe in the world around us.