Silica Biomineralization

Keywords silica condensation, polyamines, biomineralization, silica... 

Investigations on the Nature of Biomineralized Silica by X-Ray Absorption Spectroscopy and Sorption Measurements... 

In order to interpret our data in a more detailed fashion, suitable reference compounds have to be found. In this context, a comparison with synthetic mesostructured silicas like hexagonal MCM-41 [16, 17] may be of interest, because these materials already show some similarities to biominerals for example, they can be synthesized under relatively mild conditions and they can be obtained with morphologies reminiscent of biological shapes [18]. Indeed, S K XANES spectra in the multiple scattering region B of mesostructured M41S type silicas exhibit similar features as the biological samples [19]. SiK XANES spectroscopy thus may become a suitable tool to evaluate structural relationships between biomineralized silica on the one hand and its biomimetically synthesized counterparts on the other. 

The strong interest in biomineralization is due to its high effidency and the superiority of the properties of biosilica over those of silica fabricated in geological processes and industrially. It proceeds at mild, ambient conditions in... 

As mentioned earlier, biological systems have developed optimized strategies to design materials with elaborate nanostructures [6]. A straightforward approach to obtaining nanoparticles with controlled size and organization should therefore rely on so-called biomimetic syntheses where one aims to reproduce in vitro the natural processes of biomineralization. In this context, a first possibility is to extract and analyze the biological (macro)-molecules that are involved in these processes and to use them as templates for the formation of the same materials. Such an approach has been widely developed for calcium carbonate biomimetic synthesis [13]. In the case of oxide nanomaterials, the most studied system so far is the silica shell formed by diatoms [14]. 

Biomineralization. The processes controlling biomineralization are summarized in Fig. 6.1c. Organized biopolymers at the sites of mineralization are essential to these processes. In unicellular organisms these macromolecules act primarily as spatial boundaries through which ions are selectively transported to produce localized supersaturation within discrete cellular compartments. In many instances, particularity in organisms such as the diatoms that deposit shells of amorphous silica, the final shape of the mineral appears to be dictated by the ultrastrucure of the membrane-bound compartment. Thus, a diversity of mineral shapes can be biologi-... 

About twenty different skeletal minerals are reported from organisms7,8 however, only four are common (1) aragonite, (2) calcite, (3) dahllite = carbonate hydroxyapatite, and (4) opal. The remaining minerals depicted in Fig. 1 are either trace constituents or occur only in a few isolated species. It is for this reason that the article concentrates on carbonate, phosphate and silica deposition in plants and animals. For reviews on general aspects of biomineralization and discussions on individual taxonomic classes see Ref.9-47 ... 

At the onset of biomineralization the mechanism of phosphate and silica deposition is essentially the same. Both start with a highly hydrated amorphous phase having glass-like physical-chemical properties. The kinetics of crystallisation of the two differs. ACP will rapidly alter in the direction of apatite in hours or days, whereas amorphous silica requires thousands of years or higher temperatures to yield quartz. 

In conclusion, cellular extraction of carbonate, phosphate and silica appeared almost simultaneously at about the Precambrian-Cambrian boundary. Which of these three modes of biomineralization is the most ancestoral one is not clear largely due to the metastability of some biominerals, the coexistence of different minerals within some primitive organism557,558), and uncertainties of stratigraphic correlation. 

Meldrum, 2002), the elaborately shaped spicules in ascidians, and finely the sculpted coccolith shells in coccolithophores (Mann, 2001). For silica structure, the diatoms produce the most exquisite siliceous cell wall (Sumper and Brunner, 2006). Also the calcium phosphate in bone shows a highly regulated organization and arrangement. Interestingly, avians have two main biomineralization systems, one produces calcium carbonate for egg shell and the other produces calcium phosphate for bone (Bauerlein, 2000 Mann, 2001). 

Recently, Foo et al. (2006) produced some novel nanocomposites from spider silk-silica fusion (chimeric) proteins. The composite morphology and structure could be regulated by controlling processing conditions to produce films and fibers. Silk and biomineralization being natural inspiration sources will allow production of numerous new materials in various fields of application. 

Vrieling, E.G. et al., Diatom silicon biomineralization as an inspirational source of new approaches to silica production, J. Biotechnol., 70, 39, 1999. 

Sahai, N. and Tossell, J.A., Formation energies and NMR chemical shifts calculated for putative serine-silicate complexes in silica biomineralization, Geochim. Cosmo-chim. Acta, 65, 2043, 2001. 

Kroger N. and Sumper M. (2000) The biogeochemistry of silica formation in diatoms. In Biomineralization (ed. E. Baeuerlein). Wiley, Weinheim, Germany, pp. 151-170. 

Gaines et al., 1997, pp. 1568-1586). However, it is the low-temperature hydrated variety of silica, opal (Si02-nH20) which is a biomineral. 

---