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Research Overview

Silicification

Silicon, essential to life (e.g. plants or aquatic microorganisms), is the second most abundant element in the earth’s crust. It is also highly sought by industries to create materials of great importance in the developed countries (e.g. semi-conductors, glasses, catalysts). However, despite this large presence and demand, the biochemistry of silicon is still not well understood to the extent that many highly engineered structures available in nature (sponges, diatoms) still have no synthetic counterpart. Indeed, the ability of a species to capture, transport, transform silicic acid into silica at given deposition sites, in the so-called silicification process, is a marvel of engineering which to date still remains unequalled by the human kind.

Over the last fifty years¹ the understanding of silicification has become a vivid multidisciplinary issue, crucial to the geochemist, the marine biologist and the material chemist, hence extremely difficult to tackle due to the breadth of knowledge that it requires. Although the code of silicification has not yet been cracked, the key issues to resolve have become better defined². The water-soluble silicon species is silicic acid which starts autopolymerising at concentrations higher than 2mM³. Amino-rich proteins or polyamines have been shown to catalyse the oligomerisation process, however from solutions always of at least 10mM silicic acid. Yet, in species such as diatoms, silicic acid at concentrations closer to 2 μmol/L than 2mmol/L is transported through membranes inside the cell against an apparent concentration gradient, and accumulated in internal pools with no autopolymerisation. The deposition of silica is controlled by the cell and occurs in vesicles.

We are interested in understanding the mechanisms by which silicic acid is precipitated into silica under very dilute conditions. We apply a range of synthesis strategies to tackle this issue and we use numerous spectroscopic and surface techniques to assess the validity of our approach.

[1] Alexander B.G., J. Am. Chem. Soc., 1953, 75, 2887.

[2] Perry C.C., Biomineralization, 2003 (54), 291-327,

[3] Iler R. K., The Chemistry of Silica, 1979.

 Molecular Recognition in Sol-Gels

Generation of porosity within a polymeric network can be achieved by using a porogen, or a template. However, porosity can also be a consequence of simple sol-gel chemistry: as the polymer grows it become less and less soluble in the reaction solvent and consequently tends to phase-separate forming growing cavities of solvent, which, upon removal generates the porosity. Careful choice of experimental conditions controls the phase-separation both in terms of kinetics and size, thereby controlling the final porous volume. Porosity, especially if hierarchical, helps mass transfer throughout the material and also creates the high surface area crucial for subsequent reaction within the pores.

The introduction of functionalities within the walls of these pores would be a great advantage for molecular recognition and could lead to material such as membranes for instance with applications as catalysts or sensors.

C.C. Egger, C. du Fresne, D. Schmidt, J. Yang, V. Schädler, J. Sol-Gel Sci. Tech., 2008, DOI 10.1007/s1097-008-1745-9.

Zimmerman, SC; Wendland, MS; Rakow, NA, et al., Nature, 2002, 418(6896), 399-403.