Amine-Catalyzed Molecular-Mechanism for Nano-Silica Biomineralization Controls Larger-Scale Silica Morphology
Biominerals are distinguished from inorganically grown counterparts by their unique morphologies. Organic macromolecules play a role, usually by epitaxial matching or stereochemical control between the functional groups on the macromolecule and atom positions or crystal dimensions in the mineral. This mechanism may operate in the case of biogenic CaCO3 growth . Similar epitaxy may be more difficult to find in other biominerals such as the nanoporous, amorphous silica produced by diatoms, sponges and radiolaria. Using biomimetic nanosilica as a model example, we present an alternative mechanism for biomineralization, where formation of an amorphous oligomer is followed by cluster aggregation and growth. We used 29Si Nuclear Magnetic Resonance (NMR) to follow the kinetics of monoamine- and polyamine-catalyzed hydrolysis and polymerization of an organosilicate starting compound . The amorphous silica morphologies produced were examined by Scanning Electron Microscopy (SEM). The amines represent the active portions of silica-precipitating enzymes in diatoms and sponges. Results suggest that monoamines and polyamines promote organosilicate hydrolysis via an amine base-catalyzed, nucleophilic mechanism involving a hypervalent silicon reactive intermediate with a Si-N bond [2, 4]. The morphology of silicas in the presence of polyamines from stagnant solutions versus shaken solutions was different. In addition to catalyzing hydrolysis of the organosilicate, the polyamines may also promote subsequent silica oligomer aggregation by a physical templating mechanism. The term templating is not meant to signify any steoreochemical or epitaxial match. Rather, the polyamines most likely form self-assembled structures in solution and the silica oligomers can aggregate in the spaces between the polyamine aggregates similar to the mechanism proposed previously for the synthetic material, MCM-41 . The molecular mechanisms of hydrolysis and aggregation are reflected on the larger scale in the silica morphology where amines promoting faster hydrolysis result in glassy products whereas slower hydrolyzing amines result in particulate silica . REFERNCES  Lowenstam H.A. and Weiner S. (1989) On Biomineralization, Oxford University Press, pp. 324.  Delak K.A. and Sahai N. (2005a) Chem. Materials, 17, 3221-3227.  Delak K.A. and Sahai N. (2005b) In prep. for Chem. Materials.  Sahai N. (2004) Geochim. Cosmochim. Acta 68, 227-237.  Kresge C.T., Leonowicz M.E., Roth M. E., Vartuli W. J. and Beck J. S. (1992) Nature 359, 710.