Silicification and biosilicification: Part 5. An investigation of the silica structures formed at weakly acidic pH and neutral pH as facilitated by cationically charged macromolecules

Abstract

Biosilicification in diatoms has been reported to occur at (or close to) neutral pH and it has been shown that protein molecules can act as catalysts/templates/scaffolds for this elegant materials chemistry. Here we report the formation of silica spheres from an aqueous silica precursor as facilitated by both poly- l-lysine (PLL) and poly(allylamine hydrochloride) (PAH) at pH 6.0 and under ambient conditions. It was observed by scanning electron microscopy (SEM) that there were morphological differences in the silica formed at pH 6 when compared to the silica morphologies formed using the same macromolecules at neutral pH. Notably, a bimodal distribution of silica particles was seen for both the PLL and PAH systems at pH 6. These results are compared and contrasted with those previously reported for the pH dependence of silicification and biosilicification and in particular, those obtained for the pH dependence of silica formation in the presence silaffin proteins isolated form the diatom C. fusiformis. The findings for these cationically charged macromolecules suggest that lysine may be an important amino acid in the primary sequence of proteins that catalyze the formation of silica structures in vivo.