Silicification and biosilicification - Part 7: Poly-L-arginine mediated bioinspired synthesis of silica

Precipitated silica is synthesized commercially by neutralizing sodium silicate solution under harsh conditions of pH and temperature. In contrast, the formation of ornate silica structures in biological systems (biosilicification) occurs at (or close to) pH 7 under ambient conditions and is thought to be mediated by proteins. Determination of the primary sequences of these proteins has led to the identification of various amino acids that have been proposed to be important in biosilicification. The corresponding synthetic polyamino acids are now being successfully used in bioinspired materials chemistry for developing new materials and processes. Here we report the formation of well-defined silica in vitro as facilitated by poly-l-arginine (PLAr) under ambient conditions and at neutral pH. Two different silica precursors were used in this investigation; tetramethoxysilane (TMOS) and water glass. Scanning Electron Microscopy (SEM) was used for studying the silica morphology and it was revealed that the silica spheres had typical diameters in the range 300-500 nm. The PLAr is a cationically charged macromolecule at neutral pH and is believed to act as a catalyst/template/scaffold for the formation of silica in vitro in analogous fashion to certain biomacromolecules that are able to facilitate silicification/biosilicification. These results are discussed here in the context of the role(s) of (bio) macromolecules that facilitate (bio) mineralization.