There exists a great deal of interest in silica thin films, which potentially allow us to develop various applications including heterogeneous catalysis, 1 wettability control, 2 and microelectronics 3 because of their unique physicochemical properties, such as transparency, mechanical hardness, chemical stability, and low dielectric constant. Recently, the scope of substrates, on which silica thin films are deposited, has been expanded to biological entities for exploiting the emerging fields of drug delivery, 4 cell culture, 5 biosensors, and cell encapsulation. 7-9 Although chemical deposition and thermal treatment have been considered as a simple way to fabricate silica thin films, toxic chemicals and extreme temperature are required for silica-forming processes.
10In contrast to the chemical and thermal methods, biosilicification occurring in diatoms and glass sponges has its own characteristic strategies to concentrate a small amount of silicic acid from sea water and construct the siliceous exoskeletons under mild conditions (e.g., neutral pH, room temperature, aqueous media).11,12 Despite the similarity in materials, it is known that silica-forming mechanisms in the two organisms are completely different. The biosilicification in diatoms is achieved by silaffins, highly phosphorylated silica-forming peptides extracted from the cell wall of diatoms, where ε-amino group in lysine residues are posttranslationally modified to ε-N-dimethyllysine or ε-N,N,Ntrimethyl-δ-hydroxylysine or a long-chain polyamine comprising of a few N-methylpropylamine units.11 The selfassembled structure of the cationic polypeptides in silaffins is believed to act as a catalytic template for the in vivo polycondensation of silicic acid derivatives. Inspired by silaffins, cationic synthetic polymers and polypeptides have been used as catalytic templates for fabricating silica thin films. For example, silica thin films were formed on a gold substrate by using poly((2-dimethylamino)ethyl methacrylate) (PDMAEMA) grown by surface-initiated polymerization.13,14 The positively charged PDMAEMA could attract the negatively charged silicic acids, followed by the polycondensation to silica. In a similar fashion, polyelectrolyte multilayers of poly(diallyldimethylammonium chloride) (PDADMA), formed by the layer-by-layer method, were used as a catalytic template for generating micro-patterned silica films.
15On the other hand, silicateins, silica-forming proteins extracted from the spicules of various kinds of glass sponges, catalyze the polycondensation of silicic acids by mainly using two specific amino acids: histidine and serine that are in close proximity for hydrogen bonding.12 Because of the hydrogen bonding, the nucleophilic attack of oxygen in serine to silicon alkoxide is believed to occur effectively. In contrast to silaffins, the silicatein-catalyzed formation of silica thin films was carried out with silicon alkoxide at pH 7.4. For example, Perry et al. reported the formation of uniform silica thin films by immobilizing recombinant...
