Biomimetic Synthesis Of Silica

Biomineralization is ubiquitous in nature. Biogenic silica widely exists in avariety of single-cell algae, bacteria, sponges, protozoa and higher plants. It is foundthat they have complex nanostructures and specific functions, which are usually notachieved by current synthesis technology. Simultaneously, the in vivobiomineralization process occurs in mild conditions, in stark contrast to theconventional synthesis environments such as high temperature, high pressure, andextreme pH values. Therefore, how to mimic biological mineralization process toprepare silica materials with well-defined nanostructures and functions has arousedgreat concern.It has been found that organic macromolecules such as peptides, proteins, andpolysaccharide play crucial roles in mediating the biosilicification. Accordingly, wehave designed and synthesized an amphiphilic peptide-I3K, consisting of threeconsecutive hydrophobic residues (I) and one hydrophilic residue (K). Because ofthe N-terminal acetylation and C-terminal amidation, each I3K molecule carries onepositive charge at neutral pH, which is from the side chain amino group. In aqueoussolution, I3K molecules easily undergoes the self-assembly to form well-definednanostructures. The hydrophobic isoleucine residues are sequester within assembliesbut the hydrophilic lysine residues are project on the surface of assembles andexposed to water. Due to the inherent catalytic nature of lysine with respect to silica,the I3K assembled nanostructure was employed as soft template to produce silica withdefined structure.Firstly, we synthesized I3K by using Fmoc-solid phase synthesis method. Afterpurification, the corresponding mass spectrometry and high performance liquidchromatography analyses showed that the resultant I3K product had high purity.Transmission electron microscopy and atomic force microscopy revealed that thepeptide could self-assemble uniform nanotube in aqueous solution, with the diameterof approx. 10 nm and the length typically above 1μm. The circular dichroism andinfrared spectroscopy characterization indicated that the peptide molecule predominantly adopt aβ-sheet conformation within assembles. It is very interesting tonote that the assembled nanotube displayed high stability against temperature, dilution,and organic solvents. However, it could be destructed by ultrasound treatment andadding enzyme.Subsequently, we used the I3K nanotubes as the template to mimic thebiosilicification at mild conditions. The optimal conditions were achieved byinvestigating the effect of reaction time, the type and concentration of silica sources(TEOS and APTES), pH value on the morphology of mineral products. We found thatthe uniform nano-size silica tubes could be well obtained by using TEOS as silicasource, with the ratio of TEOS to I3K and the solution pH value fixing at 0.18 and 9,respectively. Moreover, the wall thickness of silica nanotubes could be tuned throughthe mineralization time. In addition, the biomimetic synthesis mechanism ofnano-silica was proposed by using the self-assembled structure of short peptide (I3K)as template.