This work reports a mechanistic study of template-directed synthesis of silica nanomaterials utilizing self-assembled peptide nanotubes as scaffolds. An ultrashort amphiphilic peptide (I3K) underwent self-assembly in aqueous solution under ambient conditions to form long and uniform nanotubes. The assembled peptide nanotubes then were used as templates for the subsequent fabrication of silica nanotubes from tetraethoxysilane (TEOS), also under ambient conditions. In order to gain better insight into the mediation of peptide self-assembly on the formation of silica nanostructures, we have carefully investigated environmental influences including the concentrations of peptide and silica precursor, solution pH, and reaction time, with the full screening of the processes by TEM, SEM, 29Si MAS NMR, FTIR, and TG-MS. The results revealed that, while peptide nanotubes worked as scaffolds for the formation of tubular silica structures, the surfaces of these peptide nanotubes served as catalytic sites for both hydrolysis and condensation of TEOS, thereby working as templates for directing silica deposition. Because the electrostatic attraction of the negatively charged silica intermediates onto the positively charged surface of peptide nanotubes drove the templating process, tuning of such an interaction by adjusting the solution conditions (such as pH) affected silica morphological structures. Silica tended to deposit along the exterior surface of the template at undersaturation over weak acidic and neutral pH ranges, while silica intermediates overcame diffusion resistance and moved inside the tubular template over mild basic pH ranges, enabling silica precipitation along the interior surface. This work has thus demonstrated that the morphological nanostructures of silica can be controlled by adjusting the silicification conditions (such as peptide concentration and solution pH) under an ambient environment, thus avoiding harsh chemicals or extreme reaction conditions.
By manipulating the interfacial interactions between the peptide templates and the silicate species derived from TEOS and APTES, a facile biomimetic method was developed for the fabrication of silica nanostructures exhibiting "string-of-beads" and fibrillar morphologies of varied sizes.
Artificial synthesis of silica under benign conditions is usually achieved by using cationic organic matrices as templates while the anionic analogues have not received enough consideration, albeit they are also functioning in biosilica formation. In this work, we report the design and self-assembly of an anionic peptide amphiphile (I3E) and the use of its self-assemblies as templates to synthesize 1D silica nanostructures with tunable sizes. We show that short I3E readily formed long nanofibrils in aqueous solution via a hierarchical self-assembly process. By using APTES and TEOS as silica precursors, we found that the I3E nanofibrils templated the production of silica nanotubes with a wide size distribution, in which the silica size regulation was achieved by tuning the interactions among the peptide template and silicon species. These results clearly illustrate a facile method for generating silica nanomaterials based on anionic matrices.
Cationic amphiphilic peptides are highly similar to native silaffins and silicateins for biosilicification in terms of their composition, amphiphilicity, and self-assembling propensity. To understand the relationship between organic molecular structures, molecular self-assembly and silica morphogenesis during biosilicification, we have prepared a series of short self-assembling peptide amphiphiles (I3-5K, I4K2, I3-4R, and I4R2) and investigated their capability to mediate silicification under ambient conditions. I3K self-assembled into tubular nanofibrils while I4K1-2 and I5K formed solid nanofibrils in aqueous solution with their outer diameters decreasing as the number of hydrophobic or hydrophilic amino acid residues increased. Changes in molecular structure thus altered their self-assembled geometries, and the exposed surfaces and surface lysine densities under different geometries then played different mediating roles in silicification, leading to different silica deposition patterns and final silica nanostructures. The templating capacity was weakened or lost when lysine was replaced by arginine, despite the fact that I3-4R and I4R2 self-assembled into nanofibrils and nanoribbons under similar conditions.
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