Bioinspired Approach to Silica Nanoparticle Synthesis Using Amine-Containing Block Copoly(vinyl ethers): Realizing Controlled Anisotropy

Cationic polymeric template-mediated preparation of silica nanocomposites

Biosilicification allows the formation of complex and delicate biogenic silica in near-neutral solutions under ambient conditions. Studies have revealed that, during biosilicification, basic amino acid residues and long-chain polyamines of organic substrates interact electrostatically with negatively charged silicate precursors in solution, catalyzing the polycondensation of silicic acid and accelerating the formation of silica. This mechanism has inspired researchers to explore polymers bearing chemical similarity with these organic matrices as cationic templates for biomimetic silicification. Such templates can be classified into two general categories based on the physical forms applied. One is a solution of water-soluble cationic polymers, either natural or synthetic, used as is for silicification. The other category includes various microscopically shaped entities made of cationic polymer-containing molecules, in the form of micelles, vesicles, crystalline aggregates, latex particles, and microgels. Combined with controlled polymerization and other techniques, these preorganized templates can be tailor designed in terms of sizes and morphologies to allow further expansion of properties and functions. In this review, notable research progress for both categories of silicification under biomimetic conditions is discussed. With the merits of silica and cationic polymers seamlessly integrated, the potential of such versatile nanocomposites in biomedical as well as energy and environmental applications is also briefly highlighted.

Silicification of Amine-Epoxide Cationic Microgels: An In Vitro Investigation

Herein, the applicability of an unconventional, non-vinylic type of amine-epoxide microgels (MGs) to promote silica deposition from tetraethyl orthosilicate (TEOS) was investigated. Simply mixing MGs with TEOS in water at 25 °C resulted in the formation of hybrid silica-MG particles (sMGs) as a function of silicification time. The sMGs were cationic with thermal-sensitive swelling capability. Extending silicification time to 24 h was shown to increase silica content to 43%. Besides, the sMGs became structurally more rigid to resist drying-induced deformation and exhibited a rugged surface texture. Mechanistically, the aminated nature of the MGs was proved beneficial for the success of their silicification, fulfilling dual functions of the catalyst for TEOS hydrolysis and template for silica deposition. Through electrostatic adsorption, the sMGs provided a facile yet robust option for surface modifications toward bone-related applications. Surface-induced mineralization in simulated biological fluids was observed with sMG-immobilized surfaces, where the presence of hydroxyapatite was characterized in the deposited apatite. In vitro MC3T3-E1 pre-osteoblast cell studies showed that cell adhesion, morphology, and proliferation could be influenced by both sMG types and their adsorption density. Of particular interest is the finding of cells exhibiting elongated and greatly polarized morphology on the surface with high adsorption density of sMGs of 43% silica. It was postulated that the rugged appearance of such sMGs could have presented a hierarchically structured surface toward cells, an interesting aspect to be further exploited for the engineering of cell-surface interactions.

Bioinspired Scaffolding by Supramolecular Amines Allows the Formation of One- and Two-Dimensional Silica Superstructures

Silica materials attract an increasing amount of interest in (fundamental) research, and find applications in, for example, sensing, catalysis, and drug delivery. As the properties of these (nano)materials not only depend on their chemistry but also their size, shape, and surface area, the controllable synthesis of silica is essential for tailoring the materials to specific applications. Advantageously, bioinspired routes for silica production are environmentally friendly and straightforward since the formation process is spontaneous and proceeds under mild conditions. These strategies mostly employ amine-bearing phosphorylated (bio)polymers. In this work, we expand this principle to supramolecular polymers based on the water-soluble cationic cyanine dye Pinacyanol acetate. Upon assembly in water, these dye molecules form large, polyaminated, supramolecular fibers. The surfaces of these fibers can be used as a scaffold for the condensation of silicic acid. Control over the ionic strength, dye concentration, and silicic acid saturation yielded silica fibers with a diameter of 25 nm and a single, 4 nm pore. Unexpectedly, other unusual superstructures, namely, nummulites and spherulites, are also observed depending on the ionic strength and dye concentration. Transmission and scanning electron microscopy (TEM and SEM) showed that these superstructures are formed by aligned silica fibers. Close examination of the dye scaffold prior silicification using small-angle X-ray scattering (SAXS), and UV/Vis spectroscopy revealed minor influence of the ionic strength and dye concentration on the morphology of the supramolecular scaffold. Total internal reflection fluorescence (TIRF) during silicification unraveled that if the reaction is kept under static conditions, only silica fibers are obtained. Experiments performed on the dye scaffold and silica superstructures evidenced that the marked structural diversity originates from the arrangement of silica/dye fibers. Under these mild conditions, external force fields can profoundly influence the morphology of the produced silica.