Biomimetic Sol-Gel Chemistry to Tailor Structure, Properties, and Functionality of Bionanocomposites by Biopolymers and Cells

Biosilica, synthesized annually only by diatoms, is almost 1000 times more abundant than industrial silica. Biosilicification occurs at a high rate, although the concentration of silicic acid in natural waters is ~100 μM. It occurs in neutral aqueous solutions, at ambient temperature, and under the control of proteins that determine the formation of hierarchically organized structures. Using diatoms as an example, the fundamental differences between biosilicification and traditional sol-gel technology, which is performed with the addition of acid/alkali, organic solvents and heating, have been identified. The conditions are harsh for the biomaterial, as they cause protein denaturation and cell death. Numerous attempts are being made to bring sol-gel technology closer to biomineralization processes. Biomimetic synthesis must be conducted at physiological pH, room temperature, and without the addition of organic solvents. To date, significant progress has been made in approaching these requirements. The review presents a critical analysis of the approaches proposed to date for the silicification of biomacromolecules and cells, the formation of bionanocomposites with controlled structure, porosity, and functionality determined by the biomaterial. They demonstrated the broad capabilities and prospects of biomimetic methods for creating optical and photonic materials, adsorbents, catalysts and biocatalysts, sensors and biosensors, and biomaterials for biomedicine.

Tannic Acid as a Versatile Template for Silica Monoliths Engineering with Catalytic Gold and Silver Nanoparticles

Tannic acid in alkaline solutions in which sol-gel synthesis is usually performed with tetraethoxysilane is susceptible to various modifications, including formation of reactive radicals, oxidation under the action of atmospheric oxygen, self-association, and self-polymerization. Here, a precursor with ethylene glycol residues instead of ethanol was used, which made it possible to synthesize bionanocomposites of tannic acid and silica in one stage in neutral media under normal conditions without the addition of acid/alkali and organic solvents. Silica was fabricated in the form of optically transparent monoliths of various shapes with 2-4 nm pores, the radius of which well correlated with the size of a tannic acid macromolecule in a non-aggregated state. Polyphenol, which was remained in pores of silica matrix, served then as reducing agent to synthesize in situ gold and silver nanoparticles. As shown, these Au@SiO₂ and Ag@SiO₂ nanocomposites possessed localized surface plasmon resonance and high catalytic activity.

Dimensionally stable cellulosic aerogels functionalized by titania

The study is aimed at imparting dimensional stability and some functionalities to cellulosic aerogels. The polysaccharide suffers from mechanical strength loss in wetted state that restricts its application. Improvement is achieved by mean of microfibrillation of cellulosic fibers combining intense mechanical treatment with freeze-thawing. Addition of the latter decreases the number of cycles. Aerogels prepared from microfibrillated cellulose by freeze-drying hold their dimensional stability in solutions that makes possible treating them chemically without loss in shape. Here a method of directional sol-gel processing is applied to mineralize such aerogels by titania. Owing to covalent bonds to cellulose macromolecules formed via the condensation reactions, titania coating possesses good adhesion, not separating at heating when it is transferred in anatase form. Its photocatalytic activity results in self-cleaning of cellulose aerogels under outdoor sunlight irradiation. Calcination in air or carbonization in an inert gas atmosphere can serve to prepare metal oxide or composites with carbon of various shape and dimensionality.

Chitosan Hydrogels and Bionanocomposites Formed through the Mineralization and Regulated Charging

The account presents survey of our systematic studies on chitosan. Only this polysaccharide bears cationic charges, possesses antimicrobial activity and wound healing ability that make it highly appropriate for using in medicine, biomedical engineering, cosmetics, food, packaging. However, its application meets with severe limitation. Chitosan belongs to polysaccharides that do not jellify solutions. Main approaches are based on the chemical modifications and cross-linking, but these treatments impairs therewith the biocompatibility and biological activity of chitosan. We have developed approaches in which monolithic hydrogels are fabricated via the mineralization of polysaccharide by method of green sol-gel chemistry and via the formation of polyelectrolyte complex with oppositely charged counterparts in the regime of its charging by means of regulated acidification. The latter approach was also extended for the preparation of chitosan bionanocomposites and films with nanoparticles.

Bionanocomposite from self-assembled building blocks of nacre-like crystalline polymorph of chitosan with clay nanoplatelets

Films containing a new crystalline polymorph are prepared by a one-pot technique combining the formation of building blocks of clay nanoplatelets with chitosan macromolecules and their evaporation-induced self-assembly.

Chitosan bionanocomposites prepared in the self-organized regime

Bionanocomposites in the self-organized regime are prepared when chitosan is gradually charged in the course of progressive change of pH by hydrolyzing D-glucono-δ-lactone in solutions of nanoparticles bearing negative charges on their surface. This novel approach is applicable to the formation of monolithic hydrogels and films. Here bionanocomposites of chitosan with clay nanoparticles of saponite and sepiolite having various geometry and with oxidized multiwall carbon nanotubes are considered. Structural organization of hydrogels and films is studied by scanning and transmission electron microscopy as well as small angle X-ray scattering. Jellification is caused by generation of three-dimensional network from fibrils, whereas films have pronounced stratified layer (nacre-like) structure from stacked nanoparticles and aligned chitosan macromolecules. Special attention is paid to mechanical properties of films that are improved drastically with introducing nanoparticles.

Titania synthesized through regulated mineralization of cellulose and its photocatalytic activity

Cellulose mineralization by titania is brought under control via restricted hydration of fibrils that provides precise localization of the fast hydration/condensation reactions proceeding in situ.

Bionanocomposites: Green sustainable materials for the near future

Bionanocomposites are a novel class of nanosized materials. They contain the constituent of biological origin and particles with at least one dimension in the range of 1–100 nm. There are similarities with nanocomposites but also fundamental differences in the methods of preparation, properties, functionalities, biodegradability, biocompatibility, and applications. The article includes two parts. Bionanocomposite definition and classification along with nanoparticles, biomaterials, and methods of their preparation are initially reviewed. Then, novel approaches developed by our team are presented. The first approach concerns the preparation of bionanocomposites from chitosan and nanoparticles. It is based on the regulated charging of polysaccharide by the gradual shift of solution pH. When charges appear, the biomacromolecules come into the electrostatic interactions with negatively charged nanoparticles that cause the jellification of solutions. It is also applied to form films. They have a nacre-like structure from stacked planar nanoparticles separated by aligned biomacromolecules. The second approach deals with the biomimicking mineralization of biopolymers by using a novel silica precursor. Its advantage over the current sol-gel processing is in the compatibility and regulation of processes and structure of generated silica. Another example of the mineralization is presented by titania. Syntheses are performed in anhydrous ethylene glycol. Processes and structure of bionanocomposites are regulated by water that is added in an amount to only hydrate functional groups in the carbohydrate macromolecule.

Tailored hybrid hyperbranched polyglycidol-silica nanocomposites with high third-order nonlinearity

One of the most convenient techniques for optical material fabrication is the sol–gel processing. It can be performed at low temperature that enables one to entrap even relatively unstable organic substances into silica matrix at the nanometer scale, thus developing homogeneous hybrid organic–inorganic nanocomposite materials of various functionalities. Here, novel hybrid organic–inorganic nanocomposites with good optical transparency and high third-order nonlinearity were prepared biomimetically through the mineralization of dendritic macromolecules (hyperbranched polyglycidols) using a compatible ethylene glycol-containing silica precursor. The synthesis was performed at neutral pH media in aqueous solutions without addition of organic solvents at ambient conditions owing to the catalysis of processing. Polyglycidols provided also the formation of gold nanoparticles localized in their core. They served as reducing and stabilizing agents. It is shown that trace amounts of nanoparticles could regulate nonlinear properties of a nanocomposite. High nonlinearity manifests itself in a supercontinuum generation at remarkably short lengths ca. 1 mm. The phenomenon consists of filamentous intense white lighting due to the spectral broadening of initial ultrashort (femtosecond) laser pulses propagating through the material. The developed hybrid nanocomposites possessing large nonlinearity, high-speed optical response, stability under intense lighting, low-cost, and easy preparation are promising for a diverse range of applications as active components for all-optical signal processing from chemical sensing to biological cell imaging and lighting control in telecommunication.

Fabrication of organic shell-covered gold nanospheres with near-infrared absorption

A simple protocol is developed for the fabrication of stable organic shell-covered gold nanospheres with near-infrared absorption characteristics. The designed strategy mainly concentrates on two steps: (i) the polymerization of gold nanoparticles (AuNPs) induced by 1,2-ethanedithiol in water/ethanol; (ii) conjugation of the polymerized AuNPs with marocyclic compound for the formation of the organic shell. In the first step, the 1,2-ethanedithiol containing two thiols towards both ends of the chain enables the successful polymerization of AuNPs due to the forceful appetency of thiols to AuNPs. In the second step, the polymerized AuNPs are covered by the marocyclic compound attributing to the hydrogel-bonding effect between NH and SH. Because of the organic shell, the stability and dispersibility of the obtained nanospheres are improved. The optical properties of the fabricated gold nanospheres (400 nm average diameter) are well investigated by the UV-vis absorption spectroscopy which exhibit intense near-infrared absorption at 972 nm in acetone. The strategy developed in this study is promising in that excellent stability and dispersibility of gold nanospheres can be achieved in a facile and economic way.