Yury Shchipunov

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.

Bionanocomposites formed by in situ charged chitosan with clay

Bionanocomposites are formed through the electrostatic interactions of exfoliated clay nanoparticles initially dispersed in an aqueous solution with chitosan macromolecules that are gradually charged in the course of a progressive pH decrease by chemical acidulating agent to exclude the phase separation.

Hydrogels formed through regulated self-organization of gradually charging chitosan in solution of xanthan

Hydrogels are prepared by means of progressive acidification of xanthan solution containing dispersed chitosan particles in noncharged state at pH a little larger than pKa of this polysaccharide. The following pH shift causes gradual charging of chitosan macromolecules that are involved into electrostatic interactions with oppositely charged xanthan. This triggered self-regulating processes of polysaccharide self-organization into supramolecular structures. Fibrillation or capsulation was observed that depended on the charge mixing ratio between chitosan and xanthan. The jellification of solutions took place in a case of fibrillar morphology. The strongest hydrogels were found where a fine and dense network of aligning fibrils of mean diameter of around 30 nm were generated. Where oppositely charged polysaccharides self-organized into capsules, the hydrogel was not formed because of the absence of cross-links between particles that could reach few mm in diameter and had mechanically strong shell.

Synthesis of alginate based silver nanocomposite hydrogels for biomedical applications

Sodium alginate and poly(acrylamide-co-N-vinylcaprolactam-co-acrylamidoglycolic acid) based dual responsive semi-IPN hydrogels (SA-PAVA) were successfully synthesized by free radical redox polymerization. N, N′-Methylene-bis-acrylamide was used as a crosslinker and 5-fluorouracil, an anti-cancer drug, was loaded onto these semi-IPN hydrogels via equilibrium swelling method. The hydrogels were also used as templates for the production of silver nanoparticles by using NaBH4 as reducing agent. In order to understand the polymer-drug interactions, pristine, as well as drug loaded, SA-PAVA hydrogels were characterized by Fourier transform infrared spectroscopy and differential scanning calorimetry. The formation of silver nanoparticles was confirmed by UV-visible spectroscopy, thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The swelling behavior of the hydrogel was investigated in distilled water under various pH and temperature conditions. In vitro release of 5-fluorouracil from these SA-PAVA hydrogels was carried out in gastro-intestinal fluids different temperatures. The SA-PAVA hydrogel/silver nanocomposites showed excellent anti-bacterial activity towards Escherichia coli and Bacilli.

Polymerized assemblies of cationic gemini surfactants in aqueous solution.

A variety of polymerized assemblies of cationic gemini surfactants has been demonstrated as a function of the electrolyte concentration in aqueous solution. The gemini surfactant consists of two cationic monomeric surfactants linked with an ethylene spacer at the level of the quaternary ammonium groups. Polymerizable methacryloxy groups are covalently attached to the terminal of the hydrocarbon chains. In the lower electrolyte concentration region, radical polymerization results in the formation of spherical aggregates [Langmuir 22 (2006) 8293]. However, in the higher electrolyte concentration region, elongated tubular hollow assemblies are observed with transmission electron microscopy, as a result of polymerization of vesicular hollow assemblies spontaneously formed in the aqueous solution. These experimental results suggest that it is possible to prepare different shapes of polymerized assemblies by changing the electrolyte concentration.

One-pot biomimetic synthesis of monolithic titania through mineralization of polysaccharide

To obviate the common precipitation of titania after a precursor addition into aqueous or water-containing solutions, here we are using ethylene glycol as the reaction media with dissolved polysaccharide xanthan. The water is introduced in a restricted amount to provide only the hydration of polysaccharide macromolecules. An introduced precursor is involved into instant hydrolysis and following condensation reactions where contacting with the hydrating water. This resulted in titania formation on carbohydrate macromolecules like the biomineralization in living organisms. The gelling of solution proceeded without the precipitation even when the precursor was taken in concentration of ca. 3 wt.%. Polysaccharide in these syntheses served as a template. By varying the concentrations of precursor, xanthan and water, it was possible to manipulate the metal oxide morphology. Fibrillar, particulate and plate-like structures presented that depended on the synthesizing conditions. Titania prepared at ambient conditions was amorphous. When it was calcined at temperatures between 300 and 900 degrees C, crystalline anatase and rutile were found at 300 and 700 degrees C, respectively.

Water-soluble polyelectrolyte complexes of oppositely charged polysaccharides

The article is an overview of our recent study on some particular aspects of polyelectrolyte complex (PEC) formation by oppositely charged polysaccharides when they are brought into contact in aqueous solutions. This type of complexation can lead to the thickening effect, jellification or PEC precipitation that find numerous applications in a variety of fields from the regulation of rheological characteristics of solutions to fabrication of functional materials by the layer-by-layer technique. Our focus was on the rheological aspects of water-soluble PEC formation and jellification, but to gain an insight into the mechanisms of the processes involved, atomic force microscopy, scanning electron microscopy and differential scanning calorimetry were also applied. As cationic polysaccharides, chitosan and cationic derivatives of hydroxypropylcellulose including hydrophobically modified samples were taken and, as their anionic counterparts, alginates, carrageenans, xanthans and fucoidans were used. Their combination allowed us to consider the influence of charge density, hydrophobicity and flexibility–stiffness of macromolecules on the association of oppositely charged polysaccharides, the formation of temperature sensitive hydrogels and some PEC morphological features.

Titania synthesized through regulated mineralization of cellulose and its photocatalytic activity

Cellulose used for thousands of years has been rediscovered recently as a novel smart material for various nanotechnological applications. Its insoluble fibrils are functionalized by using mineralization methods developed in nanochemistry. Here they are coated by titania synthesized in one stage by a new green approach. It consists of controlling the localization of very fast hydrolysis and condensation reactions. Cellulose fibrils are placed in ethylene glycol with such an amount of water that is absorbed entirely by the hygroscopic polysaccharide. This hydrating water works as a reaction centre when the precursor reaches it. Instant hydrolysis and following condensation reactions proceeding mainly on the fibrils provide their mineralization. Titania prepared at ambient conditions is in an amorphous state. It is transferred in crystalline forms under a variety of conditions including moderate temperature (80 °C), calcination in air and cellulose carbonization in an inert atmosphere. These treatments result in photocatalytic activity. Even cellulose treated at the moderate conditions demonstrates significant self-cleaning ability consisting of fast degradation of methylene blue under outdoor sunlight irradiation. Photocatalytic activity of carbon–titania hybrids includes a side reaction of the oxidation of the carbonized fibrils. Photocatalytic properties of some of the calcinated samples, not containing organics, were comparable with a commercial photocatalyst.

Hyperbranched polyglycerol hydrogels prepared through biomimetic mineralization.

Hyperbranched polyglycerols find increasing usage in biomedicine owing to their excellent biocompatibility like polysaccharides. To prepare hydrogels, they are cross-linked mainly by treating with toxic epoxy reagents. Here we suggest a one-stage nontoxic procedure for the jellification of aqueous solutions that was previously developed for nongelable polysaccharides. It was carried out via the biomimicking mineralization. As the silica precursor, tetrakis(2-hydroxyethyl)orthosilicate containing ethylene glycol residues was employed. It could mineralize directly hydroxyl-containing macromolecules passing a stage of the sol formation. Jellification was performed in one stage in the neutral pH region at the ambient conditions. An organic solvent was not needed because of high hydrophilicity of both the precursor and polyglycerols. An as-prepared hydrogel is ready for applications because of the absence of toxic products. Its structure and mechanical properties were characterized by scanning and transmission electron microscopy as well as dynamic rheology. It was demonstrated that hyperbranched polyglycerols were encased into silica matrix that formed three-dimensional mesoporous network. A study of initial solutions of hyperbranched polyglycerols by the dynamic light scattering revealed their aggregation. This important result was confirmed by direct observations of aggregated macromolecules with high resolution scanning electron microscopy. Entrapped aggregates were also found in the silica matrix.

Chitosan bionanocomposites prepared in the self-organized regime

Abstract 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.