Irina Postnova

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.

Self-organization in the chitosan-clay nanoparticles system regulated through polysaccharide macromolecule charging. 2. Films

Formation conditions are studied for bionanocomposite films prepared by mixing cationic chitosan with negatively charged nanoparticles of a synthetic clay (saponite) followed by gradual increasing of the charge of macromolecules by decreasing the pH of a medium. The data on the swelling of the bionanocomposite films in water are used to determine the stoichiometric ratio between the concentrations of macromolecules and nanoparticles that provides the most intense electrostatic interactions stabilizing the films. Their properties and structure are investigated by means of scanning electron microscopy, dynamic thermomechanical analysis, and small-angle X-ray scattering. The films are shown to occur in a glassy state and undergo a number of phase transitions, the temperatures of which depend on the chitosan-to-saponite concentration ratio. In particular, their glass transition temperature increases from 62 to 175°C when passing to the stoichiometric composition. The bionanocomposite films are found to have a layered structure. The layers are, in turn, composed of highly uniform microsized plates 20–30 nm thick. Small-angle X-ray scattering shows a structural order with a periodicity of 1.78 nm. The structure of the bionanocomposite films is discussed.

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.

Incorporation of Quantum Dots into a Silica Matrix Using a Compatible Precursor

A method is developed for one-stage incorporation of cadmium-sulfide quantum dots synthesized in the presence of mercaptosuccinic acid into a silica matrix formed from a precursor containing ethylene-glycol residues—tetrakis(2-hydroxyethyl) orthosilica. This precursor has not previously been applied for this purpose. It is more compatible with diverse substances than tetraethoxysilane, which is traditionally used. Moreover, it is advantageous in its unlimited solubility in water; release of ethylene glycol, which does not precipitate quantum dots, rather than alcohol upon hydrolysis; and the feasibility of performing the sol-gel process at any pH value in a range of 2–10 without the addition of acid or alkali and without heating. When the precursor (50 wt %) is added to a dispersion of quantum dots, the system is transformed into a gel in as little as a few minutes. The synthesized hybrid materials are optically transparent. Therewith, the quantum dots incorporated into the silica matrix exhibit luminescence, with their spectral characteristics remaining almost unchanged.

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.

Nonlinear optical properties and supercontinuum spectrum of titania-modified carbon quantum dots

We have studied the nonlinear optical properties and supercontinuum spectrum of solutions of carbon quantum dots prepared by a hydrothermal process from chitin and then coated with titania. The titania coating has been shown to have an activating effect on the carbon quantum dots, enhancing supercontinuum generation in the blue-violet spectral region and enabling their nonlinear optical characteristics to be varied.

Bimodal SBA-15 and polymethylsilsesquioxane monoliths with regulated mesoporous structure and macroporosity

Two glycol-containing silanes, one of which included a methyl group attached directly to the silicon atom, were applied for a one-pot sol–gel synthesis of bimodal mesoporous SBA-15 and polymethylsilsesquioxane with macropores. It was performed in aqueous solutions of block copolymer P123 without resorting to acid or alkali at ambient condition. Ordered mesoporous SBA-15 with large surface area and high pore volume was prepared by means of tetrakis(2-hydroxyethyl)orthosilicate containing four hydrolyzable ethylene glycol residues and amorphous polymethylsilsesquioxane, by using tris(2-hydroxyethoxy)methylsilane). The former was templated by a hexagonal mesophase formed from spherical micelles of block copolymer after admixing the precursor, and the latter was prepared in a system in which this transformation did not take place. When both the silica precursors were used together, one could find distortion of the periodic mesoporous structure, aggravating with increasing the content of methyl-containing silane. Furthermore, a decrease of the diameter, surface area, and mesopore volume took place, whereas the surface area of macropores was increased and their pore volume and diameter were decreased. The introduced methyl groups made silica hydrophobic. It was demonstrated that bimodal polymethylsilsesquioxane possessed fast and selective absorbability to oil spill on the water surface.

Laser-induced photodynamic effects at silica nanocomposite based on cadmium sulphide quantum dots

In this paper we study the laser-induced modification of optical properties of nanocomposite based on cadmium sulphide quantum dots encapsulated into thiomalic acid shell which were embedded into a porous silica matrix. We found red shift of luminescence of the nanocomposite when exposed to laser radiation at λ = 405 nm. Using pump-probe method and Small-Angle X-ray Scattering technique it was found that laser radiation at λ = 405 nm also increases the absorption coefficient of the nanocomposite in 15 times due to agglomeration of quantum dots. The modification of absorption properties is fully reversible.