Vol 12, No 1-2 (2017)

Investigations of structure formation in linear and cross-linked polymer systems including one based on xanthan, an exopolysaccharide, attract theoretical and practical interest because of their widespread applications. Freeze-fracture transmission electron microscopy is used to study the structural topology formed in aqueous xanthan solutions and xanthan hydrogels. The data enable us to visualize for the first time an intact structure of xanthan formed in dilute and semidilute solutions at concentrations ranging from 0.002 to 0.5 wt %. In addition to single macromolecules, the dilute xanthan solutions are shown to contain microgels. When the concentration grows to values close to an overlap concentration (C*), a weak gel structure appears, being composed by a continuous 3D network with relatively large meshes. Macromolecular aggregation is observed in the network skeleton when switching to semidilute entangled solutions (>C**); it is accompanied by a compaction of network structure. The cross-linking of polymer chains by polyvalent Cr³⁺ ions gives a network skeleton composed of aggregated macromolecules at lower concentrations (around C*). Macromolecular aggregation in the skeleton of a network polymer structure occurring both in the absence and in presence of Cr³⁺ cations is indicative of a microphase separation into polymer-poor and polymer-rich regions.

The peculiarities of the structure and catalytic behavior of nickel nanoparticles deposited onto an Al₂O₃ surface by laser electrodispersion (LED) with subsequent activation in carbon monoxide atmosphere has been considered. The reduction of these nanoparticles by in situ treatment in a catalytic cell in Ar + 5% Н2 atmosphere at 150–450°C has been studied using X-ray photoelectron spectroscopy (XPS). It is shown that formation of metal nickel starts by reduction in hydrogen at 300°C. A comparison of catalytic activity of the Ni/Al₂O₃ systems in the catalytic oxidation of CO is carried out. It is found that the preliminary treatment of Ni/Al₂O₃ sample by carbon monoxide leads to an increase in the catalyst efficiency and decrease in the reaction temperature by 50–100°C.

Copper-based nanocomposite pseudoalloys with a chromium content of 25 to 45 wt % have been obtained via the spark plasma sintering. The treatment of powder blends with various chromium contents in the planetary mills is found to cause pronounced changes in the electrical and mechanical properties of materials. The electric resistance of materials with the increasing amount of chromium exhibits the nonlinear rise. This dependence is also correlated with the size of sintered granules, unlike the dependence of hardness, which linearly increases with the increasing chromium concentration (i.e., with the enlarging interfaces).

In this work we use the method of direct transfer of graphene grown by chemical vapor deposition (CVD) onto a transparent polymer substrate for obtaining a transparent conducting (TC) film. In order to implement this, we use a standard laminator and a film for lamination. A copper foil with the CVD graphene is glued by the laminator to the polymer film. After copper etching, a TC film with improved electrical and optical characteristics is obtained.

The effect of ionic strength on spectral properties of negatively charged semiconductor (CdSe/ZnS) nanocrystals (quantum dots, QDs) and polycationic aluminum phthalocyanines (PCs) is considered. A QD/PC complex, formed via self-assembly, remains stable throughout a wide range of ionic strength values of a solution and [PC]/[QD] concentration ratio. The efficiency of nonradiative energy transfer from QDs to PCs rises with an increase in the ionic strength of solution. The fluorescence amplification factor of PC reduces with an increase in number of PC molecules in a complex with a quantum dot, reaching negative values at high [PC]/[QD] ratios. This is probably due to the decrease in the effect of energy migration on the total PC fluorescence upon its own significant absorption ability of a large number of acceptors. These effects are of interest to develop selection principles of components for hybrid complexes stabilized with electrostatic interaction.

Covalently linked hybrid structures of semiconductor nanocrystals (CdSe/ZnS quantum dots) as an inorganic component with a fluorescence maximum at 620 nm and photosynthetic protein allophycocyanin (APC) as an organic part are created. It is found out that CdSe/ZnS quantum dots form stable complexes with APC through covalent bonding in aqueous solutions. It is shown that the efficiency of electronic excitation energy transfer (EET) in such systems may be significantly enhanced under the conditions at which the monomerization of allophycocyanin trimers occurs. In this paper the EET efficiency is evaluated under differing experimental conditions (pH, temperature, and presence of NaSCN) for hybrid systems obtained by the self-assembling of components via electrostatic interactions, as well as via covalent linking. Under the most optimal conditions, there is a 20-fold increase in the APC fluorescence after the excitation of QDs due to the more efficient EET for the covalently linked components compared to the complexes obtained by the self-assemblage. The obtained covalently linked hybrid structures unfold new opportunities for their practical use as fluorescent markers, hybrid photosensors, and structural elements in photovoltaic devices.

The addition of 20-nm gold and silver nanoparticles to a growth medium has a positive effect on the growth of biomass of a cell suspension culture of Arabidopsis thaliana (L.) Heynh. Growing the cells with silver and gold nanoparticles causes different changes in the pH of the medium: the culture medium is made markedly acidic by the addition of silver nanoparticles and alkaline by the addition of gold nanoparticles. As is found by the MTT assay, metallic nanoparticles are responsible for a slight but steady decrease in the specific respiratory activity of cells of the A. thaliana suspension culture. Growth with the nanoparticles is accompanied by an increase in the intracellular free amino acid pool (alanine, γ-aminobutyric acid, and valine), which is characteristic of responses to abiogenic stresses. Furthermore, the addition of the nanoparticles changes the composition of extracellular proteins in the A. thaliana cell culture.