Browsing by Author "Aleksandrov L."
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Item ANTIBACTERIAL ACTIVITY AND STRUCTURE OF SOL-GEL BASED SiO2/HPC/Zn NANOCOMPOSITES(2020-01-01) Angelova T.; Rangelova N.; Aleksandrov L.; Georgieva N.The present paper reports an investigation of the antibacterial activity of sol-gel derived silica/hydroxypropyl cellulose zinc doped nanocomposites. The thermal analysis shows that the crosslinking between cellulose ether and zinc ions is more thermally stable than the crosslinking of the precursors in the matrix. The optical investigation and the transmission electron microscopy (TEM) observation verify the existence of nano-dimentional zinc particles. The antibacterial properties of the composites are investigated by measuring the inhibition zones formed around the materials of a different zinc content in presence of reference strains of Gram-positive bacteria - Bacillus subtilis 3562 and Gram-negative bacteria - Escherichia coli K12 and by calculating the growth decrease percentage.Item Enzymatic decolorization of dyes by laccase immobilized on hybrid carriers(2018-01-01) Yaneva S.; Rangelova N.; Aleksandrov L.; Danalev D.The study focused on sol-gel synthesis of biopolymer-silica hybrids further used as materials for laccase immobilization. Silica was obtained from tetraethyl orthosilicate (TEOS), while pectin, derived from citrus fruits, was used as a biopolymer. The structure of the hybrids obtained was investigated by X-ray diffraction, Fourier transform infrared spectroscopy, Scanning electron microscopy and BET analysis. The results provided to conclude that the interaction between SiO2 network and the polysaccharide proceeding via H-bonds formation was successful. Laccase was immobilized by cross-linking, while the degradation ability was tested against several dyes - Rhodamine B, Methyl orange and Malachite green. The most effective decolorization was reached for Malachite green and Methyl orange.Item Eu3+ doped glasses in the Bi2O3-B2O3-WO3-Al2O3 system(2015-01-01) Aleksandrov L.; Iordanova R.; Khanna A.; Georgiev N.; Dimitriev Y.The structure of many Eu3+ doped Bi2O3-B2O3-WO3-Al2O3 glasses of a different composition is studied using infrared spectroscopic technique (IR). It is found that distorted BiO6 (of a band at 480 cm-1), BO3 (of bands at 1390, 1230, 620 cm-1), BO4 (of a band at 1020 cm-1) and WO4 polyhedra (of a band at 880 cm-1) are the main building units forming the amorphous networks. WO6 groups exist in the glass network of a high WO3 content (of bands at 880 cm-1, 620 cm-1, and a shoulder at 670 cm-1). WO3 does not stimulate formation of BO4 structural units. According to the differential thermal analysis (DTA) the glass transition temperature (Tg) of the investigated glasses is situated in the range of 400°C - 485°C. The DTA curves of all glasses show two exothermic peaks corresponding to the formation of two crystal phases. Photoluminescence emissions due to the 4f transitions 5D0→7Fj (j=0-4) of Eu3+ ions are observed. The addition of WO3 to the glass compositions decreases the emission intensity of investigated glasses.Item Glass formation and structure of glasses in the ZnO-WO3-La2O3-Al2O3 system(2015-01-01) Ataalla M.; Milanova M.; Aleksandrov L.; Iordanova R.; Staneva A.; Dimitriev Y.The glass formation region in the section 90(xZnO.yWO3.zLa2O3).10Al2O3 of a constant Al2O3 content of 10 mol % and varying amounts of WO3, ZnO and Nd2O3 from ZnO-WO3-La2O3-Al2O3 system is determined by the melt quenching technique (with cooling rates of 10 Ks-1 - 102 Ks-1). The glasses are obtained from compositions of a high WO3 content (60 mol % - 75 mol %). The amorphous state of the samples is verified by X-ray diffraction (XRD). The thermal parameters of the investigated glasses are obtained by differential thermal analysis (DTA). The glass transition temperature (Tx) is in the range 440°C - 510°C, while the crystallization temperature (Tg) varies from 520°C to 650°C. The glasses are characterized by a good thermal stability in respect to crystallization. ΔT varies in the range of 60°C - 135°C (ΔT = Tx-Tg). The glasses structure is studied by IR spectroscopy. It is established that the amorphous network is built up mainly by WO6 octahedra connected through W-O-W bridges.Item Preparation, Characterization, and Antibacterial Properties of Cu-Fibreboards(2023-11-01) Aleksandrov L.; Rangelova N.; Lazarova-Zdravkova N.; Georgieva N.; Dragnevska M.; Nenkova S.In the present study, copper modified fibreboards were prepared and their existing phase, morphology, and antibacterial behaviour were investigated. The copper content and the physical and mechanical properties of fibreboards (thickness, bending strength, and swelling) were determined. X-ray diffraction analysis (XRD) showing diffraction peaks typical for cellulose, Cu2S, and Na2SO4, depended on the preparation conditions. The average size of the Cu2S crystals varied between 20 and 50 nm. The morphology of the obtained fibreboards, as well as the size and shape of copper particles, were observed by scanning electron microscopy (SEM) and transition electron microscopy (TEM). The antibacterial activity was tested against Gram-positive (Bacillus subtilis 3562) and Gram-negative (Escherichia coli K12 407) bacteria. The tests showed that the materials had higher antibacterial activity against E. coli, which depended on their preparation conditions. Based on these results, the obtained copper fibreboards can be used as antibacterial agents in the packaging and building industry.Item Synthesis and characterization of glass doped reduced graphene oxide(2015-01-01) Shalaby A.; Aleksandrov L.; Iordanova R.; Staneva A.; Dimitriev Y.Graphene is very attractive material which has high conductivity and good electrical properties. By using glass as a matrix to incorporate the graphene, it is possible to improve the conductivity and the electrical properties of the glass. In our previous study we synthesized composite material by adding of reduced graphene oxide (RGO) to silica. In this work we continue our experiments to synthesize a composite material on a glass basis in the PbO-ZnO-B2O3 system. The differential thermal analysis of the present glass showed that the glass transition temperature (Tg) is situated at ca 415°C. The synthesized glass was doped with 20 % of reduced graphene oxide (RGO). A mixture of glass with RGO was heated at different temperature values for different time taking into consideration the glass transition temperature. The characterization is performed by X-ray phase analysis, DTA and SEM analysis. It was found according to the XRD results that graphene was kept inside the glass matrix till 600°C.Item Synthesis of amorphous and crystalline LaBWO6 using mechanochemical activation(2015-01-01) Gancheva M.; Aleksandrov L.; Iordanova R.; Dimitriev Y.This article presents data referring to the mechanochemical activated synthesis of amorphous and crystalline LaBWO6. A stoichiometric mixture of La2O3, B2O3 and WO3 of a 1:1:2 molar ratio was subjected to intense mechanical treatment in air using a planetary ball mill (Fritsch - Premium line - Pulversette No. 7). The obtained samples were analyzed by XRD, DTA, IR and UV-Vis spectroscopy. The process of mechanochemical amorphization was accomplished within 20 h at a milling speed of 500 rpm at room temperature. An additional heat-treatment of the mechanochemically obtained amorphous phase was carried out in order to obtain single crystal LaBWO6 phase. The short range order of the amorphous network was defined by BO3, BO4 and WO4 structural units. Glassy LaBWO6 was obtained by us with the application of the melt quenching method prior to these experiments. The infrared and optical spectra showed that WO4 units were more distorted in the structure of the glassy LaBWO6 when compared to those of both amorphous phases. The crystallization of LaBWO6 was achieved after thermal treatment at 690°C of the mechanochemically obtained amorphous and glass composition.