Lyubomir Aleksandrov

Preparation and characterization of SiO2/CMC/Ag hybrids with antibacterial properties

Amorphous hybrids based on sodium salt of carboxymethyl cellulose (CMC) and tetraethoxysilane (TEOS) containing silver nanoparticles were prepared by sol-gel method. The amorphous structure, morphology and antibacterial behavior were clarified. The thermal stability of obtained hybrids decreased with the increase in silver content from 0.5 to 1.5 wt%. Infrared spectra of the material suggest that the main interaction between the cellulose ether and silica network is via hydrogen bonds (bands at approximately 3,540 and 3,625 cm(-1)). According to UV-vis spectra the silver is present in two different states Ag(+) (absorption band at approximately 210 nm) and Ag(0) (band at approximately 300 nm). The different sizes of silver particles are present as clusters. It was demonstrated that these hybrids have a well pronounced antibacterial activity against B. subtilis and E. coli K12. Even the hybrid with 0.5 wt% Ag has efficient antibacterial activity for both Gram-positive and Gram-negative bacteria.

Glass Formation and Structure of the Glasses in the MoO3-Nd2O3-Bi2O3 System

Low melting glasses in the MoO3-Nd2O3-Bi2O3 system were obtained at slow (102 K/s) and high cooling rates (104-105 K/s). The amorphous state of the samples was proved by X-ray diffraction. Comparative analysis of the infrared spectra of the obtained glasses and other available molybdate spectral data was carried out. The structure of glasses with a high MoO3 content was found to consist of corner shared MoO6 units. The increasing of Nd2O3 and Bi2O3 lead to partial transformation of MoO6 to MoO4 units, due to breaking of Mo-O-Mo linkage.

Glass Formation in the MoO3-La2O3-Nd2O3 System

Novel low melting glasses in the MoO3-La2O3-Nd2O3 system were obtained at different cooling rates (102 K/s and 104-105 K/s). Characterization of the amorphous samples was made by differential thermal analysis (DTA) and X-ray absorption fine structure (XAFS) method. According to DTA data of the glass samples, the glass transition temperatures are at 325-330 0C, the crystallization started above 410 0C and the melting temperatures are at 660-720 0C. A structural model of glasses was suggested on the basis of XAFS and IR investigations. It was shown that the predominant structural units in the amorphous network of glasses containing 90 -80 mol% MoO3 are MoO6 groups. The appearance of MoO4 groups deteriorates the glass formation ability.

Synthesis and characterization of pectin/SiO 2 hybrid materials

Amorphous hybrid materials with participation of water soluble biopolymer (pectin from apple-AP) and silica derived from tetraethoxy silane (TEOS) were synthesized by sol–gel method. The influence of biopolymer content was determined. The principles of the hybrids formation were developed using structural data from Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Differential thermal analysis (DTA/TG), and 13C-Nuclear magnetic resonance (13C-NMR) analysis. The most probable model of interaction between pectin and silica network in the hybrid materials were proposed. It has been found that the reaction between the organic and the inorganic components was performed by depolymerization of the intermolecular H-bonds. The formation of new H-bonds between silica and pectin was supposed.Graphical abstract

Nd2(WO4)3

The title compound, dineodymium(III) tris[tungstate(VI)], is a member of the Eu2(WO4)3 structure family and crystallizes isotypically with other rare earth tungstates and molybdates of this formula type. The structure is a derivative of the scheelite (CaWO4) structure and can be considered as an ordered defect variant with a threefold scheelite supercell and one rare earth (RE) site unoccupied. The Nd3+ cations are coordinated by eight O atoms in form of a distorted bicapped trigonal prism. The two unique W cations are tetrahedrally surrounded by O atoms. One WO4 tetrahedron has 2 symmetry and is relatively undistorted whereas the other tetrahedron differs considerably from an ideal geometry. This is caused by an additional remote O atom at a distance of 2.149u2005(4)u2005Å. The resulting WO4 + 1 polyhedra form W2O8 dimers through edge-sharing. Together with the WO4 and NdO8 units, the three-dimensional set-up is accomplished.

Laser-assisted fabrication of gold nanoparticle-composed structures embedded in borosilicate glass

We present results on laser-assisted formation of two- and three-dimensional structures comprised of gold nanoparticles in glass. The sample material was gold-ion-doped borosilicate glass prepared by conventional melt quenching. The nanoparticle growth technique consisted of two steps – laser-induced defect formation and annealing. The first step was realized by irradiating the glass by nanosecond and femtosecond laser pulses over a wide range of fluences and number of applied pulses. The irradiation by nanosecond laser pulses (emitted by a Nd:YAG laser system) induced defect formation, expressed by brown coloration of the glass sample, only at a wavelength of 266 nm. At 355, 532 and 1064 nm, no coloration of the sample was observed. The femtosecond laser irradiation at 800 nm also induced defects, again observed as brown coloration. The absorbance spectra indicated that this coloration was related to the formation of oxygen deficiency defects. After annealing, the color of the irradiated areas changed to pink, with a corresponding well-defined peak in the absorbance spectrum. We relate this effect to the formation of gold nanoparticles with optical properties defined by plasmon excitation. Their presence was confirmed by high-resolution TEM analysis. No nanoparticle formation was observed in the samples irradiated by nanosecond pulses at 355, 532 and 1064 nm. The optical properties of the irradiated areas were found to depend on the laser processing parameters; these properties were studied based on Mie theory, which was also used to correlate the experimental optical spectra and the characteristics of the nanoparticles formed. We also discuss the influence of the processing conditions on the characteristics of the particles formed and the mechanism of their formation and demonstrate the fabrication of structures composed of nanoparticles inside the glass sample. This technique can be used for the preparation of 3D nanoparticle systems embedded in transparent materials with potential applications in the design of new optical components, such as metamaterials and in plasmonics.

Phase Transformation of RGO/SiO2 Nanocomposites Prepared by the Sol-Gel Technique

In a previous paper (Shalaby A, Yaneva V, Staneva A, Aleksandrov L, Iordanova R, Dimitriev Y, Nanoscience & nanotechnology – nanostructured materials application and innovation transfer (14), ISSN 1313-8995, 2014) we studied reduced graphene oxide (RGO)/SiO2 composite material by adding a small amount of RGO to silica in order to avoid the aggregation process and to solve the problems connected with the exfoliation and distribution of the sheets inside the composites. But from a practical point we needed to study the effect of high amounts of RGO on the composites at different temperatures. The purpose of this investigation is to study the effect of RGO on phase transformations of the composites heated at 200, 400 and 800 °C. The sol-Gel method was used to obtain the RGO/SiO2 composite by mixing high amounts of RGO with tetraethyl orthosilicate (TEOS). Data are presented for the transformation of the nanocomposites with increasing temperature in air atmosphere. RGO nanosheets were prepared by chemical exfoliation of purified natural graphite using the Hummers and Offeman method (Hummers WS, Offeman RE, J Am Chem Soc 80:1339, 1958) to obtain graphite oxide. Then the material was exfoliated to reduced graphene nanosheets by ultrasonication and reduction process using sodium borohydride (NaBH4). Characterization of the material was performed by X-ray powder diffraction (XRD), infrared (IR) spectroscopy and scanning electron microscopy (SEM) analysis. In our previous studies we found that all samples with small amounts of RGO are amorphous up to 800 °C. By increasing the amount of RGO (20 %) crystal phases appear at 200, 400 and 800 °C. The carbon phases disappear above 400 °C and cristobalite is found at 800 °C. From the IR spectra it was established that the band related to Si-OH vibration is converted to a small shoulder at 400 °C, and the bands corresponding to vibrations of water molecules around 3,448 and 1,635 cm−1 were drastically reduced by increasing the temperature. The dominant band at 1,099.2 cm−1 (800 °C) is connected with the stretching vibration of Si-O bonds in SiO4 tetrahedrons. SEM images of RGO/SiO2 nanocomposites present randomly aggregated stacks of small sheets imbedded in an amorphous matrix.