Библиотека

2021 г.

Автори: B. Stamboliyska, S. Tapanov, E. Velcheva, D. Yancheva, M. Rogozherov, Z. Glavcheva, G. Lalev, M. Dimitrov

Резюме: The purpose of the present study is to deepen our knowledge about the painting materials and technology characteristics of iconography during the Bulgarian National Revival. Our efforts have focused on studying the murals in the catholicon (main church) “The Nativity of the Virgin” of the Rila Monastery, which is one of Bulgarian leading historical monuments. A series of micro samples from the altar mural paintings were analyzed by a combination of attenuated total reflectance Fourier transform infrared (ATR-FTIR), micro-Raman spectroscopy, scanning electron microscopy energy dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and Optical microscopy (OM). The data analysis showed that the painter’s palette is made up of nine mineral pigments: smalt, chrome yellow, emerald green, vermilion, red lead, green earths, yellow ochre, sienna and calcite. The impression of rich colorful palette was achieved by using various pigment mixtures. The presence of egg binder in many paint samples suggested that the murals were executed by the traditional egg tempera technique, with the only exception of the smalt paint of the background where carbohydrate glue is present as binder.

Списание: Spectrochimica Acta Part A, 247 (2021) 119087

Автори: S. Todorova, J.L. Blin, A. Naydenov, B. Lebeau, D. Karashanova, H. Kolev, P. Gaudin, R. Velinova, L. Vidal, L. Michelin, L. Josien, D. Filkova, I. Ivanova, A. Dotzeva, K. Tenchev

Резюме: The hierarchical macro-mesoporous silica (MMS) was used for a first time as a support for catalysts for oxidation reactions. The macro-mesoporous silica was synthesized by the emulsions templating mechanism and modified separately or simultaneously using cobalt and manganese oxides. The obtained materials were characterized by different physicochemical methods and tested in the oxidation of CO and n-hexane combustion reactions. The modification of the MMS materials does not change significantly the mesopores characteristics; however, its pores are partially blocked by the oxides. For Co-MM sample agglomerates consisting of Co3O4 with average size of 100−150 nm and small spherical aggregates, encapsulated in the mesopores are formed. The amorphous manganese oxide preferentially fills up the mesopores in Mn-MM sample. Mixed oxide Co-Mn phases situated in the mesoporous network are formed in the bi-component Co-Mn samples. No significant change is observed either in the texture, or in the structural features of the catalysts after reaction.

The highest catalytic activity for Co-MM sample in CO and n-hexane oxidation is related to the predomination of Co3+ species on the surface of Co3O4 and the more accessible oxide particles located outside the mesopores. The encapsulation of mixed Co-Mn oxides particles in the pores of the macro-mesoporous silica is responsible for a lower catalytic activity in comparison with that of the mono-component cobalt sample.

Списание: Spectrochimica Acta Part A, 247 (2021) 119087 Catalysis Today, 2020

Автори: V. Petkov, S. Simeonova, M. Kandeva, R. Valov

Резюме: The objective of this study was to deposit directly chromium with diamond nanoparticles (ND) on aluminum alloys and investigate the coating surface. The chromium coatings on aluminum alloys were obtained by electrochemical deposition. The coatings were doped with ND. The diamond nanoparticles were obtained by detonation synthesis. Chromium coatings were deposited on aluminum alloys with a silicon content of 7 % and 10 %. The ND concentration in the electrolyte was 25 g/l. The surface analysis was performed by means ofAtomic force microscopy. The surface of the coating of chromium with ND on Al10Si is twice more even than that on Al7Si. The microstructure and microhardness were examined with a metallographic microscope and a microhardness tester. The microhardness of the coated samples is 9163 MPa compared to 893 MPa of uncoated aluminum samples. The thickness of the chromium coatings doped with diamond nanoparticles is between 45 – 55 μm. The coatings are dense, continuous and uniform with good adhesion to the substrate material.

Списание: Списание Archives of Foundry Engineering, 20(4):115-120 2020

Автори: Yulian Zagranyarski, Monika Mutovska, Petia Petrova, Reni Tomova, Petar Ivanov, Stanimir Stoyanov

Резюме: A synthetic approach for preparation of a new class of 1,8-naphthalimide derivatives with fused benzo and naphtho-dioxin systems is reported. The proposed reaction conditions for the first step allow selective bromination of 1,8-naphthalic anhydride to the corresponding 3,4,6-tribromo derivative, which has the potential to be versatile building block in the chemistry of naphthalimides. The dioxin-annulated products were isolated in very good yields and their applicability for further modifications by metal-catalysed coupling reactions was proven. The synthesized fluorescent dyes showed media dependant emission and their photophysical and electroluminescent properties were evaluated with respect to their application in OLED. TGA measurements showed that all compounds are stable up to above 400 °C. The best OLED test device has turn-on voltage of 8 V; maximum luminescent intensity of 3031 cd/m2, and current efficiency of 6.9 cd/A.

Списание: Dyes and Pigments, Volume 184, 2021

Автори: R. Ferreira, Ts. Petrova, A. F. Ferreira, M. Costa, I. Inaydenova, St. Atanassova-Vladimirova, B. Ranguelov

Резюме: The main purpose of the present work was to evaluate the efficiency of the gasification process of three different types of agro-forest biomass residue (rapeseed, softwood, and sunflower husks) along with the characterization of size-segregated particulates’ emissions. The experiments were carried out in a drop tube furnace (DTF), using two different gasifying agents (O2/N2 and O2/N2/CO2) at atmospheric pressure and a constant temperature of 1000 °C. In focus was the effect of biomass and the gasifying agent on syngas composition (CO, H2, CH4, and CO2), cold gas and carbon conversion efficiency, and on the emissions of by-products, such as particulate matter (PM), known for having negative environmental and health impacts. The collected particulates were characterized by SEM/EDS and XPS analysis. The results reveal that: (i) the introduction of CO2 increased the production of CO and CH4 and syngas’ lower heating value (LHV), thus leading to higher cold gas and carbon conversion efficiency; (ii) CO2 decreased the production of H2, leading to lower H2/CO ratio (between 0.25 and 0.9). Therefore, the generated syngas is suitable for the synthesis of higher hydrocarbons, (iii) CO2 lowered the emissions of char (cyclone) particles but increased the overall PM10–0.3. Submicron size PM was the dominant fraction (PM1–0.3) in O2/N2 and (PM1.6–0.3) in O2/N2/CO2. Unimodal PM size distribution was observed, except for sunflower husks gasification in O2/N2/CO2; (iv) the SEM/EDS and XPS analysis confirmed that submicron-sized PM1–0.3 contain above 80% of carbon associated to soot, due to incomplete oxidation, whereas in cyclone (char) particles, carbon decreased to about 50%. The SEM/EDS results showed that K and Cl are typical constituents of the submicron size PM, whereas the alkaline earth metals were detected mainly in fine and coarse particulates. Detailed analysis of the XPS (C1s) spectra showed that the most common oxygen-containing groups on the PM1 surface were carbonyl and carboxyl

Списание: Energies 14 (2021) 385, 1-28

Автори: R. Andreeva, E. Stoyanova, A. Tsanev and D. Stoychev

Резюме: We investigated the corrosion-protective action of chemically deposited in green Ce solution conversion Cerium oxide coatings on technically pure aluminum (Al 1050) that were subsequently immersion-treated in phosphate solutions. A comparative study was conducted regarding the effect of: the type of pretreatment of the Аl substrate; the conversion treatment of the Аl substrate in Се-containing or Р-containing solutions; the consecutive conversion treatment of the Аl substrate in Се-containing and Р-containing solutions. The obtained results indicate that the pre-treatment of the aluminum substrate and the additional phosphate treatment of the Ce-oxide conversion coating deposited on Al 1050 have a strong impact on corrosion-protective effect. We established that the phosphate treatments lead to a transformation of the chemical composition, chemical state of the elements, structure and morphology of the Ce-oxide coatings formed on Al substrates. Oxides and phosphates of Al and Ce are formed, providing an effective barrier to the diffusion of Chloride ions towards the metal surface and increased corrosion resistance (Rp) of Al 1050 to general and pitting corrosion. We have established a marked increase in Rp for one of the model systems (Al(NaOH)/CCOC(Ce+Cu)/PhL(NH4H2PO4)) during extended corrosion tests, which was related to the formation of poorly soluble surface corrosion products.

Списание: Current Advances in Chemistry and Biochemistry, ed. by A.Martiney Romero, Vol. 7, 2021

Автори: S. Todorova, J.L. Blin, A. Naydenov, B. Lebeau, D. Karashanova, H. Kolev, P. Gaudin, R. Velinova, L. Vidal, L. Michelin, L. Josien, D. Filkova, I. Ivanova, A. Dotzeva, K. Tenchev

Резюме: The hierarchical macro-mesoporous silica (MMS) was used for a first time as a support for catalysts for oxidation reactions. The macro-mesoporous silica was synthesized by the emulsions templating mechanism and modified separately or simultaneously using cobalt and manganese oxides. The obtained materials were characterized by different physicochemical methods and tested in the oxidation of CO and n-hexane combustion reactions. The modification of the MMS materials does not change significantly the mesopores characteristics; however, its pores are partially blocked by the oxides. For Co-MM sample agglomerates consisting of Co3O4 with average size of 100−150 nm and small spherical aggregates, encapsulated in the mesopores are formed. The amorphous manganese oxide preferentially fills up the mesopores in Mn-MM sample. Mixed oxide Co-Mn phases situated in the mesoporous network are formed in the bi-component Co-Mn samples. No significant change is observed either in the texture, or in the structural features of the catalysts after reaction.

Списание: Catalysis Today 361, 2021, 09205861

Автори: Nikov, Ru.G., Dikovska, A.Og., Avdeev, G.V., Atanasova, G.B., Karashanova, D.B., S. Amoruso, G. Ausanio, N.N. Nedyalkov

Резюме: In this work, we demonstrate a novel way of fabricating oriented composite nanowires consisting of arranged nanoparticles. The structures are produced by an advanced pulsed laser deposition technology involving the simultaneous ablation of two metal targets, Fe and Ag. The depositions are carried out in air at atmospheric pressure in the presence of a magnetic field by using nanosecond laser pulses delivered by a Nd:YAG laser system operating at its fundamental wavelength. Samples are thus produced composed by iron oxide and silver at different percentage ratios. Their morphology represents nanowires with few tens of microns in length and an orientation predominantly following the direction of the magnetic force lines. The study on the optical properties of the structures thus produced revealed plasmon resonance behavior in their transmission spectra with its position depending on the ratio between the two building materials. UV–vis spectra were also obtained with polarized light. Higher transmission was measured for light polarized perpendicular to the length of the nanowires, compared to the case of polarization parallel to the nanowires. The structures can find applications in the design of novel polarization and magneto-optics devices, as well as in nanoelectronics and spintronics.

Списание: Materials Today Communications 26, 2021, 101717

Автори: T.I. Milenov, I.A. Avramova, A. Dikovska, D. Karaivanova, P. Terziyska, S.K. Kolev, D. Karashanova, B. Georgieva, D. Dimov, V. Atanasov, E.P. Valcheva

Резюме: The study aims to explore the effect of low fluence UV-C radiation on the structural quality of thin carbon films. We have modified single to few-layered nano-sized graphene-like films deposited by pulsed laser deposition (PLD) on ~300 nm SiO2/Si substrates and additionally different hydrogenated amorphous carbon a-C:H, tetrahedral amorphous carbon (ta-C:H) and amorphous carbon (a-C) thin carbon films. The modification was carried out by irradiation of the samples with UV-C lamps (Hg lamps λ= 254 nm wavelength and fluence of about 2 × 10−3 W/cm−2) for 5- 30 min in air-atmosphere for graphene-like and up to 60 min for thin carbon films. The irradiated graphene-like films were oriented either at about 2 arcdeg to the impinging light, i.e. the light was almost parallel to the honey-comb plane of graphene film or perpendicular to the UV-C light. The thin carbon films were treated only by a light directed perpendicularly to the films’ surface for 60 min. Films were studied before and right after UV-C modification by ellipsometry or profilometry, X-ray photoelectron and Raman spectroscopies to clarify the influence of the UV-C treatment. The most pronounced influence of the UV-C irradiation on the structural quality of the films was established on a-C:H and ta-C:H films where the sp3 and the oxygen-containing radicals content decrease moderately and drops significantly, respectively. The influence of the UV-C irradiation directed almost parallel to the films’ surface on the structural quality of the graphene-like films was slightly higher than that directed perpendicularly. No significant influence on the quality of a-C films synthesized or a-C:H films annealed at high temperatures (1020–1050) °C was observed.

Списание: Surfaces and Interfaces 24, 2021, 101073

Автори: Nikolov, A.S., Stankova, N.E., Karashanova, D.B., Nedyalkov, N.N., Pavlov, E.L., Koev, K. Tz., Najdenski, Hr., Kussovski, V., Avramov, L.A., Ristoscu, C., Badiceanu, M., Mihailescu, I. N.

Резюме: This paper reports on the production of Ag nanoparticles (AgNPs) in a water solution based on a two-phase pulsed laser procedure in view of therapeutic ophthalmological applications requiring AgNPs size of ≤10 nm with a narrow size distribution. Nanoparticles of this size scale are capable of penetrating the complex ocular barriers, thus ensuring effective non-invasive drug delivery to the retina. Moreover, the ocular irritation, which is currently associated to the conventional ocular drug administration routes, would be avoided.

In the first phase, AgNPs larger than 20 nm were fabricated via laser ablation of a Ag target in water by irradiation with the fundamental wavelength (λ = 1064 nm) generated by a Nd:YAG laser. During the second phase, to reduce the mean size of the as-obtained nanoparticles and properly adjust the size distribution, the water colloids were additionally irradiated with the ultraviolet harmonics (355 nm and 266 nm) of the same laser source. The effect of the key laser parameters – wavelength, fluence and laser exposure time – upon the nanoparticles morphology was studied. The most suitable post-ablation treatment of initial colloids was obtained by a consecutive irradiation with the third (λ = 355 nm) and the fourth (λ = 266 nm) harmonics of the fundamental laser wavelength. By using this approach, a synergistic effect between two mechanisms of light absorption by AgNPs was induced. As a result, contaminant-free colloids of AgNPs with sizes of less than 10 nm and quite a narrow size distribution with a standard deviation of 1.6 nm were fabricated.

The toxic effect of the as-produced AgNPs on Gram-positive and Gram-negative bacteria and Candida albicans was explored. The most efficient action was reached against Pseudomonas aeruginosa and Escherichia coli.

A potential application was proposed of the synthesized AgNPs colloidal aqueous solutions with antimicrobial action as a non-invasive method for ocular infections prevention and treatment.

Списание: Optics & Laser Technology 138 (2021) 106850

Автори: Lazarova, K.; Boycheva, S.; Vasileva, M.; Zgureva, D.; Babeva, T.

Резюме: Thin films consisting of a sol-gel Nb2O5 matrix doped with zeolite Na-X synthesized from fly ash through ultrasonic-assisted double stage fusion-hydrothermal alkaline activation were deposited by the spin-coating method. In order to improve the optical quality and sensing properties of the thin films zeolites were wet-milled for 60, 120 and 540 seconds prior to incorporation in the film. The liquid adsorption ability of thin films were tested by measuring the reflectance spectra prior to and after exposure to liquid acetone and the change in the reflection coefficient ∆R of the films was calculated. The influence of milling time of zeolites on the sensing and optical properties of the films was studied.

Списание: Eng. Proc. 2021, 6, 55., https://sciforum.net/manuscripts/10068/manuscript.pdf

Автори: A. Stoilova, G. Mateev, D. Nazarova, L. Nedelchev, E. Stoykova, B. Blagoeva, N. Berberova, S. Georgieva, P. Todorov

Резюме: The paper presents a study of the diffraction efficiency of polarization holographic gratings recorded in thin films of the azopolymer PAZO (poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt]) doped with Cu(II) 3-amino-5,5′-dimethylhydantoin (CLP) and Ni(II) 3-amino-5,5′-dimethylhydantoin (NLP) at three different concentrations, namely 1, 2 and 5 wt.%. The influence of the dopants composition and concentration on the parameters of the polarization holographic gratings recorded in the thin composite films has been discussed. The gratings are recorded with a He-Cd gas laser with wavelength 442 nm. The polarization of the recording beams was left and right circular and the recording angle was 20°. Along with the anisotropic grating in the volume of the media, surface relief is also formed. The diffraction efficiency kinetics is probed at 635 nm and the height of the relief gratings is determined by AFM. Diffraction efficiency (η) higher than 30 % was achieved for the hybrid samples, as well as 585 nm surface relief height.

Списание: Journal of Photochemistry and Photobiology A: Chemistry, Volume 411, 113196, 2021., https://www.sciencedirect.com/science/article/pii/S1010603021000691

Автори: Koseva, V. Nikolov, P. Tzvetkov, M. Gancheva, P. Ivanov, P. Petrova, R. Tomova

Резюме: Glasses with different compositions were prepared from the oxide system CaO-GeO2-Li2O-B2O3 in order to find out compositions, where after thermal treatment of the initial glasses crystallize Ca2GeO4, Ca5Ge3O11 or Li2CaGeO4 as a main phase. Glass-ceramics were obtained and crystallizing phases were determined. Ca2GeO4, Ca5Ge3O11 and Li2CaGeO4 regions of existence in the glass-ceramics were defined. The infrared spectra of pure and Eu3+, Tb3+ and Dy3+ doped glasses show some variations between glass structures depending on the glass composition. Excitation and emission spectra of the doped glasses were studied. All luminescence spectra contain the characteristic transitions of Eu3+, Tb3+ and Dy3+ ions. Some difference can be seen in the Eu3+ doped and Tb3+ doped glass spectra depending on the glass composition. Differences are slight in the Dy3+ doped glass spectra. CIE coordinates of Eu3+ doped glasses, Tb3+ doped glasses and Dy3+ doped glasses are located in the red, green-yellow-pink and yellow regions.

Списание: Journal of Photochemistry and Photobiology A: Chemistry, Volume 411, 113196, 2021., https://www.sciencedirect.com/science/article/pii/S0022309320305536

Автори: Lazarova, K.; Bozhilova, S.; Ivanova, S.; Christova, D.; Babeva, T.

Резюме: Thin spin-coated polymer films of amphiphilic copolymer obtained by partial acetalization of poly (vinyl alcohol) are used as humidity-sensitive media. They are deposited on polymer substrate (PET) in order to obtain a flexible humidity sensor. Pre-metallization of substrate is implemented for increasing the optical contrast of the sensor, thus improving the sensitivity. The morphology of the sensors is studied by surface profiling, while the transparency of the sensor is controlled by transmittance measurements. The sensing behavior is evaluated through monitoring of transmittance values at different levels of relative humidity gradually changing in the range 5–95% and the influence of up to 1000 bending deformations is estimated by determining the hysteresis and sensitivity of the flexible sensor after each set of deformations. The successful development of a flexible sensor for optical monitoring of humidity in a wide humidity range is demonstrated and discussed.

Списание: Sensors 2021, 21, 3674., https://www.mdpi.com/1424-8220/21/11/3674

Автори: Гергинова, Д.; Митрев, Я.; Симова, Св., Приложение на ЯМР метаболомика за анализ на пчелен мед от мурсалски чай

Резюме:

Списание: Списание на БАН 4, 9 (2021)

Автори: M. Dangalov, P. Petrov, N. G. Vassilev

Резюме: Two new bis-carbene (Pd(NHC)2Cl2 and Pd(NHC)(NHC`)Br2) complexes derived from substituted 1,8-naphthalimides were synthesized. All complexes were fully characterized by NMR spectra, which indicate trans orientation of the NHC ligands. 1H and 13C NMR spectra of complex Pd(NHC)2Cl2 are influenced by existence of trans-syn and trans-anti rotamers. The dynamic behaviour in solution of the complex Pd(NHC)2Cl2 was studied by 1H NMR spectra in the temperature range of 293–403 K. The rotational barrier is experimentally estimated to 20 kcal/mol at 298 K. The DFT calculations reproduce the experimental data very well. The analysis of DFT calculations suggests electronic origin of the restricted rotation around Pd-Ccarbene bond. The electron-donating properties of reported system are situated between unsaturated and saturated five-membered ring NHC ligands.

Списание: Synthesis, Structure elucidation and DFT study, Journal of Molecular Structure 1230, 129944 (2021)

Автори: Popova, M.; Szegedi, Á.; Oykova, M.; Lazarova, H.; Koseva, N.; Mihályi, M.R.; Shestakova, P.

Резюме:Mono- and bimetallic Ni-, Ru- and Pt-modified hierarchical ZSM-5 materials were prepared by impregnation technique and characterized by X-ray diffraction (XRD), N2 physisorption, temperature-programmed reduction (TPR–TGA), ATR–FTIR and solid state NMR spectroscopy. Formation of finely dispersed nickel, ruthenium and platinum species was observed on the bimetallic catalysts. It was found that the peculiarity of the used zeolite structure and the modification procedure determine the type of formed metal oxides and their dispersion and reducibility. The samples’ acidity was studied via FTIR spectroscopy of adsorbed pyridine. The changes in the zeolite structure were studied via solid-state NMR spectroscopy. The catalysts were investigated in a gas-phase hydrodeoxygenation, transalkylation and dealkylation reaction of model lignin derivative molecules for phenol production.

Списание: Molecules 2021, 26, 3576., https://www.mdpi.com/1420-3049/26/12/3576

Автори: Grigorova, E., Tzvetkov, P., Todorova, S., Markov, P., Spassov, T.

Резюме: Composites based on Mg2Ni with 5% activated carbon from apricot stones (ACAP) have been prepared by ball milling and subsequent annealing in hydrogen atmosphere. The purpose of the primary metal (Mg, Ni, and V) milling was to reduce the particle size and achieve a good contact between them, without forming intermetallic compounds. During hydriding/dehydriding at 300 °C the amount of the Mg2Ni phase progressively increased, and after 10 cycles about 50% Mg2(Ni,V) was achieved. The hydrogenation produced mainly Mg2NiH4, but small amounts of MgH2 and VHx were also detected in the powder mixture. Relatively high hydrogen storage capacity and fast hydriding/dehydriding kinetics of the Mg2.1Ni0.7V0.3—5 wt.% ACAP composite were determined both from hydrogen gas phase and electrochemically.

Списание: Materials 2021, 14 (8), art. no. 1936. DOI: 10.3390/ma14081936

Автори: Yordanov, S., Jivov, B., Simeonova, S., Lakov, L., Dyakova, V., Aleksandrova, M., Gacheva, M., Stambolova, I.

Резюме: 

Списание: International Journal “NDT Days” Volume IV ISSN: 2603-4018 eISSN: 2603-4646

Автори: E. Lilov, V. Lilova, Ch. Girginov, S. Kozhukharov, S.Nedev, A.Tsanev, D.Yancheva, V.Velinova, D.Ilieva

Резюме: The kinetics of film formation on zinc in water solutions of sodium hydroxide was investigated. The concentrations of the forming electrolyte were varied between 0.001 and 0.08 mol dm−3. The anodic polarization was carried out in galvanostatic regime with current densities between 5 and 35 mA cm−2. A point at which the rate of film formation changes was found on the kinetic curves. The dependencies of the slopes of the kinetic curves before and after this point, the position of this point, as well as the breakdown voltage on the concentration of the forming electrolyte, on the applied current density and on the temperature were investigated. By means of X-ray diffractometry and Raman spectrometry was found, that the formed film consists of polycrystalline zinc oxide. Further investigations using ATR-FTIR and XPS confirmed these observations and suggested the possible presence of small quantities of impurities in the layers. SEM and AFM studies show that in the initial stages of anodic polarization a film is formed, which fills the concavities of the metal surface and afterwards the electrode is covered with hexagonal plate-like crystals. Subsequently the surface becomes smoother again. Some thermodynamic considerations, as well as some additional experiments with sandwich structures formed on the electrode surface suggest that the shift in the kinetic curves is caused by a change in the specific resistivity of the layer, occurring at a temperature of about 305 K.

Списание: Materials Chemistry and Physics 263, (2021) 124298

Автори: V. Mihaylova, B. Todorov , V. Lyubomirova, R. Djingova

Резюме: A high-performance liquid chromatography method is developed and validated for determination of imidacloprid, cypermethrin and chlorpyrifos ethyl in water samples. Separation is performed with a chromatographic column C18 (Microsorb-MV, 100-5, C18, 150 × 4.6 mm) using a mobile phase consisting of methanol/water (70:30, v/v) at a flow rate of 0.5 mL min-1 and UV detection at 205 nm. The method exhibits good linearity in the range of 1-1000 μg L-1 for the analyzed pesticides. The percentage recovery of the method at three concentration levels (10, 100, and 1000 μg L-1) is within 98.6 to 101.6% for the three pesticides. The limit of quantification is low (0.51, 1.20 and 1.50 μg L-1 for imidacloprid, cypermethrin and chlorpyrifos ethyl, respectively) which enables their determination in water samples at low concentration levels. A stability test of imidacloprid, cypermethrin and chlorpyrifos ethyl was also performed to predict their stability in surface water under variation of ambient temperature.

Списание: Bulgarian Chemical Communications, брой 53 (1) 2021, 55-60

Автори: I. Belovezhdova, B. Todorov, M. Bogdanov

Резюме: 1-methyl-3-octylimidazolium saccharinate {[C8C1im][Sac]} – a hydrophobic but polar ionic liquid (IL) capable of capturing and liberating water in a controllable manner was employed as an extraction phase in the liquid–liquid extraction for the recovery of radioactive elements (241Am, 137Cs and 60Co) from aqueous solutions. The influence of factors such as IL/aqueous source ratio, pH and presence of complex-forming agents (citric acid, oxalic acid) on the extraction yield was comprehensively studied. The radionuclides were shown to possess different behavior under the same conditions, the latter being attributed, and further proved by theoretical speciation analysis, to the formation of species capable of interacting in a specific manner with the IL-phase. The results obtained showed the possibility for controllable and selective transfer between the IL and aqueous phases and resulted in the development of a simple procedure for extraction and concentration of 241Am. The method developed proved successful in the monitoring of this radionuclide in drinking water samples.

Списание: Separation and Purification Technology 262 (2021) 118303

Автори: M. Popova, D. Gerginova, B. Trusheva, S. Simova, A. N. Tamfu, O. Ceylan, K. Clark and V. Bankova

Резюме: Recently, the honey and propolis of stingless bees have been attracting growing attention because of their health-promoting properties. However, studies on these products of African Meliponini are still very scarce. In this preliminary study, we analyzed the chemical composition of honey, two cerumen, and two resin deposits (propolis) samples of Meliponula ferruginea from Tanzania. The honey of M. ferruginea was profiled by NMR and indicated different long-term stability from Apis mellifera European (Bulgarian) honey. It differed significantly in sugar and organic acids content and had a very high amount of the disaccharide trehalulose, known for its bioactivities. We suggested trehalulose to be a potential marker for African stingless bee honey analogously to the recent proposal for Meliponini honey from Asia, South America, and Australia and demonstrated its easy discrimination by 13C NMR. Propolis and cerumen were studied by GC-MS (gas chromatography-mass spectometry). The samples contained mainly terpenoids (di-and triterpenes) but demonstrated qualitative and quantitative differences. This fact was an indication that possibly M. ferruginea has no strict preferences for resins used to construct and protect their nests. The antimicrobial and anti-quorum sensing properties of the two materials were also tested. These first results demonstrated that the honey, cerumen, and propolis of African stingless bees were rich in biologically active substances and deserved further research.

Списание: Foods 2021, 10, 997., https://www.mdpi.com/2304-8158/10/5/997

Автори: Blin, Jean-Luc, Laure Michelin, Bénédicte Lebeau, Anton Naydenov, Ralitsa Velinova, Hristo Kolev, Pierrick Gaudin, Loïc Vidal, Anna Dotzeva, Krasimir Tenchev and Silviya Todorova

Резюме: Reported here are new data on the structural and catalytic properties of a series of mono-component cobalt and bi-component Co–Ce catalysts supported on SBA-15 (Santa Barbara Amorphous-15)). The catalysts performance has been evaluated by tests on combustion of methane, propane, and n-hexane. It was established that the preparation of the Co–Ce catalysts by the ‘two-solvent’ technique does not significantly change the mesoporous structure, however, its pores are clogging with the Co and Ce guest species. Cobalt and cerium are uniformly distributed and preferentially fill up the channels of SBA-15, but oxide agglomerates located on the surface are observed as well. The highest activity of the mono-component cobalt sample is explained by its higher reducibility as a result of lower interaction of the cobalt oxide with the SBA-15. The fine dispersion of cobalt and cerium oxide and their strong interaction in the channels of the SBA-15 molecular sieve, leads to the formation of difficult-to-reduce oxide phases and, consequently, to lower catalytic activity compared to monocomponent cobalt oxide catalyst. The synthesised mesoporous structure can prevent the agglomeration of the oxide particles, thus leading to the successful development of a new and stable catalyst for decreasing greenhouse gas emissions.

Списание: Catalysts, 11 (2021) 366

Автори: Dimitrov, D., Z. Chen, V. Marinova, D. Petrova, C.Y. Ho, B. Napoleonov, B. Blagoev, V. Strijkova, K. Y. Hsu, S. H. Lin, and J. Y. Juang

Резюме: In this work, highly conductive Al-doped ZnO (AZO) films are deposited on transparent and flexible muscovite mica substrates by using the atomic layer deposition (ALD) technique. AZO-mica structures possess high optical transmittance at visible and near-infrared spectral range and retain low electric resistivity, even after continuous bending of up to 800 cycles. Structure performances after bending tests have been supported by atomic force microscopy (AFM) analysis. Based on performed optical and electrical characterizations AZO films on mica are implemented as transparent conductive electrodes in flexible polymer dispersed liquid crystal (PDLC) devices. The measured electro-optical characteristics and response time of the proposed devices reveal the higher potential of AZO-mica for future ITO-free flexible optoelectronic applications.

Списание: Nanomaterials, 11 (2021) 1011

Автори: Lazarova, K.; Bozhilova, S.; Ivanova, S.; Christova, D.; Babeva, T.

Резюме: Humidity-sensitive polymer, namely poly(vinyl alcohol-co-vinyl acetal) was deposited on a flexible poly(ethylene terephthalate) (PET) substrate pre-covered with sputtered Au:Pd thin layer in order to develop an optical flexible humidity sensor. Spin-coating method was applied for the thin polymer film deposition. The optical and sensing properties of the device were studied after repeated bending deformation in the range 25−1000 times. Transmittance measurements at different levels of relative humidity were conducted in order to examine the sensing properties of probed flexible sample. The influence of the number of bending deformations on performance of the sensor is studied and the possibility for its successful application is demonstrated and discussed.

Списание: Eng. Proc. 6 (2021) 6 (ИОМТ-БАН)

Автори: Popova, M., Á. Szegedi, M. Oykova, H Lazarova, N. Koseva, M. R. Mihályi, P. Shestakova

Резюме: Mono- and bimetallic Ni-, Ru- and Pt-modified hierarchical ZSM-5 materials were prepared by impregnation technique and characterized by X-ray diffraction (XRD), N2 physisorption, temperature-programmed reduction (TPR–TGA), ATR–FTIR and solid state NMR spectroscopy. Formation of finely dispersed nickel, ruthenium and platinum species was observed on the bimetallic catalysts. It was found that the peculiarity of the used zeolite structure and the modification procedure determine the type of formed metal oxides and their dispersion and reducibility. The samples’ acidity was studied via FTIR spectroscopy of adsorbed pyridine. The changes in the zeolite structure were studied via solid-state NMR spectroscopy. The catalysts were investigated in a gas-phase hydrodeoxygenation, transalkylation and dealkylation reaction of model lignin derivative molecules for phenol production.

Списание: Molecules, 26 (2021) 3576 (ИОХЦФ-БАН)

Автори: Stoykova, E., N. Berberova, B. Blagoeva, D. Nazarova, L. Nedelchev, A. Machikhin

Резюме: Industrial inspection of processes by capture of speckle patterns often requires detection of a small activity area buried in a background. This work presents analysis of sensitivity of the dynamic speckle method by processing simulated and experimental correlated in time 8 bit encoded speckle patterns. Simulation of the patterns was done for an exponentially decreasing temporal correlation function of intensity fluctuations by Fresnel propagation of a monochromatic wave reflected from a delta-correlated in time phase screen and captured at different diameters and focal distances of the optical sensor objective lens. For the experiment, we used a 3D printed flat object with hollow sections that was covered with a transparent film and a droplet of a polymer solution and monitored the process of their drying. Both normalized and non-normalized processing algorithms were used.

Списание: Proc. SPIE 11782, Optical Measurement Systems for Industrial Inspection XII, 117821U (20 June 2021)

Автори: Stoykova, Elena, Dimana Nazarova, Marin Nenchev, and Margarita Deneva

Резюме: A conventional Fizeau wedge (FW) is built of two flat reflecting surfaces inclined at a small angle. They form a gap with linearly increasing thickness normally to the wedge ridge. Such FWs with apex angles of 5–100 microradians and 5– 1000 micrometers thickness find application in optical metrology, spectroscopy and laser spectral control. The apex angle, the reflection coefficients and the refractive index in the gap form a unique interference pattern on both FW’s sides. To benefit from a large free spectral range of thin wedges and high spectral resolution of thick wedges, recently we proposed a stack of two FWs with matched parameters. Matching provides the same change of the resonant wavelength at the same lateral displacement of both wedges. The aim of this study is to develop a technique for calculation of the resultant transmission of the stack of two matched wedges at plane wave illumination that is based on determination of the number and optical path differences of the rays leaving the stack at a given point on its rear surface. Only rays with non-negligible contribution as an amplitude are taken into account. Numerical simulation and experimental verification are provided.

Списание: Proc. SPIE 11783, Modeling Aspects in Optical Metrology VIII, 117830A (20 June 2021)

Автори: Todorova, S., J.L. Blin, A. Naydenov, B. Lebeaud, D. Karashanova, H. Kolev, P. Gaudin, R. Velinova, L. Vidald, L. Michelind, L. Josiend, D. Filkova, I. Ivanova, A. Dotzeva, K. Tenchev

Резюме: The hierarchical macro-mesoporous silica (MMS) was used for a first time as a support for catalysts for oxidation reactions. The macro-mesoporous silica was synthesized by the emulsions templating mechanism and modified separately or simultaneously using cobalt and manganese oxides. The obtained materials were characterized by different physicochemical methods and tested in the oxidation of CO and n-hexane combustion reactions. The modification of the MMS materials does not change significantly the mesopores characteristics; however, its pores are partially blocked by the oxides. For Co-MM sample agglomerates consisting of Co3O4 with average size of 100−150 nm and small spherical aggregates, encapsulated in the mesopores are formed. The amorphous manganese oxide preferentially fills up the mesopores in Mn-MM sample. Mixed oxide Co-Mn phases situated in the mesoporous network are formed in the bi-component Co-Mn samples. No significant change is observed either in the texture, or in the structural features of the catalysts after reaction.

The highest catalytic activity for Co-MM sample in CO and n-hexane oxidation is related to the predomination of Co3+ species on the surface of Co3O4 and the more accessible oxide particles located outside the mesopores. The encapsulation of mixed Co-Mn oxides particles in the pores of the macro-mesoporous silica is responsible for a lower catalytic activity in comparison with that of the mono-component cobalt sample.

Списание: Catalysis Today, 361 (2021) 94-101

Автори: Stoykova, E.; Nazarova. D.; Nedelchev, L.; Park, J

Резюме: Application of the JPEG compression scheme to dynamic speckle patterns captured for visualization of faster or slower changes in 3D objects is analyzed. Compression efficiency is proven by simulation and experiment.

Списание: OSA Imaging and Applied Optics Congress, DM1, B5, 2021

Автори: Nedelchev, L.; Mateev, G.; Strijkova, V.; Salgueiriño, V.; Schmool, D.S.; Berberova-Buhova, N.; Stoykova, E.; Nazarova, D.

Резюме: We employ two approaches to tune the properties of concurrently inscribed volume polarization and surface relief gratings in nanocomposite thin films containing the azopolymer PAZO (poly[1-4-(3-carboxy-4-hydrophenylazo)benzensulfonamido]-1,2-ethanediyl, sodium salt]) and goethite (α-FeOOH) nanorods. The first one is applied on the stage of sample preparation by varying the concentration of the goethite nanorods from 0% to 15%. Then, different angles between the recording beams are set in the holographic scheme, which allow us to obtain gratings with spatial periods in the range from 0.86 to 2.51 µm. Surface relief modulation close to 300 nm is achieved as well as total diffraction efficiency in the ±1 diffracted orders of more than 50%. The influence of the incorporated goethite nanorods on the properties of both volume birefringence and the surface relief grating are discussed

Списание: Photonics 8 (2021) 306 (ИОМТ-БАН)

Автори: Blagoeva, B.; Nedelchev, L.; Nazarova, D.; berberova-Buhoba, N.; Stoykova, E.; Park, J.

Резюме: Photoinduced chiral structures in four azopolymers (three amorphous and one LC) are quantified by the azimuth rotation angle Δθ. Both kinetics of Δθ and its dependence on the input ellipticity are presented.

Списание: OSA Imaging and Applied Optics Congress, DM6E.7, 2021 (ИОМТ-БАН)

Автори: Atanassova, M.; Angelov, R.; Gerginova, D.; Karashanova, D.

Резюме: Study of the liquid-liquid extraction of Ag(I) ions with a series of phosphine oxides (trioctylphosphine, tributylphosphine, triphenylphosphine, and octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide) and phosphorus-containing calix[4]arene by the well-known method of slope analysis and determination of the process parameters are presented employing CHCl3 and an ionic liquid, 1-alkyl-3- methylimidazolium bis(trifluoromethylsulfonyl)imide ([C1C4im+ ][Tf2N]) as diluents. The ligand effect on the complexation properties of Ag(I) is quantitatively assessed. The silver-containing extracts are examined by 1 H, 31P NMR as well as by SEM and TEM approaches in order to viewing the cation coordination and ligands’ binding mode inside, in the organic phase solution at micro-scale level. A conclusion can be made about aggregation in micelle formation of the calix[4]arene under study upon extraction of Ag(I) ion. A method employing thermogravimetric analysis (TGA) is also exploited for the determination of the solubility of CMPO ligand diluted in [C1C4im+ ][Tf2N]. Final conclusions are given highlighting the role of the diluent, molecular or ionic, in complexation processes and selectivity involving ligands with P=O donor group and various metal s-, p-, d- and f-cations. The Ag(I) ions have shown the best extraction behavior compared to the other ions in the +1 oxidation state. Indeed, the calix[4]arene derivative bearing four P=O functions at the lower rim, shows strongly enhanced extraction as well as pronounced separation abilities towards lanthanoids in an ionic liquid environment.

Списание: Journal of Molecular Liquids, 2021, 335, 116287

Автори: Berberova-Buhova, Nataliya, Lian Nedelchev, Georgi Mateev, Elena Stoykova, Velichka Strijkova, Dimana Nazarova

Резюме: This study is aimed to determine the influence of the size of gold (Au) nanoparticles (NP) doped in azopolymer PAZO (poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt]) on the photoinduced birefringence of the nanocomposite thin films as well as on the diffraction efficiency of polarization holographic gratings recorded in them. For this reason, we used spherical Au NP with five different dimensions – 10, 20, 30, 40 and 50 nm. Concentrations of the NP in the nanocomposites varied from 0 a.u. (non-doped samples) to 16 a.u. where 1 a.u. corresponds to 0.015 wt %. Birefringence was induced with He–Cd laser (λ = 442 nm) and measured at 635 nm using DPSS laser. We observe an enhancement of photoinduced birefringence, which is most pronounced for the Au NP with size 20 nm. The polarization holographic gratings were inscribed with two orthogonal circular polarizations (left- and right-handed). AFM measurements were performed to determine the properties of the resulting surface relief gratings. Our results indicate that both the diffraction efficiencies and surface relief modulation of the gratings inscribed in the nanocomposite thin films are higher compared to those in the non-doped samples.

Списание: Optical Materials, 121 (2021) 111560, https://www.mdpi.com/2073-4344/11/3/366

Автори: Bozova, N.; Petrov, P.D

Резюме: Biopolymer materials have been considered a “green” alternative to petroleum-based polymeric materials. Biopolymers cannot completely replace synthetic polymers, but their application should be extended as much as possible, exploiting the benefits of their low toxicity and biodegradability. This contribution describes a novel strategy for the synthesis of super-macroporous 2-hydroxyethylcellulose (HEC) cryogels. The method involves cryogenic treatment of an aqueous solution of HEC and citric acid (CA), freeze drying, and thermally induced crosslinking of HEC macrochains by CA in a solid state. The effect of reaction temperature (70–180 °C) and CA concentration (5–20 mass % to HEC) on the reaction efficacy and physico-mechanical properties of materials was investigated. Highly elastic cryogels were fabricated, with crosslinking carried out at ≥100 °C. The storage modulus of the newly obtained HEC cryogels was ca. 20 times higher than the modulus of pure HEC cryogels prepared by photochemical crosslinking. HEC cryogels possess an open porous structure, as confirmed by scanning electron microscopy (SEM), and uptake a relatively large amount of water. The swelling degree varied between 17 and 40, depending on the experimental conditions. The degradability of HEC cryogels was demonstrated by acid hydrolysis experiments.

Списание: Molecules, 26 (2021) 6370 (ИП-БАН)

Автори: Daskalova, Albena, Filipov, Emil, Angelova, Liliya, Stefanov, Radostin, Tatchev, Dragomir, Avdeev, Georgi, Sotelo, Lamborghini, Christiansen, Silke, Sarau, George, Leuchs, Gerd, Iordanova, Ekaterina and Buchvarov, Ivan

Резюме: The use of laser processing for the creation of diverse morphological patterns onto the surface of polymer scaffolds represents a method for overcoming bacterial biofilm formation and inducing enhanced cellular dynamics. We have investigated the influence of ultra-short laser parameters on 3D-printed poly-ε-caprolactone (PCL) and poly-ε-caprolactone/hydroxyapatite (PCL/HA) scaffolds with the aim of creating submicron geometrical features to improve the matrix biocompatibility properties. Specifically, the present research was focused on monitoring the effect of the laser fluence (F) and the number of applied pulses (N) on the morphological, chemical and mechanical properties of the scaffolds. SEM analysis revealed that the femtosecond laser treatment of the scaffolds led to the formation of two distinct surface geometrical patterns, microchannels and single microprotrusions, without triggering collateral damage to the surrounding zones. We found that the microchannel structures favor the hydrophilicity properties. As demonstrated by the computer tomography results, surface roughness of the modified zones increases compared to the non-modified surface, without influencing the mechanical stability of the 3D matrices. The X-ray diffraction analysis confirmed that the laser structuring of the matrices did not lead to a change in the semi-crystalline phase of the PCL. The combinations of two types of geometrical designs—wood pile and snowflake—with laser-induced morphologies in the form of channels and columns are considered for optimizing the conditions for establishing an ideal scaffold, namely, precise dimensional form, mechanical stability, improved cytocompatibility and antibacterial behavior.

Списание: Materials (MDPI) 14 (2021) 7513 (ИФХ-БАН)

Автори: Ivanov, R., Czibula, C., Teichert, C., Bojinov, M., Tsakova, V.

Резюме: Five types of carbon-based screen-printed electrodes (SPE) – carbon (C110), mesoporous carbon (MC), single walled carbon nanotubes (SWCNT)-, carbon nanotubes (CNT)-, and carbon nanofibers (CNF)-modified electrodes – are characterized with respect to surface roughness, true surface area (TSA), electrochemical activity and electrochemical impedance (EIS) behavior and further used for electroless deposition of Pd. Structural characterization was performed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Investigation of the surfaces with AFM revealed an increasing root mean square (RMS) roughness in the order C110, SWCNT, CNT, CNF, and MC. EIS studies demonstrate significant difference in the resistance of the SPEs with largest value found for the carbon electrode (C110), followed by the carbon nanofiber (CNF)-modified electrode. MC, CNT- and SWCNT-modified electrodes exhibit close, significantly lower resistance values. The capacitance of the carbon-based SPEs is found to be rather high for all electrodes even if accounting for the TSA which has been determined with AFM. This implies a significant effect of nanoroughness and/or of a large internal porous structure especially for CNF and MC SPEs.

Pd electroless deposition occurs at all carbon-based SPEs after reductive pre-treatment in supporting electrolytes (0.5 mol dm−3 H2SO4 or 0.1 mol dm−3 LiClO4) without using a solute chemical reductant in the metal plating solution. The amount and characteristics of the deposited Pd is estimated by anodic stripping voltammetry, EDX analysis, and SEM. It is established that the type and amount of Pd deposit depends on the structure of the carbon-based electrodes with nanostructured carbon coatings (consisting of CNT, SWCNT, or CNF) providing higher amounts of deposited Pd (in the range of 4.4 to 5.8 µg cm−2) and smaller size of the obtained metal particles.

Списание: J. Electroanal. Chem., 897 (2021) 115617 (ИФХ-БАН)

Автори: Lazarova, K.; Boycheva, S.; Vasileva, M.; Zgureva-Filipova, D.; Georgieva, B.; Babeva, T.

Резюме: In this study, thin composite films of a sol–gel Nb2O5 matrix doped with coal fly ash Na-X zeolites were deposited by the spin-coating method. Fly ash of lignite coal collected from the electrostatic precipitators of one of the biggest TPPs in Bulgaria was used as a raw material for obtaining zeolites. Zeolite Na-X was synthesized by ultrasonic-assisted double stage fusion-hydrothermal alkaline conversion of coal fly ash. In order to improve the optical quality and sensing properties of the deposited thin films, synthesized zeolites were wet-milled for 60, 120, and 540 s prior to film deposition. The surface morphology of zeolite powders was studied both by scanning electron microscopy and transmission electron microscopy, while their porosity was investigated by N2-physisorption. Refractive index, extinction coefficient, and thickness of the films were determined through fitting of their reflectance spectra. The sensing ability of thin films towards acetone vapors was tested by measuring the reflectance spectra prior to and during exposure to the analyte, and the change in the reflection coefficient ∆R of the films was calculated. The influence of milling time of zeolites on the sensing and optical properties of the films was assumed and confirmed.

Списание: Nanomaterials 11 (2021), 2399 (ИОМТ-БАН)

Автори: Mantovska, D.; Zhiponova, M.; Georgiev, M.; Grozdanova, T.; Gerginova, D.; Alipieva, K.; Simova, S.; Popova, M.; Kapchina-Toteva, V.; Yordanova, Z.

Резюме: Micropropagation of rare Veronica caucasica M. Bieb. was achieved by successful in vitro cultivation of mono-nodal segments on MS medium supplemented with 1.0 mg L–1 6-benzylaminopurine (BA) and then transferring the regenerated plants on hormone free basal MS medium for root development. In vitro multiplicated plants were successively acclimated in a growth chamber and a greenhouse with 92% survival. The number of plastid pigments and the total phenolics content in in vitro cultivated and ex vitro adapted plants were unchanged, and no accumulation of reactive oxygen species (ROS) was detected by staining with 3-3′-diaminobenzidine (DAB) and 2′,7′-dichlorofluorescein diacetate (DCF-DA). Nuclear Magnetic Resonance (NMR) fingerprinting allowed for the identification of the major alterations in metabolome of V. caucasica plants during the process of ex situ conservation. Iridoid glucosides such as verproside, aucubin and catalpol were characteristic for in vitro cultivated plants, while in ex vitro acclimated plants phenolic acid–protocatechuic acid and caffeic acid appeared dominant. The successful initiation of in vitro and ex vitro cultures is an alternative biotechnological approach for the preservation of V. caucasica and would allow for further studies of the biosynthetic potential of the species and the selection of lines with a high content of pharmaceutically valuable molecules and nutraceuticals.

Списание: Molecules, 2021, 26(19), 5888 (ИОХЦФ-БАН)

Автори: Marinova, Vera, Stefan Petrov, Blagovest Napoleonov, Jordan Mickovski, Dimitrina Petrova, Dimitre Dimitrov, Ken-Yuh Hsu and Shiuan-Huei Lin

Резюме: Graphene has attracted considerable interest as a prospective material for future electronics and opto-electronics. Here, the synthesis process of large area few layers graphene by Atmospheric Pressure Chemical Vapor Deposition (APCVD) technique is demonstrated. Quality assessments of graphene are performed and confirmed by Raman analysis and optical spectroscopy. Next, graphene was transferred on Polyethylene Terephthalate (PET) substrates and implemented as transparent conductive electrode in flexible Polymer Dispersed Liquid Crystal (PDLC) devices. Their electro-optical properties, such as voltage-dependent transmittance and flexibility behavior are measured and discussed. The stability of the sheet resistance after 1200 bending tests of graphene/PET structure is demonstrated. The obtained results open a great potential of graphene integration into the next generation Indium Tin Oxide (ITO) free flexible and stretchable optoelectronics.

Списание: Mater. Proc., 4 (1) (2021) 63 (ИОМТ-БАН)

Автори: Nikolova, Y., G. M. Dobrikov, Z. Petkova, P. Shestakova

Резюме: A series of squaric acid amides (synthesized in 66–99% isolated yields) and a set of chiral aminoalcohols were comparatively studied as ligands in a model reaction of reduction of α-chloroacetophenone with BH3•SMe2. In all cases, the aminoalcohols demonstrated better efficiency (up to 94% ee), while only poor asymmetric induction was achieved with the corresponding squaramides. A mechanistic insight on the in situ formation and stability at room temperature of intermediates generated from ligands and borane as possible precursors of the oxazaborolidine-based catalytic system has been obtained by 1H DOSY and multinuclear 1D and 2D (1H, 10/11B, 13C, 15N) NMR spectroscopy of equimolar mixtures of borane and selected ligands. These results contribute to better understanding the complexity of the processes occurring in the reaction mixture prior to the possible oxazaborolidine formation, which play a crucial role on the degree of enantioselectivity achieved in the borane reduction of α-chloroacetophenone.

Списание: Molecules 2021, 26, 6865 (ИОХЦФ-БАН)

Автори: Ravutsov, М., Y. Mitrev, P. Shestakova, H. Lazarova, S. Simeonov, M. Popova

Резюме: The post-synthesis procedure for cyclic amine (morpholine and 1-methylpiperazine) modified mesoporous MCM-48 and SBA-15 silicas was developed. The procedure for preparation of the modified mesoporous materials does not affect the structural characteristics of the initial mesoporous silicas strongly. The initial and modified materials were characterized by XRD, N2 physisorption, thermal analysis, and solid-state NMR. The CO2 adsorption of the obtained materials was tested under dynamic and equilibrium conditions. The NMR data revealed the formation of different CO2 adsorbed forms. The materials exhibited high CO2 absorption capacity lying above the benchmark value of 2 mmol/g and stretching out to the outstanding 4.4 mmol/g in the case of 1-methylpiperazin modified MCM-48. The materials are reusable, and their CO2 adsorption capacities are slightly lower in three adsorption/desorption cycles.

Списание: Nanomaterials, 11 (2021) 2831

Автори: Shopska M., H. Kolev, K. Aleksieva, I. Shtereva, K. Tenchev, S. Todorova, M. Fabian, G. Kadinov

Резюме: Supported bimetallic catalysts of (Co + Pd)/SiO2 system were studied in carbon monoxide hydrogenation. Comparative analysis showed that depending on precursor treatment mode the catalysts ranged in different rows of activity in CO conversion and selectivity to methane. Samples of best performance were obtained after pretreatment in Ar flow. Highly selective catalysts were synthesized by reduction at 450 °C determining low metal dispersion, high extent of alloying, and agglomeration. A low H2,100C/COstrong adsorbed gas ratio was ascribed to a great amount of bimetallic particles and concerned with a diminished number of sites for multiply bonded CO species. Metal dispersion was low due to large Co particles, which enhanced CO dissociation and hydrogenation to CH4. In presence of bimetallic particles the reaction CO + 3H2 = CH4 + H2O was hampered. A decreased H2O formation influenced the WGS reaction. Catalyst samples activated at higher temperatures had better selectivity. During the process, formation of bidentate carbonate species was registered. It was supposed that palladium impeded creation of the latter species and following decomposition to CO2. Active catalyst samples were prepared by reduction at 300 °C leading to higher unreduced cobalt quota and metal dispersion, and decreased alloy particle formation. Higher H2,100C/COstrong ratio values were assigned to pure Co and Pd particle segregation, i.e. availability of sites for multiply bonded CO species favoring a higher activity in CO dissociation and further hydrogenation. A higher amount of CO species on these samples was conducive to CH4 formation, but also to CO2 production. The latter reaction was facilitated by unreduced cobalt.

Списание: Reaction Kinetics, Mechanisms and Catalysis, 134 (2021) 303–330

Автори: Stankova, N., Nikolov, A., Iordanova, E., Yankov, G., Nedyalkov, N., Atanasov, P., Tatchev, D., Valova, E., Kolev, K., Armyanov, S., Karashanova, D., Fukata, N.

Резюме: We report on a new approach toward a laser-assisted modification of biocompatible polydimethylsiloxane (PDMS) elastomers relevant to the fabrication of stretchable multielectrode arrays (MEAs) devices for neural interfacing technologies. These applications require high-density electrode packaging to provide a high-resolution integrating system for neural stimulation and/or recording. Medical grade PDMS elastomers are highly flexible with low Young’s modulus < 1 MPa, which are similar to soft tissue (nerve, brain, muscles) among the other known biopolymers, and can easily adjust to the soft tissue curvatures. This property ensures tight contact between the electrodes and tissue and promotes intensive development of PDMS-based MEAs interfacing devices in the basic neuroscience, neural prosthetics, and hybrid bionic systems, connecting the human nervous system with electronic or robotic prostheses for restoring and treating neurological diseases. By using the UV harmonics 266 and 355 nm of Nd:YAG laser medical grade PDMS elastomer is modified by ns-laser ablation in water. A new approach of processing is proposed to (i) activate the surface and to obtain tracks with (ii) symmetric U-shaped profiles and (iii) homogeneous microstructure This technology provides miniaturization of the device and successful functionalization by electroless metallization of the tracks with platinum (Pt) without preliminary sensitization by tin (Sn) and chemical activation by palladium (Pd). As a result, platinum black layers with a cauliflower-like structure with low values of sheet resistance between 1 and 8 Ω/sq are obtained.

Списание: Polymers 13 (2021) 3004

Автори: Stoilova, Ani, Dean Dimov, Yordanka Trifonova, Vanya Lilova, Blaga Blagoeva, Dimana Nazarova and Lian Nedelchev

Резюме: Composite films based on the azopolymer poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt] doped with GeTe4-Cu chalcogenide particles have been prepared through electrospray
deposition. Polarization microscopy was applied to study the films morphology. The transmittance coefficient (T), the reflectance coefficient (R), the refractive index (n), the extinction coefficient (k), the optical absorption coefficient
(α) and the optical band gap (Eopt g ) of the composite films and of the pure azo polymer film were determined. A significant change in the optical properties of the composite films in comparison to the non-doped azo polymer film
has been observed.

Списание: Eur. Phys. J. Appl. Phys. 95 (2021) 30301

Автори: Stoilova, Olya, Nevena Manolova and Iliya Rashkov

Резюме: Electrospinning was successfully used for the one-step fabrication of poly(methyl methacrylate) (PMMA) fibers loaded with an inorganic photocatalyst—titanium oxide (TiO2). By tuning the PMMA/TiO2 ratio and the electrospinning conditions (applied voltage, needle tip-to-collector distance, and flow rates), PMMA/TiO2 composites with selected organic/inorganic ratios, tailored designs, and targeted properties were obtained. The morphology of the electrospun composites was affected by the amount of TiO2 incorporated into the PMMA fibers. In addition, the inorganic photocatalyst had an impact on the wettability, thermal stability, and optical properties of the electrospun composites. In particular, the surface wettability of the composites was strongly influenced by UV light irradiation and from hydrophobic became superhydrophilic. Moreover, PMMA/TiO2 composites had enhanced tensile strength in comparison with those of bare PMMA mats. The electrospun PMMA/TiO2 composites showed excellent photocatalytic efficiency against the model organic pollutant—methylene blue—which is very promising for the future development of membranes that are highly efficacious for photocatalytic water treatment.

Списание: Polymers 13(2021) 3923

Автори: Tsoncheva, T., Ivanova, R., Velinov, N., Kovacheva, D., Spassova, I., Karashanova, D., Petrov, N.

Резюме: Carbon foams with different surface functionality and tailored texture characteristics were prepared from mixtures containing coal tar pitch and furfural in different proportions. The obtained materials were used as a host matrix for the preparation of zinc- and cobalt-mixed ferrite nanoparticles. The texture, morphology, phase composition, and the related redox and catalytic properties of the obtained composites were characterized by low-temperature nitrogen physisorption, XRD, SEM, HRTEM, FTIR, Mössbauer spectroscopy, TPR and catalytic decomposition of methanol to syngas. The impact of the carbon support on the formation of Co- and Zn-mixed ferrites was discussed in detail using KIT-6 silica-based modifications as reference samples. The catalytic behavior of the ferrites was considered in a complex relation to their composition, morphology, location in the porous matrix and metal ions distribution in the spinel sub-lattices. The higher amount of furfural in the carbon foam precursor promoted the formation of cobalt-rich, more accessible and highly active methanol decomposition to syngas spinel particles.

Списание: Symmetry 13 (2021) 1532

Автори: Velikova, Nina, Ivanka Spassova

Резюме: Bifunctionalized mesoporous silica materials were prepared by sol–gel method applying newly proposed sequence of addition of the used silanols in the systems tetraethylortosilicate (TEOS): Tris[3-(trimethoxysilyl)propyl] isocyanurate (ISC): bis[(3- trimethoxysilyl)propyl]amine (BTPA), TEOS: ISC: (3-mercaptopropyl)trimethoxysilane (MPTMS) (TEOS: ISC: MPTMS) and TEOS: BTPA: MPTMS. The bi-functionalized hybrid silicas were synthesized by co-condensation reaction between TEOS and silsesquioxane precursors in acidic media. Soft template approach for pore formation was applied with structural directing agent Pluronic P123. Mesitylene and KCl were used for improving the materials’ texture. New sequences of addition of the silanol precursors into the reaction mixture were applied in order to achieve better distribution of the functional groups on the materials surface and for prevention of entrapment of the functional groups in the pore walls. The synthesized bi-functional hybrid mesoporous silicas were investigated by FTIR, N2-physisorption, DTA/TG-MS, SEM, XPS and XRD techniques. CO2 adsorption properties of the synthesized bi-functionalized hybrids were investigated. I was found that the sequence of addition of silanol precursors, the type of the silsesquioxane precursors and the presence of isocyanurate groups have significant influence on the materials texture, morphology and CO2 sorption properties. The presence of isocyanurate groups in the hybrid silica framework significantly improves the textural characteristics and CO2 sorption capacities. The determined heats of adsorption evidenced CO2 physisorption on the active sites of the hybrid materials.

Списание: J Sol-Gel Sci. Technol. (2021)

Автори: Voyslavov, Ts.; Mladenova, E.; Balkanska, R.

Резюме: A new approach for the botanical origin determination of monofloral bee honey is developed. The methodology combines mineral content and physicochemical parameters determination with intelligent statistics such as self-organizing maps (SOMs). A total of 62 monofloral bee honey samples were analysed, including 31 linden, 14 rapeseed, 13 sunflower, and 4 acacia. All of them were harvested in 2018 and 2019 from trusted beekeepers, after confirming their botanical origin, using melissopalynological analysis. Nine physicochemical parameters were determined, including colour, water content, pH, electrical conductivity, hydroxymethylfurfural content, diastase activity, specific optical rotation, invertase activity, and proline. The content of thirty chemical elements (Ag, Al, As, B, Ba, Bi, Ca, Cd, Co, Cr, Cs, Cu, Fe, Ga, In, K, Li, Mg, Mn, Na, Ni, P, Pb, Rb, S, Se, Sr, Te, V, and Zn) was measured using ICP-OES, ICP-MS, and FAAS as instrumental techniques. The visualisation of the SOMs shows an excellent separation of honey samples in five well-defined clusters—linden, rapeseed, acacia, sunflower, and polyfloral honey—using the following set of 16 descriptors: diastase activity, hydroxymethylfurfural content, invertase activity, pH, specific optical rotation, water content, Al, B, Cr, Cs, K, Na, Ni, Rb, V, and Zn.

Списание: Molecules 26 (2021) 7219

Автори: Б. Стамболийска, С. Тъпанов, Е. Велчева, Д. Панталеева, С. Стоянов

Резюме: Художествените ценности в църквата, за съжаление, постоянно са изложени на тежките условия на високопланинския климат, което неизбежно влияе отрицателно върху тяхното състояние. Това налага предприемане на своевременни мерки за опазване и реставрация на застрашените художествени произведения. А за да се проведе адекватно реставрацията на една творба, е необходимо да бъдат изяснени всички аспекти на нейната технология и реставрационната ѝ проблематика. Анализът на техниката и технологията често се явява ключ за успешното решаване на реставрационните задачи. С тази цел беше проведено аналитично проучване за изясняване на технологичните характеристики на стенописите в църквата „Успение на св. Иван Рилски“ и за идентифициране на използваните живописни материали и техники.

Списание: Списание на БАН 4 (2021) 29-36

Автори: Y. Tavitian, D.Y. Yancheva, N. D. Todorov

Резюме: Three Persian oil paintings on canvas from the collection of the National Gallery, Sofia, representing a Musician playing Daf, Musician playing Santour and Royalty/Prince, dating from the Persian Qajar era, created as marouflaged canvas murals, but dismounted, relined and converted to easel paintings at some point, were studied for the purpose of dating, conservation and technological evaluation. A series of micro-samples and cross sections were investigated by optical microscopy, energy dispersive X-ray spectroscopy in a scanning electron microscope (SEM-EDS), attenuated total reflectance Fourier transform infrared (ATR-FTIR), Raman spectroscopy and with non-destructive methods as X-ray, IR and UV-fluorescence imaging in order to characterize the protective coatings or varnishes, paint layers, primers, ground layers, lining adhesive components and canvases. The non-destructive investigation methods, the samples and cross sections studies with the analytical methods applied contributed to identify the composition and stratigraphy, identical for both paintings with Musicians showing previous historical restorations. The data collected from the analysis of the Royalty/Prince painting also proved previous interventions, complex stratigraphy and presence of overpaintings. The integrated materials are similar for the three artworks. The analysis revealed a rich blend of predominant traditional materials and techniques with little new European impact, which allows dating the paintings back to the first quarter of the XIX century. The study of the composition of the paintings determines the direction of the conservation and restoration approaches.

Списание: Eur. Phys. J. Plus 136 (2021) 733

Автори: E. Lilov, V. Lilova, Ch. Girginov, S. Kozhukharov, S. Nedev, A. Tsanev, D. Yancheva, V. Velinova, D. Ilieva

Резюме: The kinetics of film formation on zinc in water solutions of sodium hydroxide was investigated. The concentrations of the forming electrolyte were varied between 0.001 and 0.08 mol dm−3. The anodic polarization was carried out in galvanostatic regime with current densities between 5 and 35 mA cm−2. A point at which the rate of film formation changes was found on the kinetic curves. The dependencies of the slopes of the kinetic curves before and after this point, the position of this point, as well as the breakdown voltage on the concentration of the forming electrolyte, on the applied current density and on the temperature were investigated. By means of X-ray diffractometry and Raman spectrometry was found, that the formed film consists of polycrystalline zinc oxide. Further investigations using ATR-FTIR and XPS confirmed these observations and suggested the possible presence of small quantities of impurities in the layers. SEM and AFM studies show that in the initial stages of anodic polarization a film is formed, which fills the concavities of the metal surface and afterwards the electrode is covered with hexagonal plate-like crystals. Subsequently the surface becomes smoother again. Some thermodynamic considerations, as well as some additional experiments with sandwich structures formed on the electrode surface suggest that the shift in the kinetic curves is caused by a change in the specific resistivity of the layer, occurring at a temperature of about 305 K.

Списание: Mater. Chem. Phys., 263 (2021) 124298

Автори: К. Anichina, M. Argirova, R. Tzoneva, V. Uzunova, A. Mavrova, D. Vuchev, G. Popova-Daskalova, F. Fratev, M. Guncheva, D. Yancheva

Резюме: In the present study, fifteen benzimidazolyl-2-hydrazones 7a-7o of fluoro-, hydroxy- and methoxy-substituted benzaldehydes and 1,3-benzodioxole-5-carbaldehyde were synthesized and their structure was identified by IR, NMR, and elemental analysis. The compounds 7j 2-(3-hydroxybenzylidene)-1-(5(6)-methyl-1H-benzimidazol-2-yl)hydrazone and 7i 2-(3-hydroxybenzylidene)-1-(1H-benzimidazol-2-yl)hydrazone have exerted the strongest anthelmintic activity (100% after 24 h incubation period at 37 °C) against isolated muscle larvae of Trichinella spiralis in an in vitro experiment. The in vitro cytotoxicity assay towards MCF-7 breast cancer cells and mouse embryo fibroblasts 3T3 showed that the studied benzimidazolyl-2-hydrazones exhibit low to moderate cytotoxic effects. The ability of the studied benzimidazolyl-2-hydrazones to modulate microtubule polymerization was confirmed and suggested that their anthelmintic action is mediated through inhibition of the tubulin polymerization likewise the other known benzimidazole anthelmitics. It was also shown that the four most promising benzimidazolyl-2-hydrazones do not affect significantly the AChE activity even at high tested concentration, thus indicating that they do not have the potential for neurotoxic effects. The binding mode of compounds 7j and 7n in the colchicine-binding site of tubulin were clarified by molecular docking simulations. Taken together, these results demonstrate that for the synthesized benzimidazole derivatives the anthelmintic activity against T. spiralis and the inhibition of tubulin polymerization are closely related.

Списание: Chemico-Biological Interactions, 345 (2021) 109540

Автори: V. Mihaylova, V. Lyubomirova, R. Djingova

Резюме: Water is essential to human life. Our bodies need a certain amount of water intake on a daily basis to function appropriately. Several health benefits have been attributed to the mineral and trace element content of mineral and spring waters. Although the quality of drinking water depends to a large extent on its microelement composition, only limited data are available about trace element content in Bulgarian bottled spring waters. In this study, using ICP-MS a simultaneous determination of 69 chemical elements in bottled spring waters has been performed and the results were compared to the previous analysis obtained for bottled mineral waters sold on the Bulgarian market. The data for both types of water (mineral and spring) prove that all determined elements are below Bulgarian Regulation No 9 of maximum admissible levels. Only Fe in one brand of spring water is slightly higher than Bulgarian health-based value but lower than EPA. Multivariate statistics (cluster, discriminant and factor analysis) were used to reveal groups of similarity among the investigated mineral and spring waters. For everyday use, along with the alternation of different brands of mineral water also change of the water type might be recommended.

Списание: Bulgarian Chemical Communications, 53 (3) (2021) 371 – 379

2020 г.

Автори: K. Lazarova, Bozhilova, S.; Ivanova, S.; Christova, D.; Babeva, T.

Резюме: In this paper, the possibility to design flexible humidity sensors by spin-coating of moisture-sensitive polymer on three types of substrates—poly(ethylene terephthalate) (PET), polylactide (PLA) and composite polysiloxane is investigated. The optical properties, surface morphology and roughness of the substrates covered with polymer are studied by transmittance measurements and surface profiling, respectively. Thin polymer films of amphiphilic copolymerobtained by partial acetalization of poly(vinyl alcohol) are used as humidity sensitive media. The sensing properties are probed through transmittance measurements at different levels of relative humidity (RH). The influence of substrate type is studied by comparing the hysteresis of flexible sensors with those that are deposited on glass substrates.

Списание: Engineering Proceedings MDPI, 42, 16, 2020

Автори: D. Gerginova, S. Simova, M. Popova, M. Stefova, J. P. Stanoeva, V. Bankova

Резюме: Bulgaria and North Macedonia have a long history of the production and use of honey; however, there is an obvious lack of systematic and in-depth research on honey from both countries. The oak honeydew honey is of particular interest, as it is highly valued by consumers because of its health benefits. The aim of this study was to characterize honeydew and floral honeys from Bulgaria and North Macedonia based on their NMR profiles. The 1D and 2D 1H and 13C-NMR spectra were measured of 16 North Macedonian and 22 Bulgarian honey samples. A total of 25 individual substances were identified, including quinovose, which was found for the first time in honey. Chemometric methods (PCA—principal component analysis, PLS-DA—partial least squares discriminant analysis, ANOVA—analysis of variance) were used to detect similarities and differences between samples, as well as to determine their botanical and geographical origin. Semiquantitative data on individual sugars and some other constituents were obtained, which allowed for the reliable classification of honey samples by botanical and geographical origin, based on chemometric approaches. The results enabled us to distinguish oak honeydew honey from other honey types, and to determine the country of origin. NMR was a rapid and convenient method, avoiding the need for other more time-consuming analytical techniques.

Списание: Molecules 25, 4687-4698 (2020)

Автори: Maya G. Shopska, Iskra Z. Shtereva, Hristo G. Kolev, Krasimir K. Tenchev, Silviya Z. Todorova, Georgi B. Kadinov

Резюме: The paper is focused on evaluation of active centres and adsorbed species type impact on 10%Co+0.5%Pd)/Al2O3 catalyst system performance aiming selectivity optimization. Application of different sets of precursor pretreatment and reductions resulted in catalysts exhibiting high CO conversion or high methane selectivity. Sample of high selectivity was prepared by pretreatment in hydrogen and the performance was determined by lower amount of strongly adsorbed CO, strongly adsorbed carbonate species, higher amount of reduced metal and bimetallic particles. A more active system was formed by pretreatment in air leading to larger amount of unreduced metal and CO-bridged species on the surface, stable coverage of hydroxyl groups on the support, and medium-strength sites for adsorption of carbonates. Ratios of adsorbed H/СО and strongly/weakly СО species appeared as important criteria for catalyst efficiency together with supported metal state, amount of unreduced ions, bimetallic particle formation, and alumina’s ability to adsorb CO and CO2.

Списание: Croatica Chemica Acta, Vol. 93 No. 2, 2020, https://hrcak.srce.hr/247328

Автори: R. Georgiev, Georgieva, B., Lazarova, K., Vasileva, М., Babeva, T.

Резюме: Present study aims at development of thin flms of sol–gel Ta2O5 with tunable refractive index that could be used as an active medium for volatile organic compound (VOCs) optical detection. In order to tune the refractive index, mesoporosity is introduced through soft templating approach where series of co-polymers (Pluronics) are utilized as structuredirecting agents. Thin flms are prepared by spin-coating method using tantalum sol synthesized from TaCl5 as a precursor and ethanol as a solvent. The temperature annealing at 320 °C fnalizes the tantalum oxide lattice formation and decomposes the template being used. The morphology and elemental composition of the flms are studied by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy in STEM, respectively, and the amorphous status of the flms is confrmed by selected area electron diffraction. Non-linear curve ftting method and non-destructive UV–Vis refectance measurements are used for determination of optical parameters of the flms. The free volume fraction in the flms is estimated through modeling of refractive index by Bruggeman efective medium approximation. The flms are exposed to acetone vapors as a commonly studied VOC. The reaction of the flm due to vapors’ exposure is recorded as a change in the refection spectra of the flm and the adsorbed acetone quantity is estimated. The infuence of template composition on optical and sensing properties of Ta2O5 flms is revealed and discussed.

Списание: Opt Quant Electron 52, 437 (2020)

Автори: D. Tsyganov, N. Bundaleska, J. Henriques, E. Felizardo, A. Dias, M. Abrashev, J. Kissovski, A. M. Botelho do Rego, A. M. Ferraria, and E. Tatarova

Резюме: An experimental and theoretical investigation on microwave plasma-based synthesis of free-standing N-graphene, i.e., nitrogen-doped graphene, was further extended using ethanol and nitrogen gas as precursors. The in situ assembly of N-graphene is a single-step method, based on the introduction of N-containing precursor together with carbon precursor in the reactive microwave plasma environment at atmospheric pressure conditions. A previously developed theoretical model was updated to account for the new reactor geometry and the nitrogen precursor employed. The theoretical predictions of the model are in good agreement with all experimental data and assist in deeper understanding of the complicated physical and chemical process in microwave plasma. Optical Emission Spectroscopy was used to detect the emission of plasma-generated “building units” and to determine the gas temperature. The outlet gas was analyzed by Fourier-Transform Infrared Spectroscopy to detect the generated gaseous by-products. The synthesized N-graphene was characterized by Scanning Electron Microscopy, Raman, and X-ray photoelectron spectroscopies.

Списание: Materials, MDPI, 2020, 13(18), 4213 (IF 3.057)

Автори: Petar Todorov, Stela Georgieva, Petia Peneva, Rusi Rusew, Boris Shivachevc and Anton Georgiev

Резюме: Herein, the synthesis and characterization of four novel 5,50 -diphenylhydantoin Schiff bases containing different aromatic species are presented. Their structure–property relationship was studied by X-ray, optical and electrochemical methods as well as DFT calculations in terms of their E/Z photoisomerization and enol/keto phototaumerization. The big challenge in photoinduced motion is achieving control and stability over the two isomers. Solvent-driven bidirectional photoswitching behavior was studied in nonpolar 1,4-dioxane and polar aprotic DMF. T-type photochromism in 1,4-DOX and opposite behavior in DMF as P-type switches (bistable system) were observed. The obtained results lead to a conclusion that by variation of the solvent environment a direct control over the bidirectional switching behaviour from T-type to P-type can be achieved.

Списание: New J. Chem., Royal Society of Chemistry, 2020, 44, 15081

Автори: N. Burdzhiev, A. Ahmedova, B. Borrisov, R. Graf

Резюме: Tautomerization of 2-phenylimidazolecarbaldehydes has not been studied in detail so far, although this process is a well-known phenomenon for imidazole derivatives. That is why we focus our study on a series of 2-phenylimidazolecarbaldehydes and their parent alcohols that were synthesized and studied by detailed 1H and 13C NMR in solution and in the solid state. The apparent problem is that the fast tautomerization impedes the full structural description of the compounds by conventional 13C NMR measurements. Indeed, the 13C NMR spectra in solution exhibit poor resolution, and in most cases, signals from the imidazole ring are not detectable. To avoid this problem, we used 13C CP-MAS NMR as an alternative spectroscopic method for unambiguous spectroscopic characterization of the studied series of 2-phenylimidazoles. The data were analyzed in combination with quantum chemical DFT-GIAO methods by considering the tautomerization process and the intermolecular interactions. The DFT (B3LYP/6-31G(d,p)) calculations allowed to identify and suggest the preferred tautomer in the gas phase and in DMSO solvent, which for alcohols are (2-phenyl-1H-imidazol-4-yl)methanol and its analogs, and for the aldehydes are the 2-phenyl-1H-imidazole-5-carbaldehydes. The gas-phase calculated energy differences between the two possible tautomeric forms are in the range 0.645–1.415 kcal/mol for the alcohols and 2.510–3.059 kcal/mol for the aldehydes. In the DMSO solvent, however, for all compounds, the calculated energy differences go below 1.20 kcal/mol. These data suggest that both tautomeric forms of the studied 2-phenylimidazoles can be present in solution at room temperature. Our data from detailed 2D NMR measurements in the solid state (1H-13C HETCOR and 1H-1H double-quantum coherence MAS NMR) suggested that also in the solid state both tautomers coexist in different crystalline domains. This fact does not obscure the 13C CP-MAS NMR spectra of the studied 2-phenyl substituted imidazoles and suggests this spectroscopic method as a powerful tool for a complete structural description of tautomeric systems with aromatic conjugation.

Списание: Molecules, MDPI, 2020, 25(17), 3770

Investigation of photophysical, electrochemical and electroluminescent properties of Iridium(III)bis[2 phenylbenzo[d] thiazolato N,C2′ ] quinolin 8 olate for white organic light emitting diodes application

Автори: P. Ivanov, P. Petrova, R. Tomova

Резюме: The complex Iridium(III)bis[2-phenylbenzo[d]thiazolato-N,C2 ′]-quinolin-8-olate ((bt)2Irq) was synthesized and identifed by 1 H NMR and IR spectroscopy. Its photophysical, electrochemical and electroluminescent properties for White Organic Light-Emitting Diodes (WOLED) application were investigated. It was established that (bt)2Irq irradiates yellow-orange light in CH2Cl2 (DCM) solution and solid flm with maximum peaks, respectively, at 578 and 666 nm and shoulders at 628 and 720 nm. The complex was used as stand-alone emitter and a dopant in the matrixes of hole transporting layer (HTL) or electroluminescent layer (EL) in the range from 0 to 16.5 w%. The base OLED structure was Anode/HTL/EL/ETL/Cathode, where HTL is N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) included in a matrix of poly(N-vinylcarbazole) (PVK), EL is Aluminium (III) bis(2-methyl-8-quninolinato)-4-phenylphenolate (BAlq) and ETL is an electron transporting layer of Tris-(8-hydroxyquinoline) aluminium (Alq3) or Zinc (II) bis(2-(hydroxyphenyl) benzothiazole (Zn(bt)2). It is found that the electroluminescent spectra of both types OLEDs were the sum of the greenish-blue and yellow-orange emissions of BAlq and Ir complex and CIE coordinates of the emitted light remain within the white range over wide dopant concentrations. The best current efciency and CIE coordinates close to the ideal white (0.3300; 0.3300) demonstrate devices with 2 wt% doped HTL (6 Cd/A, (0.2735; 0.3613)) and 16.5 w% doped EL (5.1 Cd/A, (0.2721; 0.3284)). It was found that both types of doped devices operate through a charge trapping mechanism.

Списание: Journal of Materials Science: Materials in Electronics 31 (2020) 15707–15717

Автори: N. Bundaleska, A. Dias, N. Bundaleski, E. Felizardo, J. Henriques, D. Tsyganov, M. Abrashev, E. Valcheva, J. Kissovski, A. M. Ferraria, A. M. Botelho do Rego, A. Almeida, J. Zavašnik, U. Cvelbar, O. M. N. D. Teodoro, Th. Strunskus, and E. Tatarova

Резюме: The ability to change the secondary electron emission properties of nitrogen-doped graphene (N-graphene) has been demonstrated. To this end, a novel microwave plasma-enabled scalable route for continuous and controllable fabrication of free-standing N-graphene sheets was developed. High-quality N-graphene with prescribed structural qualities was produced at a rate of 0.5 mg/min by tailoring the high energy density plasma environment. Up to 8% of nitrogen doping levels were achieved while keeping the oxygen content at residual amounts (~1%). The synthesis is accomplished via a single step, at atmospheric conditions, using ethanol/methane and ammonia/methylamine as carbon and nitrogen precursors. The type and level of doping is afected by the position where the N-precursor is injected in the plasma environment and by the type of precursors used. Importantly, N atoms incorporated predominantly in pyridinic/pyrrolic functional groups alter the performance of the collective electronic oscillations, i.e. plasmons, of graphene. For the frst time it has been demonstrated that the synergistic efect between the electronic structure changes and the reduction of graphene π-plasmons caused by N doping, along with the peculiar “crumpled” morphology, leads to sub-unitary (< 1) secondary electron yields. N-graphene can be considered as a prospective low secondary electron emission and plasmonic material.

Списание: Nature – Scientific Reports 10, 13013 (2020);

Автори: Dancho Yordanov, Vera Deneva, Anton Georgiev, Aurelien Crochet, Katharina M. Fromm, Liudmil Antonov

Резюме: The paper presents the synthesis and characterization of two 4-substituted phthalimide 2-hydroxy-Schiff bases containing salicylic (4) and 2-hydroxy-1-naphthyl (5) moieties. The structural differences of 2-hydroxyaryl substituents, resulting in different enol/keto tautomeric behaviour, depending on the solvent environment were studied by absorption UV–Vis spectroscopy. Compound 5 is characterized by a solvent-dependent tautomeric equilibrium (KT in toluene = 0.12, acetonitrile = 0.22 and MeOH = 0.63) while no tautomerism is observed in 4. Ground state theoretical DFT calculations by using continuum solvation in MeOH indicate an energy barrier between enol/keto tautomer 5.6 kcal mol−1 of 4 and 0.63 kcal mol−1 of 5, which confirms the experimentally observed impossibility of the tautomeric equilibrium in the former. The experimentally observed specific solvent effect in methanol is modeled via explicit solvation. The excited state intramolecular proton transfer (ESIPT) was investigated by steady state fluorescence spectroscopy. Both compounds show a high rate of photoconversion to keto tautomers hence keto emissions with large Stokes shifts in five alcohols (MeOH, EtOH, 1-propanol, 1-butanol, and 1-pentanol) and various aprotic solvents (toluene, dichlormethane, acetone, AcCN). According to the excited state TDDFT calculations using implicit solvation in MeOH, it was found that enol tautomers of 4 and 5 are higher in energy compared to the keto ones, which explains the origin of the experimentally observed keto form emission.

Списание: Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy,237,118416, 2020

Автори: Koseva, P. Tzvetkov, P. Ivanov, P. Petrova, R. Tomova, A. Yordanova, V. Nikolov

Резюме: Powder samples of Ca2GeO4 doped with 0.5, 1, 2 and 3 at.% Tb3+ were prepared via solid-state synthesis technique. The obtaining of pure phase at all dopant concentrations was proved through XRD analyses. In the emission and excitation spectra of the obtained powders, the characteristic peaks of Tb3+ ion were observed. The Tb3+ excitation spectrum in the range from 300 to 500 nm shows characteristic transitions of Tb3+, attributed to the f–f transitions. The strongest peak is located at 379 nm corresponding to the 7F6 → 5D3 transition. The main emission peak of Tb3+ is 5D4 → 7F5 transitions at 545 nm, corresponding to green color. Other transitions are located at 416 (5D3 → 7F5), 437 (5D3 → 7F4), 458 (5D3 → 7F3), 488 (5D4 → 7F6), 588 (5D4 → 7F4), 621 (5D4 → 7F3), 651 (5D4 → 7F2) and 675 (5D4 → 7F1) nm. The optimum emission is observed for 2 at.% Tb3+ ion concentration. At this concentration dominate also (5D4 → 7F4) and (5D4 → 7F3) transitions. CIE coordinates of the samples show different emission colors depending on the active ion concentration. The obtained results confirm that as-prepared terbium-doped materials could be used like green, yellow and reddish phosphors.

Списание Journal of Optics 49 (2020) 403–407,

Автори V. Lyutov and V. Tsakova

Резюме The redox behavior of polyaniline (PANI) doped with polysulfonic acids is studied in slightly acidic and neutral solutions (from pH 4 to pH 7). The dopants used are polystyrene sulfonic acid (PSSA), poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPSA) and poly-(4,4’-(2,2′-disulfonic acid)-diphenylene-tere-phthalamide) (t-PASA). Thin PANI layers are used in order to identify clearly the redox peak structure. Based on deconvolution of the reductive currents the pH dependence of the constituent peaks is studied and pathways of the reductive transitions are suggested. With increasing pH a loss of electroactivity is observed for all three dopants but to a different extent: the largest for PAMPSA- and the smallest PSSA-doped PANIs. In the same time a stabilization of the conductive emeraldine salt state is observed at pH 7 for all PANIs with largest extent of stabilization in the PANI/PAMPSA case. The electrocatalytic properties of the polysulfonic acids-doped PANIs are comparatively studied for ascorbic acid (AA) and dopamine (DA) oxidation at pH 7. It is found that all three types of PANI are good electrocatalysts for both reactions with advantage of the PAMPSA-doped layers showing higher absolute oxidative currents for both reactions. The results concerning DA oxidation demonstrate the possibility to involve polysulfonic acid-doped PANIs in electrocatalytic reactions in neutral solutions occurring at high positive potentials beyond the peak potential of the PANI oxidative transition

Списание: Journal of Solid State Electrochemistry (2020) 24:3113-3123

Автори: T. Todorova, P. Shestakova, T. Petrova, M. Popova, H. Lazarova, Y. Kalvachev

Резюме: The post-synthesis treatment of AlZSM-5 and GaZSM-5 zeolites by etching with buffer solution of ammonium fluoride and 0.25 M HF acid was carried out. The treatment is applied in order to obtain secondary pores in crystals and to provide easier access to the catalytically active zeolite centers. Hydrothermal synthesis of these zeolites was performed from systems containing tetrapropylammonium bromide as a template. The reaction parameters of synthesis conditions for both zeolites were optimized in order to obtain pure crystalline phases. The zeolites obtained were characterized by X-ray diffraction, IR spectroscopy, scanning electron microscopy, physical adsorption–desorption of nitrogen and solid-state NMR spectroscopy. It has been found that the result materials remain with high crystallinity while new mesoporous are created in their structure. The volume due to the presence of mesopores increases by up to 67% from the total volume, which is a drastic increase compared with the parent solids. The investigations with FTIR spectroscopy of low-temperature CO adsorption and solid-state NMR spectroscopy show that the treatment is slightly selective toward dissolution of heteroatom (aluminum or gallium). This phenomenon is observed for the first time for gallium-substituted zeolites. In order to study the influence of the metal atom in zeolite structure and the efficacy of the acid attack on the catalytic activity, the samples obtained were tested in reaction of m-xylene and toluene transformation.

Списание:  J. Mater. Sci. 55, 13799-13814 (2020)

Автори: St. Todorova, V. Rangelova, L. Mihaylov, T. Spassov

Резюме: MmNi5 hydrogen storage alloy with an average particle size of 120 μm was subjected to hydriding treatment at 50oC, under 40 atm. pure hydrogen atmosphere. As a result, a large density of particle cracks was formed, which led to an increase in the particle surface area. The enlarged particles surface, in turn, led to substantially improved hydrogen sorption kinetics compared to the as-received (untreated) alloy. It was also found that hydrogen-induced decrepitation of the MmNi5 powder results in enhancement of the hydrogen diffusivity mainly due to shortening of its diffusion distances.

Списание: International Journal of ELECTROCHEMICAL SCIENCE

Int. J. Electrochem. Sci., 15 (2020) 4900 – 4907, doi: 10.20964/2020.06.29

Автори: Elena Stoykova, Dimana Nazarova, Lian Nedelchev, Kwan-Jung Oh, Joongki Park

Резюме: Analysis of dynamic speckle patterns enables visualization of faster or slower changes for 3D objects. Compression of data in this type of metrology by coarse quantization at non – uniform illumination is studied by simulation and experiment.

Списание: Imaging and Applied Optics Congress 2020, Optical Society of America (OSA), paper HTh5H.5 

Автори: L. Nedelchev, B. Blagoeva, G. Mateev, D. Nazarova, and E. Stoykova

Резюме: Characteristics of polarization holographic gratings (PHG) in non-covered azopolymer thin films and azopolymer films covered with glass slide are compared. An improvement of the polarization properties of the PHG in the covered films is established.

Списание: Imaging and Applied Optics Congress, Technical Digest, Optical Society of America, 2020,

Автори: E. Haladjova, M6. Smolíček, I. Ugrinova, D. Momekova, P. Shestakova, Z. Kroneková, J. Kronek, S. Rangelov

Резюме: Poly(2-methyl-2-oxazoline)-polyethylenimine (PMeOx-co-PEI) copolymers differing by degree of polymerization (DP = 50 and 200) and PEI content (from 37 to 99 mol%) were synthesized by living cationic ring-opening polymerization of 2-methyl-2-oxazoline, followed by partial hydrolysis. Upon mixing with DNA in a wide range of N/P ratios, they formed well-defined polyplex particles of small size (typically below 100 nm) and narrow size distribution. The polyplexes demonstrated good colloidal stability and very low in vitro cytotoxicity. The copolymers exhibited buffering capacity of over 50% relative to that of the reference PEI implying effective endo-lysosomal escape of the polyplexes. Increased cellular internalization of both PCR fragments and plasmid DNA, attributable to the strongly positive ζ potential and small size of the polyplexes, was observed. In spite of these favorable prerequisites, the transfection efficiency was low (below 20% relative to the control PEI) and was attributed to retarded swelling of the polyplex particles, endo-lysosomal rupture, and DNA release.

Списание: Journal of Applied Polymer Science. 137 (45), 49400 (2020) 

Автори: K. Lazarova, Bozhilova, S., Christova, D., Babeva

Резюме: The optical and humidity-sensing properties are studied of hydrophobically modified PVA, namely poly(vinyl alcohol-co-vinyl acetal)s of varied copolymer composition. Copolymers are synthesized by reacting PVA with acetaldehyde in aqueous solution, thus introducing cyclic acetal functionalities in the polymer chain. Thin polymer films are deposited by spin-coating and their optical properties are studied as a function of the copolymer composition, i.e., degree of acetalization. Reflectance measurements at different relative humidities in the range 5 – 95 % RH are conducted in order to probe the sensing behavior. The comparison with neat PVA thin films confirms the influence of the copolymer’s acetal fraction on the optical and sensing properties and degree of hysteresis. The feasibility of applying poly(vinyl alcohol-co-vinyl acetal) thin films for optical sensing of humidity is demonstrated and discussed.

Списание: Journal of Physics: Conference Series, 1492, 2020.

Автори: Dimitre Dimitrov, Che-Liang Tsai, Vera Marinova, Stefan Petrov, Dimitrina Petrova, Blagovest Napoleonov, Blagoy Blagoev, Velichka Strijkova, Ken Yuh Hsu and Shiuan Huei Lin

Резюме: The integration of high uniformity, conformal and compact transparent conductive layers into next generation indium tin oxide (ITO)-free optoelectronics, including wearable and bendable structures, is a huge challenge. In this study, we demonstrate the transparent and conductive functionality of aluminum-doped zinc oxide (AZO) thin films deposited on glass as well as on polyethylene terephthalate (PET) flexible substrates by using an atomic layer deposition (ALD) technique. AZO thin films possess high optical transmittance at visible and near-infrared spectral range and electrical properties competitive to commercial ITO layers. AZO layers deposited on flexible PET substrates demonstrate stable sheet resistance over 1000 bending cycles. Based on the performed optical and electrical characterizations, several applications of ALD AZO as transparent conductive layers are shown—AZO/glass-supported liquid crystal (LC) display and AZO/PETbased flexible polymer-dispersed liquid crystal (PDLC) devices.

Списание: Coatings 10, 539; (2020)  

Автори: M. V. Abrashev,  V. G. Ivanov,  B. S. Stefanov,  N. D. Todorov, J. Rosell, and  V. Skumryev

Резюме: Synthetic powder, ore samples, and mineral single crystals of goethite (αα-FeOOH) were investigated with polarized Raman spectroscopy at temperatures from 293 K to 473 K. The symmetry of the vibrational modes, observed in different scattering configurations, was determined unequivocally. The assignment of the Raman-active modes to definite atomic vibrations is supported by two types of lattice-dynamical calculations: empirical shell model and ab initio density functional theory. The temperature dependencies of the line shape parameters of some Raman-active vibrations at temperatures near to the antiferromagnetic–paramagnetic phase transition infers for a significant spin–lattice coupling in this compound. The most informative in this aspect is the B3gB3g phonon at 387cm−1387cm−1, which overlays a broad scattering background and displays a pronounced asymmetric Fano-line shape. The asymmetry increases in the paramagnetic state above the Néel temperature (TN=393KTN=393K), indicating a strong interaction of this mode with the underlying excitation continuum. The origin of the excitation background is discussed in light of our experimental results and the existing data for the magnetic structure and transport properties of αα-FeOOH. By using the molecular-orbital dimer approach, we calculate the spin–phonon coupling constants for the B3gB3g mode as J′=−0.2eV/ÅJ′=−0.2eV/Å and J″=+0.18eV/Å2J″=+0.18eV/Å2, respectively. Thus, we rationalize that, most probably, the scattering background stems from magnetic excitations, and the asymmetric shape of the B3gB3g band is a result of a linear spin–phonon coupling of this mode with the Fe–O1–Fe spin dimers. Another mechanism, a phonon interaction with thermally activated charge carriers above TNTN, is also considered.

Списание: Journal of Applied Physics 127, 205108 (2020);

Автори: Tanya Tsoncheva, Ivanka Spassova, Gloria Issa, Radostina Ivanova, Daniela Kovacheva, Daniela Paneva, Daniela Karashanova, Nikolay Velinov, Boiko Tsyntsarski, Biliana Georgieva, Momtchil Dimitrov, Nartzislav Petrov

Резюме: This work is focused on the preparation of Ni0.5M0.5Fe2O4 mixed ferrites (M=Zn or Cu), supported on nanoporous carbons materials. The carbon supports are obtained from various waste residues, such as peach stones from the canning industry and by-products from the low rank coals pyrolysis. X-ray difraction (XRD), nitrogen physisorption, high-resolution transmission electron microscopy (HRTEM), Moessbauer spectroscopy and temperature programmed reduction (TPR) analyses as well as mesoporous silica with tridimensional structure type KIT-6 silica based reference samples are used for detail characterization of the obtained materials. It was established that the loaded on the carbon supports phase is a complex mixture of fnely dispersed ferrite, substituted magnetite, metal (Cu, Fe, FeNi alloy) and ZnO particles. Their dispersion and composition depend on the texture characteristics of the carbon support, which could be easily controlled by the waste precursor used. The existence of mesoporosity in the carbon host matrix provokes the formation of more fnely dispersed and easily reducible spinel particles, which ensures higher initial catalytic activity, but fast deactivation of the catalysts. The formation of activated carbon mesoporosity is facilitated by the presence of cellulose and hemicellulose in the biomass or the addition of furfural to the coal tar pitch precursor. The Ni0.5Zn0.5Fe2O4 modifcations demonstrate higher potential as catalysts for hydrogen production via methanol decomposition.

Списание: Waste and Biomass Valorization, 2020

Автори: Katerina Lazarova, Silviya Boycheva, Marina Vasileva, Denitza Zgureva and Tsvetanka Babeva

Резюме: In this study, solid waste from coal combustion in thermal power plants (TPPs) was used for the synthesis of zeolite Na-X samples. They were prepared by the long-term alkaline atmospheric conversion of coal ash collected from the electrostatic precipitators in the TPP “AES Galabovo”. When used in the form of thin films/layers, the optical detection of volatile organic compounds (VOCs) is possible due to a change in their reflectance spectra and color. In order to improve the sensing properties of synthesized zeolites, they were wet milled for 60 s and both milled and unmilled zeolites were used as dopants for the niobium oxide matrix in the form of thin films deposited by the spin-coating method on a silicon substrate. The surface morphology and structure of both zeolite powders were studied by scanning electron microscopy, while their size was determined by dynamic light scattering (DLS) spectra. Optical constants (refractive index, n, and extinction coefficient, k) and the thickness of the films were calculated from reflectance measurements. The change in the reflection coefficient ∆R of the films was determined from measured reflectance spectra prior to and after exposure to probe acetone molecules. An increase in the reaction of the films with milled zeolites to acetone, compared to the samples with unmilled zeolites, is demonstrated.

Списание: Materials Proceedings 2020, 2, 3.

Автори: Silviya Boycheva, Denitza Zgureva, Katerina Lazarova, Tsvetanka Babeva, Cyril Popov, Hristina Lazarova and Margarita Popova

Резюме: Fly ash (FA) from lignite coal combusted in different Thermal Power Plants (TPPs) was used for the synthesis of zeolites (FAZs) of the Na-X type by alkaline activation via three laboratory procedures. FAZs were characterized with respect to their morphology, phase composition and surface properties, which predetermine their suitability for applications as catalysts and adsorbents. FAZs were subsequently modified with metal oxides (CuO) to improve their catalytic properties. The catalytic activity of non-modified and CuO-modified FAZs in the total oxidation of volatile organic compounds was investigated. FAZs were studied for their potential to retain CO2, as their favorable surface characteristics and the presence of iron oxides make them suitable for carbon capture technologies. Thin films of FAZs were deposited by in situ crystallization, and investigated for their morphology and optical sensitivity when exposed to pollutants in the gas phase, e.g., acetone. This study contributes to the development of novel technological solutions for the smart and valuable utilization of FA in the context of the circular economy and green energy production.

Списание: Materials 2020, 13(9), 2014 

Автори: Anton Georgiev, Dancho Yordanov, Deyan Dimov, Ivailo Zhivkov, Dimana Nazarova, Martin Weiter

Резюме: Herein, we report the synthesis and E→Z photoswitching behavior of two 4-substituted azomethine phthalimides containing anthracenyl and 4-(dimethylamino)phenyl moieties (EAMP1 and EAMP2). These compounds represent newly synthesized and unstudied photoswitches with dual fluorescence properties as E-isomers and at photostationary state (PSS) depending on the solvent polarity. Steady-state fluorescence measurements were performed in various solvents and the results show strong sensitivity on the environmental polarity. The kinetics of E→Z photoswitching to PSS was studied in AcCN by visible light activation at 410 nm (EAMP1) and long wavelength UV-light activation at 350 nm (EAMP2). The quantitative and qualitative performance of the switching behavior was evaluated by the degree of photoisomerization (R) and the rate constant (k). It was found for EAMP1 R = 6.95 %, k = 8.87 × 10−4 s −1 and EAMP2 R = 88.72 %, k = 4.00 × 10−4 s −1 , respectively. The reason for the lower photoconversion of EAMP1 compared to the EAMP2 was analyzed through optimization of the molecular geometry of E- and Z-isomers in the ground state (S0) and first excited state (S1) by DFT/ TD-DFT calculations with B3LYP/6-31+G(d,p) level of theory using IEFPCM in AcCN. It was found that Eisomers in the S0 have nonplanar conformation, while the Z-isomer of EAMP1 prefers twisted conformation and the Z-isomer of EAMP2 T-shaped conformation is energetically advantageous compared to the twisted one. The reason is the weak H…..π noncovalent interaction (NCI) between 4-(dimethylamino)phenyl moiety and phthalimide ring. Moreover, the Z-isomer of EAMP2 is unusual stable up to 600 min at room temperature in dark compared to the EAMP1, which undergoes full Z→E relaxation for less than 60 min at the same conditions. The Z→E relaxation of EAMP2 is achieved for 90 min at 60 °C. The fluorescence E→Z switching behavior was studied by emission measurements in AcCN and 1,4-DOX as E-isomers and at PSS in room and liquid nitrogen (77 K) temperatures. In the polar and nonpolar solvents, red-shifted emissions with increased fluorescence quantum (Φfl) yields have been observed at PSS compared to the E-isomers. The molecular rotor behavior was studied in the binary mixture of glycerol:ethanol and the results show a sensitivity of the emission bands depending on the environmental viscosity. Time-resolved fluorescence decay measurements were performed in AcCN and 1,4-DOX as E-isomers and at PSS to estimate the mechanism of fundamental fluorescence bands. We found that dyes at PSS have longer lifetime (τ) compared to the E-isomers, especially in less polar 1,4-DOX.

Списание: Journal of Photochemistry and Photobiology A: Chemistry, Volume 393, 2020 

Автори: Blagoeva, B., Nedelchev, L., Mateev, G., Stoykova, E., Nazarova, D.

Резюме: We present a study of the diffraction efficiencies of polarization holographic gratings recorded in thin films of the azopolymer PAZO (poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt]). Two series of layers have been prepared using two different solvents – distilled water and methanol. The gratings are inscribed by two plain waves with orthogonal circular polarizations (left and right circular) from a He-Cd gas laser (442 nm) at recording angle 20°, corresponding to grating period 1.3 μm. Higher diffraction efficiency is obtained for the thin film samples spin-coated from the methanol solution for thicknesses below 600 nm. Diffraction efficiency higher than 27% was achieved, as well as surface relief height more than 500 nm.

Списание: Photosensitive Materials and their Applications, Proceedings, Volume 11367; 113671G (2020),

Автори: Mariela Tsanova-Stamatova, Neshka Manchorova-Veleva, Tsvetanka Babeva

Резюме: 3D profilometry is a highly technological method successfully used in dentistry for evaluating materials’ roughness. The aim of our in vitro study was to estimate the effect of mechanical and chemical agents on surface roughness and microscopic structure of zirconia polycrystals by means of 3D profilometry. Seven groups of yttria-stabilized KATANA™Zirconia (Kuraray Noritake Dental Inc.) samples were prepared: one control group (no pretreatment); four groups sandblasted with 50 µm and 250 µm Al2O3 particles at a pressure of 2 and 4 bars and two groups treated with HF (hydrofluoric acid) of 10% and 40% concentration. Each specimen was with diameter and thickness of 11.8 mm and 5.0 mm, respectively. 3D optical profilometry was used to study the surface topography and roughness of the samples. By comparison with control group it was demonstrated that the most suitable treatment that leads to the highest surface roughness values is sand blasting with 50 µm Al2O3 particles at a pressure of 4 bars. This method can be suggested as an effective method for surface pretreatment of zirconia.

Списание: Comptes rendus de l’Academie bulgare des Sciences, 73, (11), No 11, 1510-1516, 2020, DOI: 10.7546/CRABS.2020.11.04

Автори: D. Tsyganov, N. Bundaleska, A. Dias, J. Henriques, E. Felizardo, M. Abrashev, J. Kissovski, A. M. Botelho do Rego, A. M. Ferraria, and E. Tatarova

Резюме: Free-standing N-graphene was synthesized using a microwave plasma-based method at atmospheric pressure conditions through a single step and in a controllable manner. Using ethanol and ammonia as precursors, N-graphene with low relative amount of bonded oxygen and low level of saturated sp3 carbon bonds was produced. Adjusting the injection position of the nitrogen precursor in the plasma medium leads to selectivity in terms of doping level, nitrogen configuration and production yield. A previously developed theoretical model, based on plasma thermodynamics and chemical kinetics, was further updated to account for the presence of nitrogen precursor. The important role of HCN attachment to the graphene sheets as the main process of N-graphene formation is elucidated. The model predictions were validated by experimental results. Optical Emission Spectroscopy was used to detect the emission of plasma generated ‘‘building units’’ and to determine the gas temperature. The plasma outlet gas was analyzed by Fourier-Transform Infrared Spectroscopy to detect the generated gaseous by-products. The synthesized N-graphene was characterized by Scanning Electron Microscopy, Raman and X-ray photoelectron spectroscopies.
Списание: PCCP, Royal Society of Chemistry, 2020, 22, 4772, (IF 3.567).

Автори: R. Georgiev, K. Lazarova, M. Vasileva, T. Babeva

Резюме: Vapor responsive all Nb2O5 Bragg stacks were prepared using spin-coating of alternating dense and porous flms with quarter-wavelength thickness. The required porosity in the flms was achieved by soft-templating method using micellar solution of Pluronic copolymer as an organic template. The surface morphology and structure of the flms were studied by transmission electron microscopy and selected area electron difraction, respectively, while optical characterization was performed by combination of ellipsometric and refectance measurements. Vapor sensing response of the flms was tested by refectance measurements of the samples prior to and after exposure to acetone vapors selected as probe molecules. Incorporation of “defect” layer in the multilayered structures was frstly simulated theoretically and then realized experimentally aiming at improvement of the optical response to volatile organic compounds. Furthermore, the potential of all niobia stacks consisting of alternating dense and porous sol–gel Nb2O5 flms as chemical sensors with optical read-out is demonstrated and discussed.

Списание: Optical and Quantum Electronics, 52:114, 2020

Автори: Valentina Lyubomirova, Veronika Mihaylova, Rumyana Djingova

Резюме: The accurate determination of macroelements (Na, K, Ca, Mg and Si) in potable waters with inductively-coupled plasma mass spectrometry (ICP-MS) is difficult due to their high concentrations. In the present study, optimisation of cell-based ICP-MS for application of a bandpass parameter (RPa) for signal reduction was performed to extend their linear ranges. Individual values of the RPa for each isotope have been selected. A method for the determination of macroelements without dilution using optimised RPa values has been developed and applied for potable water analysis. The accuracy was evaluated by analyses of a surface water reference material.

Списание: Spectroscopy Europe, 32 (5), 2020

2019 г.

Автори: Katerina Lazarova, Silvia Bozhilova, Siika Ivanova, Darinka Christova and Tsvetanka Babeva

Резюме: Hydrophobically modified poly(vinyl alcohol)s of varied copolymer composition were tested as active media for optical sensing of humidity. Copolymer thin films were deposited on silicon substrate using water-methanol solution in a volume ratio of 20:80 and concentration of 1 wt%. Films were subjected to low (60 °C) and moderate (180 °C) temperature annealing in order to study the temperature influence on optical and humidity sensing properties. Refractive index, extinction coefficient along with thickness of the films were determined by non-linear minimization of the goal function comprising measured and calculated reflectance spectra at normal light incidence. The humidity sensing ability of the films was studied through reflectance measurements at different humidity levels in the range 5–95 %RH. The influence of temperature annealing on optical and sensing properties was demonstrated and discussed.

Списание: Proceedings 2019 (MDPI), 42, 16; doi:10.3390/ecsa-6-06555

Автори N.M. Ferreira, A.R. Sarabando, S. Atanasova-Vladimirova, R. Kukeva, R. Stoyanova, B.S. Ranguelov, F.M. Costa

Резюме Mg-Al-Si-O glassy systems have a great importance in a wide range of industrial applications, specifically as an electrolyte for molten oxide electrolysis processes in steelmaking. Understanding how the iron oxidation state of the raw material (Fe2+/Fe3+) and its corresponding amount influence this glassy system’s properties will be the aim of the current work. Iron oxides (as Fe2O3 or Fe3O4) were used to dope Mg-Al-Si-O system obtaining amorphous materials through an unconventional method: Laser Floating Zone (LFZ). Above 8% mol of Fe formation of magnetic phases or iron clusters, were observed in the glass matrix. Samples with Fe2O3 showed a higher crystal concentration, when compared with Fe3O4. The electron paramagnetic resonance measurements show a strong dependence on the iron source (Fe3O4 or Fe2O3). In addition, the magnetization decreases linearly with iron content, independently of iron oxidation state, except for samples with a higher concentration of Fe2O3(15% mol), due to sample crystallization. Moreover, with Fe3O4 as raw material there is an improvement (~250 times) of the electrical conductivity when compared with Fe2O3. The results show that the presence of Fe2+ on the glass influences the electrical conductivity, which could have impact in the efficiency of molten oxide electrolysis process.

Списание Ceramics International, 45 (17) Part А, 21379-21384, doi.org/10.1016/j.ceramint.2019.07.125

Автори Miroslav V. Abrashev, Petko Chernev, Paul Kubella, Mohammad Reza Mohammadi, ChiaraPasquini, Holger Dau and Ivelina Zaharieva,

Резюме Catalysis of the oxygen evolution reaction (OER) by earth-abundant materials in the near-neutral pH regime is of great interest as it is the key reaction for non-fossil fuel production. To address the pertinent stability problems and insufficiently understood structure–activity relations, we investigate the influence of moderate annealing (100–300 °C for 20 min) for two types of electrodeposited Mn oxide films with contrasting properties. Upon annealing, the originally inactive and structurally well-ordered Oxide 1 of birnessite type became as OER active as the non-heated Oxide 2, which has a highly disordered atomic structure. Oxide 2 also improved its activity upon heating, but more important is the stability improvement: the operation time increased by about two orders of magnitude (in 0.1 M KPi at pH 7). Aiming at atomistic understanding, electrochemical methods including quantitative analysis of impedance spectra, X-ray spectroscopy (XANES and EXAFS), and adapted optical spectroscopies (infrared, UV-vis and Raman) identified structure–reactivity relations. Oxide structures featuring both di-μ-oxo bridged Mn ions and (close to) linear mono-μ-oxo Mn3+–O–Mn4+ connectivity seem to be a prerequisite for OER activity. The latter motif likely stabilizes Mn3+ ions at higher potentials and promotes electron/hole hopping, a feature related to electrical conductivity and reflected in the strongly accelerated rates of Mn oxidation and O2 formation. Poor charge mobility, which may result from a low level of Mn3+ ions at high potentials, likely promotes inactivation after prolonged operation. Oxide structures related to the perovskite-like ζ-Mn2O3 were formed after the heating of Oxide 2 and could favour stabilization of Mn ions in oxidation states lower than +4. This rare phase was previously found only at high pressure (20 GPa) and temperature (1200 °C) and this is the first report where it was stable under ambient conditions.

Списание J. Mater. Chem. A, 2019, 7, 17022

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Автори L. Nedelchev, D. Ivanov, B. Blagoeva, D. Nazarova

Резюме Azopolymers are highly efficient materials, able to register the polarization state of light. They have broad range of applications from diffractive optical elements with unique polarization properties to biophysics. We present a study of birefringence kinetics, induced with five different pump lasers with wavelengths 355, 442, 491, 514 and 532 nm in thin films of widely used amorphous azopolymer PAZO – poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzene sulfon amido]-1,2-ethanediyl, sodium salt]. During the real-time measurement, the value of the photoinduced birefringence (Δn) is determined at 635 nm, using a DPSS laser. When saturation is reached, the spectrum of the birefringence is measured. Our results indicate that the investigated azopolymer can be used for applications in a wide wavelength range.

Списание, Journal of Photochemistry & Photobiology A: Chemistry, 376, 1–6, doi.org/10.1016/j.jphotochem.2019.02.009

Автори G. Mateev, L. Nedelchev, A. Georgiev and D. Nazarova

Резюме We present a study of the photoinduced birefringence in nanocomposite films of the azopolymer PAZO (poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfon amido]-1,2-ethanediyl, sodium salt]) doped with TiO2 nanoparticles (NP) with different concentrations before and after thermal annealing. The NP represent nanopowder with primary particle size 21 nm. The concentration of the NP was varied from 0% (non-doped azopolymer film) to 5 wt%. The thermal process, applied to the nanocomposite films, includes 1 h heating at 200°C. Previous studies of PAZO show that the polymer is stable up to 270°C. We study the dependence of the maximal birefringence induced with He-Cd laser (λ= 442 nm) on the concentration of the TiO2 NP in the azopolymer thin films as well as thermal effect on the absorbance spectra of the thin films. As indicated by our results, the birefringence is higher for the thermally annealed samples. An increase of the photoinduced birefringence is also observed for the nanocomposite layers with 1% NP for the non-annealed films, and with 2% NP for the annealed films.

Списание Open Materials Science, 4, 1–5 doi.org/10.1515/oms-2019-0001

Автори K. Lazarova, D. Christova , R. Georgiev, B. Georgieva and T. Babeva

Резюме Thin films with nanometer thicknesses in the range 100–400 nm are prepared from double hydrophilic copolymers of complex branched structures containing poly(N,N-dimethyl acrylamide) and poly(ethylene oxide) blocks and are used as humidity sensitive media. Instead of using glass or opaque wafer for substrates, polymer thin films are deposited on Bragg stacks and thin (30 nm) sputtered Au–Pd films thus bringing color for the colorless polymer/glass system and enabling transmittance measurements for humidity sensing. All samples are characterized by transmittance measurements at different humidity levels in the range from 5% to 90% relative humidity. Additionally, the humidity induced color change is studied by calculating the color coordinates at different relative humidity using measured spectra of transmittance or reflectance. A special attention is paid to the selection of wavelength(s) of measurements and discriminating between different humidity levels when sensing is performed by measuring transmittance at fixed wavelengths. The influence of initial film thickness, sensor architecture, and measuring configuration on sensitivity is studied. The potential and advantages of using top covered Bragg stacks and polymer/metal thin film structures as humidity sensors with simple optical read-outs are demonstrated and discussed.

Списание Nanomaterials, 9, 875, doi:10.3390/nano9060875

Автори B. Blagoeva, G. Mateev, D. Nazarova, L. Nedelchev

Резюме In this work we study the dependence of the photoinduced birefringence on the initial temperature of the layer, as well as the possibility of recording and termal erasure in thin azopolymer films. To determine the temperature dependence, the sample is placed on a hot stage, which can maintain or vary the temperature of the sample with a high precision. Birefringence is induced with a linearly polarized light from a laser at 442 nm, and its value is determined by measuring the Stokes parameters of a probe laser beam after the sample. The birefringence is erasured by placing the sample on a hot stage.

Списание Journal of Physics and Technology 3(1), 3-7

Автори D. Ivanov, V. Strijkova, L. Nedelchev, D. Nazarova and E. Borisova

Резюме This study presents optical and chemical methods to visualize healthy and malignant sections of histological samples, by using polarized light imaging and staining with different contrast agents. This approach complements the diagnostic conclusions made by the physicians and improves qualitatively healthy versus tumor tissue differentiation, which may be in practical use for pathologistsin their diagnostic conclusions.

Списание Journal of Physics and Technology, 3(1) 14-17

Автори G. Mateev, D. Nazarova and L. Nedelchev

Резюме In this article, we investigate the photoinduced birefringence in commercially available azo-polymer PAZO doped with TiO2 spherical nanoparticles (21 nm size). The films are prepared by spin-coating water solution of the polymer mixed with the nanoparticles with different concentrations from 0 to 10 wt %. To induce the birefringence we use He-Cd laser with wavelength 442 nm. For measurement of the effect, probe laser with wavelength 635 nm is used. The experimental setup includes also polarimeter, which measures the Stokes parameters in real time. We observe increase of the photo-induced birefringence in azopolymer films doped with TiO2 nanoparticles. The maximal value of n is achieved for the sample with 1 wt. % concentration.

Списание Journal of Physics and Technology, 3(1) 18-21

Автори S. Kozhukharov, V. Zhelev and S. Tchaoushev

Книга Advances in Materials Science Research Vol. 37, Ed. By: M. Wythers, NOVA Sci. Publ. pp.101-166; ISBN: 978-1-53615-038-4

Автори Stephan V. Kozhukharov and Christian A. Girginov

Книга Phenomena and Theories in Corrosion Science. Methods of Prevention, ed. By A. Gergely NOVA Sci. Publ. pp. 3 – 150; ISBN 978-153-615253-1