Browsing by Author "Stankov S."

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    COMPARATIVE ANALYSIS OF ANTIBACTERIAL ACTIVITY AND PHYSICOCHEMICAL PROPERTIES OF PEPPERMINT AND CORNMINT ESSENTIAL OILS AND THEIR MAIN COMPOUND MENTHOL
    (2023-01-01) Gandova V.; Fidan H.; Iliev I.; Lasheva V.; Stankov S.; Stoyanova A.; Yavorov N.
    The aim of the present paper was to study and present a comparative analysis of the antibacterial activity and physicochemical properties of commercial mint essential oils from two different species - peppermint (Mentha piperita Huds. (L.)) and cornmint (Mentha arvensis L.). Peppermint oils exhibited weak antibacterial activity, but were more pronounced against Gram-positive bacteria Staphylococcus aureus (1.3 - 2.0 mm) and Gram-negative bacteria Escherichia coli (1.2 - 1.9 mm) and Klebsiella sp. (2.6 mm). The essential oil of the species M. arvensis did not exhibit antimicrobial activity against the tested cultures. L-menthol, which is a commercial sample isolate, exhibited activity against all tested microorganisms, with the exception of Gram-positive bacteria Listeria monocytogenes and Bacillus cereus. The diameter of the inhibition zones was the largest against Gram-positive bacteria Bacillus subtilis (3.4 mm), and the smallest was against Gram-positive bacteria Staphylococcus aureus (1.6 mm) and Gram-negative bacteria Klebsiella sp. (1.4 mm). The surface tension, density and refractive index of different mint essential oils were determined experimentally. The surface energy and surface heat capacity were calculated based on the calculations of surface tension. All experiments and calculations were provided at a temperature range between 6℃ and 30℃. A dependence between surface tension and temperature was not observed.
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    MICROWAVE-ASSISTED SYNTHESIS OF SILVER NANOPARTICLES AS CATALYST FOR AIR GAS-DIFFUSION ELECTRODES
    (2022-01-01) Mladenova B.; Stankulov T.; Stankov S.; Boukoureshtlieva R.; Momchilov A.; Karsheva M.; Hinkov I.
    In this work, silver nanoparticles were synthesized from silver nitrate aqueous solution by using microwave irradiation. The morphology of the synthesized nanoparticles was studied by UV-Vis spectroscopy and by transmission electron microscopy showing spherical shapes with diameters of about 10 nm. The X-ray diffraction analysis exhibits their crystalline nature with face-centered cubic structure. Afterward, silver nanoparticles were integrated into activated carbon Norit NK to obtain catalytic composites. These composites were incorporated in the active layer of air gas-diffusion electrodes in order to study the catalytic activity for oxygen reduction reaction. For this purpose, a non-aggressive electrolyte solution of NaCl was used as an electrolyte. The obtained electrochemical results show a stable operation of all electrodes. Higher performance was found for composites containing 2 % wt. silver nanoparticles.
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    Plant mediated synthesis of silver nanoparticles using extracts from Tilia cordata, Matricaria chamomilla, Calendula officinalis and Lavandula angustifolia flowers
    (2018-01-01) Mladenova B.; Diankov S.; Karsheva M.; Stankov S.; Hinkov I.
    In this work, a simple, rapid and efficient green method for silver nanoparticles synthesis was developed by using extracts from four medicinal plants: flowers of Tilia cordata, Matricaria chamomilla, Calendula officinalis and Lavandula angustifolia, acting as reducing and capping agents. Silver nitrate was used as a precursor. The effect of the sunlight exposure and/or the sonication treatment on the synthesis process was investigated. The ultraviolet-visible spectroscopy (UV-Vis) studies showed surface Plasmon resonance peaks between 432 nm and 448 nm, characteristic of spherical silver nanoparticles. The morphology of the nanoparticles produced was investigated by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The proposed method could be easily implemented for any kind of scientific or industrial application due to its cost effective nature.
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    SONOCHEMICAL SYNTHESIS OF SILVER NANOPARTICLES FOR GAS-DIFFUSION ELECTRODES APPLICATION
    (2024-01-01) Mladenova B.; Stankulov T.; Stankov S.; Karsheva M.; Hinkov I.; Momchilov A.; Boukoureshtlieva R.
    The aim of the current study is to explore the application of silver nanoparticles (AgNPs) as a catalyst in air gas-diffusion electrodes (AGDE). AgNPs have been successfully synthesized through an original sonochemical method. The effects of reagent concentrations, temperature and reaction time were studied as well. Composites of AgNPs and activated carbon (Norit NK) were prepared by using two adsorption methods to demonstrate the catalytic activity. Method 1 involves homogenizing preliminary prepared AgNPs colloid solution and Norit NK followed by evaporation of the suspension. One-step route was used for Method 2, i.e. all components were mixed altogether and sonicated, resulting in the AgNPs formation directly on the surface and inside the pores of Norit NK. Additionally, some of the samples were thermally treated at 300°C for 1h under air and argon. The composites were characterized by X-ray diffraction (XRD), scanning electron microscopy with energy dispersive X-Ray analysis (SEM/EDX) and transmission electron microscopy (TEM). The catalytic activity of the obtained composites regarding oxygen reduction reaction (ORR) was investigated in AGDE using an aqueous 4M NaCl electrolyte. The thermally treated composite obtained via Method 2 appears to show improved electrochemical catalytic activity regarding ORR in comparison to the Method 1 samples.