Browsing by Author "Martinov B."
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Item BIOLOGICAL ACTIVITY OF GRAPHENE NANOCOMPOSITES WITH ZINC OXIDE, Cu AND Ag NANOPARTICLES(2023-01-01) Martinov B.; Pavlova E.; Ivanova I.A.; Yocheva L.; Kostadinova A.; Staneva A.The aim of this research is to obtain collagenе nanocomposites based on graphene, graphene oxide, GO, zinc oxide and metal nanoparticles and to evaluate their pro-, antioxidant and biological activities by luminescent and standard microbiological assays. The antimicrobial activity of graphene composites with added nanosized zinc oxide, silver and copper nanoparticles was tested on Firmicutes bacteria Staphylococcus epidermidis (ATCC 1228) and Gracillicutes Escherichia coli (ATCC 25922). The method of diffusion in agar was used in three variants - spot diffusion, well and paper-disc diffusion. The spot and diffusion disc approaches of the method have shown better effect than the well diffusion for testing the effect of graphene composites on bacteria. The composites with high ZnO content had the best antimicrobial properties against the tested bacteria. The cytotoxicity of the nanocomposites using normal MDCK and A549 epithelial sarcoma cells were tested for 24 h at a concentration of 100 mg mL-1. Cancer cells were found to be more sensitive than normal to the graphene composites, proving antitumor activity. The pro and antioxidant effects of the tested nanomaterials depend on the pH level. At physiological conditions, in the Fenton's system, all but RGO+Cu do not appear to be suitable as an implant nanomaterial. In the H2O2 oxidation system all materials present stable antioxidant effects; only ZnO+RGO+Cu is close to control prooxidant levels. When the nanomaterials are tested for oxidation by O2.- radicals, ZnO+RGO and Zn+RGO+Cu show prooxidant effects, as the prooxidant activity is kept for ZnO+RGO even at physiological acidity pH 7.4.Item Collagen/(ZnTiO3/SiO2) composites of an wide spectrum antimicrobial activity(2020-01-01) Vladkova T.; Staneva A.; Albu-Kaya M.; Martinov B.; Ivanova I.The aim of this study is to prepare collagen composites of an increased wide spectrum antimicrobial activity using zinc titanate embedded in a silane matrix (ZnTiO3 /SiO2). Aiming to improve the predictability between the patient outcome and the in vitro antimicrobial testing, four Gram-negative, two Gram-positive bacterial strains, two fungi and seven mixed cultures are used in this study. A higher antimicrobial activity of Collagen/(ZnTiO3/SiO2) composites as compared to that of Collagen/ZnTiO3 ones is found against all test mono-microbial strains. Collagen/(ZnTiO3 /SiO2) composites demonstrate also an well expressed action against the mixed test bacterial cultures, in the most cases a better one compared to that against the bacteria participating in the corresponding mixed bacterial cultures. The increased antimicrobial effect is due to a more homogeneous distribution of the submicron ZnTiO3 aggregates embedded in a silane matrix along the collagen fibrils and a formation of a specific, snail-like structure depicted by SEM. The specific activity against different microbial cells and mixed bacterial cultures is ascribed to features of the microbial cells: size, shape, cell membrane and walls as well as to different exopolymeric substances (EPSs) and their concurrent adsorption in cases of mixed bacterial cultures, as well as the EPSs concurrent adsorption being similar to the concurrent protein adsorption from blood plasma known as Vroman's effect. The wide spectrum high activity against fungi, mono-and multi-specie bacterial cultures makes the studied Collagen/(ZnTiO3 /SiO2) composites a promising antimicrobial biomaterial.Item DIELECTRIC PROPERTIES OF NEW COMPOSITES BASED ON GRAPHENE OXIDE AND NANO-SIZED ZnO(2021-01-01) Staneva A.; Martinov B.; Slavov S.; Karashanova D.; Mateeva J.; Melo B.M.G.; Costa L.C.The objectives of this study were to obtain a complete series of graphene oxide and nano sized zinc oxide and to make a structural and phase characterization of the resulting composites. XRD, SEM, EDX, TEM and impedance analyses were used to investigate these materials as energy storage devices. The complex dielectric permittivity was measured for all samples as a function of frequency. All dielectric characteristics were studied (frequencies from from 100 Hz to 1 MHz and temperatures from 150 to 400 K). The composite GO/ZnO - 80/20 is closer to the desired properties.Item EFFECT OF GRAPHENE OXIDE AND REDUSED GRAPHENE OXIDE ON ELECTRICAL PROPERTIES OF YBCO AND YBCO/Ag COMPOSITES(2020-01-01) Staneva A.; Mateeva J.; Martinov B.; Blagoev B.; Nurgaliev T.Series of bulk composites based on YBCO superconductors, silver and graphene oxide/reduced graphene oxide are synthesized. The aim of this study is to determine the effect of GO and RGO on the superconducting properties of YBCO composites. Series of YBCO and YBCO/Ag phases are synthesized by the ceramic method. GO and RGO are obtained by the modified Hammer’s method. Series of YBCO, YBCO/Ag composites with GO or RGO are studied. The temperatures of the superconducting transition of the obtained composites are determined and the effect of GO/ RGO is investigatedItem Electric properties of new composite materials based on RGO, nanosized ZnO and Cu nanoparticles(2021-02-16) Martinov B.; Slavov S.; Staneva A.; Dimitrov D.; Mateeva J.Graphene nanostructures combine properties that make them an attractive material for various technical applications. The objective of this study is to obtain a series composites based on Reduced graphene oxide (RGO), nano-sized ZnO and Cu nanoparticles. RGO is synthesized by a modified Hammer's method and chemical reduction after sonication. ZnO nanoparticles are obtained by the Sol-Gel method. The composites' phases and structures are characterized by XRD, SEM, TEM, and EDX analysis. It is shown that ZnO is monophasic with a particle size of 48 nm. A series of composite materials based on RGO, ZnO and Cu nanoparticles is obtained by ultrasonic treatment. Electrical and dielectric properties are measured by Impedance analyser and RLC bridge. For comparison, the conductivity for 100% RGO is 309.513 S/m and for 100 % Cu nanosized is 0.0056 S/m. Therefore, it should be initially concluded that increasing the RGO in the mixture will increase the conductivity, and increasing nanosized Cu will decrease it. The presence of cluster regions in the composites with different conductivity is found: 1) nano grains of conductive copper which are partially or completely surrounded by 2) low-conductivity shells of copper oxide and a minimal number of RGO sheets. The present work shows promising results for future research for the creation of new composite materials with controlled shape and size of copper oxide - RGO clusters. Dielectric measurements for RGO/ZnO composites are made in range from 5 mass% to 100 mass% content of RGO, the composites show variable conductivity. For lower concentration of RGO, selected samples are dielectrics.Item NANOCOMPOSITE BIFILLERS BASED ON GRAPHENE OXIDE AND ORGANOBENTONITE(2022-01-01) Mateeva J.; Yoleva A.; Djambazov S.; Martinov B.Nanocomposites and in particular polymer nanocomposites play a significant role in adsorption treatment of wastewaters. The use of nanobifillers in polymer nanocomposites is relatively less explored area in materials science. Graphene oxide (GO) and its nanocomposites have drawn increasing interests in the application of water treatment, especially for the removal of metal-containing pollutants. GO has large surface area, due to its sheet-like structure. Recently, GO and clay minerals like bentonite (montmorillonite) find an application as adsorbents because of their good adsorption capacity and low cost. GO nano-sheets were synthesized using a modified Hummers’ method from graphite powder. A natural bentonite from Bulgaria deposit was activated and organically modified by using octadecylamine (ODA) to obtain organobentonite (OB). Nanocomposites based on organobentonite and graphene oxide (OB: GO = 10: 1 and OB: GO = 5: 1) were prepared through a facile ultrasonic-assisted method and exhibited excellent synergetic adsorption behavior for methylene blue (MB). The synthesized nanocomposite bifillers were characterized by XRD and FTIR and their adsorption capacities to methylene blue (MB) were determined.Item Preparation and antimicrobial activity of collagen/(RGO/ZnO/TiO2/SiO2) composites(2020-01-01) Staneva A.; Albu-Kaya M.; Martinov B.; Ivanova I.; Vladkova T.A serial investigation is initiated aiming to explore the biological activity of some newly synthetized chemical compounds for the development of novel antimicrobial collagen based biomaterials. Collagen/ZnTiO3, Collagen/RGO, Collagen/(Ag/RGO), Collagen/(Ag/RGO/SiO2) and Collagen/(ZnTiO3/SiO2) composites have been so far studied and all of them demonstrate a specific antimicrobial activity against Gram-negative and Gram-positive bacteria and in some cases against fungi. The aim of this investigation is to develop new antimicrobial collagen biomaterials using RGO, ZnO, and TiO2 embedded in TEOS as another antimicrobial agent, combining the biological activity of RGO, ZnO and TiO2 with the dispersing effect of SiO2. The new Collagen/(RGO/ZnO/TiO2/SiO2) composites demonstrate an antimicrobial activity dependent on the agent loading level. It is specific in respect to Gram-negative, Grampositive bacteria and fungi. An optimal balance between the antimicrobial activity and the cytotoxicity is achieved by varying the concentration of the antimicrobial agent, RGO/ZnO/TiO2/SiO2, in Collagen/(RGO/ZnO/TiO2/SiO2) composites. It is suggested that the mechanism of the antimicrobial action includes the simultaneous proceeding of (i) metal ions chelation; (ii) free oxygen radicals formation due to the interactions between the microbial cells and the antimicrobial agent; (iii) mechanical demolition of the cell walls and membranes by RGO crystal nanoparticles. The broad spectrum antibacterial and anti-fungal activity combined with the low cytotoxicity at an optimal Collagen/ Antimicrobial agent ratio makes the studied Collagen/(RGO/ZnO/TiO2/SiO2) composites a promising antimicrobial material increasing the medical biomaterials assortment. © 2020 by the authors.Item Synthesis and characterization of ybco and ybco/ag superconducting ceramic composites containing reduced graphene oxide(2019-01-01) Mateeva J.; Staneva A.; Martinov B.; Blagoev B.; Nurgaliev T.The high-temperature ceramic superconductors and nanomaterials are among the most promising groups of materials that are intensively studied. Graphene is a new material with unique electrical, optical and biological properties. The high mobility of graphene charges implies the possibility of using it as a suitable additive improving the superconductors' physical characteristics. The idea of this study is to synthesize series of bulk composites based on YBCO superconductors, silver and RGO with superconducting properties. The YBCO phase is obtained by a ceramic technology. It includes Ag aiming better sintering. Graphene is synthesized by a modified Hammer's method. XRD is applied to verify the obtaining of the starting phases. The synthesized composites are characterized by XRD, SEM and EDX analysis. All composites prepared are superconductive and have a Meissner effect.