Browsing by Author "Kovacheva D."

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    Characterization of art materials and degradation processes in the exterior wall paintings of the main church of Rila Monastery, Bulgaria
    (2023-09-01) Stamboliyska B.; Tapanov S.; Kovacheva D.; Atanasova-Vladimirova S.; Ranguelov B.; Yancheva D.; Velcheva E.; Stoyanov S.; Guncheva M.; Fischer D.; Lederer A.
    The present study focused on the characterization of the art materials and the degradation processes in the exterior (exonarthex) mural paintings of the main church of the Nativity of the Virgin in Rila monastery, Bulgaria, which is believed to be the last large-scale example of Eastern Orthodox wall painting. For the first time the art materials used to create a unique and colourful polychrome decoration of the outer gallery and the possible degradation products, caused by atmospheric influence - permanent exposure to open air and moisture - were revealed by a multi-technique approach. The mineral pigments were identified by means of attenuated total reflectance Fourier transform infrared (ATR-FTIR), micro-Raman spectroscopy, scanning electron microscopy energy dispersive X-ray spectroscopy (SEM-EDS) and X-ray diffraction (XRD). The natural yellow, red ochre, and green earth pigments, as well as some synthetic ones such as ultramarine and vermillion, were found in stable condition. Minium and emerald green pigments showed chemical transformations due to adverse environmental conditions which lead to chromatic changes of wall paintings. Black discolouration occurred due to the conversion of orange minium to black plattnerite (PbO2) and white discolouration – due to its transformation to white lead carbon oxide (PbCO3). The copper acetoarsenite (Cu(CH3COO)2.3Cu(AsO2)2) in the emerald green pigment showed partial transformation to arsen-containing mineral phases clinoclase and lindackerite, which fortunately did not affect much the colour appearance. Gypsum and calcium oxalate were found in the majority of the microsamples as decay products. Analysis of the binders by enzyme-linked immunosorbent assay (ELISA) implied the use of the Orthodox Church post-Byzantine egg-tempera technique. The registered Ca metal oxalates in accordance with ELISA results suggested binder chemical degradation induced by external factors. Most of the used painting materials are close to the those found in other Eastern Orthodox Byzantine and post-Byzantine monuments which indicates that the wall painting decoration of the main church of Rila monastery continues the post-Byzantine traditions. On the other hand, the study showed that the exonarthex wall paintings of the main charge of Rila monastery bear some new features as the religious artists supplemented the colourful scheme by emerald green as a new pigment and replaced smalt by the brighter synthetic ultramarine.
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    Exploring the impact of different soft reducers in Reduced Graphene Oxide synthesis
    (2019-01-01) Kichukova D.; Staneva A.; Kovacheva D.; Tzoneva I.
    The most common way for the synthesis of bulk graphene is based on exfoliation of graphite. This method involves oxidation of graphite using highly oxidizing agents and subsequently reducing graphene oxide (GO) to graphene. Reduced graphene oxide (RGO) is prepared from reduction of GO by thermal, chemical or electrical treatments. Different reducing agents are widely used for the chemical reduction. However, many of the commonly used reducing agents are hazardous. To decrease the level of toxicity, ``green`` materials recently were applied. The present study reports the preparation of RGO with the use of mild and nontoxic reducing agents. The obtained materials were characterized by X-Ray powder diffraction, Raman and FTIR spectroscopy, as well as by SEM and TEM electron mycroscopy. The results show that at the conditions choosen for the experiment glycerol, ethylene glycol and citric acid do not reduce GO. The use of succrose, acetic acid and oxalic acid in the rection process resulted in an incomplete reduction of GO, The use of urea, L-ascorbic acid and glycine leaded to the removal of interlayer functional groups in GO where the complete reduction of GO is acheaved in the case of the use of L-ascorbic acid and glycine. The results show that L-ascorbic acid and glycine reduce graphene oxide effectively but, depending on the type of the reducer, the morphological properties of the obtained RGO are different.
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    Facile Synthesized Cu–RGO and Ag–RGO Nanocomposites with Potential Biomedical Applications
    (2022-06-01) Kichukova D.; Spassova I.; Kostadinova A.; Staneva A.; Kovacheva D.
    In the present study, we report on the facile prepared nanocomposites of reduced graphene oxide RGO with Cu and Ag. The synthesis was performed through an environmentally friendly and easy method by simultaneous reduction in solutions containing Cu2+ or Ag+ and graphene oxide (GO) using zinc powder as a reducing agent in aqueous acidic media. The composites are characterized by powder X-ray diffraction, low-temperature nitrogen adsorption, X-ray photoelectron and FTIR and Raman spectroscopies, as well as Scanning and Transmission electron microscopies. The antibacterial activity of the composites was tested for Staphylococcus aureus, Escherichia coli and antifungal activity for Candida albicans. The cytotoxicity of the materials was studied towards two types of eukaryotic cells—MDCK II and A549 cell lines. The composites obtained consist of homogeneously distributed Cu and Ag nanoparticles on the surface of graphene sheets and manifest good antimicrobial activity and high cytotoxicity. The results clearly show that both metal–RGO composites can be successfully used as antimicrobial and anticancer agents.
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    Organic/inorganic bioactive materials part III: In vitro bioactivity of gelatin/silicocarnotite hybrids
    (2009-10-19) Radev L.; Fernandes M.; Salvado I.; Kovacheva D.
    In this work we present our experimental results on synthesis, structure evolution and in vitro bioactivity assessment of new gelatin/ silicocarnotite hybrid materials. The hybrids were obtained by diluting gelatin (G) and silicocarnotite (S) ceramic powder with G:S ratios of 75:25 and 25:75 wt.% in hot (40°C) water. The hybrids were characterized using XRD, FTIR, SEM/EDS and XPS. FTIR depicts that the ``red shift`` of amide I and COO- could be attributed to the fact that the gelatin prefers to chelate Ca2+ from S. The growth of calcium phosphates on the surface of the hybrids synthesized and then immersed in 1.5 SBF for 3 days was studied by using of FTIR, XRD and SEM/EDS. According to FTIR results, after an immersion of 3 days, A and B-type CO3HA can be observed on the surface. XRD results indicate the presence of hydroxyapatite with well defined crystallinity. SEM/EDS of the precipitated layers show the presence of CO3HA and amorphous calcium phosphate on the surface of samples with different G/S content when immersed in 1.5 SBF. XPS of the G/S hybrid with 25:75 wt.% proved the presence of Ca-deficient hydroxyapatite after an in vitro test for 3 days. © Versita Warsaw and Springer-Verlag Berlin Heidelberg 2009.
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    Synthesis of Antibacterial Nanocomposites with Reduced Graphene Oxide and Silver Nanoparticles
    (2021-01-01) Staneva A.; Kichukova D.; Kovacheva D.
    A technological scheme for the production of nanocomposite materials based on silver nanoparticles and graphene has been developed. It involves the preparation of graphene oxide and the subsequent in-situ preparation of both reduced graphene oxide (RGO) and silver nanoparticles in the composites by the addition of AgNO3and reduction using sodium borohydride. The obtained composites were characterized by X-ray diffraction, Raman spectroscopy, and Scanning Electron Microscopy. It has been shown that the silver nanoparticles have an average size of about 35 nm and are evenly dispersed in the composites. According to Raman spectroscopy data, the number of graphene layers is in the range of 1-2 for the developed composites as well as in pure RGO. All composite materials with silver nanoparticles and graphene have antibacterial properties against Staphylococcus aureus and Escherichia coli. As the content of silver nanoparticles in the composites increases, the antibacterial activity against the two studied bacteria increases too. The activity of all samples against Escherichia coli was higher than against Staphylococcus aureus.