Browsing by Author "Hinkov I."
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Item Carbon dioxide capture by adsorption (review)(2016-01-01) Hinkov I.; Lamari F.D.; Langlois P.; Dicko M.; Chilev C.; Pentchev I.The present paper reviews the different types of adsorbents that could be used for CO2 capture from flue gases. They include carbon-based adsorbents, zeolites, molecular sieves, metal-organic frameworks, hydrotalcite-like compounds and advanced adsorbents. Their possibilities are described and confronted. In particular, it has been demonstrated that classical adsorbent materials need further functionalization or impregnation with different nitrogen-containing species in order to become suitable for CO2 capture. The different methods for CO2 capture by adsorption cyclic processes such as Pressure Swing Adsorption (PSA), Vacuum Swing Adsorption (PSA), Thermal Swing Adsorption (TSA), Electric Swing Adsorption (ESA) as well as the combination of TSA and chemical reaction, known as Thermal Swing Sorption-Enhanced Reaction (TSSER), are also mentioned in the cited literature.Item COMPARATIVE STUDY OF POLYPHENOLIC COMPOSITION AND REDUCING PROPERTIES OF EXTRACTS OF CANNABIS SATIVA L., CANNABIS INDICA AND CANNABIS HYBRID(2024-01-01) Vetskov N.; Kaloyanov N.; Hinkov I.; Diankov S.; Pashova K.; Gerasimova V.; Funeva-Peycheva M.; Yankovska-Stefanova T.; Garbev A.Cannabis Sativa L., Cannabis Indica and Cannabis Hybrid are widespread plant species that find legal use in many countries around the world. The first of them has found considerable application in medicine, cosmetics, building construction, textile and food industries. Cannabis Indica and Cannabis Hybrid are used as raw materials for the extraction of oils that are used as pain relievers as well as for recreation purposes. The aim of the present study is to compare the polyphenolic composition of extracts from the three types of plants, by the maceration method of dry green leaf mass consisting of inflorescences and leaves. Ethanol (99.9 %) is used as an extractant, and the duration of the maceration process is seven days, at room temperature and stirring. The polyphenol composition is determined using HPLC-DAD method. The following polyphenol composition is found in extracts of Cannabis Sativa L.: epigallocatechin, rutin, myricetin, quercetin and kaempferol. On the other hand, quercetin and kaempferol are detected in the Cannabis Indica samples, while epigallocatechin, quercetin and kaempferol are present in the hybrid sort. In addition, green synthesis of silver nanoparticles by using sunlight or microwave irradiation was carried out to evaluate the reduction properties of three cannabis extracts. The least quantity of silver nanoparticles was obtained in the presence of Cannabis Sativa L. extracts. From the other two species of cannabis, microwave irradiation produces more silver nanoparticles, with smaller sizes, while exposure to direct sunlight produces fewer particles, but with larger sizes.Item Conventional Ultrasound And Microwave-Assisted Extraction Of Phenolic Compounds From Mavrud Grape, Pomace And Seeds(2020-01-01) Chimshirova R.; Karsheva M.; Diankov S.; Hinkov I.This research is dedicated to the valorization of wine industry residual products as a source of valuable compounds of interest to the food, pharmaceutical and cosmetic industries. Whole grape seeds and grape pomace of organic Mavrud of Bulgarian origin, harvested in 2016 were used. The aqueous ethanolic extracts were prepared by three different techniques: conventional extraction via magnetic stirring, ultrasound-assisted extraction (US) and microwave-assisted extraction (MW). Microwaves seemed to be more efficient in extracting the desired substances through the hard shell of the seeds. The polyphenolic content of the seed extracts, expressed as GAE (gallic acid equivalent) was remarkably higher: 58.57 mg GAE g-1 dw (first stage of MW-assisted extraction) compared to the conventional extraction with a magnetic stirrer – 7.03 mg GAE g-1 dw. The results obtained for the US extraction were similar - 49.73 mg GAE g-1 dw, but the processing time in the microwave field was considerably shorter. The US-assisted extraction was not so convenient for the grape pomace extracts - 20.47 mg GAE g-1 dw were obtained for 240 min, compared to 75.02 mg GAE g-1 dw for 20 min in the microwave field. Three analyzes were performed to determine the total polyphenolic content, antioxidant activity and anthocyanin content. The MW-assisted extraction was the most suitable for leaching of polyphenolic compounds and the extracts exhibited a higher antioxidant activity. Contrarily it has been observed that the anthocyanin content of the extracts produced via MW-assisted extraction was lower compared to the conventional and US-assisted techniques. Both techniques revealed time and energy-saving and are considered as eco-procedures.Item Erratum: Computer-aided design of graphene and 2D materials synthesis via magnetic inductive heating of 11 transition metals (J. Phys. D: Appl. Phys. (2021) 55 (105302) DOI: 10.1088/1361-6463/ac357d)(2022-04-14) Dhaouadi E.; Hinkov I.; Pashova K.; Challab N.; Roussigné Y.; Abderrabba M.; Farhat S.There was a typo during the production of our paper. The alpha factor has been omitted from equation (3). The equation should appear as follows: ρ = ρ0 (1 + αT).Item Extraction of valuable compounds from Bulgarian St. John'S Wort (Hypericum Perforatum L.). Antioxidant capacity and total polyphenolic content(2019-01-01) Chimshirova R.; Karsheva M.; Diankov S.; Hinkov I.Ethnobotanical reports claim that St. John's Wort has important biological and chemical properties and can be used for the treatment of many diseases. Most studies related to Hypericum perforatum are focused on its antidepressant effects, but it has also been examined for wound healing, antiviral and antimicrobial activity. In this study the experiments are carried out at ambient temperature at constant stirring rate. The solvent is a 50 vol. % water-ethanolic solution, the solid-to-liquid ratio is 1:10 g mL-1, and the average particle size is 0,75 mm. The results show that the antioxidant capacity and the total polyphenolic content are superior compared to those reported by other authors. The liquid extracts have been stored in a refrigerator for 9 months, then they have been examined again and it has been noticed that their antioxidant effect and total polyphenolic content are preserved. After drying at 50°C, storage and regeneration, the quality of the extracts did not change, which is important issue for long term transportation and storage.Item Graphene synthesis by electromagnetic induction heating of oxygen-rich copper foils(2023-02-01) Dhaouadi E.; Alimi W.; Konstantakopoulou M.; Hinkov I.; Abderrabba M.; Farhat S.We report in this article an optimized synthesis of high-quality monolayer graphene by chemical vapor deposition (CVD) using methane as carbon source. The synthesis occurs on a centimeter-sized copper substrate previously oxidized in air at 180 °C then heated by electromagnetic induction in a controlled atmosphere of argon and hydrogen in which the steady-state temperature of ∼1050 °C is reached after only ∼2 min heating from room temperature. The rapidity of the heating and cooling process highlights the advantage of using the electromagnetic induction heating method in order to achieve the synthesis of graphene. When applied to CVD, electromagnetic inductive heating only heats the metallic substrate avoiding energy losses in the reaction medium. Therefore, inductive heating has great potential for large-scale and rapid manufacturing of graphene and 2D materials. This work includes an experimental study that consists in comparing the quality of the synthesized graphene over different substrates ranging from oxygen-free copper foil to oxidized copper with the aim to reduce the defects in the graphene and also to increase the domain size. We find that copper with an oxidized surface can drastically reduce graphene nucleation density thereby increasing the graphene domain sizes. In addition, we demonstrated experimentally and by numerical simulations that the presence of a thin layer of copper oxide does not disturb the mechanism of induction heating allowing the growth of high-quality graphene films by inductive heating of copper oxides. Graphene quality was studied by Raman spectroscopy, and scanning electron microscopy (SEM) which respectively showed very little defective monolayer graphene (ID / IG < 0.2) and non-defective graphene subdomains of average size ranging from ∼5 μm to∼10 μm which is comparable, if not better, to traditional thermal CVD method. These results provide directions for effective control of the defects and layers of graphene.Item Graphene synthesis by electromagnetic induction heating: Domain size and morphology control(2024-04-01) Dhaouadi E.; Alimi W.; Hinkov I.; Abderrabba M.; Farhat S.In this paper we discuss the effect of hydrogen and methane content during low-pressure chemical vapor deposition (LPCVD) of graphene on inductively heated copper foils. By increasing the H2/CH4 ratio by a factor of 5 from 25 to 125, different graphene morphologies ranging from dendritic fractals to compact hexagonal islands are obtained. In addition, increasing the hydrogen concentration allows the nucleation rate to be slowed down by a factor of ∼10 thereby high-quality regular hexagonal graphene single crystals of significant size of 0.1 mm are found. From these measurements, we estimate the activation energy for graphene nucleation in low-pressure CVD (2 eV) and propose a phenomenological law for graphene nucleation. As compared to conventional CVD methods, considerable advantages of inductive heating are outlined, and some fundamental aspects of this approach are discussed.Item Graphene synthesis by inductively heated copper foils: Reactor design and operation(2020-04-01) Pashova K.; Dhaouadi E.; Hinkov I.; Brinza O.; Roussigné Y.; Abderrabba M.; Farhat S.We report on the design of a reactor to grow graphene via inductively heating of copper foils by radio frequency (RF) magnetic fields. A nearly uniform magnetic field induced by Helmholtz-like coils penetrates the copper foil generating eddy currents. While the frequency of the current is being rapidly varied, the substrate temperature increases from room temperature to ~1050 °C in 60 s. This temperature is maintained under Ar/H2 flow to reduce the copper, and under Ar/H2/CH4 to nucleate and grow the graphene over the entire copper foil. After the power cut-off, the temperature decreases rapidly to room temperature, stopping graphene secondary nucleation. Good quality graphene was obtained and transferred onto silicon, and coated with a 300 nm layer of SiO2 by chemical etching of the copper foil. After synthesis, samples were characterized by Raman spectroscopy. The design of the coils and the total power requirements for the graphene induction heating system were first estimated. Then, the effect of the process parameters on the temperature distribution in the copper foil was performed by solving the transient and steady-state coupled electromagnetic and thermal problem in the 2D domain. The quantitative effects of these process parameters were investigated, and the optimization analysis results are reported providing a root toward a scalable process for large-sized graphene.Item Graphene synthesis by microwave plasma chemical vapor deposition: Analysis of the emission spectra and modeling(2019-04-03) Pashova K.; Hinkov I.; Aubert X.; Prasanna S.; Bénédic F.; Farhat S.In this article, we report on some of the fundamental chemical and physical processes responsible for the deposition of graphene by microwave plasma-enhanced chemical vapor deposition. The graphene is grown by plasma decomposition of a methane and hydrogen mixture (CH4/H2) at moderate pressures over polycrystalline metal catalysts. Different conditions obtained by varying the plasma power (300-400 W), total pressure (10-25 mbar), substrate temperature (700 °C-1000 °C), methane flow rate (1-10 sccm) and catalyst nature (Co-Cu) were experimentally analyzed using the in situ optical emission spectroscopy technique to assess the species rotational temperature of the plasma and the H-atom relative concentration. Then, two modeling approaches were developed to analyze the plasma environment during graphene growth. As a first approximation, the plasma is described by spatially averaged bulk properties, and the species compositions are determined using kinetic rates in the transient zero-dimensional (0D) configuration. The advantage of this approach lies in its small computational demands, which enable rapid evaluation of the effects of reactor conditions and permit the identification of dominant reactions and key species during graphene growth. This approach is useful for identifying the relevant set of species and reactions to consider in a higher-dimensional model. The reduced chemical scheme was then used within the self-consistent two-dimensional model (2D) to determine auto-coherently the electromagnetic field, gas and electron temperatures, heavy species, and electron and ion density distributions in the reactor. The 0D and 2D models are validated by comparison with experimental data obtained from atomic and molecular emission spectra.Item 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.Item One-step synthesis of graphene, copper and zinc oxide graphene hybrids via arc discharge: Experiments and modeling(2020-04-01) Kane A.; Hinkov I.; Brinza O.; Hosni M.; Barry A.H.; Cherif S.M.; Farhat S.In this paper, we report on a modified arc process to synthetize graphene, copper and zinc oxide graphene hybrids. The anode was made of pure graphite or graphite mixed with metals or metal oxides. After applying a controlled direct current, plasma is created in the interelectrode region and the anode is consumed by eroding. Continuous and abundant flux of small carbon, zinc or copper species, issued from the anode at a relatively high temperature, flows through the plasma and condenses in the vicinity of a water-cooled cathode leading to few-layered graphene sheets and highly ordered carbon structures. When the graphite rod is filled with copper or zinc oxide nanoparticles, few layers of curved graphene films were anchored with spherical Cu and ZnO nanoparticles leading to a one-step process synthesis of graphene hybrids, which combine the synergetic properties of graphene along with nanostructured metals or semiconducting materials. The as-prepared samples were characterized by Raman spectroscopy, X-ray diffraction (XRD), spatially resolved electron energy loss spectroscopy (EELS), energy filtered elemental mapping and transmission electron microscopy (TEM). In addition to the experimental study, numerical simulations were performed to determine the velocity, temperature and chemical species distributions in the arc plasma under specific graphene synthesis conditions, thereby providing valuable insight into growth mechanisms.Item Optimization of extraction of antioxidants from Bulgarian Mavrud by-Products(2018-01-01) Dimcheva V.; Karsheva M.; Diankov S.; Hinkov I.Grape pomace, an abundant by-product from the wine industry, is released in large quantities but remains unused. This investigation is focused on the optimization of extraction from grape pomace and grape seeds discharged as wastes from the Bulgarian Mavrud winery. The optimized parameters refer to the solid to the solvent ratio effect (1/10 g mL-1, 1/20 g mL-1, 1/30 g mL-1, 1/40 g mL-1, 1/50 g mL-1) and the solvent concentration (water or aqueous ethanol solutions) in relation on the total polyphenol, flavonoid and anthocyanin contents. The Trolox equivalent antioxidant capacity is also determined. All analyses are performed spectrophotometrically. All extraction runs are carried out at constant extractive parameters chosen previously - temperature, sample particle size, extraction manner. The optimal conditions are selected for the Mavrud seeds and pomace - 50 % ethanol in water solution, 1:20 g ml-1 solid to solvent ratio. The selection of the best extraction conditions is of great importance because it could alter the characteristics of the final extract having an economic impact on the extraction evaluation of the Bulgarian bio-Mavrud by-products.Item 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.Item Rheological properties of cosmetic compositions containing rosemary and grapefruit pulp and seeds extracts(2014-01-01) Petkova-Parlapanska K.; Nancheva V.; Diankov S.; Hinkov I.; Karsheva M.Modern cosmetic formulations represent a combination of synthetic and natural ingredients. Knowledge of the rheological behavior allows managing the structure and the quality of the products by means of supplement addition, regime change or mechanical and technological processing adjustment. The extraction from rosemary and grapefruit pulp and seeds with ethanol-in-water solutions is carried out in a thermostated shaker at 20°C. The total polyphenol content (TPC) and the antioxidant capacity (AOC) of the extracts are determined. The cosmetic compositions prepared are based on stearic acid, with 3-ethanolamine as emulsifier. Extracts are used as a preservative, instead of controversial lately methyl-parabens. The rheological behavior of the cosmetic formulations is studied. It is found that all compositions exhibited thixotropic flow behavior. The cosmetic emulsion containing an alcoholic extract of grapefruit seeds has different rheological behavior which may be described by the Bingham rheological model. This effect can be explained by the composition of the extract and its influence on the pH. It is found that the extract from grapefruit seeds has acidic properties and affects the pH of the medium. The cosmetic formulation with rosemary extract exhibits power low and thixotropic behavior. The purity of the cosmetic formulations was tested against E. Coli bacteria. It is found that no bacterial colonies are observed neither after one day, nor after three months of storage. It can be concluded that the natural extracts from rosemary and grapefruit seeds and pulp exhibit good antibacterial properties, being quite satisfactory substitute of the parabens.Item SILVER NANOPARTICLES INCORPORATED INTO POROUS COLLAGEN SCAFFOLDS(2022-01-01) Samichkov V.; Mladenova B.; Diankov S.; Foteva T.; Hinkov I.In this work, we present a simple, rapid and cost-effective microwave-assisted method for silver nanoparticles synthesis by chemical reduction using an aqueous solution of silver nitrate, ethanol and ammonia. The effect of reagent concentrations and the heating time was investigated. Samples were analyzed through UV-Vis spectroscopy and transmission electron microscopy. Furthermore, the obtained silver nanoparticles were mixed with collagen dispersions to prepare porous scaffolds by vacuum-sublimation drying. Silver nanoparticles can adhere to the bacterial cell wall subsequently causing its death. The microbiological study showed that the collagen scaffolds with incorporated silver nanoparticles exhibited antibacterial activity against Bacillus subtilis NBIMCC 3562 and Escherichia coli NBIMCC 407 bacteria and could find potential applications in medicine.Item 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.Item THE EFFECT OF RADIATION PRESERVATION ON MACROMOLECULE CONTENT, ANTIMICROBIAL ACTIVITY AND RADICAL - SCAVENGING POTENTIAL OF PORTULACA OLERACEA L. LEAVES EXTRACT(2024-01-01) Karamalakova Y.; Petkova-Parlapanska K.; Hinkov I.; Vetskov N.; Georgieva E.; Nikolova G.The Portulaca oleracea L. (P. oleracea) phytochemical composition (contains omega-3 fatty acids, quercetin, rutin, gallo tannins, proteins) is responsible for its biological effects, e.g. antioxidative, antibacterial, anti-inflammatory and antiradiation activity. The plant has a C4 metabolism and it is known for its tolerance to different stressors, such as salinity, no water, high - temperature conditions, and 9 kGy radiation. The purpose of the present investigation was focused on the inhibitory effects of P. oleracea leaves extracts against 5 - 10 kGy radiation - induced abiotic stress, and assess to total phenol, flavonoid, and tannins content; antibacterial potential (Staphylococcus aureus ATCC25923, Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853) and antioxidant capacity. The free dried, powdered P. oleracea samples were 60Co irradiated, 50 % ethanol solvent (v/v) extracted and compared to 0 kGy extract. 10 kGy irradiated extract provide to be potentially effective against bacterial strains and possessed stable antioxidant activity, towards DPPH (p ˂ 0.002), ABTS•+ (p ˂ 0.05), FRAP (p ˂ 0.05) and NO (p ˂ 0.002). In addition, highly sensitive Electron Paramagnetic Resonance (EPR) was used to evaluate antiradical capacity. Single, symmetrical signal (g = 2.0023) was recorded in 10 kGy irradiated P. oleracea extract, comparable to the EPR signal in non-irradiated sample and depended directly on the antiradical potential. Moreover, in vitro P. oleracea inhibited the superoxide anion (•O2 −), hydroxyl (•OH), alkyl radicals, and exhibited antioxidant properties against 10 kGy irradiation.Item Ultrasound-assisted green synthesis of silver nanoparticles and their incorporation in antibacterial cellulose packaging(2015-04-01) Popov V.; Hinkov I.; Diankov S.; Karsheva M.; Handzhiyski Y.The antimicrobial activity of nanoparticles (NPs) depends of the surface area in contact with microorganisms. The large surface area of the nanoparticles enhances their interaction with the microbes. In this work, a green, simple, rapid, and efficient ultrasound-assisted reduction method for silver nanoparticles (AgNP) synthesis is presented. For the synthesis, an aqueous solution of silver nitrate, ethanol, and ammonia was used. The adopted method can be easily implemented for any kind of scientific or industrial application due to its cost-effective nature. The effect of sonication time on the nanoparticle formation was investigated. Silver nanoparticles were analyzed through transmission electron microscopy and UV-vis spectroscopy. Antimicrobial additives can be incorporated in mass in different matrixes (polymeric or cellulosic), which is a convenient methodology to achieve antimicrobial activity. In this work, silver nanoparticles were incorporated in cellulose using an ultrasonic bath technique. The most important aspect of cellulose containing silver nanoparticles prepared by this method is its high antimicrobial efficiency. The microbiological study was carried out by a standard agar technique. The analysis showed that cellulose with incorporated silver nanoparticles exhibited strong antimicrobial activity against Escherichia coli bacteria. This makes it a promising antibacterial material for food packaging.