Miha Drofenik

University of Maribor, Maribor, Maribor, Slovenia

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Publications (238)445.4 Total impact

  • [Show abstract] [Hide abstract] ABSTRACT: A proton transfer compound, (ABTH)+(PydcH)− (1), obtained from 2-aminobenzothiazole (ABT) and 2,6-pyridinedicarboxylic acid (Pydc) as well as its Eu(III), Tb(III), and Cu(II) complexes (ABT)3[Eu(Pydc)3]⋅5H2O (2), (ABT)3[Tb(Pydc)3]⋅5H2O (3) and (ABT)[Cu(Pydc)(PydcH)]⋅3H2O (4) were obtained under ambient conditions and structurally verified by single-crystal X-ray diffraction analyses and further characterized by elemental analyses, powder X-ray diffraction (PXRD), infrared spectroscopy (IR), thermogravimetric analysis (TGA), and magnetic measurements. Compounds 2-4 are the first known solids containing complex anions with Pydc ligands, 2-aminobenzothiazole cations (ABT), and solvate water molecules. During the synthesis of 3, a secondary phase with the formula ABTCl∙H2O was obtained and characterized by elemental analysis and single-crystal X-ray diffraction. The asymmetric unit of 5 consists of six symmetry independent ABT cations, six chlorides, and six water molecules. The two lanthanide complexes showed characteristic emissions of Eu3+ and Tb3+ ions. The good solubilities of these complexes in water and their luminescence properties make them attractive luminescent labels of biological molecules.
    No preview · Article · Apr 2016 · Journal of Coordination Chemistry
  • [Show abstract] [Hide abstract] ABSTRACT: There is increasing demand for the production of large quantities of aqueous suspensions of magnetic iron-oxide nanoparticles. Amino acids are one possible type of inexpensive, nontoxic, and biocompatible molecules that can be used as the surfactants for the preparation of stable suspensions. This preparation can be conducted in a simple, one-step process based on the co-precipitation of Fe3+/Fe2+ ions in the presence of the amino acid. However, the presence of this amino acid changes the mechanism of the magnetic nanoparticles' formation. In this investigation we analyzed the influence of aspartic amino acid (Asp) on the formation of magnetic iron-oxide nanoparticles during the co-precipitation. The process of the nanoparticles' formation was followed using a combination of TEM, x-ray diffractometry, magnetic measurements, in-situ FT-IR spectroscopy, and chemical analysis, and compared with the formation of nanoparticles without the Asp. The Asp forms a coordination complex with the Fe3+ ions, which impedes the formation of the intermediate iron oxyhydroxide phase and suppresses the growth of the final magnetic iron-oxide nanoparticles. Slower reaction kinetics can lead to the formation of nonmagnetic secondary phases. The aspartic-acid-absorbed nanoparticles can be dispersed to form relatively concentrated aqueous suspensions displaying a good colloidal stability at an increased pH.
    No preview · Article · Apr 2016 · Journal of Magnetism and Magnetic Materials
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    [Show abstract] [Hide abstract] ABSTRACT: Three new mononuclear complexes of nickel(II) pyridinecarboxamides (picolinamide - pia, nicotinamide - nia, isonicotinamide - isn), [Ni(OCOCH3)(pia)2(H2O)]·(CH3COO)·2H2O (1), [Ni(nia)2(H2O)4](CH3COO)2·2H2O (2) and [Ni(OCOCH3)2(isn)2(H2O)]·C3H7NO (3) have been synthesized. The turquoise (1) and green (2,3) crystals of the compounds were prepared by the reaction of nickel(II) acetate tetrahydrate and pyridinecarboxamides in different solvents by standard method under reflux (1) and by solvothermal method (2, 3). The compounds were characterized structurally by single-crystal X-ray diffraction analysis, magnetically and spectrally by FT-IR spectroscopy. All complexes consist of six coordinated central Ni2+ ion with pyridinecarboxamides, acetate ligands and/or water molecules. The interactions between building units in the crystal structures include intra- and intermolecular hydrogen bonds in all compounds and π–π interactions in complexes 1 and 2. Magnetic properties of the compounds were measured between 2 K and 300 K giving the results: μeff =3.0 BM for compound 1, μeff =2.8 BM for 2 and 2.6 BM for compound 3 in paramagnetic region. Experimental values of the IR spectra are comparable with literature data and are in good agreement with results of the structural analysis.
    Full-text · Article · Mar 2016
  • [Show abstract] [Hide abstract] ABSTRACT: Abstract A series of nickel-copper alloy magnetic nanoparticles with a range of Curie points from 51°C to 63°C were prepared by the reduction of intimately mixed nickel and copper oxides in a silica matrix using the sol-gel method. The silica matrix was subsequently removed with an etching solution, assisted by sonication. The alloy nanoparticles were characterized using X-ray diffraction (XRD) analysis, thermogravimetric analysis (TGA/SDTA), thermomagnetic analysis (TMA), transmission electron microscopy (TEM), magnetic measurements (SQUID, vibrating-sample magnetometer) and specific absorption rate measurements (SAR). The synthesized nanoparticles show a size in the range 15-20 nm, exhibited superparamagnetic behavior with a blocking temperature (TB) of approximately 135 K and a room-temperature magnetization of 3-9 emu/g, depending on the composition. The nanoparticles showed a relatively high effective anisotropy constant (Keff) and a significant heating ability in an alternating magnetic field. The synthesis method is straightforward and allows the preparation of homogeneous Ni-Cu alloy nanoparticles with a relatively narrow particle size distribution.
    No preview · Article · Jul 2015 · Journal of Alloys and Compounds
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    [Show abstract] [Hide abstract] ABSTRACT: The incorporation of magnetic barium hexaferrite nanoparticles in a transparent polymer matrix of poly(methyl methacrylate) (PMMA) is reported for the first time. The barium hexaferrite nanoplatelets doped with Sc3+, i.e., BaSc0.5Fe11.5O12 (BaHF), having diameters in the range 20 to 130 nm and thicknesses of approximately 5 nm, are synthesized hydrothermally and stabilized in 1-butanol with dodecylbenzenesulfonic acid. This method enables the preparation of monolithic nanocomposites by admixing the BaHF suspension into a liquid monomer, followed by in-situ, bulk free-radical polymerization. The PMMA retains its transparency for loadings of BaHF nanoparticles up to 0.27 wt.%, meaning that magnetically and optically anisotropic, monolithic nanocomposites can be synthesized when the polymerization is carried out in a magnetic field. The excellent dispersion of the magnetic nanoparticles, coupled with a reasonable control over the magnetic properties achieved in this investigation, is encouraging for the magneto-optical applications of these materials.
    Full-text · Article · Jun 2015 · Scientific Reports
  • Gregor Ferk · Miha Drofenik · Darko Makovec · Irena Ban
    [Show abstract] [Hide abstract] ABSTRACT: A magnetic fluid (MF) consisting of water-soluble maghemite nanoparticles, stabilized by a polymerized bilayer of fatty acid, was prepared. The simplicity of the synthesis without intermediate purification phases allows the preparation of MF. The structure and composition of the nanoparticles were analyzed by transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, thermal gravimetric analysis (TGA) and magnetic measurements. The hydrodynamic diameter of the coated nanoparticles in the magnetic liquid and their stability against dilution was monitored by dynamic light scattering (DLS).
    No preview · Article · May 2015 · Materials Letters
  • [Show abstract] [Hide abstract] ABSTRACT: The coordination compounds of pyridine-2,6-dicarboxylic acid and two lanthanide(III) ions, Ho3+ and Dy3+, were hydrolytically synthesized in aqueous solutions at a slightly basic pH, and then characterized by thermogravimetric analysis, IR spectroscopy, magnetic measurements as well as X-ray powder and single-crystal diffraction analysis. The elemental analyses were performed to check the purity of the compounds. The formula for these compounds is identified as Na3[Ln(Pydc)3]⋅14H2O (Ln = Ho, 1; Ln = Dy, 2) in agreement with the X-ray structural analysis and all the other experimental data. The absence of the 1709 cm−1 band corresponding to ν(СO) in the IR spectra of the compounds evidences the deprotonating of the carboxyl group. The very strong inductive effect of the metal ion that is readily coordinated by the carboxylate group of the zwitterionic ligand is responsible for the formation of the product. The single-crystal X-ray structural analysis revealed that compounds 1 and 2 are isostructural. Their structure can be described as interchanging layers of complex anions [Ln(Pydc)3]3 (Ln = Ho and Dy for 1 and 2, respectively) and layers of hydrated sodium cations. In complex anions the holmium and dysprosium atoms are coordinated by three crystallographically independent pyridinedicarboxylate ligands in tridentate-chelate mode, via one O atom of both carboxylate groups and the ring N atom. The coordination number is nine and the coordination polyhedron is a tricapped trigonal prism with O atoms at the corners.
    No preview · Article · Jan 2015 · Journal of Molecular Structure
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    [Show abstract] [Hide abstract] ABSTRACT: The synthesis and characterization of two new cobalt(II) coordination compounds with nicotinamide (nia) and isonicotinamide (isn) are reported. The products were characterized magnetically, structurally by single-crystal X-ray diffraction analysis and spectrally by FT-IR spectroscopy. Using the reaction of cobalt(II) acetate tetrahydrate and nicotinamide in methanol we obtained light-red crystals of the mononuclear complex [Co(nia)(2)(H2O)(4)](CH3COO)(2)center dot 2H(2)O (1). The synthesis in a system cobalt(II) acetate dihydrathe, isonicotinamide and dimethylformamide-methanol mixture gave a new dinuclear coordination compound with the formula [Co-2(CH3COO)(4)(isn)(4)]center dot 2C(3)H(7)NO (2). In both compounds a trans arrangement of pyridinecarboxamide ligands was found. Intermolecular hydrogen bonds in the crystal structures of both complexes are discussed. The magnetic properties were studied between 2 K and 300 K giving the result mu(eff) = 4.6 BM for 1 and mu(eff) = 4.7 BM for 2 in the paramagnetic region.
    Full-text · Article · Nov 2014 · Journal of Molecular Structure
  • Sašo Gyergyek · Miha Drofenik · Darko Makovec
    [Show abstract] [Hide abstract] ABSTRACT: Magnetic CoFe2O4 nanoparticles were synthesized by co-precipitation at 80 °C. This co-precipitation was achieved by the rapid addition of a strong base to an aqueous solution of cations. The investigation of the samples that were quenched at different times after the addition of the base, using transmission electron microscopy (TEM) coupled with energy-dispersive X-ray spectroscopy (EDXS) and X-ray powder diffractometry, revealed the formation of a Co-deficient amorphous phase and Co(OH)2, which rapidly reacted to form small CoFe2O4 nanoparticles. The nanoparticles grew with the time of aging at elevated temperature. The colloidal suspensions of the nanoparticles were prepared in both an aqueous medium and in a non-polar organic medium, with the adsorption of citric acid and ricinoleic acid on the nanoparticles, respectively. The measurements of the room-temperature magnetization revealed the ferrimagnetic state of the CoFe2O4 nanoparticles, while their suspensions displayed superparamagnetic behaviour.
    No preview · Article · Sep 2014 · Acta Chimica Slovenica
  • Igor Zajc · Miha Drofenik
    [Show abstract] [Hide abstract] ABSTRACT: We have identified a PTCR anomaly in undoped BaTiO3 (BT) ceramics. This anomaly was ascribed to a disconnection of the semiconducting grains, due to dimensional changes of the BT grains at the Curie point, in a composite composed of two constituent BT phases, one with a low electrical resistivity and the other with a high electrical resistivity. The composite exhibits a significant PTCR effect of three orders of magnitude at the Curie temperature. Using a similar approach, a PTCR anomaly was observed in a composite where copper was used as the conductive matrix, while BT particles were used as the second constitutive phase in the composite, acting as a circuit-breaker, and disconnecting the continuous conducting phase at the Curie temperature.
    No preview · Article · Jul 2014 · Ceramics International
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    [Show abstract] [Hide abstract] ABSTRACT: Nickel–copper (NiCu) alloy magnetic nanoparticles with a narrow size distribution were prepared by reducing a Ni, Cu-oxide mixture in a silica matrix, obtained with a sol–gel method. In order to obtain the NiCu nanoparticles, the silica matrix was treated with an etching solution in an argon atmosphere. By selecting the nominal NiCu alloy composition of 67.5:32.5, a Curie temperature close to 65 °C was achieved. The nanoparticles were superparamagnetic with a magnetization of 8 emu/g at 293 K and exhibited a significant heating ability in an alternating magnetic field. The simplicity of the synthesis allows the preparation of NiCu alloy nanoparticles on a large scale. The method could also be used for the preparation of other mixed-alloy nanoparticles.
    Full-text · Article · Jun 2014 · Materials Letters
  • [Show abstract] [Hide abstract] ABSTRACT: This article reports the synthesis of novel, rare-earth coordination complexes with nicotinic acid. Three compounds with the general formula Ln2[(C5H4NCOO)6(H2O)4] (Ln = Yb, 1; Ln = Gd, 2; Ln = Nd, 3) were prepared from relatively cheap and readily available reactants. Their compositions and structure were characterized by IR spectroscopy and single-crystal X-ray diffraction. The magnetic and thermogravimetric properties were also studied. The complexes consist of centrosymetric, dimeric molecules having all six nicotinato ligands coordinated with the central atom in the bidentate mode. The coordination environment of the Ln3+ for all three compounds is 8. Here we describe the crystal structure of Yb and Gd complexes with nicotinic acid.
    No preview · Article · Feb 2014 · Central European Journal of Chemistry
  • [Show abstract] [Hide abstract] ABSTRACT: Mg1+xFe2-2xTixO4 ferrite nanoparticles for Self-Regulating Magnetic-Fluid Hyperthermia were synthesized by the co-precipitation of Fe3+, Mg2+ and Ti4+ ions using NaOH. The hydroxide precursors were heated to 1000 °C in air and the product, (Mg,Ti)-ferrite, was milled in the presence of a surfactant in a high-energy mill, resulting in the magnetic fluid. The magnetic particles were characterized with transmission electron microscopy, X-ray diffractiometry and magnetic measurements. The Mg1+xFe2-2xTixO4 nanoparticles with x=0.37 exhibited a room-temperature magnetization of 8 emu/g at 1 T and a Curie temperature (Tc) of 46 °C. The heating ability of the nanoparticles in an AC magnetic field was measured using a calorimeter.
    No preview · Article · Jan 2014 · Journal of Magnetism and Magnetic Materials
  • [Show abstract] [Hide abstract] ABSTRACT: Magneto-electric (ME) ceramic composites of cobalt ferrite (CoF) and lead zirconate titanate (PZT) were prepared by mechanical mixing of the constituent powders followed by cosintering. The cosintering conditions for nano-sized CoF and submicrometer-sized PZT powders were studied in detail. It was found that the CoF powder needs to be presintered at 700°C for 2 h to minimize the differences in the sintering kinetics of the constituent powders. Despite the low cosintering temperatures (900°C–1000°C) the interdiffusion of the cations from both phases was confirmed with energy-dispersive X-ray analysis and X-ray diffraction. Efforts were made to optimize the cosintering conditions to prepare dense ceramic ME composites, which showed the converse ME effect.
    No preview · Article · Jan 2014 · Journal of the American Ceramic Society
  • Irena Ban · Janja Stergar · Miha Drofenik · Gregor Ferk · Darko Makovec
    [Show abstract] [Hide abstract] ABSTRACT: A chemical and a physical method have been applied for the preparation of chromium-nickel alloy nanoparticles. These particles were designed to be used for controlled magnetic hyperthermia applications. Microemulsions with Ni2+ and Cr3+ and/or NaBH4 as precursors were prepared using the isooctane/CTAB, n-butanol/H2O system. The samples of CrxNi1-x nanoparticles with the desired composition were obtained after the reduction of their salts with NaBH4 and afterwards heat treated in a TGA in a N2 atmosphere at various temperatures. The CrxNi1-x materials were also prepared by mechanical milling. Utilizing a ball-to-powder mass ratio of 20 : 1 and selecting the proper alloy compositions we were able to obtain nanocrystalline CrxNi1-x particles. Thermal demagnetization in the vicinity of the Curie temperature of the nanoparticles was studied using a modified TGA-SDTA method. The alloy's phase composition, size and morphology were determined with XRD measurements and TEM analyses.
    No preview · Article · Dec 2013 · Acta Chimica Slovenica
  • Alenka Mertelj · Darja Lisjak · Miha Drofenik · Martin Čopič
    [Show abstract] [Hide abstract] ABSTRACT: More than four decades ago, Brochard and de Gennes proposed that colloidal suspensions of ferromagnetic particles in nematic (directionally ordered) liquid crystals could form macroscopic ferromagnetic phases at room temperature. The experimental realization of these predicted phases has hitherto proved elusive, with such systems showing enhanced paramagnetism but no spontaneous magnetization in the absence of an external magnetic field. Here we show that nanometre-sized ferromagnetic platelets suspended in a nematic liquid crystal can order ferromagnetically on quenching from the isotropic phase. Cooling in the absence of a magnetic field produces a polydomain sample exhibiting the two opposing states of magnetization, oriented parallel to the direction of nematic ordering. Cooling in the presence of a magnetic field yields a monodomain sample; magnetization can be switched by domain wall movement on reversal of the applied magnetic field. The ferromagnetic properties of this dipolar fluid are due to the interplay of the nematic elastic interaction (which depends critically on the shape of the particles) and the magnetic dipolar interaction. This ferromagnetic phase responds to very small magnetic fields and may find use in magneto-optic devices.
    No preview · Article · Dec 2013 · Nature
  • [Show abstract] [Hide abstract] ABSTRACT: Magnetic particles with a controlled Curie temperature were prepared by reducing a Ni,Cu-hydrazine complex that was synthesized in a compartmentalized state of reverse micelles. The planned Curie temperature of 43 °C was achieved by a thermally activated homogenization of as-prepared alloy particles embedded in a NaCl salt environment. The particles were superparamagnetic with a blocking temperature of 16.5 K and a room-temperature magnetization of 2.5 emu/g. The particles exhibited a therapeutic Curie temperature that is suitable for self-regulating magnetic hyperthermia.
    No preview · Article · Nov 2013 · Journal of Alloys and Compounds
  • Matjaž Kristl · Nuša Hojnik · Sašo Gyergyek · Miha Drofenik
    [Show abstract] [Hide abstract] ABSTRACT: There is a growing interest in the synthesis of nanostructured copper sulfides due to their ability to form compounds with various stoichiometries. We report a sonochemical route for the preparation of copper sulfides with different compositions in aqueous solutions, using different, general and convenient copper sources such as copper acetate, copper hydroxide or basic copper carbonate and thiourea or thioacetamide as sulfur precursors under ambient air. Phase analysis, purity and morphology of the products were studied by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). The results revealed that nanoparticles of covellite, CuS, with crystallite sizes between 7 and 18 nm can be obtained by using different precursors and complexing agents and that chalcocite, Cu2S, can also be prepared sonochemically.
    No preview · Article · Mar 2013 · Materials Research Bulletin
  • B. Denac · M. Kristl · S. Gyergyek · M. Drofenik
    [Show abstract] [Hide abstract] ABSTRACT: Ternary cadmium chalcogenides were prepared using different methods: hydrothermal, microwave, and sonochemical synthesis in ethylenediamine (en) and different en/water solutions, as well as by direct reactions of elemental precursors and binary cadmium chalcogenides. The obtained ternary nanoparticles CdSexTe1-x (x = 0.1 and x = 0.6) were characterized by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). The formation and transformation of ternary phases was studied by thermal analysis (TGA). The as-prepared chalcogenides were nanosized with different shapes and crystallite sizes between 6 and 35 nm, depending on the preparation method.
    No preview · Article · Mar 2013 · Chalcogenide Letters
  • Darja Lisjak · Petra Jenus · Miha Drofenik
    [Show abstract] [Hide abstract] ABSTRACT: We have studied the assembly of cobalt ferrite nanoparticles in a magnetic field. Nanoparticles of different sizes, i.e., 5-40 nm, were synthesized with coprecipitation or under hydrothermal conditions. Their saturation magnetization values ranged between 31 and 68 Am2 /kg, respectively, resulting in a strong magnetic attraction and agglomeration between the nanoparticles. In aqueous ferrofluids this was prevented by the adsorption of citric acid on the nanoparticles' surfaces. The estimated interaction energies show that the DLVO (Derjaguin, Landau, Verwey, Overbeek) theory fails to explain the stability of cobalt ferrite ferrofluids and that the solvation interaction cannot be neglected. Cobalt ferrite ferrofluids were deposited on alumina substrates and dried in a magnetic field of 0.03 or 0.5 T. The homogeneity of the deposits decreased with the increasing saturation magnetization of the nanoparticles and/or with the increasing magnetic field, until columnar structures were formed from the nanoparticles with Ms ≥ 55 Am2/kg under a magnetic field of B = 0.5 T.
    No preview · Article · Nov 2012 · IEEE Transactions on Magnetics

Publication Stats

3k Citations
445.40 Total Impact Points

Institutions

  • 2001-2015
    • University of Maribor
      • Faculty of Chemistry and Chemical Engineering
      Maribor, Maribor, Slovenia
  • 2012
    • EN-FIST Centre of Excellence
      Lubliano, Ljubljana, Slovenia
  • 1973-2012
    • Jožef Stefan Institute
      • • Department of Synthesis of Materials
      • • Department of Advanced Materials
      Lubliano, Ljubljana, Slovenia
  • 2011
    • Institute of Mathematics, Physics and Mechanics
      Lubliano, Ljubljana, Slovenia
  • 2005
    • Kolektor Group
      Idria, Idrija, Slovenia
  • 1974-1995
    • University of Ljubljana
      • Faculty of Chemistry and Chemical Technology
      Lubliano, Ljubljana, Slovenia