Biomedical subjects

Epameinondas Leontidis

Publications and source records attributed to Epameinondas Leontidis.

6 recordsLinked to original sources

PubMed2006

Controlled production of ZnO nanoparticles from zinc glycerolate in a sol-gel silica matrix.

The controlled production of ZnO nanoparticles within an amorphous silica matrix is achieved using a new methodology consisting of four stages. First, precursor zinc glycerolate nanoparticles are produced within reversed micelles of glycerol in heptane stabilized by the surfactant Aerosol-OT (bis-ethylhexyl sodium sulfosuccinate, AOT). The surface of these nanoparticles is then modified by exchanging AOT with bis-trimethoxysilyl-ethane (BTME). The surface-modified nanoparticles are copolymerized with tetramethoxysilane (TMOS) to provide a composite silica material, in which the nanoparticles are apparently dissolved, producing a uniform distribution of zinc in the silica matrix. Finally, the conversion of zinc to ZnO is achieved by heating the material at 700 degrees C, leading to a uniform dispersion of very small (<10 nm) ZnO particles within the amorphous matrix. The fluorescence spectrum of the ZnO particles within the matrix is blue-shifted, as expected from the strong quantum confinement achieved. The properties of the system at all stages in this synthetic process are monitored using TEM, XRD, fluorescence and FT-IR spectroscopy. Glycerol forms complexes with many metal ions, so the present procedure may be generalized to provide uniform distributions of metal ions and subsequently metal oxide nanoparticles in amorphous silica.

Emulsions↗

PubMed2006

Formation mechanism of nanotubes comprising layers of PbS nanoparticles in polymer-surfactant solutions.

The crystallization of PbS in aqueous solutions containing the surfactant sodium dodecyl sulfate (SDS) and hydrophilic polymers resulted in a novel type of metastable nanotubes, the walls of which consist of layers of ordered PbS nanoparticles, apparently separated by layers of surfactant molecules. Information on the mechanism of formation of these structures was obtained by focusing on the roles of the polymer, and of the insoluble lead dodecyl sulfate (Pb(DS)2) present in the system. TEM investigations of the early stages of crystallization revealed the coexistence of PbS and Pb(DS)2 precipitates, the latter being surprisingly important for nanotube formation, and allowed to follow the evolution of layered structures from combination of the two types of crystals. Six different hydrophilic polymers have been used, which interact with SDS with varying strengths. Surprisingly, and in contrast to previous hypotheses, layered nanostructures were observed in all polymer solutions, regardless of the strength of polymer-surfactant interactions. This indicates that, although the presence of a polymer is necessary, polymer-SDS interactions are not a driving force for the formation of the layered structures and nanotubes. On the contrary, the interactions between the polymer chains and the growing particles appear to be of the utmost importance. Results presented here can be interpreted in terms of two alternative mechanisms for layered nanostructure and nanotube formation.

Crystallization↗

PubMed2005

NMR investigation of the interaction of vanadate with carbasilatranes in aqueous solutions.

Reaction of vanadate with carbasilatranes [methoxy{N,N',N' '-2,2',3-[bis(1-methylethanolato)(propyl)]amino}silane (1), methoxy{N,N',N' '-2,2',3-[bis(1-ethanolethanolato)(propyl)]amino}silane (2), and {N,N',N' '-2,2',2-[bis(ethanolato)(glycolpropyl ether)]amino}silane (3)] in aqueous solution results in the formation of vanadosilicates and five-coordinated chelate vanadium(V) complexes as evidenced by 51V, 1H, and 13C NMR spectroscopy. Chiral carbasilatrane S,S-1 was characterized in the solid state by X-ray diffraction, revealing a trigonal bipyramidal geometry around the metal ion, with one unidentate methoxy group and one atrane nitrogen atom at the axial positions and one carbon and two atrane oxygen atoms at the equatorial plane of the bipyramid. Crystal data (Mo Kalpha; 100(2) K) are as follows: orthorhombic space group P2(1)2(1)2(1); a = 8.8751(6), b = 9.7031(7), c = 14.2263(12) A; Z = 4. The complexation of vanadium either with 1 or 2 is stereoselective yielding approximately 94% of the complex containing ligand in the S,R-configuration. The lower ability of the S,S- and R,R-diastereoisomers of 1 and 2 to ligate vanadate was attributed to stereochemical factors, dictating a square pyramidal geometry for the chelated complexes. A dynamic process between the vanadium chelate complexes and the respective carbasilatranes was evaluated by 2D {1H} EXSY NMR spectroscopy. These spectra show that the vanadate complexes with the open carbasilatranes exchange more slowly with the free ligand compared to the respective alcohol aminate complexes.

Journal Article↗

PubMed2005

Attraction of iodide ions by the free water surface, revealed by simulations with a polarizable force field based on Drude oscillators.

Recent theoretical and experimental studies have shown that polarizable anions, such as iodide and bromide, preferentially accumulate close to the surface of electrolyte solutions. This finding is in sharp contrast to the previously prevailing idea that salts are dielectrically excluded from the free water surface and opens up new avenues for research in specific salt effects. In this work, we have verified the ability of a recently introduced polarizable water model, SWM4-DP, to reproduce this behavior, by simulations of a NaI/water slab, corresponding to a 1.2 M solution. The water and ion polarizabilities are modeled by classical Drude oscillator particles. As revealed by the simulations, a double layer is formed close to the free water surface, with the iodide ions located closer to the interface and the sodium ions at a neighboring, interior layer. Near the surface, all solution species acquire an induced dipole moment, that is perpendicular to the surface and points toward the exterior. The double charge layer causes ordering of water at a subsurface region. Simulations with a simpler system of a single iodide ion in a water slab show that the surface position is stabilized by induced charge interactions; in contrast, the charge-dipole interactions between the iodide permanent charge and the water permanent dipole moment favor the bulk position. Thus, the polarizabilities of ion and water are essential for explaining the increased preference of iodide for the air-water interface, in accordance with other studies.

Computer Simulation↗

PubMed2004

Study of copper sulfide crystallization in PEO-SDS solutions.

The crystallization of copper sulfide in aqueous supersaturated solutions in the presence of the polymer poly(ethylene oxide), PEO, and the surfactant sodium dodecyl sulfate, SDS, was investigated. In these systems, copper sulfide precipitation competes with the reaction between copper cations and dodecyl sulfate anions. The competition of the two reactions may affect the reaction products significantly; therefore it is important to study the properties of the surfactant salt, copper dodecyl sulfate (Cu(DS)2), in detail. The thermodynamic solubility constant of Cu(DS)2 was measured at 8 degrees C and was equal to (2.4 +/- 0.4) x 10(-10) M3. The Krafft point of Cu(DS)2 and its solubility curve (precipitation temperature for a range of concentrations) were also measured. The latter was found to be very close to room temperature. Temperature is thus a very significant parameter in these systems and must be carefully controlled in all experiments. The crystallization of copper sulfide in PEO-SDS solutions was investigated in solutions with compositions above and below the solubility curve. Copper sulfide nanoparticles predominate and are stabilized at temperatures above the solubility curve. Surprisingly, at temperatures below the solubility curve CuxS coexists with Cu(DS)2, which appears in the form of lamellar crystals. The system is further complicated by the presence of at least two different types of copper sulfides corresponding to different oxidation states of copper. Our results suggest that the predominance of Cu(DS)2 at lower temperatures is due to its limited solubility and is modified by the CuI/CuII redox equilibrium in combination with the solution pH.

Journal Article↗

PubMed2002

Speed selection mechanism for propagating fronts in reaction-diffusion systems with multiple fields.

We introduce a speed selection mechanism for front propagation in reaction-diffusion systems with multiple fields. This mechanism applies to pulled and pushed fronts alike, and operates by restricting the fields to large finite intervals in the comoving frames of reference. The unique velocity for which the center of a monotonic solution for a particular field is insensitive to the location of the ends of the finite interval is the velocity that is physically selected for that field, making thus the solution approximately translation invariant. The fronts for the various fields may propagate at different speeds, all of them being determined though through this mechanism. We present analytic results for the case of piecewise parabolic potentials, and numerical results for other cases.

Journal Article↗