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Electrochemistry, biosensor development, microelectrodes Our research activity also concentrates on the development, investigation and application of selective electrochemical sensors and electroanalytical methods. In this work we deal with ion selective electrodes, with chemically modified electrodes of electrocatalytic property or enhanced selectivity. We develope enzyme or bacterial sensors. We work out methods for the preparation of selective microelectrodes capable of giving information about the local concentration of different species with high spatial resolution. In our work the potential application field of these sensors is also intesively studied. Recently a scanning electrochemical microscope was put together in our laboratory. The new technique allows us to investigate different heterogeneous catalytic processes, corrosion reactions and other interfacial events. For more information please ask Prof. Géza Nagy. Research activity in field of Theoretical Chemistry and Molecular Luminescence Quantum chemical investigations on the dynamics of hydrogen halide elimination from vinyl halides : Influence of the molecular environment The gas phase dissociation reactions of halogenated ethylene (vinyl halides, H2C=CHX, X=F, Cl, Br or I) have provided a fruitful testing ground for unimolecular reaction theories. Especially those reactions where HX molecules eliminate from vinyl halides have particular interest. The dissociation of HX molecules from vinyl halide results in C2H2 product in two different forms: vinylidene intermediate (a carbene-type product with two unpaired electrons, namely: three-center elimination) and acetylene (a stable product, namely : four-center elimination). The practical importance of these reactions relates to their polymer formation ability (as the well known PVC). In this project the dynamics of three-center versus four-center dissociative reactions of vinyl halides (H2C=CHX, X=F, Cl, Br or I) was investigated theoretically by Langevin molecular dynamics method in the absence and in the presence of frictional interaction between the molecules and their environment. The results show a redistribution of the vibrational energy on the bonds of vinyl halide molecules under the effect of frictional interaction. The three-center HX elimination channel related to the fragmentation process of vinyl halide molecules shows lower activation energy than the four-center elimination channel. However, the environment-induced redistribution modifies the reaction rates of the dissociation channels by increasing the transition probability on the four-center elimination channel. This unexpected phenomenon is most pronounced in case of vinyl-iodide and shows a slight decrease by moving towards vinyl-fluoride. It has been shown, that the existence of friction may not guarantee statistical behaviour of the dissociation processes of these asymmetrically substituted olefins. This environment-induced property could explain some unusual experimental findings and it was already successfully tested on the unexpected reactivity difference obtained experimentally between iodo-alkene moieties of steroids possessing remote lactame or cycloalkane structural units. References :
For more information please write to Dr. Sándor Kunsági-Máté.
Solubilization of C60 fullerene in aqueous media The electron accepting ability of the fullerenes is known to inhibit the activity of redox enzymes (e.g. nitric oxide synthase, thus providing a therapeutic approach for some neurodegenerative disorders) and a large number of fullerene derivatives are competitive inhibitors of the human immunodeficiency virus (HIV) protease. However, the very low solubility of fullerenes in aqueous solvents is a major difficulty hindering their application in clinical practice. Therefore several strategies have been developed to increase the water solubility of fullerene derivatives. One of them is the inclusion of C60 within water-soluble hosts such as cyclodextrins and calixarenes. Other approaches such as covalent functionalization of C60 with hydrophilic, ionic or non-ionic organic moieties or electrochemical/ chemical reduction of C60 to a water-soluble anion have also been utilized to overcome the natural water-repulsion of C60 fullerene. In our related work the inclusion complexes of sulfonated thiacalix[4]arene (1) and calix[6]arene (2) sodium salts with C60 fullerene were investigated by photoluminescence (PL) and quantum-chemical methods. The stoichiometries of calixarene/C60 complexes were found to be 2:1 for 1 and 1:1 for 2. The thermodynamic properties of the complexation were successfully determined. Related quantum-chemical investigations show that C60 fullerene is included in a cavity composed of two half-bowl molecules of 1. The C60 fullerene ball is located deep within the cavity of 2 and the negatively charged sulfonate arms probably inhibit the formation of the bowl-shaped capsule that was observed in the case of 1. This observation makes these compounds promising candidates to overcome the natural water-repulsive character of C60 fullerene. References :
For more information please write to Dr. Sándor Kunsági-Máté.
Model for the incorporation of excess arsenic into the GaAs layers grown at low temperature Bulk II-IV and III-V single crystal semiconductors provide an appealing coupling of chemical, electrical and optical properties that can be exploited in the design of chemical sensors. Particularly, GaAs layers grown by molecular beam epitaxy (MBE) at a substrate temperature lower than 400°C are crystalline, but non-stoichiometric due to an appreciable content of excess As, Asex, (LT-GaAs). It is this excess arsenic that gives rise to the very interesting electrical and optical properties of LT-GaAs layers, namely high resistivity, high breakdown voltage, and very short lifetimes of photogenerated carriers. The Asex content itself has some partly still unexplained specifics. It shows an inverse dependence on the growth temperature. A higher As/Ga flux ratio and lower substrate temperature (200°C) increase the arsenic content of the grown layer up to 1.5%. The Asex concentration in the LT-GaAs layers is much higher than that derived from the equilibrium phase diagram (0.1% at most), therefore it cannot simply be considered as a thermodynamic consequence. In the model describing the origin of excess arsenic content in low-temperature grown GaAs layers developed with cooperation of the research group of professor Horst P. Strunk (University of Erlangen Nuremberg, Institut for Microcharacterization) during the last ten years, formation of an interstitial As atom was identified as precursor to excess arsenic formation. Using ab-initio methods, we calculated the interaction energy of an As2 molecule above the As terminated GaAs surface. When an As2 molecule oriented perpendicular to the As-terminated surface, a metastable position below the growing surface was obtained for the lower As atom of the As2 molecule. This metastable conformation also was found near differently oriented surface steps in accordance with the fact that a substrate misorientation has a strong influence on the excess As content. Our growing sequence for the formation of As interstitials is in agreement with both theoretical and experimental results, and we propose it as a very probable way for excess As formation during the low-temperature MBE growth of GaAs layers. Our results are applicable in the more effective growing of LT-GaAs layers. References :
For more information please write to Dr. Sándor Kunsági-Máté.
The predictive behavior of the phase transition temperatures of imidazolium based ionic liquids The applicability of ionic liquids in synthetic or separation science highly depends on the temperature range within they are really liquid. Therefore, the phase transitions are the most important property of these materials. Accordingly, the solid-solid and solid-liquid phase transitions of 1-methyl-3-tatradecylimidazolium hexafluorophosphate ([C14mim]+[PF6]-) ionic liquid was studied by density functional calculations and semiempirical AM1 molecular dynamic analysis. Our results show that the recently discovered anomalous structural change observed experimentally during crystalline to crystalline phase transition can be explained at molecular level with a conversion of two stable conformation of the [C14mim]+[PF6]- pair. The solid-liquid phase transition temperature of the imidazolium cation-based ionic liquids was found to be affected by the length of alkyl side chain of the imidazol ring. In our present work a close relationship was found between the melting point of imidazolium cation-related ionic liquids and the dynamics of atomic vibrations in the side alkyl chain of different length of the cation. References :
For more information please write to Dr. Sándor Kunsági-Máté.
Effect of the entropy compensation on the stability of host-guest complexes applied in selective chemical sensors The p -p interaction-based inclusion complexation of calix[6]arene hexasulfonate as host with neutral aromatic guest molecules was studied in aqueous media using both experimental and theoretical methods. Experimental measurements (PL and DSC) showed an increased complex stability when the electron density on the guest s aromatic rings increased, although the enthalpy change lowered. The evaluation of the thermodynamic parameters of the complex formation (enthalpy, entropy and Gibbs free energy) highlighted the excessive role of the entropy term in this particular case. To obtain atomistic view on the complex formation, DFT/B3LYP/6-31++G method and molecular dynamics simulation were performed. The stability of the complex and the complex formation were evaluated considering the Hammett parameters of the substituents of the guest. The entropy term of the complex formation was studied by simulation of the molecular rearrangement of solvent water molecules around the calixarene host when the phenol derivatives with their differently polarized aromatic rings enter into the calixarene cavity. The results show a structure of higher order of the water molecules when the aromatic ring of the entering guest molecule is electron-deficient. These results can contribute to the development of selective and sensitive chemical sensors for aromatic organic analytes. References :
For more information please write to Dr. Sándor Kunsági-Máté. The photochemical research in the department is focused on the investigation of the correlations between the conformation and the optical or electrochemical properties of different molecules. Commercial and home made highly automatized up to date instruments are at disposal for the experimental work. All the computerized measuring stations are connected to an internal network. The phenomenon of fluorescence-polarization can be monitored by a self-developed spectrofluorometer using front-face excitation and detection. Polarization studies showed that the self-determined fine structure of polarization spectra are characterized by molecular vibrations. Extending the Kasha-rules, quantitative relationship was deduced between the polarization spectra and molecular vibrations. The theoretical polarization spectra of several dies were calculated using this theory and fairly good agreement was found with the corresponding experimental spectra. Since the molecular geometry plays inportant role in polarization of fluorescence the quantum-chemical calculations were performed for highly-symmetrical light emitting semiconductor surfaces. In this work first, the (001) surface of GaAs was investigated in international cooperation. The results clearly indicated that the potential energy-field over the GaAs surface is determined and pre-oriented by the crystallic structure. The basic research of fluorescence and molecular-structure resulted also in practical application: Investigations of photochemical reactions of metal-complexes and equillibria study of fluorescent dies in micellar media helped to develope novel analytical methods for trace-metal and detergent analysis. Spectroscopical data were often compared by independent electrochemical methods, e.g. several ion-selective electrodes were developed for ionic detergents. Investigations of fluorescence based optical chemical sensors are also in progress. An annual meeting in Pécs, "Hungarian Conference of Luminescence Spectroscopy" (OLSI) has been organized since 1986 by our group. The topics of the meeting : research in luminescence, in photochemistry and its biological, analytical and industrial applications. The printed version of the lectures is published in "Actual Questions in the Luminescence Research" (Eds.: N.Marek, K.Szabó). This book contains the full content (in Hungarian) of the papers presented at the meeting and abstracts are given in English. For more information please write to Dr. Sándor Kunsági-Máté. |
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