University of North Florida
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Stuart Chalk, Ph.D.
Department of Chemistry
University of North Florida
Phone: 1-904-620-1938
Fax: 1-904-620-3535
Email: schalk@unf.edu
Website: @unf

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Gyorgy Marko Varga

Abbrev:
Marko Varga, G.
Other Names:
Address:
AstraZeneca, Cell and Molecular Biology, Research and Development, Box 34, S-22187 Lund, Sweden
Phone:
+46-46-336887
Fax:
+46-46-337383

Citations 8

"Development Of Enzyme-based Amperometric Sensors For The Determination Of Phenolic Compounds"
Trends Anal. Chem. 1995 Volume 14, Issue 7 Pages 319-328
György Marko-Varga*, Jenny Emnéus, Lo Gorton and Tautgirdas Ruzgas

Abstract: The development of biosensor-based techniques for the determination of phenolic compounds in surface waters is described. The enzyme's catalytic cycle must first be considered, and then the incorporation of enzyme electrodes into simple flow injection or integrated sample handling units. The catalytic properties of laccases, peroxidases and tyrosinases for the construction of sensors with narrow or broad selectivity can be exploited. (30 references).
Phenols Surface Sensor Amperometry Review

"Silicon Microstructures For High-speed And High-sensitivity Protein Identifications"
J. Chromatogr. B 2001 Volume 752, Issue 2 Pages 217-232
Thomas Laurell, Johan Nilsson and György Marko-Varga

Abstract: Silicon microtechnology has been used to develop a microstructure toolbox in order to enable high accuracy protein identification. During the last 2 years we developed and applied monocrystalline silicon structures and established new automated protein analysis platforms, The development of a high throughput protein platform is presented where fully automated protein identifications are performed. It includes the reduction and alkylation of the protein sample in a standard 96- or 384-well plate format prior to injection of 1 µL samples into the continuous flow based microtechnology platform. The processed sample is transferred to a microchip nanovial array target using piezoelectric microdispensing. identification is made by MALDI-TOF MS and a database search. After the initial sample reduction and alkylation period of 50 min the platform can digest and process protein samples at a speed of 100 samples in 210 min. An optional configuration of the platform, operating the dispenser in the static mode, enables on-target enrichment of low abundant proteins and peptides e.g. from 2DE samples. This makes detection at the low attomole level possible.

"Phenol Oxidase-based Biosensors As Selective Detection Units In Column Liquid Chromatography For The Determination Of Phenolic Compounds"
J. Chromatogr. A 1994 Volume 675, Issue 1-2 Pages 65-78
Fidel Ortegaa, Elena Domíngueza, Elisabeth Burestedtb, Jenny Emnéusb, Lo Gortonb and György Marko-Vargab,*

Abstract: Five ways of incorporating monophenol monooxygenase in C paste and two ways of coating graphite rods with the enzyme were tested. The most stable and sensitive electrode was that formed by covalent attachment of the enzyme to rods. The 3 mm outside diameter rods were first activated by heating at 700°C for 92 s and treated with carbodiimide. The rods were dipped in a tyrosinase solution in 0.1 M phosphate buffer of pH 6, containing glutaraldehyde for 16 h at 4°C. For use, the electrodes were press-fitted into PTFE tubes so that only the flat end was exposed and mounted in a wall-jet flow-through amperometric cell. The catechol formed in the enzymatic reaction was detected at -0.05 V vs. Ag/AgCl, with use of a Pt counter electrode. For testing in the flow injection mode, the carrier was 0.1 M phosphate buffer of pH 6 at 0.7 ml/min. Calibration graphs were linear for 25 µL injections from 10 nM- to 20 µM-catechol, with a detection limit of 2.3 nM. The electrode was applied to the analysis of waste water (details given).
Phenols Waste LC Electrode Electrode Electrode Sensor

"High Performance Liquid Chromatography Separation Of Some Mono- And Disaccharides With Detection By A Post-column Enzyme Reactor And A Chemically Modified Electrode"
J. Chromatogr. A 1987 Volume 408, Issue 1 Pages 157-170
György Marko-Varga

Abstract: The effluent from a chromatographic column was mixed with nicotinamide adenine dinucleotide coenzyme (NAD+) buffer and passed through a packed-bed reactor containing immobilized glucose dehydrogenase. Oxidation of the carbohydrates emerging from the column produced an equivalent amount of reduced coenzyme (NADH), which was detected electrochemically using an electrode modified with 7-dimethylamino-1,2-benzophenoxazine (Meldola Blue). Separation was effected in three different chromatographic systems containing a protonated ion exchanger, a calcium(II)-saturated or a lead(II)-saturated ligand exchange column. Separation, detection and k? values are reported for glucose, 2-deoxyglucose, xylose, mannose, cellobiose, lactose, ribose and glucosamine. The detection limit was 2 ng for a 20-l injection of glucose and the response was linear up to 6300 ng. Samples from fermentation of penicillin were analyzed for lactose and glucose with the described detector. A comparison with the recordings from a refractive index detector showed that the selectivity of the enzymes and the modified electrodes are necessary for the determination of glucose and lactose.
Monosaccharides Disaccharides Food Amperometry HPLC Electrode Immobilized enzyme Post-column derivatization

"Effect Of HY-zeolites On The Performance Of Tyrosinase-modified Carbon-paste Electrodes"
Electroanalysis 1996 Volume 8, Issue 12 Pages 1121-1126
György Marko-Varga *, Elisabeth Burestedt, Carl Johan Svensson, Jenny Emnéus, Lo Gorton, Tautgirdas Ruzgas, Mareike Lutz, Klaus K. Unger

Abstract: Tyrosinase-modified graphite pastes containing varying amounts of HY-zeolite 6, 25, 56 and 200 were prepared either by simple mixing or by adsorption of the enzyme on the zeolite particles. The performance of the electrodes were investigated by cyclic voltammetry of 1 mM catechol in 0.25 M phosphate buffer of pH 7 using SCE as reference electrode and a Pt-wire electrode as the counter electrode. The potential was cycled from -0.2 to 0.5 V at 0.05 V/s. FIA measurements were performed using 0.1 M phosphate buffer of pH 6 as the carrier solution and applying a potential of -0.05 V to the prepared electrode. Highest response was obtained with the electrode containing the most hydrophilic HY-zeolite 6 to which kanamycin was added.
Electrode Voltammetry

"Column Liquid Chromatography In Combination With Immobilized Enzymes And Electrochemical Detection And Its Applications In Some Industrial Processes"
Electroanalysis 1992 Volume 4, Issue 4 Pages 403-427
György A. Marko-Varga

Abstract: A review, with 206 references. The first part of the paper provides a review of the pre- and post-column derivatization systems used in column liquid chromatography (CLC) in combination with immobilized enzyme reactors (IMERs) and electrochemical detection (EC). In the second part, an outline of important factors to consider in the optimization of CLC-IMER-EC systems are presented. Three industrial applications are described utilizing enzyme-based detection or enzymes for sample handling purposes. In two of the cases, CLC-IMER were used in combination with amperometric detection using chemical modified electrodes (CME). These applications were performed in the author's lab., and the high selectivity and sensitivity of these systems as well as the problems encountered in these complex samples will be discussed.
Fermentation broth Industrial HPLC Electrochemical analysis Review Post-column derivatization Pre-column derivatization Immobilized enzyme Optimization

"Flow Injection Analysis Of Phenolic Compounds With Carbon Paste Electrodes Modified With Tyrosinase Purchased From Different Companies"
Anal. Lett. 1996 Volume 29, Issue 7 Pages 1055-1068
Lindgren, A.;Ruzgas, T.;Emmeus, J.;Csoregi, E.;Gorton, L.;Marko Varga, G.

Abstract: Tyrosinase-modified C paste electrodes were prepared by several methods: (i) direct mixing of tyrosinase powder with graphite and oil vs. adsorption of tyrosinase from solution on to graphite powder, drying and mixing with oil; (ii) preparation of tyrosinase-modified paste doped with mediator vs. mediatorless; and (iii) preparation of C paste using porous graphite vs. non-porous vitreous C powder. Each design was evaluated for nine phenolic compounds including six substituted catechols. Samples (220 µL) were aspirated into a flow injection system (schematic given) and injected into a 0.1 M phosphate buffer of pH 6 carrier stream (0.8 ml/min). After mixing, detection was at tyrosinase-modified paste electrodes inserted into a flow-through wall-jet electrochemical cell, with Pt and Ag/AgCl as counter and reference electrodes, respectively, operated at -0.05 V vs. AgCl. The tyrosinase-modified C paste electrode response was limited by the rate of the enzymatic reaction; higher responses were obtained for direct mixing of the enzyme powder on to the paste and mediator-doped pastes exhibited a higher response for some phenols.
Phenols Catecholamine, derivatives

"Development Of An Offline Noncompetitive Flow Immunoassay For The Determination Of"
Anal. Biochem. 2000 Volume 279, Issue 1 Pages 46-54
E. Burestedta, S. Kjellströma, J. Emnéusa and G. Marko-Varga

Abstract: A noncompetitive flow immunoassay system (FIA) for the analysis of interleukin-8 (IL-8) in cell samples was developed. Affinity interaction assays based on offline incubation of excess labeled antibodies and antigen (IL-8) were carried out. The residual unbound labeled antibody was trapped in an immunoaffinity column with immobilized IL-8 while the immunocomplex, labeled antibody/IL-8, was detected by a fluorescence detector. Two fluorophores, FLUOS and Cy5.5, were conjugated with IL-8 antibody. Optimization and comparison between the two fluorescent labeled antibodies were performed with regard to pH, antibody concentration, flow rate, injection volume, and association time. Additionally, a horseradish peroxidase enzyme label was used for the conjugation to the anti-IL-8. The enzyme substrate reaction was optimized with respect to temperature and length of the substrate reaction coil. The detection limits were found to be 200 amol using the FLUOS-labeled anti-IL-8 and 1 fmol using the Cy5.5 fluorescence label. The developed FIA technique was applied for the analysis of IL-8 in cell samples. Matrix-assisted laser desorption/ionization time-of- flight mass spectrometry was used to identify IL-8 in the cell samples. (C) 2000 Academic Press.