Potassium selective chemically modified field effect transistors based on AlGaN/GaN two-dimensional electron gas heterostructures.

We investigate the use of the AlGaN/GaN high electron mobility transistor (HEMT) as a novel transducer for the development of ion-selective chemically modified HEMT sensors (ChemHEMTs). For this, polyvinyl chloride (PVC) membrane doped with ion-selective ionophores is deposited onto the area of the gate for the chemical recognition step, while the AlGaN/GaN HEMT is used as the transducer. In particular, the use of a valinocycin doped membrane with thickness of 50 microm generates a sensor with excellent analytical characteristics for the monitoring of K(+). The K(+)-ChemHEMT has sensitivity of 52.4 mV/pK(+)in the linear range of 10(-5) to 10(-2)M, while the detection limit is in the order of 3.1 x 10(-6)M. Also, the sensor shows selectivity similar to valinomycin-based ISEs, while the signal stability over time and the measurement to measurement reproducibility are very good.

Oxidative stability and radical scavenging activity of extra virgin olive oils: an electron paramagnetic resonance spectroscopy study.

The oxidative stability of extra virgin olive oils (EVOO) from the Greek island of Crete was evaluated by electron paramagnetic resonance (EPR) spectroscopy and the spin trapping technique. The spin trap N-t-butyl-alpha-phenylnitrone (PBN) was added to the olive oil samples and the production of free radicals was monitored during heating at 70 degrees C. Induction time for the accelerated oxidation of virgin olive oils at 70 degrees C was determined. The EPR results were compared with the oxidative stability values provided by the Rancimat method at 110 degrees C and high linear correlations were found (r=0.922). EPR spin trapping provides a sensitive and rapid method for evaluating the oxidative stability of EVOO. The same samples of Greek extra virgin olive oils were also examined for their radical scavenging activity (RSA) toward the stable galvinoxyl radical by EPR spectroscopy. The decrease of the intensity of the EPR signal upon incubation time was followed. Both oxidative stability and radical scavenging activity of EVOO samples were correlated to their content in polyphenols and tocopherols.

Novel carbon materials in biosensor systems.

In this work, novel carbon materials are evaluated as transducers, stabilizers and mediators for the construction of amperometric biosensors. It is shown that materials such as fullerenes and carbon nanotubes are promising materials as electrochemical mediators and enzyme stabilizers. Additionally porous carbon and porous glassy carbon are excellent transducers for amperometric measurements, while they provide cavities adequate for enzyme immobilization. At the same time, the sensitivity to peroxide is shown to depend on the activation procedures. Treatment that introduces oxygen groups increases the sensitivity of the carbon-based sensor to hydrogen peroxide considerably. These materials are used for the construction, mediation and stabilization of glucose biosensor.

Improved operational stability of biosensors based on enzyme-polyelectrolyte complex adsorbed into a porous carbon electrode.

A novel porous active carbon is utilized in order to adsorb the diethylaminoethyl-dextran (DEAE-dextran)-enzyme stabilized complexes, for the construction of highly stable biosensors. The interaction of DEAE-dextran with the examined enzymes increases dramatically the operational stabilization of the sensors, without adverse effects on the enzyme activity. At the same time, the porous active carbon allows for high enzyme loading, good electrical contact and low resistance throughout the sensing element. Glucose oxidase and horseradish peroxidase are used as model enzymes in this study to construct biosensors, with very good reproducibility (less than 5% RSD). As a result, the glucose sensor exhibits very long operational stability (over a period of 5 months), while the hydrogen peroxide sensor retains its initial activity after several weeks.

Salicylate-selective membrane electrode based on tin(IV) tetraphenylporphyrin.

The response properties of a new solvent/polymeric membrane electrode with unique selectivity toward anionic salicylate are reported. The electrode is prepared by incorporating 5, 10, 15, 20-tetraphenyl(porphyrinato)tin(IV) dichloride (Sn[TPP]Cl2) into a plasticized poly(vinyl chloride) membrane. The resulting sensor exhibits an anti-Hofmeister selectivity pattern, with high specificity for salicylate over lipophilic inorganic anions (perchlorate, periodate, thiocyanate, iodide, etc.) and biological organic anions (citrate, lactate, acetate). Moderate selectivity over structural analogues of salicylate (3- and 4-hydroxybenzoate, benzoate) is also observed. Radiotracer uptake experiments using [14C]salicylate clearly show that the metal center of the metalloporphyrin is critical for selective salicylate transport in the membrane phase. Minimal response to chloride ions makes the new electrode potentially useful for estimating salicylate levels in biological samples.