Single-step microwave digestion of food and biological samples for the quantitative conversion of Se into the +4 oxidation state.

An improved single step microwave digestion procedure is described for providing the fast and easy exhaustive mineralisation of biological samples concomitantly with the quantitative conversion of any type of selenium compounds into Se(IV). In such a way, digested samples are directly suitable for the subsequent Se analysis at trace and ultratrace levels by both spectrometric methods such as HG-ICP-MS or HG-ICP-OES and differential pulse cathodic stripping voltammetry (DPCSV). It is based on the use, under suitably optimised microwave irradiation conditions, of a digestion mixture with a carefully tailored composition such that its redox potential is made lower than that allowing Se(IV) to be oxidized to Se(VI), but high enough to permit total destruction of biological or, in general, organic matrices. It consists of a nitric acid (65%, w/w) and hydrogen peroxide (30%, w/w) mixture in a volume ratio 5:1, frequently adopted for the mineralisation of organic and biological samples, but added simply with 0.25 g mL(-1) of NaCl. Successful application of the procedure, in terms of both repeatability and accuracy, to the quantification of selenium by the instrumental methods above in standard compounds and in a certified biological sample proved its good performance. The application to the Se determination in human blood plasma and in a wide variety of foods is also reported.

A glucose/hydrogen peroxide biofuel cell that uses oxidase and peroxidase as catalysts by composite bulk-modified bioelectrodes based on a solid binding matrix.

An improved composite bulk-modified bioelectrode setup based on a solid binding matrix (SBM) has been used to develop a glucose/hydrogen peroxide biofuel cell. Fuel is combined through a catalytically promoted reaction with oxygen into and oxidized species and electricity. The present work explores the feasibility of a sugar-feed biofuel cell based on SBM technology. The biofuel cell that utilizes mediators as electron transporters from the glucose oxidation pathway of the enzyme directly to electrodes is considered in this work. The anode was a glucose oxidase (GOx, EC 1.1.3.4)/ferrocene-modified SBM/graphite composite electrode. The cathode was a horseradish peroxidase (HRP, EC 1.11.1.7)/ferrocene-modified SBM/graphite composite electrode. The composite transducer material was layered on a wide polymeric surface to obtain the biomodified electrodic elements, anodes and cathodes and were assembled into a biofuel cell using glucose and H(2)O(2) as the fuel substrate and the oxidizer. The electrochemical properties and the characteristics of single composite bioelectrodes are described. The open-circuit voltage of the cell was 0.22 V, and the power output of the cell was 0.15 microW/cm(2) at 0.021 V. The biofuel cell proved to be stable for an extended period of continuous work (30 days). The reproducibility of the biotransducers fabrication was also investigated. In addition, an application of presented biofuel cell, e.g. the use of hydrolyzed corn syrup as renewable biofuels, was discussed.

Urea biosensor based on amperometric pH-sensing with hematein as a pH-sensitive redox mediator.

The natural dye hematein in water solution was used as a pH-sensitive redox-active mediator for amperometric pH-sensing. The electrochemical characteristics were studied using cyclic voltammetry and chronoamperometry. Several types of urea biosensors were constructed with urease on the surface of platinum and graphite composite electrodes or in the bulk of the graphite composite. They were used for the amperometric urea determination at a working potential of 0 mV (versus SCE) using 0.5 mM hematein. Detection limits and response linearity was in the micromolar range depending on the biosensor type, concentration and pH of buffers used. An interference study of various cations, anions, and substances, which may be present in real samples demonstrated good selectivity for the determination of urea. The biosensors showed good operational (>3 h) and storage (>3 months) stability. The results of urea determination in blood and urine obtained by biosensor correlated well with those obtained by a spectrophotometric reference method.

Selective and sensitive biosensor for theophylline based on xanthine oxidase electrode.

Milk and microbial xanthine oxidases (XOs) were used for the construction of amperometric enzyme electrodes. Substrate specificity differences of these enzymes were studied. Of the two enzymes, only the microbial XO was found to oxidize theophylline, but not theobromine and caffeine. The substrate specificity of microbial XO was affected by pH, where the optimum for xanthine was 5.5, while for theophylline it was in the range from 6.5 to 8.5. The theophylline biosensor showed a low detection limit of 2 x 10(-7) M and signal linearity up to 5 x 10(-5) M. The sensitivity of the microbial XO electrode to theophylline could be selectively eliminated by immersion in alkaline phosphate solution, thus allowing for the construction of a blank electrode for differential measurements. The feasibility of this approach has been demonstrated by the determination of free (unbound) and total theophylline in blood samples. The biosensor exhibited good operational (>6 h) and shelf (>3 months) stability when trehalose was used as a stabilizer of the biocatalytic layer.

Amperometric biosensors based on solid binding matrices applied in food quality monitoring.

Solid binding matrix (SBM) based composite transducers have been used for development of series of multibiosensor systems applicable in various fields. Here we present two hybrid three-channel multibiosensors for simultaneous amperometric operation in food quality control, i.e. glucose/fructose/ethanol multibiosensor, based on glucose oxidase/fructose dehydrogenase/alcohol dehydrogenase surface-modified enzyme electrodes and L-lactate/L-malate/sulfite multibiosensor, based on L-lactate dehydrogenase/L-malate dehydrogenase/sulfite oxidase surface-modified enzyme electrodes. Different parameters have been studied in order to optimize the response of the multibiosensor systems. The multibiosensor showed a good sensitivity, linear range and storage stability. The multibiosensors were used for the determination of glucose, fructose, ethanol, L-lactate, L-malate and sulfite in samples of wine, resulting in a good agreement with data certified by the supplier. Comparison of various designs, surface-modified, bulk-modified and thick-cover, of SBM based biosensors is studied on the example of fructose biosensor.

Continuous flow immunosensor for atrazine detection.

The hapten atrazine was detected under continuous flow conditions using a micro-column which contained immobilized monoclonal antibodies (Ab) against atrazine and atrazine labeled with alkaline phosphatase (An*). The equilibrium of the antibody-hapten system, was achieved by a continuous flow of the tracer An* through the micro-column containing the immobilized antibodies. The activity of the tracer was monitored continuously, after the micro-column, by an amperometric detector using p-hydroquinone phosphate as substrate. When pulses of unlabeled atrazine (An) were added to the An* flowing continuously through the micro-column, An* bound to the antibody was displaced, with a consequent change of the detector signal. By this method atrazine concentrations in the range 9-180 micrograms/l were monitored under conditions of continuous operation. Since the equilibrium condition for the system Ab-An* was continuously restored by the flow of An* through the micro-column the regeneration of the antibody was not required.