Milan Sýs

Determination of vitamin E in margarines and edible oils using square wave anodic stripping voltammetry with a glassy carbon paste electrode

A new electroanalytical method for determination of vitamin E in the form of the total content of tocopherols present in margarines and edible oils has been developed. The method is based on extraction of these biologically active compounds into silicone oil, acting as lipophilic binder of glassy carbon paste electrode, with subsequent electrochemical detection by square wave anodic stripping voltammetry (SWASV) in 0.1M HNO3. The values of vitamin E contents were expressed as mass equivalent of α-tocopherol known as the most active form of this lipophilic vitamin. The linear ranges for α-tocopherol determination were 5×10-7-4×10-5 and 5×10-8-1×10-5molL-1 with the detection limits of 1×10-7 and 3.3×10-9molL-1 for 5 and 15min accumulation, respectively. The results have shown that SWASV with extraction step is very sensitive method for the determination of vitamin E, being comparable to reversed-phase high performance liquid chromatography chosen as reference method.

Improvement of enzyme carbon paste-based biosensor using carbon nanotubes for determination of water-soluble analogue of vitamin E

The catalytic oxidation of a synthetic water-soluble analogue of vitamin E (α-tocopherol, Trolox) by tyrosinase enzyme in the presence of molecular oxygen was studied using electrochemical techniques. This specific enzymatic reaction was exploited for the preparation of a biosensor based on the amperometric reduction of the electroactive product (α-tocoquinone) formed. An electroactive surface of the transducers used was covered with a thin conductive layer of Nafion containing tyrosinase. Significant progress in sensitivity towards polyphenolic compounds such as Trolox was achieved at CPE with carbon nanotubes immobilised on its surface (CPE/CNTs) as electric transducers. The biosensor so developed can be used for the direct determination of total phenolic content (TPC). This important nutrition value can be expressed as the mass equivalent of Trolox, i.e. Trolox equivalent antioxidant capacity (TEAC), which could be used as an alternative to the evaluations currently used based on spectrophotometric methods such as total radical-trapping antioxidant parameter (TRAP), ferric reducing-antioxidant power (FRAP) or 1,1-diphenyl-2-picrylhydrazyl spectrometric assay (DPPH). The effects of the enzyme amount in the Nafion layer (3.0 µg), the influence of the nanoparticles present, the optimal pH value suitable for enzymatic activity (7.0), and the kinetics of enzymatic and electrochemical reactions were studied using cyclic voltammetry (CV). The determination of optimal conditions for amperometry in batch configuration (working potential, speed of stirring, volume of sample, calibration curve, etc.) was not a target of this electrochemical study.

Application of screen-printed carbon electrode modified with lead in stripping analysis of Cd(II)

Abstract In the work presented, a lead film electrode was prepared in situ on a screen-printed carbon support using a reversibly deposited mediator (Zn) and applied to the determination of Cd(II) by anodic stripping voltammetry. The electrochemical method for lead film formation is based on a co-deposition of a metal of interest (Pb), with a reversibly deposited zinc mediator, followed by oxidation of zinc, with additional deposition of lead at the appropriate potential. It serves to increase the density of lead particles, promoting lead film growth, and consequently helps to improve the electrochemical properties of the electrode. This was confirmed by microscopic and voltammetric studies. The obtained detection limit of Cd(II) is equal to 6.6 × 10−9 mol L−1 (−1.6 V for 180 s and then −0.95 V for 5 s). The presented procedure was successfully applied to cadmium determination in Bystrzyca River water samples.

Electrochemical Study and Determination of All-trans Retinol at Carbon Paste Electrode Modified by Surfactant

Summary The oxidation mechanism of all-trans-retinol (vitamin A1) and its several esters in non-aqueous, aqueous organic mixture, and pure aqueous media was investigated by cyclic voltammetry. The oxidation occurred in several irreversible steps. The calculated highest density of electrons in retinoid molecules which are delocalized over carbon atoms of the five conjugated double bonds (C5-C14) was found in the part of the molecule involved in oxidation processes. The most sensitive oxidation peak (at +0.8 V vs. Ag/AgCl) was used for development of new direct voltammetric method based on differential pulse voltammetry for the determination of retinol at carbon paste electrode modified with surfactant sodium dodecyl sulfate (CPE/SDS). The results show that 30% (by mass) of modifier SDS exhibited optimal sensitivity and shape of voltammograms. Compared to commonly used glassy carbon electrode (GCE), the CPE/SDS showed significant progress in the retinol electroanalysis. The linear ranges for retinol determination were 1.5·10−6–1.8·10−4 M for CPE/SDS and 4.4·10−6–7.0·10−4 M for GCE with the detection limits of 1.3·10−6 and 4.6·10−7 M, respectively.

Tyrosinase electrochemical biosensors monitoring medicinally significant substances

Nowadays in biosensing, development of analytical techniques is focused on improvement for monitoring biologically active species in living organisms, especially in real-time analysis. This article provides an overview of applications of electrochemical tyrosinase biosensors in the analysis of medicinally significant substances, also known as biomarkers. At the beginning, special attention is paid to characterisation of the tyrosinase enzyme, explanation of reaction mechanisms of tyrosinase with various types of electrochemical transducers and techniques needed for stable immobilization of this biocatalyst on the transducer surface used. In this case, amperometric transducers represent the most frequently used type of electrochemical sensing because they usually provide sensitive current response to the presence of analyte in the sample. Many scientific works suggest that these very selective bioanalytical devices could find application in the clinical diagnosis of various serious diseases because they represent the effective analytical tools for diagnosis of neurodegenerative disorders, detection of microbial pathogens responsible for foodborne illness and diagnosis of cytopathology, especially melanoma cancer. Additionally, various kinds of electrochemical tyrosinase biosensors are presented which were developed to determine the catalytic activity of other enzymes significant for human metabolism, medicaments and their metabolic products, several hormones, amino acids and proteins. Despite all of their advantages, it is necessary to state that tyrosinase biosensors are still not used in the routine laboratory practice due to their relatively short service life which is not longer than one month unfortunately. Never-ending development of catalytic polymers imitating the tyrosinase active site could be one of ways to solve this serious drawback.