Chronocoulometry of wine on multi-walled carbon nanotube modified electrode: Antioxidant capacity assay.

Phenolic antioxidants of wine were electrochemically oxidized on multi-walled carbon nanotubes modified glassy carbon electrode (MWNT/GCE) in phosphate buffer solution. Three oxidation peaks were observed at 0.39, 0.61 and 0.83V for red dry wine and 0.39, 0.80 and 1.18 V for white dry wine, respectively, using differential pulse voltammetry at pH 4.0. The oxidation potentials for individual phenolic antioxidants confirmed the integral nature of the analytical signals for the wines examined. A one-step chronocoulometric method at 0.83 and 1.18 V for red and white wines, respectively, has been developed for the evaluation of wine antioxidant capacity (AOC). The AOC is expressed in gallic acid equivalents per 1L of wine. The AOC of white wine was significantly less than red wine (386 ± 112 vs. 1224 ± 184, p<0.0001), as might be expected. Positive correlations were observed between gallic acid equivalent AOC of wine and total antioxidant capacity, based on coulometric titration with electrogenerated bromine (r=0.8957 at n=5 and r=0.8986 at n=4 for red and white wines, respectively).

Chronoamperometric estimation of cognac and brandy antioxidant capacity using MWNT modified glassy carbon electrode.

Cognac and brandy components are electrochemically oxidized on multi-walled carbon nanotube modified glassy carbon electrode at 0.44 and 0.59 V in 0.1 М phosphate buffer solution pH 3.0. Voltammetric behavior of the main antioxidant constituents of cognac (ellagic and gallic acids, syringaldehyde, coniferaldehyde, vanillin, 5-hydroxymethylfurfural and furfural) has been investigated. The peak at the less positive potential of cognacs is caused by oxidation of gallic acid as well as syringaldehyde- and coniferaldehyde. The second peak corresponds to ellagic acid oxidation. One-step chronoamperometry at 0.59 V for 75 s has been applied for the cognac and brandy antioxidant capacity (AOC) evaluation. Ellagic acid, being the main antioxidant of cognac, has been used as a reference substance. The chronoamperometric response of ellagic acid is linear in the range of 0.66-52.8 µM with the limit of detection and quantification at 0.19 and 0.63 µM, respectively. AOC in ellagic acid equivalents per 100mL of cognac and brandy for different denominations (11 cognacs and 11 ordinary and vintage brandies) has been estimated. AOC of cognacs and brandies increases with the age of the beverages. Positive correlations (r=0.9134-0.9703) with common parameters characterizing antioxidant properties of beverages, in particular antiradical activity, total phenolics content, total antioxidant capacity and ferric reducing power have been observed.

Coulometric titration with electrogenerated oxidants as a tool for evaluation of cognac and brandy antioxidant properties.

Stoichiometric coefficients for reactions of cognac antioxidants with coulometric titrants (electrogenerated bromine and hexacyanoferrate(III) ions) have been found. Ellagic and gallic acids react with both titrants while aldehydes (vanillin, syringic and coniferaldehyde) - with electrogenerated bromine only. Furfurals do not show significant reactivity toward both oxidants. Cognac and brandy total antioxidant capacity (TAC) and ferric reducing power (FRP) based on reactions with electrogenerated bromine and hexacyanoferrate(III) ions, respectively, have been evaluated. Both parameters for cognacs are statistically significant higher than for brandies and grow with the age increase. Beverages under investigation has shown relatively high antiradical activity toward 2,2-diphenyl-1-picrylhydrazyl (7-92% and 5-93% for cognacs and brandies, respectively). Total phenolics content has been evaluated by Folin-Ciocalteu method. Older beverages represent the higher phenolics content caused by more time of extraction from oak barrels. Positive correlations (r=0.8077-0.9617) have been observed for TAC and FRP with antiradical activity and total phenolics content.

Electrochemical DNA sensors based on electropolymerized materials.

The use of electropolymerized materials in the DNA sensors is reviewed with particular emphasis on their functions and specific interactions with DNA and oligonucleotides. Polyaniline, pollypyrrole, polythiophenes and polymeric forms of phenazines play significant role in the immobilization and signal transduction of DNA sensors for the detection of hybridization events, DNA-protein and other specific interactions on the sensor surface. The mechanism of electropolymerization and the influence of oligonucleotides are also considered for various types of polymers. The DNA sensor performance is classified in accordance with the biological targets and composition of the surface layer.

Surfactant media for constant-current coulometry. Application for the determination of antioxidants in pharmaceuticals.

Effect of surfactant presence on electrochemical generation of titrants has been evaluated and discussed for the first time. Cationic (1-dodecylpyridinium and cetylpyridinium bromide), anionic (sodium dodecyl sulfate) and nonionic (Triton X100 and Brij(®) 35) surfactants as well as nonionic high molecular weight polymer (PEG 4000) do not react with the electrogenerated bromine, iodine and hexacyanoferrate(III) ions. The electrogenerated chlorine chemically interact with Triton X100 and Brij(®) 35. The allowable range of surfactants concentrations providing 100% current yield has been found. Chain-breaking low molecular weight antioxidants (ascorbic acid, rutin, α-tocopherol and retinol) were determined by reaction with the electrogenerated titrants in surfactant media. Nonionic and cationic surfactants can be used for the determination of antioxidants by reaction with the electrogenerated halogens. On contrary, cationic surfactants gives significantly overstated results of antioxidants determination with electrogenerated hexacyanoferrate(III) ions. The use of surfactants in coulometry of α-tocopherol and retinol provides their solubilization and allows to perform titration in water media. Simple, express and reliable coulometric approach for determination of α-tocopherol, rutin and ascorbic acid in pharmaceuticals using surfactant media has been developed. The relative standard deviation of the measurements does not exceed of 5%.

Voltammetric determination of ß-carotene in raw vegetables and berries in Triton X100 media.

Electrochemical behavior of ß-carotene in polar organic media and solubilized systems has been investigated. ß-Carotene is irreversibly oxidized at 500 and 920 mV on glassy carbon electrode in 0.1 M LiClO(4) ethanol containing 10% of CH(2)Cl(2). Effect of surfactants (cationic, nonionic and anionic) on voltammetric characteristics of ß-carotene oxidation has been evaluated. High concentrations of surfactants facilitate the electrooxidation of ß-carotene independently of surfactant type. The increase of oxidation current by 7-11% has been obtained in the presence of nonionic surfactant. The best results have been observed in 10 mM Triton X100 media. The peak current showed a linear dependence with the ß-carotene concentration over the range 10-380 µM. The calculated detection limit was 2.5 µM and the quantification limit was 8.3 µM. Liquid extraction of ß-carotene with dichloromethane from raw vegetables and berries has been developed. Quantitative determination of ß-carotene in real samples using cyclic voltammetry in Triton X100 media combined with preliminary extraction has been carried out. The results obtained are in good agreement with data of nutrient database for standard references.

Application of constant-current coulometry for estimation of plasma total antioxidant capacity and its relationship with transition metal contents.

Simple and express coulometric method for the evaluation of the total antioxidant capacity (TAC) of human plasma based on the reaction with electrogenerated bromine is applied. TAC of plasma from patients with different ethiology of chronic renal failure was observed. The levels of antioxidant capacity for venous and arterial plasma are authentically different (15+/-1 kCl/L versus 11.7+/-0.7 kCl/L, p<0.01). The application of Vitamin E and ximedon as an antioxidant treatment significantly increase TAC level of plasma. Free liposoluble antioxidants in plasma in alpha-tocopherol units was determined. Redox potential of plasma is measured and its correlation with lg(TAC) is obtained. Transition metal contents of Fe, Cu, Mn, Ni, and Cr in plasma of patients with chronic renal failure is significantly higher than that for a control group. Correlation analysis has shown negative linear regression between TAC value and transition metals concentration in plasma. This confirms interrelation of processes with participation of free radicals, antioxidants and transition metals as donors of electrons in chain radical processes. Moreover, it shows utility of common parameters, TAC for example, for estimation of efficiency of antioxidant defense system in living organism, in particular its antioxidant status.

The application of coulometry for total antioxidant capacity determination of human blood.

New coulometric method for estimation of blood and plasma total antioxidant capacity (TAC) based on using electrogenerated bromine was proposed. TAC of blood from patients with chronic renal disease undergoing long-term hemodialysis was investigated. Statistical significant changes in TAC level of venous and arterial blood were found. Catalase activity and low density lipoproteins (LDL) concentrations were determined. Linear correlation between TAC and parameters mentioned was found. Contribution from some individual antioxidants was investigated. The developed method for TAC assay is expressive, simple, stable and reliable, and successfully could be used for TAC determination of some biological fluids. This method could be applied in clinic for estimation of blood TAC from patients.

Cholinesterase sensors based on screen-printed electrodes for detection of organophosphorus and carbamic pesticides.

Cholinesterase sensors based on screen-printed electrodes modified with polyaniline, 7,7',8,8'-tetracyanoquinodimethane (TCNQ), and Prussian blue have been developed and tested for detection of anticholinesterase pesticides in aqueous solution and in spiked grape juice. The influence of enzyme source and detection mode on biosensor performance was explored. It was shown that modification of the electrodes results in significant improvement of their analytical characteristics for pesticide determination. Thus, the slopes of the calibration curves obtained with modified electrodes were increased twofold and the detection limits of the pesticides were reduced by factors of 1.6 to 1.8 in comparison with the use of unmodified transducers. The biosensors developed make it possible to detect down to 2 x 10(-8) mol L(-1) chloropyrifos-methyl, 5 x 10(-8) mol L(-1) coumaphos, and 8 x 10(-9) mol L(-1) carbofuran in aqueous solution and grape juice. The optimal conditions for grape juice pretreatment were determined to diminish interference from the sample matrix.