G. K. Ziyatdinova

Determination of captopril in pharmaceutical forms by stripping voltammetry

A voltammetric procedure is developed for determining captopril in pharmaceuticals. It is based on the preliminary electrochemical accumulation of the captopril oxidation product on a platinum electrode in a 0.1 M HNO3 solution at 1.2 V versus a saturated silver-silver chloride electrode. The reduction current of the oxidation product is a linear function of captopril concentration in the range from 1.2 × 10−6 to 3.2 × 10−4 M. The determination limit is 9.2 × 10−7 M. The relative standard deviation is between 1 and 4%.

Voltammetric evaluation of the antioxidant capacity of tea on electrodes modified with multi-walled carbon nanotubes

The characteristics of the voltammograms of tea polyphenols on a glassy carbon electrode modified with multi-walled carbon nanotubes (MCNT-GCE) were evaluated. With the use of atomic force microscopy, it was found that MCNTs are oriented as rows 0.8–1.0 μm wide with alternating hills to 586 nm in height. Polyphenols other than of tannin are reversibly oxidized at the first step. Corresponding electrode reaction schemes are proposed. A voltammetric procedure for the estimation of the antioxidant capacity (AOC) of tea based on the oxidation of its polyphenol compounds was developed. The voltammograms of tea exhibited clearly defined peaks and oxidation steps whose potentials depend on the type of tea. The area of oxidation peaks was chosen as the parameter that characterizes antioxidant properties. The AOC of tea was expressed in terms of catechin equivalents per 100 mL of a beverage. 27 tea samples were analyzed. It was found that the AOC of green tea is 79% higher than that of black tea (290 ± 40 and 54 ± 22 mg/100 mL, respectively, P < 0.05). The AOC of oolong tea (70 ± 5 mg/100 mL) is considerably lower than that of green tea and statistically insignificantly higher than that of black tea. The AOC of white tea is comparable with the AOC of green tea (255 ± 11 and 290 ± 40 mg/100 mL, respectively, P > 0.05).

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.

Reactions of superoxide anion radical with antioxidants and their use in voltammetry

Kinetic parameters were calculated for the electrochemical reduction of oxygen at a glassy-carbon electrode with the generation of superoxide radical anions in a 0.05 M solution of (C2H5)4NI in dimethylformamide in the presence of fat-soluble antioxidants, retinol and α-tocopherol. A procedure based on the protonation of the radical anion with antioxidant molecules is proposed for the voltammetric determination of antioxidants to determine milligram amounts of retinol and α-tocopherol in model solutions (RSD = 1–2%). The calibration graphs for retinol and α-tocopherol are linear in the concentration ranges 9.7 × 10−5–2.3 × 10−3 and 6.2 × 10−4–3.1 × 10−3 M, respectively. The detection limits for retinol and α-tocopherol are 4.8 × 10−5 and 4.1 × 10 −4 M, respectively. The procedure was applied to the determination of the active component (retinol and α-tocopherol) in pharmaceuticals.

Natural phenolic antioxidants in bioanalytical chemistry: state of the art and prospects of development

Published data of the last decade on the chemical analysis and related problems of investigation of phenolic antioxidants as components of complex biomedical and food objects are generalized. The physicochemical properties of these compounds are presented in accordance with the up-to-date classification of their structures. Particular attention is paid to the development of new and advancement of traditional methods for determination of natural phenolic antioxidants in relation to public health protection problems. The interdisciplinary character of the considered problems is emphasized and the ways for problem solution are outlined. The bibliography includes 380 references.

Complex electrochemical and impedimetric evaluation of DNA damage by using DNA biosensor based on a carbon screen-printed electrode

DNA biosensor (DNA/SWCNT-COOH-CHIT/SPCE) composed of dsDNA adsorptive layer on a carboxylated single-walled carbon nanotubes–chitosan composite deposited at a commercial carbon based screen-printed electrode has been prepared and applied to a complex investigation of damage to DNA by the Fenton type cleavage agent (hydroxyl radicals formed in the mixture of Cu2+, H2O2 and ascorbic acid) and copper(II)–quercetin system in 0.1 M PBS pH 7.0 under aerobic conditions. The dsDNA damage detection is performed by using square-wave voltammetry (SWV), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in 1 × 10−7 M thioridazine and 1 × 10−3 M K4[Fe(CN)6]/K3Fe(CN)6 in the 0.1 M phosphate buffer solution, pH 7.0. Initial enhancement of the intrinsic guanine and adenine moieties SWV response over that of original dsDNA one indicates opening of the helix structure in the first stage of damage. At the same time, decrease in the intercalated thioridazine response confirms damage of the helix structure in parallel to deep degradation of the DNA chain and its removal from the electrode surface as indicated by the CV and EIS measurements in the presence of the [Fe(CN)6]3−/4−redox indicator in solution.

Voltammetric determination of curcumin in spices

Curcumin is oxidized on a glassy carbon electrode in 0.1 M LiClO4 in ethanol at the potential 0.74 V. The calibration graph is linear in the range 9.9 × 10−6−1.07 × 10−4 M curcumin; the detection limit is 4.1 × 10−6 M. Curcumin is determined in model solutions. The relative standard deviation does not exceed 0.05. A procedure is proposed for the voltammetric determination of curcumin in spices. It was shown that single extraction by ethanol quantitatively recovers curcumin from spices.

Voltammetric determination of papaverine and drotaverine

Isoquinoline derivatives (papaverine and drotaverine) are oxidized at a graphite electrode in a 0.1 M sulfuric acid solution to give voltammetric waves at 1.1 V for papaverine and at 1.05 and 1.28 V for drotaverine. Determination limits and linearity ranges of currents as functions of papaverine and drotaverine concentrations are estimated. Microgram amounts of papaverine and drotaverine are determined in model solutions (RSD = 1–4%). A procedure for the direct determination of papaverine and drotaverine in pharmaceuticals is proposed.

Voltammetric sensing and quantification of eugenol using nonionic surfactant self-organized media

Triton X100 and Brij® 35 based self-organized systems (at a concentration of 0.1 M) provide eugenol solubilization in water media. Eugenol is irreversibly oxidized on a glassy carbon electrode at 780 and 700 mV in 0.1 M LiClO4 in 0.1 M Triton X100 and Brij® 35 micellar media, respectively. Electrochemical oxidation of eugenol is a diffusion-controlled process that is confirmed by the linear dependence of peak currents on the v1/2 with R2 = 0.9968 and 0.9989 in Triton X100 and Brij® 35 media, respectively, and involves 2.0 ± 0.1 electrons corresponding to the formation of o-quinone. The eugenol calibration graph is linear in the range 15–1230 μM with an estimated detection limit of 3.8 μM and a quantification limit of 12.6 μM. The addition of ethanol (10% v/v) to a 0.1 M Triton X100 micellar medium leads to the cathodic shift of eugenol oxidation potential of 50 mV. Under these conditions, the oxidation current linearly depends on the eugenol concentration in the range 0.02–1.0 mM with a detection limit of 0.01 mM. The recovery of eugenol determination in test solutions is in the range 99.0–101.2%. The preliminary extraction of eugenol with ethanol is used for its voltammetric determination in spices. Quantitative determination of eugenol in essential oils in a Triton X100 micellar medium has been carried out. The results obtained for real samples are in good agreement with data from independent spectrophotometric methods.

Application of surfactants in voltammetric analysis

The possibilities of applying surfactants to the voltammetry of various biologically active compounds are generalized. Attention is focused on micellar solutions and surfactant-modified electrodes in the determination of a number of organic compounds, including antioxidants. It is demonstrated that the use of surfactants makes it possible to improve analytical characteristics and, in some cases, to simultaneously determine different analytes.