Endzhe Ziganshina

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.

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

Electrochemical behavior of β-carotene in polar organic media and solubilized systems has been investigated. β-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 β-carotene oxidation has been evaluated. High concentrations of surfactants facilitate the electrooxidation of β-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 β-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 β-carotene with dichloromethane from raw vegetables and berries has been developed. Quantitative determination of β-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.

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 sterically hindered phenols in surfactant-based self-organized media

Tert-butylhydroquinone (TBHQ), butylhydroxyanisole (BHA), and butylhydroxytoluene (BHT) are irreversibly oxidized on a glassy-carbon electrode in a medium of acetonitrile-Britton-Robinson buffer solution (1: 9, pH 3.0) at 0.61, 0.79, and 1.1 V, respectively, to give quinones. Micellar media of Triton X100,Brij® 35, and sodium dodecyl sulfate affect their oxidation. The splitting and suppression of the oxidation steps for sterically hindered phenols (SHP) is observed in the presence of nonionic surfactants. Sodium dodecyl sulfate considerably enhances anodic currents and also shifts positive potentials to lower values. The conditions of the differential-pulse voltammetric determination of SHP in a 0.1 M sodium dodecyl sulfate supporting electrolyte are found. The calibration graphs are linear in the ranges 2.02–1010, 2.34–1170, and 6.15–615 μM and the detection limits (S/N = 3) are 0.23, 0.18, and 3.5 μM for TBHQ, BHA, and BHT, respectively. The possibility of determining SHP in TBHQ-BHT and BHA-BHT mixtures in wide ranges of component concentration is demonstrated. A method is developed for the extraction-voltammetric determination of BHT in vegetable oils.

Simultaneous determination of TBHQ and BHA on a MWNT-Brij® 35 modified electrode in micellar media

A voltammetric method for the simultaneous detection and quantification of tert-butylhydroquinone (TBHQ) and butylated hydroxyanisole (BHA) was developed using a multi-walled carbon nanotube (MWNT)-Brij® 35 modified glassy carbon electrode in Brij® 35 micellar medium. Significant improvements of TBHQ and BHA voltammetric characteristics were observed on the modified electrode. Electrochemical oxidation of TBHQ and BHA is a diffusion-controlled process that is confirmed by the linear dependence of peak currents on υ1/2 with R2 = 0.9983 and 0.9901, respectively, and involves two electrons corresponding to the formation of p-quinones. Under conditions of differential pulse voltammetry, a linear dynamic range of 1.0–1000 μmol L−1 for TBHQ and 0.50–7.50 and 10.0–750 μmol L−1 for BHA were obtained with the detection limits of 0.26 and 0.15 μmol L−1, respectively. The approach developed was successfully applied for the quantification of TBHQ and BHA in linseed oil samples with recoveries from 99% to 103% after a simple and rapid extraction with Brij® 35 micellar medium.

Determination of the Antioxidant Capacity of the Micellar Extracts of Spices in Brij® 35 Medium by Differential Pulse Voltammetry

It was established that the micellar extracts of spices are electrochemically active on a glassy carbon electrode modified with cerium dioxide nanoparticles in a 0.02 M Brij® 35 in the presence of a phosphate buffer solution (pH 7.4) under the conditions of differential pulse voltammetry. The number of oxidation steps and their potentials vary over a wide range depending on the type of spice. A number of the oxidation peaks of the micellar extracts of spices were identified based on the oxidation potentials of the following individual antioxidants: gallic acid, ferulic acid, p-coumaric acid, caffeic acid, rosmarinic acid, thymol, eugenol, vanillin, syringaldehyde, capsaicin, rutin, quercetin, catechin, tannin, and curcumin. The contribution of the main antioxidants to the amperometric response of the extracts was confirmed by the standard addition method. A procedure for the voltammetric determination of the antioxidant capacity of the extracts of spices based on the oxidation of their antioxidants was developed. The antioxidant capacity of spices was evaluated from the total area of the oxidation steps in units of gallic acid, whose analytical range, detection limit, and determination limit were 50–2490, 11.9, and 39.6 μM, respectively. Twenty types of spices were analyzed. Positive correlations of the antioxidant capacity with the ferric reducing power and the antioxidant activity (r = 0.8971 and 0.9127, respectively at rcrit = 0.497) were found.

Voltammetric Determinations of Thymol on an Electrode Modified by Coimmobilized Carboxylated Multiwalled Carbon Nanotubes and Surfactants

Thymol is oxidized at glassy carbon electrodes (GCEs) modified with coimmobilized carboxylated multiwalled carbon nanotubes (MWCNT-COOH) and surfactants of various nature in a Britton–Robinson buffer solution. The effect of the nature and concentration of surfactants in the composition of the electrode surface modifier on the amperometric response of thymol was evaluated. It was found that the best voltammetric characteristics are achieved in the case of an anionic 0.10 mM sodium dodecyl sulfate (SDS) (a decrease in oxidation potential by 50 mV and an increase in oxidation currents 2.2-fold in comparison with MWCNT-COOH/GCE). The electrooxidation of thymol at MWCNT-COOH–SDS/GCE proceeds irreversibly with the participation of one electron and one proton and is controlled by the adsorption of the analyte. The electrode response is linear in the ranges 0.500–17.0 and 17.0–150 μM of thymol with the limits of detection 0.14 μM and determination 0.48 μM. The developed method is tested on thymol-containing pharmaceutical preparations. The voltammetry data are compared with the results of an independent spectrophotometric determination.

Chronoamperometric determination of synthetic phenolic antioxidants in Brij® 35 micellar medium

Abstracttert-Butylhydroquinone (TBHQ) and tert-butylhydroxyanisole (BHA) are oxidized on a glassy carbon electrode modified with multiwalled carbon nanotubes in a 0.1 M LiClO4 supporting electrolyte in the medium of 1 mM Brij®35 at 0.27 and 0.47 V, respectively. A method for their chronoamperometric determination is developed. It is shown that a steady-state electrolysis is achieved within 100 s. The analytical range is 2.50–1000 µM for TBHQ and 1.50–100 and 250–1000 µM for BHA with the detection limits (S/N = 3) 0.64 and 0.38 µM, respectively. The relative standard deviation in the determination of TBHQ and BHA in model solutions does not exceed 6%. The method was tested on micellar extracts of linseed oils. The accuracy measure is 100 ± 1%, which points to the absence of matrix effects in the determination of TBHQ and BHA.