Recent Advances in Electrochemical Sensors for Sulfur-Containing Antioxidants

Sulfur-containing antioxidants are an important part of the antioxidant defense systems in living organisms under the frame of a thiol-disulfide equilibrium. Among them, l-cysteine, l-homocysteine, l-methionine, glutathione, and α-lipoic acid are the most typical representatives. Their actions in living systems are briefly discussed. Being electroactive, sulfur-containing antioxidants are interesting analytes to be determined using various types of electrochemical sensors. Attention is paid to the chemically modified electrodes with various nanostructured coverages. The analytical capabilities of electrochemical sensors for sulfur-containing antioxidant quantification are summarized and discussed. The data are summarized and presented on the basis of the electrode surface modifier applied, i.e., carbon nanomaterials, metal and metal oxide nanoparticles (NPs) and nanostructures, organic mediators, polymeric coverage, and mixed modifiers. The combination of various types of nanomaterials provides a wider linear dynamic range, lower limits of detection, and higher selectivity in comparison to bare electrodes and sensors based on the one type of surface modifier. The perspective of the combination of chromatography with electrochemical detection providing the possibility for simultaneous determination of sulfur-containing antioxidants in a complex matrix has also been discussed.

Cerium(IV) and Iron(III) Oxides Nanoparticles Based Voltammetric Sensor for the Sensitive and Selective Determination of Lipoic Acid

A novel voltammetric sensor based on CeO2·Fe2O3 nanoparticles (NPs) has been developed for the determination of lipoic acid, playing an essential role in aerobic metabolism in the living organism. Sensor surface modification provides a 5.6-fold increase of the lipoic acid oxidation currents and a 20 mV anodic shift of the oxidation potential. The best voltammetric parameters have been obtained for the 0.5 mg mL-1 dispersion of CeO2·Fe2O3 NPs. Scanning electron microscopy (SEM) confirms the presence of spherical NPs of 25-60 nm, and their aggregates evenly distributed on the electrode surface and formed porous coverage. This leads to the 4.4-fold increase of the effective surface area vs. bare glassy carbon electrode (GCE). The sensor shows a significantly higher electron transfer rate. Electrooxidation of lipoic acid on CeO2·Fe2O3 NPs modified GCE is an irreversible diffusion-controlled pH-independent process occurring with the participation of two electrons. The sensor gives a linear response to lipoic acid in the ranges of 0.075-7.5 and 7.5-100 μM with the detection limit of 0.053 μM. The sensor is selective towards lipoic acid in the presence of inorganic ions, ascorbic acid, saccharides, and other S-containing compounds. The sensor developed has been tested on the pharmaceutical dosage forms of lipoic acid.

Fast quantification of α-lipoic acid in biological samples and dietary supplements using batch injection analysis with amperometric detection

Batch injection analysis (BIA) with amperometric detection, using a pyrolytic graphite electrode modified with cobalt phthalocyanine (PG/CoPc), was employed for determination of α-lipoic acid (ALA) in pharmaceutical product and in synthetic urine samples. The proposed BIA method is based on the application of a potential of +0.9V vs. Ag/AgCl, KCl sat, enabling quantification of ALA over a concentration range from 1.3×10(-6) to 1.0×10(-4)molL(-1), with a detection limit of 1.5×10(-8)molL(-1). A sampling rate of 180 injections per hour was attained and measurements of the reproducibility of successive injections (100µmolL(-1) ALA on the same electrode) showed a RSD of 2.11% for 40 successive injections. The new sensor was utilised for ALA quantification in a dietary pharmaceutical supplement and in synthetic urine and the results obtained for both samples were compared with parallel analysis using high performance liquid chromatography (HPLC), the method recommended by the United States Pharmacopeia. The results obtained were similar (at a 95% confidence level) and in the case of the synthetic urine sample (prepared with a known amount of ALA) the recovery was situated between 98.0% and 102.6%.