Simultaneous Determination of Ferulic Acid and Vanillin in Vanilla Extracts Using Voltammetric Sensor Based on Electropolymerized Bromocresol Purple

Natural phenolic antioxidants are one of the widely studied compounds in life sciences due to their important role in oxidative stress prevention and repair. The structural similarity of these antioxidants and their simultaneous presence in the plant samples stipulate the development of methods for their quantification. The current work deals with the simultaneous determination of vanillin and its bioprecursor ferulic acid using a voltammetric sensor for the first time. A sensor based on the layer-by-layer deposition of the polyaminobenzene sulfonic acid functionalized single-walled carbon nanotubes (f-SWCNTs) and electropolymerized bromocresol purple has been developed for this purpose. The best response of co-existing target analytes was registered for the polymer obtained from the 25 µM dye by 10-fold potential cycling from 0.0 to 1.2 V with the scan rate of 100 mV s−1 in 0.1 M phosphate buffer (PB), pH 7.0. Scanning electron microscopy (SEM), cyclic voltammetry and electrochemical impedance spectroscopy (EIS) confirmed the effectivity of the sensor developed. The linear dynamic ranges of 0.10–5.0 µM and 5.0–25 µM for both analytes with the detection limits of 72 nM and 64 nM for ferulic acid and vanillin, respectively, were achieved in differential pulse mode. The sensor was applied for the analysis of vanilla extracts.

Graphene and Chitosan Composite Film Modified Electrode as a Sensitive Voltammetric Sensor for Tyrosine Detection in Food and Biological Samples

A voltammetric sensor made from a graphene and chitosan modified glassy carbon electrode (GR-CTS/GCE) was fabricated for accurate analysis of tyrosine (Tyr) in both food and biological samples. The surface morphology of the electrode and the properties of the electrode-electrolyte interface were determined by scanning electron microscopy and cyclic voltammetry. Compared with a bare GCE, the synergistic effect of GR and CTS is obvious. The peak current increases 35.6 times. The experimental conditions were optimized by second derivative linear sweep voltammetry (SDLSV) and Tyr was quantitatively analyzed on the electrode. The study shows that the oxidation peak current of Tyr obtained in 0.1 M pH 2.7 phosphate buffer is proportional to its concentration between 0.006-0.8 and 0.8-10.0 μM, with the low detection limit being 4.0 nM (signal/noise = 3). Excellent anti-interference ability was demonstrated by investigating the voltammetric response of Tyr in mixtures containing other biomolecules. In addition, the sensor exhibited good stability and repeatability. Through the detection of Tyr in milk and serum samples, the effectiveness of the sensor was studied, and the results were satisfactory.

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 СеO2·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 СеO2·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 СеO2·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.

Electroanalysis of Ascorbic Acid in Fresh Orange Juice Using a PTH/MWCNTCOOH-RGO/CS/CuO Film Modified Carbon Paste Electrode

The incorporation, characterization, and analytical ability of an ascorbic acid (AA) voltammetric sensor, based on the PTH/MWCNTCOOH-RGO/CS/CuO modified carbon paste electrode (CPE), were studied. The nanocomposite was prepared via an in situ chemical polymerization route. The structural characterization, surface morphology, and thermal analysis of the modified polymers were confirmed. The kinetics and mechanism of the oxidation process of AA on the modified electrode were studied via scan rate analysis. The quantitative detection of AA was achieved effectually by a square wave voltammetry (SWV) technique. The proposed voltammetric sensor exhibited high performance in 0.1 M phosphate-buffered solution (PBS) at pH 3. In the pH range 3 to 7 AA undergoes irreversible oxidation via transfer of 2H+/2e-. The proposed sensor showed a wide linear range (5 µM to 2 mM) and a low detection limit (0.613 µM). Additionally, the reproducibility and stability (at the 1 mM level) expressed in terms of relative standard deviation (RSD) was 2.13% and 1.98% respectively. The practicality of the proposed sensor was confirmed by the successful detection of ascorbic acid in fresh orange juice and the recovery was between 107.9% and 92.63%, with excellent accuracy.