Medicinal plants are an important source of bioactive compounds with a wide spectrum of practically useful properties. Various types of antioxidants synthesized in plants are the reasons for their application in medicine, phytotherapy, and aromatherapy. Therefore, reliable, simple, cost-effective, eco-friendly, and rapid methods for the evaluation of antioxidant properties of medicinal plants and products on their basis are required. Electrochemical methods based on electron transfer reactions are promising tools to solve this problem. Total antioxidant parameters and individual antioxidant quantification can be achieved using suitable electrochemical techniques. The analytical capabilities of constant-current coulometry, potentiometry, various types of voltammetry, and chrono methods in the evaluation of total antioxidant parameters of medicinal plants and plant-derived products are presented. The advantages and limitations of methods in comparison to each other and traditional spectroscopic methods are discussed. The possibility to use electrochemical detection of the antioxidants via reactions with oxidants or radicals (N- and O-centered) in solution, with stable radicals immobilized on the electrode surface, via oxidation of antioxidants on a suitable electrode, allows the study of various mechanisms of antioxidant actions occurring in living systems. Attention is also paid to the individual or simultaneous electrochemical determination of antioxidants in medicinal plants using chemically modified electrodes.

Plants of the Lamiaceae family have been used for thousands of years in cooking, as well as phyto- and aromatherapy. Their essential oils are characterized by high antioxidant and other types of biological activities. In our study, the phytochemical profile and quantification of the essential oil components of thyme, marjoram, and sage were analyzed by gas chromatography with mass-spectrometric detection (GC-MSD). The antioxidant properties of the samples were evaluated using total antioxidant parameters (total antioxidant capacity (TAC), ferric reducing power (FRP), antioxidant activity (AOA) towards 2,2-diphenyl-1-picrylhydrazyl (DPPH•), and total phenolics by Folin–Ciocalteu method). The obtained FRP was 46–321-fold lower than TAC, which is consistent with the contents of phenolics identified in the samples. Terpenes, isopropylmethylphenols, and eugenol turned out to be the major components of all essential oils and determined their TAC and AOA. The Folin–Ciocalteu method was applicable to the thyme essential oil only. Its FRP, which is based on the reaction of phenolic antioxidants with electrogenerated ferricyanide ions, agreed well with the total phenolic contents (329 ± 17 and 334 ± 15 mg of carvacrol per mL, respectively). The thyme essential oil had the highest antioxidant parameters, while sage showed the weakest antioxidant properties. Positive correlations (r = 0.8846–0.9964) of the antioxidant parameters were obtained.

Eugenol is the major component of cloves and clove-based products, which are widely used in food, the pharmaceutical industry, and medicine. Its content can be considered a marker of the sample quality. Therefore, simple, sensitive, and reliable methods for eugenol quantification are required. Glassy carbon electrodes (GCEs) modified with carboxylated multi-walled carbon nanotubes (MWCNTs-COOH) and electropolymerized pyrogallol red were developed for the determination of eugenol in essential oils. The working conditions (supporting electrolyte, pH, monomer concentration, and electrolysis parameters) of pyrogallol red were found using voltammetric characteristics of eugenol. The electrodes developed were studied using scanning electron microscopy, voltammetry, and electrochemical impedance spectroscopy. The effectivity of the electrode surface modification was demonstrated. Eugenol determination was performed in differential pulse mode, whose parameters have been optimized. The linear response of the electrodes towards eugenol was obtained in the range of 0.75–100 µM with a detection limit of 0.73 µM. The high selectivity of eugenol determination was observed in the presence of typical inorganic and organic compounds, including essential oils components. The approach developed was tested on commercial clove, cinnamon, and nutmeg essential oils. Validation with the independent method showed similar accuracy and the absence of systematic errors in eugenol determination.

Essential oils have been known about since ancient times and have been used in aromatherapy. Nowadays, their applications also cover the medicine and food industries due to their wide spectrum of bioactivity, including antioxidant properties. Thus, the estimation of essential oils’ antioxidant properties is of practical interest. Phenolic compounds and terpenes are the major antioxidants present in essential oils according to gas chromatography-mass spectrometry (GC-MS). Total antioxidant parameters are a good alternative to the characterization of individual components by GC-MS, allowing the avoidance of a time-consuming and expensive procedure. Coulometric titration with electrogenerated bromine and ferricyanide ions has been used for the estimation of the total antioxidant capacity and the ferric reducing power of essential oils for the first time. Data on the reaction of individual antioxidants (volatile phenolics and terpenes) with coulometric titrants confirm the applicability of the method for the characterization of essential oils’ antioxidant properties. The essential oils clove, cinnamon, nutmeg, lavender, ginger, anise, basil, bergamot, jasmine, ylang-ylang, marjoram, neroli, rosemary, thyme, and clary sage of various trademarks (total 27 samples) are investigated. The data are compared to the standard parameters (antioxidant activity towards 2,2-diphenyl-1-picrylhydrazyl and the total phenolic contents). Positive correlations with coefficients from 0.7051 to 0.9558 confirm the accuracy of the coulometric approach. Moreover, the ferric reducing power reflecting the phenolic antioxidant content can be used for all samples of essential oils, while the Folin–Ciocalteu method for total phenolic contents is applicable to four essential oils only. Another advantage of coulometry titration is the possibility of automation and its rapidity, making it an attractive tool for screening purposes in routine practice.

Essential oils are widely used in aromatherapy, food, and pharmaceutical industries. They contain a range of electroactive natural phenolic antioxidants like eugenol, trans-anethole, thymol, carvacrol, and vanillin. Therefore, the sensitive voltammetric determination of these compounds is of practical interest. Voltammetric sensors based on the layer-by-layer combination of carbon nanotubes and electropolymerized dyes were developed. Pyrogallol red, mixture of phenol red and p-coumaric acid, thymolphthalein, bromocresol purple were used as monomers. The created sensors were used in the quantification of target analytes using differential pulse voltammetry in a Britton–Robinson buffer. The detection limits in the range of 3.7 × 10⁻⁸–7.3 × 10⁻⁷ M were achieved.