Screening of the essential oils antioxidant capacity using electrode modified with carboxylated multi-walled carbon nanotubes

Essential oils are of high interest in analytical chemistry due to their bioactive properties and wide application area (in aromatherapy, medicine, and food industry). Gas chromatography with mass- spectrometric detection (GC-MS) is the golden standard in their characterization and investigations. On the other hand, the presence of volatile phenolics and terpenoids make it possible to use electrochemical methods for the characterization and screening of essential oils using antioxidant parameters in particular antioxidant capacity. Unfortunately, essential oils are fully out of consideration in modern electroanalysis from this point of view. Voltammetric behavior of essential oils (from 15 types of plant material) at the electrode modified with carboxylated multi-walled carbon nanotubes have been studied for the first time. All samples are electrochemically active in neutral medium under conditions of differential pulse voltammetry. There are well-pronounced signals on the voltammograms in the ranges of 0.0-0.75 and 0.75-1.5 V caused by electrooxidation of phenolic constituents and terpenoids, respectively that is confirmed by oxidation potential of individual standard compounds. Moreover, clear oxidation peaks for 9 essential oils are registered in the range of 0.75-1.5 V only that agrees well with GC-MS data on their major constituents. Two-step chronoamperometric method has been developed for the evaluation of the essential oils antioxidant capacity. Two anodic potentials of 0.80 and 1.4 V have been chosen for these purposes. The electrolysis steady-state is achieved at 75 s of electrolysis. The antioxidant capacity has been expressed as current consumed per 1 mL of essential oil. Screening of 37 samples of essential oils by their antioxidant capacity has been performed. The data obtained are compared to the standard antioxidant parameters (antioxidant activity towards 2,2-diphenyl-1-picrylhydrazyl and total phenolics).