Screening of Essential Oil Antioxidant Capacity Using Electrode Modified with Carboxylated Multi-Walled Carbon Nanotubes †
Analytical Chemistry Department, Kazan Federal University, Kremleyevskaya, 18, 420008 Kazan, Russia
*
Author to whom correspondence should be addressed.
†
Presented at the 9th International Electronic Conference on Sensors and Applications, 1–15 November 2022; Available online: https://ecsa-9.sciforum.net/ .
Eng. Proc. 2022, 27(1), 48; https://doi.org/10.3390/ecsa-9-13181
Published: 1 November 2022
(This article belongs to the Proceedings of The 9th International Electronic Conference on Sensors and Applications)
Abstract
:Essential oils are of interest in analytical chemistry due to their bioactive constituents and wide application area. The voltammetric behavior of essential oils (from 15 types of plant material) at an electrode modified with carboxylated multi-walled carbon nanotubes was studied for the first time. Oxidation peaks at 0.0–0.75 and 0.75–1.5 V were obtained on differential pulse voltammograms in a neutral medium, caused by the electrooxidation of phenolic constituents and terpenoids. A two-step chronoamperometric method was developed for the evaluation of the antioxidant capacity of the essential oils. Screening of 37 samples of essential oils was performed. The data agree with the standard antioxidant parameters.
1. Introduction
Essential oils are of great interest in analytical chemistry due to their bioactive properties and wide application area (in aromatherapy, medicine, and the food industry) [1,2,3]. Gas chromatography with mass-spectrometric detection is the gold standard in their characterization and investigation [1,4]. 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 almost out of consideration in modern electroanalysis from this point of view. The only example is the voltammetric method for the estimation of the antioxidant properties of the Mentha species based on the reaction of the antioxidants with a superoxide anion radical [5]. Therefore, simple, reliable, and cost-effective electrochemical approaches for the evaluation of the total antioxidant parameters of essential oils are of practical interest.
The current work is focused on the development of an electrochemical approach for the evaluation of the antioxidant capacity of essential oils using the electrooxidation of their antioxidants at an electrode modified with carboxylated multi-walled carbon nanotubes. The practical applicability of the method developed is shown by the screening of the 37 samples of essential oils from 15 types of plant material. A comparison of the data obtained with standard parameters (antioxidant activity by reaction with 2,2-diphenyl-1-picrylhydrazyl and total phenolic content by the Folin–Ciocalteu method) has been performed.
2. Materials and Methods
Commercially available essential oils from various plant materials (clove, cinnamon, nutmeg, lavender, ginger, anise, basil, bergamot, jasmine, ylang-ylang, marjoram, neroli, rosemary, thyme and clary sage) of various trademarks were studied. Their ten-fold dilution with ethanol was used exactly before the measurements.
Thymol of 99.5% purity (Sigma, Steinheim, Germany), 98% carvacrol, 99% eugenol (Aldrich, Steinheim, Germany), 97% phythol (Sigma-Aldrich, Steinheim, Germany), 99% benzyl alcohol, 96% limonene, 98% α-pinene, 98% β-pinene, 98% D-carvone, 75% camphene, as well as α-fenchene, myrcene, 99.5% L-menthol, and 97% L-borneol (Acros Organics, Geel, Belgium) were used. Their 10 mM stock solutions were prepared by dissolving an accurately weighed portion in 5.0 mL of ethanol (rectificate). Other reagents were of chemical purity and used as received.
Carboxylated multi-walled carbon nanotubes (diameter 9.5 nm, length 1.5 μm and carboxylation degree >8%) from Aldrich (Steinheim, Germany) were used as an electrode surface modifier. Their 1.0 mg mL−1 suspension in 1% sodium dodecylsulfate (Panreac, Barcelona, Spain) was obtained by sonication for 30 min in an ultrasonic bath (WiseClean WUC-A03H (DAIHAN Scientific Co., Ltd., Wonju-si, Korea)).
Potentiostats/galvanostats μAutolab Type III (Eco Chemie B.V., Utrecht, The Netherlands) with GPES 4.9.005 software and a 10 mL glassy electrochemical cell were used for the electrochemical measurements. Working glassy carbon electrodes (GCE) of 3 mm diameter (BASi® Inc., West Lafayette, IN, USA), or modified electrodes, an Ag/AgCl reference electrode, and a platinum wire as an auxiliary electrode, were used.
GCE modification was performed by the drop casting of 2 µL of carboxylated multi-walled carbon nanotube suspension.
The pH measurements were performed using an “Expert-001” pH meter (Econix-Expert Ltd., Moscow, Russia) and a glassy electrode.
3. Results and Discussion
The voltammetric behavior of essential oils (from 15 types of plant material) at the electrode modified with carboxylated multi-walled carbon nanotubes has been studied. All samples are electrochemically active in a neutral medium (phosphate buffer pH 7.0) 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 (Table 1). Clear oxidation peaks for nine essential oils are registered in the range of 0.75–1.5 V only.
Investigation of the voltammetric characteristics of individual antioxidants has shown that thymol and carvacrol are oxidized at 0.53 and 0.54 V, respectively. Eugenol shows two oxidation steps at 0.13 and 0.40 V. Among the terpenoids considered, only limonene and α-pinene are electrochemically active at 1.27 and 1.09 V, respectively. Phythol, β-pinene, D-carvone, camphene, α-fenchene, myrcene, L-menthol, benzyl alcohol, and L-borneol are silent on the voltammograms under the conditions of the experiment. Thus, oxidation peaks of essential oils in the range of 0.0–0.75 V are caused by the oxidation of phenolic antioxidants, while the terpenoids are oxidized in the second range of 0.75–1.5 V.
On the basis of the data obtained, a 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. A typical view of the chronoamperogram is shown in Figure 1.
The antioxidant capacity has been expressed as a current at 0.8 V reflecting the contents of phenolics (AOC0.8) and the total current at 1.4 V (AOCtotal) recalculated per 1 mL of essential oil. The antioxidant capacity screening of 37 essential oil samples from 15 types of plant material has been performed (Figure 2).
The data obtained have been compared to the standard antioxidant parameters (antioxidant activity towards 2,2-diphenyl-1-picrylhydrazyl [6] and total phenolics [7]). Statistically significant correlations have been found. A comparison of total antioxidant capacity with antioxidant activity towards 2,2-diphenyl-1-picrylhydrazyl has been performed for all the studied samples and has shown a positive correlation with r = 0.4458 that is more than the critical value of 0.3245. The antioxidant capacity at 0.8 V correlates with the total phenolic content by the Folin–Ciocalteu method (r = 0.7969 at n = 12). It should be noted that the spectrophotometric approach was applicable for clove, cinnamon, nutmeg, and thyme essential oils only. The turbid systems were obtained for other oils after the addition of photometric reagents. The chronoamperometric approach successfully overcomes this limitation which can be considered as a significant advantage.
Thus, the data obtained clearly demonstrated the applicability of electrochemical methods on a carbon nanotube modified electrode, and in particular two-step chronoamperometry, for the evaluation of essential oil antioxidant capacity and sample screening. Simplicity, rapidity, cost-efficiency, and the reliability of the method, as well as the possibility of miniaturization, make it an attractive tool for such purposes as an alternative to chromatography for the fast screening of the essential oils.
Author Contributions
Conceptualization, G.Z.; methodology, G.Z. and A.K.; validation, A.K. and G.Z.; formal analysis, G.Z., and A.K.; investigation, A.K. and G.Z.; writing—original draft preparation, G.Z. and A.K.; writing—review and editing, G.Z.; visualization, A.K. and G.Z.; supervision, G.Z. All authors have read and agreed to the published version of the manuscript.
Funding
This research received no external funding.
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Data Availability Statement
The data presented in this study are available upon request from the corresponding author.
Conflicts of Interest
The authors declare no conflict of interest.
References
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Figure 1.
Typical view of the background subtracted chronoamperogram on an example of 10 µL of 10-fold diluted rosemary essential oil. E1 = 0.8 V, E2 = 1.4 V.
Figure 1.
Typical view of the background subtracted chronoamperogram on an example of 10 µL of 10-fold diluted rosemary essential oil. E1 = 0.8 V, E2 = 1.4 V.
Figure 2.
Antioxidant capacity of the essential oils based on chronoamperometric data. Samples 1–5—clove, 6–10—cinnamon, 11—nutmeg, 12–15—lavender, 16—ginger, 17 and 18—anise, 19 and 20—basil, 21–24—bergamot, 25–28—jasmine, 29—ylang-ylang, 30 and 31—marjoram, 32—neroli, 33 and 34—rosemary, 35—thyme, 36 and 37—clary sage essential oils.
Figure 2.
Antioxidant capacity of the essential oils based on chronoamperometric data. Samples 1–5—clove, 6–10—cinnamon, 11—nutmeg, 12–15—lavender, 16—ginger, 17 and 18—anise, 19 and 20—basil, 21–24—bergamot, 25–28—jasmine, 29—ylang-ylang, 30 and 31—marjoram, 32—neroli, 33 and 34—rosemary, 35—thyme, 36 and 37—clary sage essential oils.
Table 1.
Essential oil oxidation potentials.
| Essential Oil | Oxidation Potential (V) |
|---|---|
| Clove | 0.13; 0.39 |
| Cinnamon | 0.14; 0.41; 1.33 |
| Nutmeg | 0.14; 0.41; 0.64; 0.96; 1.36 |
| Lavender | 1.30 |
| Ginger | 1.31 |
| Anise | 0.96; 1.25; 1.35 |
| Basil | 1.21 |
| Bergamot | 0.94; 1.14; 1.33 |
| Jasmine | 0.14; 0.43; 0.80; 1.22 |
| Ylang-Ylang | 1.31 |
| Marjoram | 1.01; 1.36 |
| Neroli | 0.89; 1.33 |
| Rosemary | 1.17; 1.35 |
| Thyme | 0.56; 1.4 |
| Clary sage | 1.29 |
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MDPI and ACS Style
Kalmykova, A.; Ziyatdinova, G. Screening of Essential Oil Antioxidant Capacity Using Electrode Modified with Carboxylated Multi-Walled Carbon Nanotubes. Eng. Proc. 2022, 27, 48. https://doi.org/10.3390/ecsa-9-13181
AMA Style
Kalmykova A, Ziyatdinova G. Screening of Essential Oil Antioxidant Capacity Using Electrode Modified with Carboxylated Multi-Walled Carbon Nanotubes. Engineering Proceedings. 2022; 27(1):48. https://doi.org/10.3390/ecsa-9-13181
Chicago/Turabian StyleKalmykova, Alena, and Guzel Ziyatdinova. 2022. "Screening of Essential Oil Antioxidant Capacity Using Electrode Modified with Carboxylated Multi-Walled Carbon Nanotubes" Engineering Proceedings 27, no. 1: 48. https://doi.org/10.3390/ecsa-9-13181
