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Proceeding Paper

Electrode Modified with Manganese Dioxide Nanorods for the Simultaneous Voltammetric Determination of Food Colorants †

by
Liliya Gimadutdinova
* and
Guzel Ziyatdinova
Analytical Chemistry Department, Kazan Federal University, Kremleyevskaya, 18, Kazan 420008, Russia
*
Author to whom correspondence should be addressed.
†
Presented at the 3rd International Electronic Conference on Applied Sciences, 1–15 December 2022; Available online: https://asec2022.sciforum.net/.
Eng. Proc. 2023, 31(1), 12; https://doi.org/10.3390/ASEC2022-13837
Published: 9 December 2022
(This article belongs to the Proceedings of The 3rd International Electronic Conference on Applied Sciences)
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Abstract

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Direct voltammetric determination of brilliant blue FCF and tartrazine in soft and isotonic sports drinks.

Abstract

Synthetic colorants, in particular tartrazine and brilliant blue FCF, are widely used in food chemistry and technology although they can give negative health effects of various severities. Therefore, sensitive, selective, simple, and reliable methods for the quantification of these dyes are required. A glassy carbon electrode (GCE) modified with manganese dioxide nanorods (MnO2 NR) dispersed in cetylpyridinium bromide gives a sensitive response to tartrazine and brilliant blue FCF in mixtures. Electrode modification provides a 7.9-fold increase in the electroactive surface area and a 72-fold decrease in electron transfer resistance. Simultaneous voltammetric quantification of colorants was performed in phosphate buffer pH 7.0 in differential pulse mode. The linear dynamic ranges of 0.10–2.5 and 2.5–15 µM of tartrazine and 0.25–2.5 and 2.5–15 µM of brilliant blue FCF were obtained with the limits of detection of 43 and 41 nM, respectively. The advantage of the sensor developed is the high selectivity of response in the presence of typical interferences (inorganic ions, saccharides, ascorbic and sorbic acids) and other food colorants (riboflavin, indigo carmine, and sunset yellow). The practical applicability of the approach is shown in soft and isotonic sports drinks and is validated by comparison to chromatography.

1. Introduction

Synthetic colorants, in particular tartrazine and brilliant blue FCF, are widely used in food chemistry and technology, although they can give negative health effects of various severities [1,2]. Both tartrazine and brilliant blue FCF are used as colorants for alcoholic and non-alcoholic beverages, candies, jellies, ice cream, etc. [3]. The average daily intake of tartrazine and brilliant blue FCF is regulated at 7.5 mg/kgbw [4] and 6 mg/kgbw [5]. Therefore, colorant contents in foodstuffs have to be controlled.
Sensitive and selective, simple, and reliable methods for the quantification of these dyes are required. Voltammetric sensors suit well for such purposes. A wide range of electrochemical sensors has been developed for the determination of tartrazine. Brilliant blue FCF is a seldom studied colorant in electroanalytical chemistry. Most sensors are based on the application of surface modifiers, such as carbon nanomaterials [6,7,8], metal [9,10] and metal oxide [11] nanoparticles, polymeric coverages [12,13], and a combination of various modifiers [14,15,16,17,18].
Tartrazine and brilliant blue FCF are often used together for the production of green-colored foodstuffs and beverages. Therefore, simultaneous voltammetric determination of these colorants is of practical interest. However, this topic did not receive enough attention. Just three voltammetric approaches have been reported [19,20,21]. All of them are based on the application of modified electrodes (carbon black–polyethylene composite electrode [19], ionic liquid-modified expanded graphite paste electrode [20], and multi-walled carbon nanotube paste electrode [21]).
Electrochemically inert metal oxide nanomaterials (TiO2, In2O3, CeO2, ZnO, Fe3O4, etc.) are perspective modifiers of the electrode surface [22]. Their effectivity has been successfully demonstrated in the example of antioxidants [23,24,25], pharmaceuticals [26], and pollutants [27,28]. There is almost no application of such electrodes to food colorants excluding recent works focused on the tartrazine determination on CeO2 [29] and TiO2 [30,31] nanoparticles-modified electrodes. Thus, the development of a voltammetric method based on the oxidation of tartrazine and brilliant blue FCF at the metal oxide nanomaterials-based electrodes is needed. In the current work, manganese dioxide nanorods (MnO2 NR) dispersed in surfactant have been successfully applied as electrode surface modifiers. The electrode created provides well-resolved oxidation peaks of tartrazine and brilliant blue FCF allowing their simultaneous quantification.

2. Materials and Methods

Tartrazine (85% purity) was obtained from Sigma (St. Louis, MO, USA) and 85% brilliant blue FCF from Sigma-Aldrich (Steinheim, Germany). Ascorbic acid of 99% purity (Sigma, Steinheim, Germany), 9% sunset yellow and 99% vanillin (Aldrich, Steinheim, Germany), 99% sorbic acid, 98% riboflavin, and 85% indigo carmine (Sigma-Aldrich, Steinheim, Germany) were used for the interference study. In addition, 10 mM standard solutions of all compounds were prepared in distilled water. Other reagents were c.p. grade. The laboratory temperature was (25 ± 2 °C).
MnO2 NR (99%, diameter × L = 5–30 nm × 80–100 nm) from Sigma-Aldrich (Steinheim, Germany) were used as an electrode surface modifier. A 1 mg mL−1 suspension was prepared in 1.0 mM cetylpyridinium bromide using sonication for 40 min in an ultrasonic bath (WiseClean WUC-A03H (DAIHAN Scientific Co., Ltd., Wonju-si, Republic of Korea). A standard surfactant solution with a concentration of 1.0 mM was prepared from 98% cetylpyridinium bromide (Aldrich, Steinheim, Germany) by dissolving it in distilled water.
Voltammetric measurements were conducted on the potentiostat/galvanostat μAutolab Type III (Eco Chemie B.V., Utrecht, The Netherlands) and NOVA 1.7.8 software. Electrochemical impedance spectroscopy (EIS) was performed on the potentiostat/galvanostat Autolab PGSTAT 302N with the FRA 32M module (Eco Chemie B.V., Utrecht, The Netherlands) and the NOVA 1.10.1.9 software. A glassy electrochemical cell of 10 mL volume was used for electrochemical measurements. The tree-electrode system consisted of a working glassy carbon electrode (GCE) of 3 mm diameter (CH Instruments, Inc., Bee Cave, TX, USA), or a modified electrode, an Ag/AgCl reference electrode, and a platinum wire as an auxiliary electrode. After polishing on 0.05 µm alumina slurry, working electrode surface modification was performed by drop casting of 5 µL of MnO2 NR suspension.
The pH measurements were carried out using the “Expert-001” pH meter (Econix-Expert Ltd., Moscow, Russian Federation) with a glassy electrode.
A MerlinTM (Carl Zeiss, Oberkochen, Germany) high-resolution field emission scanning electron microscope was applied for the electrode surface morphology characterization and operated at a 5 kV accelerating voltage and a 300 pA emission current.

3. Results and Discussion

3.1. Voltammetric Characteristics of Colorants at Bare and Modified Electrodes

Tartrazine and brilliant blue FCF are electrochemically active on bare GCE in phosphate buffer pH 7.0. Single-step oxidation proceeds irreversibly which is typical for these colorants. The corresponding voltammetric characteristics are summarized in Table 1.
Simultaneous detection of tartrazine and brilliant blue FCF at the bare GCE is impossible due to the full overlap of the oxidation peaks. Oxidation currents are low in spite of relatively high concentrations of colorants. Modifying with a MnO2 NR electrode was used to solve this problem. The oxidation potential of colorants at the modified electrode is significantly changed (Table 1). The difference in oxidation potential achieves 210 mV, making it possible to detect colorants simultaneously. There are two well-defined oxidation peaks on the voltammograms of the colorant’s mixture with a peak potential separation of 180 mV. Furthermore, oxidation currents at the modified electrode are statistically significantly increased which confirms the higher sensitivity of the colorant’s response and the effectivity of the suggested modifier.

3.2. Morphology, Effective Surface Area, and Electron Transfer Properties of the Modified Electrode

Scanning electron microscopy data confirm the presence of a modifier on the GCE surface (Figure 1). A sponge-like structure from the intertwined nanorods with a width of 15–20 nm included in the surfactant film has been obtained for MnO2 NR (Figure 1b).
Electrochemical investigation of redox peaks of 1.0 mM [Fe(CN)6]4− ions has shown that the modified electrode demonstrates a significant increase in the effective surface area compared to bare GCE (70 ± 2 mm2 vs. 8.9 ± 0.3 mm2 for bare GCE). This explains the increase in the colorant’s oxidation currents at the modified electrode.
EIS in the presence of 1.0 mM [Fe(CN)6]4−/3− as a redox probe was used for the characterization of the electron transfer properties of the electrodes. The 72-fold decrease (72 ± 3 kOhm vs. 1.0 ± 0.2 kOhm for GCE) of the charge transfer resistance clearly confirms the increase in the electron transfer rate at the modified electrode. The constant phase element was increased 29-fold compared to bare GCE which is caused by the porous structure of the modified electrode surface as well as by the increase in the total surface charge due to the presence of a cationic surfactant.
The data obtained confirm once more the effectivity of MnO2 NR as an electrode surface modifier.

3.3. Simultaneous Determination of Tartrazine and Brilliant Blue FCF

Differential pulse voltammetry was used for the simultaneous quantification of tartrazine and brilliant blue FCF in phosphate buffer pH 7.0. Well-resolved oxidation peaks of colorants at 0.77 and 0.97 V for tartrazine and brilliant blue FCF, respectively, were observed on the voltammograms (Figure 2). Oxidation currents increase linearly with the growth of the colorant’s concentration in the ranges of 0.10–2.5 and 2.5–15 µM for tartrazine and 0.25–2.5 and 2.5–15 µM for brilliant blue FCF with detection limits of 0.043 and 0.041 µM, respectively. The limits of detection are worse than for the other electrodes for the determination of tartrazine and brilliant blue FCF [19,20] but improved vs. multi-walled carbon nanotubes–carbon paste electrode [21]. Nevertheless, simultaneous determination is impossible at the carbon ink film-modified carbon black–polyethylene composite electrode [19] as far as detection is performed at the various pH for tartrazine and brilliant blue FCF. Method [20] requires pre-concentration for 500 s complicating the measurement procedure.
Voltammograms for the non-equimolar mixtures of colorants indicate their independent oxidation in the first linear range. Therefore, calibration graphs obtained for equimolar mixtures are universal and can be used independently of the colorant’s concentration ratio in the sample. Simple dilution can be applied in the case of high contents of the colorants in real samples.
The accuracy of the method developed has been shown on the model mixtures of colorants at five concentration levels. The relative standard deviation of the determination does not exceed 3% confirming the high reproducibility of the electrode response (the electrode was renewed before each measurement). The recovery value is in the range of 99–100% confirming the high accuracy of the sensor developed.
Foodstuffs are characterized by a multi-component composition which can affect the response of colorants. The selectivity test has shown that typical interferences (inorganic ions (1000-fold excess of K+, Mg2+, Ca2+, NO3−, Cl−, and SO42−), saccharides (100-fold excess of glucose, rhamnose, and sucrose), 10-fold excess of ascorbic acid, and electrochemically silent sorbic acid), equimolar level of vanillin, and other food colorants (50-fold excess of riboflavin, 10-fold excess of indigo carmine, and equimolar level of sunset yellow) do not affect the response of tartrazine and brilliant blue FCF. Thus, the high selectivity of the electrode created towards tartrazine and brilliant blue FCF is an important advantage over other electrodes [19,20,21].
Practical application of the electrode has been demonstrated on soft and isotonic sports drinks. Sample 1 is free of tartrazine while samples 2–4 contain both colorants but the concentration of brilliant blue FCF is too low and cannot be determined by voltammetry. Standard addition method data confirm that oxidation peaks of real samples belong to the colorants. The results of soft and isotonic sports drinks analysis are presented in Figure 3. Voltammetric data agree well with that obtained by high-performance liquid chromatography [32]. t- and F-tests confirm the absence of systematic errors of determination and similar precision of both methods.

4. Conclusions

An electrode modified with MnO2 NR was developed for the determination of tartrazine and brilliant blue FCF for the first time. The simultaneous determination of colorants in the ranges of 0.10–2.5 and 2.5–15 µM of tartrazine and 0.25–2.5 and 2.5–15 µM of brilliant blue FCF was achieved using the electrode created. The high selectivity of the electrode response to target colorants is a major advantage of the approach developed. The voltammetric method developed is simple, highly selective, express, and reliable and can be used for beverage quality control.

Author Contributions

Conceptualization, G.Z.; methodology, G.Z.; validation, L.G. and G.Z.; investigation, L.G.; writing—original draft preparation, G.Z.; writing—review and editing, G.Z.; visualization, L.G.; 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.

Acknowledgments

The authors thank Aleksei Rogov (Laboratory of Scanning Electron Microscopy, Interdisciplinary Center for Analytical Microscopy, Kazan Federal University) for the scanning electron microscopy measurements.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Morphology of the electrode surface by scanning electron microscopy data: (a) bare GCE; (b) MnO2 NR/GCE.
Figure 1. Morphology of the electrode surface by scanning electron microscopy data: (a) bare GCE; (b) MnO2 NR/GCE.
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Figure 2. Baseline-corrected differential pulse voltammograms of equimolar mixtures of tartrazine and brilliant blue FCF at the MnO2 NR/GCE in phosphate buffer pH 7.0. ΔEpulse = 75 mV, tpulse = 25 ms, υ = 20 mV s−1.
Figure 2. Baseline-corrected differential pulse voltammograms of equimolar mixtures of tartrazine and brilliant blue FCF at the MnO2 NR/GCE in phosphate buffer pH 7.0. ΔEpulse = 75 mV, tpulse = 25 ms, υ = 20 mV s−1.
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Figure 3. Brilliant blue FCF (sample 1) and tartrazine (samples 2–4) contents in the soft and isotonic sports drinks.
Figure 3. Brilliant blue FCF (sample 1) and tartrazine (samples 2–4) contents in the soft and isotonic sports drinks.
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Table
Table 1. Voltammetric characteristics of 10 µM tartrazine and brilliant blue FCF in phosphate buffer pH 7.0.
Table 1. Voltammetric characteristics of 10 µM tartrazine and brilliant blue FCF in phosphate buffer pH 7.0.
ElectrodeTartrazineBrilliant Blue FCF
Eox (V)Iox (µA)Eox (V)Iox (µA)
Bare GCE0.940.07 ± 0.010.940.07 ± 0.01
MnO2 NR/GCE0.810.42 ± 0.011.020.40 ± 0.04
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MDPI and ACS Style

Gimadutdinova, L.; Ziyatdinova, G. Electrode Modified with Manganese Dioxide Nanorods for the Simultaneous Voltammetric Determination of Food Colorants. Eng. Proc. 2023, 31, 12. https://doi.org/10.3390/ASEC2022-13837

AMA Style

Gimadutdinova L, Ziyatdinova G. Electrode Modified with Manganese Dioxide Nanorods for the Simultaneous Voltammetric Determination of Food Colorants. Engineering Proceedings. 2023; 31(1):12. https://doi.org/10.3390/ASEC2022-13837

Chicago/Turabian Style

Gimadutdinova, Liliya, and Guzel Ziyatdinova. 2023. "Electrode Modified with Manganese Dioxide Nanorods for the Simultaneous Voltammetric Determination of Food Colorants" Engineering Proceedings 31, no. 1: 12. https://doi.org/10.3390/ASEC2022-13837

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