Elsevier

Talanta

Volume 187, 1 September 2018, Pages 156-164
Talanta

The performance of cathodically pretreated boron-doped diamond electrode in cationic surfactant media for enhancing the adsorptive stripping voltammetric determination of catechol-containing flavonoid quercetin in apple juice

https://doi.org/10.1016/j.talanta.2018.05.016Get rights and content

Highlights

  • Usefulness of CPT-BDD in cationic surfactant media for Quercetin determination.

  • Helping to understand the flavonoid/surfactant interactions for antioxidant activity.

  • Extremely high sensitivity (LOD 4.4 ×10−10 M) over most of the chemically modified electrodes.

  • Practical applicability in apple juice samples.

Abstract

In the present paper, an electroanalytical methodology was developed for the determination of an important catechol-containing flavonoid derivative, quercetin using adsorptive stripping voltammetry at a cathodically pretreated boron-doped diamond electrode. In cyclic voltammetry, the compound showed a couple of oxidation/reduction peak at low positive potentials, and additional two oxidation peaks at more positive potentials. The sensitivity of the stripping voltammetric measurements was significantly improved when the cationic surfactant, cetyltrimethylammonium bromide (CTAB) was present in the electrolyte solution. Using square-wave stripping mode, a highly linear analytical curve was obtained for quercetin determination in 0.1 M acetate buffer solution (pH 4.7) containing 3 × 10−4 M CTAB at + 0.37 V (vs. Ag/AgCl) (after 30 s accumulation at open-circuit condition) in the range of 0.5–200 ng mL−1 (1.7 × 10−9–3.3 × 10−7 M), with a detection limit of 0.132 ng mL−1 (4.4 × 10−10 M). As an example, the practical applicability of proposed method was successfully tested with the measurement of quercetin concentration in commercial apple juice samples.

Introduction

Flavonoids are one of the major groups of phenolic compounds widely present in many fruits and vegetables, and products derived from them, such as wine and tea. These compounds have important physiological roles for the plant survival, such as UV protection, antifungal and phytoalexin (plant chemical defense) properties. Moreover, they are responsible for color and taste on the plant life cycle. In turn, flavonoids are of importance to human health, though they must be supplied by vegetal intake. Over the last two decades clinical researches have shown their potential health benefit effects on the prevention of chronic disorders such as certain forms of cancer (lung and breast cancer), cardiovascular disease, inflammatory and many degenerative diseases. Most of these biological effects are attributed to their antioxidant and/ or radical scavenging abilities as they contain phenolic groups [1], [2].

Most of flavonoids are flavonols (3-hydroxy derivatives); among them quercetin (3,3′,4′,5,7-pentahydroxyflavone) is the dominant one present in many medicinal plants, fruits and vegetables such as the leaves of green and black tea, cowberry, red and yellow onions, cranberries, black currant, apples, red grapes, and celery in its free form (aglycones) or in the form of glycosides. It is also introduced into the composition of many dietary supplements and certain medications. A broad spectrum of beneficial properties have been described for quercetin, including protection against various diseases such as osteoporosis, certain forms of cancer, pulmonary and cardiovascular diseases but also against aging. However, taking very high doses of quercetin may lead to kidney cancer [3], [4].

Taking into account that quercetin is often used as a reference material for determining the antioxidant activity of various substances, there is an increase for its determination in various plant samples, foods, dietary supplements, and pharmaceutical preparations. In the last decade, several analytical methods have been reported in the literature for the determination of quercetin, either alone or simultaneously with other flavonoids in different matrices (natural sources, marketed food products, dietary supplements, biological fluids, etc.). These particularly include spectrophotometry [5], [6], [7], [8], luminescence [9], gas chromatography [10], [11], but mostly liquid chromatography [12], [13], [14], [15], [16] and capillary electrophoresis [17], [18], [19], [20] with different types of detection. Spectrophotometric methods are relatively fast, however they suffer from insufficient selectivity for individual and/or simultaneous determination of quercetin and other flavonoids in case of analyzing natural sources (vegetables and plants), foods and biological fluids. Among the instrumental techniques, chromatographic approaches offer a high degree of sensitivity and selectivity; however, these are considered as being long lasting, expensive and often too laborious.

Electroanalytical methods, such as the voltammetric ones, are simple, cheap and rapid in comparison with chromatographic methods, and have been widely applied due to their high sensitivity. Besides, it may be helpful to investigate the electrochemical behavior of quercetin in order to understand its antioxidant role better as radical scavenger, of consequence, to use its additional therapeutic potential to protect the biological targets from oxidative stress. Despite the electroactive nature of quercetin, bare solid electrodes, such as glassy carbon electrode [21], carbon paste electrode [22], [23] and screen printed electrode [24] have rarely been used for its voltammetric analysis. This is mainly attributed to the fact that during the electrochemical oxidation of phenolic compounds at conventional bare solid electrodes it can be observed a noticeable decrease in the current owing to the formation of a polymeric film on the surface of electrode that may promote surface poisoning. This is why a large number of papers in the literature involve the use of modified electrodes for the voltammetric quantification of quercetin [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36].

Recently, the boron-doped diamond (BDD) as a new and excellent carbon electrode material can be used to overcome this difficulty. Due to its outstanding electrochemical and mechanical properties without the need of any chemical modifications, BDD electrode, nowadays, is well established for electroanalytical applications [37]. However, for many analytes, these properties of BDD electrode are extremely dependent on its surface termination (oxygen or hydrogen), which can be modified by appropriate electrochemical pretreatment (anodic or cathodic). These procedures enhance the particular voltammetric signals, and ensure repeatable and reproducible response of analytes, removing or reducing the substances adsorbed on the surface during measurements [38], [39], [40].

On the other hand, one possible way of enhancing the sensitivity and selectivity of the voltammetric method is the use of surfactants. Additionally, next to the electrode material, the medium containing surfactant can prevent electrode fouling. Currently our research group has reported some papers dealing with the application of surfactant effect (anionic and/or cationic) on the surface of BDD electrode for the electroanalysis of environment, pharmaceutical, and food samples [41], [42], [43], [44], [45], [46]. The surviving literature shows some studies dealing with the interactions of quercetin with surfactant micelles. In those reports, the average location site of quercetin in different micelles was explained by cyclic voltammetry on glassy carbon electrode [47], [48]. As far as we know, no literature data were found for the electrochemical determination of quercetin when surfactant is present in the electrolyte solution. However, in very recent report, a cationic surfactant (cetyltrimethylammonium bromide, CTAB) was used in preparing carbon paste electrode modified with iron nanoparticles decorated multi-wall carbon nanotubes to establish a voltammetric methodology for quercetin determination [34].

In our previous paper [44], it has been shown that the hydrophobic surface of cathodically pretreated BDD electrode accumulates less soluble compound, pterostilbene, from solutions to the electrode surface which is strengthened through the enhancement effects of cationic surfactant. In keeping with this knowledge in mind, and in continuation of our earlier reports, the main objective of the current paper consisted of the evaluation of redox behavior of quercetin on a cathodically pretreated BDD electrode in the presence of CTAB. The practical applicability of proposed method was demonstrated in determination of quercetin in the commercial apple juice.

Section snippets

Chemicals

Standard quercetin was purchased from Sigma-Aldrich and used as received. Other reagents used were of analytical grade, and their solutions were prepared with deionized water further purified via a Milli-Q unit (Millipore). Since quercetin possesses poor aqueous solubility with strong hydrophobic property [49], its stock standard solutions (1 mg mL−1) was prepared in ethanol, stored in dark bottles at 4 °C when not in use. The working solutions of lower concentrations were freshly prepared by

Investigation of the electrochemical behavior of quercetin on the boron doped diamond electrode

To the best of our knowledge, no work using BDD electrode to determine quercetin has been reported. Thus, in order to characterize the voltammetric behavior of quercetin on BDD electrode, the experiments by means of CV and AdSV were performed in solutions at different pHs with or without surfactant.

Conclusions

So far previously published papers dealing with determination of natural flavonoid quercetin are predominantly focused on the application of time and cost demanding chemically modified electrodes. Herein, the coupling of modification-free CPT-BDD electrode with AdSV technique in the presence of cationic surfactant, CTAB was introduced for the first time to develop a novel and alternative electroanalytical method for this analyte. The proposed method was applied for the quercetin quantification

Acknowledgements

This work is produced from master's thesis of Abdullah A. Abdullah (Van Yüzüncü Yıl University, Institute of Natural and Applied Sciences).

Conflicts of interest

The authors declare that there is no scientific or financial conflict of interest.

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