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Novel Potentiometric 2, 6-Dichlorophenolindo-Phenolate

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Novel Potentiometric 2, 6-Dichlorophenolindo-Phenolate sensors Article Novel Potentiometric 2,6-Dichlorophenolindo-phenolate (DCPIP) Membrane-Based Sensors: Assessment of Their Input in the Determination of Total Phenolics and Ascorbic Acid in Beverages Nada H. A. Elbehery 1, Abd El-Galil E. Amr 2,3,* , Ayman H. Kamel 1,* , Elsayed A. Elsayed 4,5 and Saad S. M. Hassan 1,* 1 Chemistry Department, Faculty of Science, Ain Shams University, Cairo 11556, Egypt; [email protected] 2 Pharmaceutical Chemistry Department, Drug Exploration & Development Chair (DEDC), College of Pharmacy, King Saud University, Riyadh 11451, Saudi Arabia 3 Applied Organic Chemistry Department, National Research Centre, Dokki, Giza 12622, Egypt 4 Zoology Department, Bioproducts Research Chair, Faculty of Science, King Saud University, Riyadh 11451, Saudi Arabia; [email protected] 5 Chemistry of Natural and Microbial Products Department, National Research Centre, Dokki, Cairo 12622, Egypt * Correspondence: [email protected] (A.E.-G.E.A.); [email protected] (A.H.K.); [email protected] (S.S.M.H.); Tel.: +966-565-148-750 (A.E.-G.E.A.); +20-1000743328 (A.H.K.); +20-1222162766 (S.S.M.H.) Received: 23 March 2019; Accepted: 26 April 2019; Published: 2 May 2019 Abstract: In this work, we demonstrated proof-of-concept for the use of ion-selective electrodes (ISEs) as a promising tool for the assessment of total antioxidant capacity (TAC). Novel membrane sensors for 2,6-dichlorophenolindophenolate (DCPIP) ions were prepared and characterized. The sensors membranes were based on the use of either CuII-neocuproin/2,6-dichlorophenolindo-phenolate ([Cu(Neocup)2][DCPIP]2) (sensor I), or methylene blue/2,6-dichlorophenolindophenolate (MB/DCPIP) (sensor II) ion association complexes in a plasticized PVC matrix. The sensors revealed significantly enhanced response towards DCPIP ions over the concentration range 5.13 10 5–1.0 10 2 and × − × − 5.15 10 5–1.0 10 2 M at pH 7 with detection limits of 6.3 and 9.2 µg/mL with near-Nernstian × − × − slope of 56.2 1.7 and 51.6 2 mV/decade for sensors I and II, respectively. The effects of − ± − ± plasticizers and various foreign common ions were also tested. The sensors showed enhanced selectivity towards DCPIP over many other phenolic and inorganic ions. Long life span, high potential stability, high reproducibility, and fast response were also observed. Method validation was also verified by measuring the detection limit, linearity range, accuracy, precision, repeatability and between-day-variability. The sensors were introduced for direct determination of TAC in fresh and canned juice samples collected from local markets. The obtained results agreed fairly well with the data obtained by the standard method. Keywords: 2,6-dichlorophenolidophenolate (DCPIP); total antioxidant capacity (TAC); ascorbic acid; potentiometry; ion-selective electrodes; flow analysis 1. Introduction The organism’s status regarding cell damage is defined as oxidative stress caused by enhanced release of oxygenated free radicals [1]. These excessive amounts of free radicals arising physiologically Sensors 2019, 19, 2058; doi:10.3390/s19092058 www.mdpi.com/journal/sensors Sensors 2019, 19, 2058 2 of 13 during cellular aerobic metabolism, may lead to disruption of a living cell or to molecular and cellular DNA damage [2]. There are different types of oxygen-centered free radicals, collectively called reactive oxygen species (ROS). These radicals comprise superoxide anion radical (O ), hydroxyl 2·− (HO ), peroxyl (ROO ) and alkoxyl (RO ) radicals, and nitric oxide (NO ). Hydroxyl (HO ) and alkoxyl · · · · · (RO ) free radicals are characterized by their high reactivity towards biomolecules, while, superoxide · anion (O ) and nitric oxide (NO) have lower reactivity, as previously reported [3]. In addition to the 2·− aforementioned radical oxygenated species, singlet oxygen, hydrogen peroxide, or hypochlorous acid are also known as non-radical reactive oxygen species [4]. These oxygen- containing free radicals are implicated in cancer, cardiovascular diseases, cataracts, and dysfunctions of the immunity system which develop especially with age [5]. Oxidative processes can be inhibited by antioxidants [6,7]. Natural antioxidants, especially plant sourced ones, are now considered the best choice for safe food antioxidants. Epidemiological studies confirm that the rate of consumption of fruits and vegetables is inversely proportional to the development of oxidative stress-related disorders. This has been attributed to the presence of compounds with high antioxidant activity [8–11]. One of the main classes of bio-compounds found in natural sources are phenolics (flavonoids or non-flavonoids). These compounds are associated with the health benefits resulting from the inhibition of the oxidation of low-density lipoprotein [9,12–16]. Antioxidants have high chemical diversity and are found in complex mixtures in many real samples. It is difficult to successfully use any single separation technique for these complex mixtures. Moreover, the effect of a complex mixture of these antioxidants is almost inevitably different from the sum of the contributions of the individual antioxidants in the mixture [17]. According to these facts, the useful concept of total antioxidant capacity (TAC) has appeared. TAC considers the collective strengths of all antioxidants found in the sample instead of being concerned with the individual strengths of each antioxidant [18]. Different methods for TAC assessment are reported in the literature [6,19–26]. These methods vary widely in both the chemical concept and the numerical values obtained for TAC. Among of these methods, we may mention the ferric reducing antioxidant power (FRAP) [19], the Trolox equivalent antioxidant capacity assay (TEAC) [20], and the oxygen radical absorbance capacity (ORAC) [21]. All of the methods are carried out under widely different conditions and often produce quite different TAC assay results [22–26]. This makes any attempt to compare data obtained with different procedures ill-advised. Recently, great attention has been directed to apply electrochemical tools to determine TAC. These tools have many numerous advantages. They are rapid, highly sensitive, and simple, requiring small sample volumes for analysis and low cost instrumentation. In voltammetry, a confirmed disadvantage results from the poor reliability of TAC values for the samples. This result unreliability is attributed to the strong dependence of the value obtained on the reaction mechanism at the electrode’s surface [27]. In amperometry, very strict control of the experimental conditions is required, so, the selectivity of the method depends on the specificity of the reaction between the oxidized or reduced form of the redox couple and the analyte [28–30]. Potentiometry as an electroanalytical tool offers advantages when compared with voltammetry or amperometry because it does not apply current or potential modulation [6,23,24,28,29,31–35]. The apparatus used in potentiometry is simple, easy to use, and the sample is viewed as the single independent variable [36,37]. Application of flow-through potentiometric measurements offers rapid and reproducible evaluation of TAC suitable for several aqueous plant extracts. TAC was estimated by a potentiometric tool via recording the potential generated from the reaction of K3[Fe(CN)6]/K4[Fe(CN)6] as a result of its interaction with the sample antioxidants upon using platinum electrode as a detector [38,39]. A potentiometric assay for non-invasively providing the skin oxidant/antioxidant balance, relied, as measured analytical signal, on the potential change of the 3 4 ferricyanide/ferrocyanide [Fe(CN)6]− / [Fe(CN)6]− mediator system applied to the skin by means of a conductive gel. The change in the Pt electrode potential was caused by the shift in the oxidized form to the reduced form ratio of the mediator system. A decrease in the potential value indicates Sensors 2019, 19, 2058 3 of 13 the antioxidant activity of the analyzed medium [40]. Another potentiometric antioxidant capacity assessment was reported in literature and applied in biological samples, food and drinks. It was based on the use of the free radical generator 2,20-azobis(2-amidinopropane) dihydrochloride (AAPH) with K3[Fe(CN)6]/K4[Fe(CN)6] as a system mediator [41]. The sample underwent reaction with AAPH, so the antioxidants’ concentration is lowered, as a consequence of the interaction with peroxyl radicals generated from AAPH decay in phosphate buffer solution pH = 7.40 at 37 ◦C. Another potentiometric assay for TAC was reported using an I2/I− mediator redox couple in 0.1 M, pH = 6.7 phosphate buffer. A Pt electrode was used as an indicator electrode [42]. 2,6-Dichlorophenolindophenol (DCPIP) is a chemical used as a redox. DCPIP can also be used as an indicator for the assessment of ascorbic acid (vitamin C). If vitamin C, which is a good reducing agent, is present, the blue dye, which turns pink in acid conditions, is reduced to a colorless compound by ascorbic acid. Pharmacological experiments suggest that DCPIP may serve as a pro-oxidant chemotherapeutic targeting human cancer cells in an animal model of human melanoma; DCPIP-induced cancer cell death occurs by depletion of intracellular glutathione and upregulation of oxidative stress [43]. The goal of this work was to develop a method for the potentiometric determination of ascorbic acid and phenolic antioxidants, and to evaluate their contribution to the total antioxidant capacity of beverages. The method is based on the preparation and
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