Determination of Caffeine at a Nafion‐Covered Glassy Carbon Electrode

Abstract

A simple differential pulse voltammetric method based on a Nafion-covered glassy carbon electrode was developed for the quantitative determination of caffeine in cola beverages. The modified electrode exhibits a clear improvement of the current response compared to that observed at a bare glassy carbon electrode. The method allows quantifying the analyte in the 9.95 Â 10 À7 -1.06 Â 10 À5 M range, with a detection limit of 7.98 Â 10 À7 M in 0.1 M sulfuric acid solutions. The developed procedure was tested by recovery studies. The results are described and discussed in the light of the existing literature.Keywords: Caffeine, Nafion, Voltammetry, Polymer-coated electrodes, Cola beverages DOI: 10.1002/elan.200603679 Caffeine (1,3,7-trimethylxanthine) is a natural alkaloid exerting many physiological effects, such as stimulation of the central nervous system, diuresis and gastric acid secretion [1,2]. It is widely distributed in plant products and beverages [3] and its quantification is mainly of pharmaceutical and alimentary concern.Many methods, including spectrophotometry, chromatography and biosensing (among the most recent, see for example [4 -9]), were proposed to this aim. Usually, these methods are generally more expensive, time-consuming and complicated than electroanalytical ones [10,11]. However, the major drawback of the electrochemical determination of caffeine at the more common electrode materials (e.g., metals, glassy carbon) is that its oxidation occurs at a very positive potential, thus overlapping with the discharge of the background medium [10], generally not exactly reproducible. Several solutions were proposed, including the use of electrodes with a wider anodic potential range [10,11] or modified electrodes [12], or unusual combination of solvent/ supporting electrolyte [13].In this paper, we describe a method based on the modification of a glassy carbon electrode (GC) surface by deposition of Nafion , a perfluorinated sulfonate polymer widely applied in electroanalysis (see for example [14 -17]), since it was proved to have a good affinity towards caffeine [18].Preliminary cyclic voltammetric experiments were performed to study the caffeine voltammetric behavior at the Nafion-covered electrode (NCE). As previously reported [10], the oxidation system is characterized by an anodic peak in the positive-going step and by the absence of any cathodic peak on the reverse scan, indicating that the oxidation is irreversible.The oxidation mechanism was already elucidated (see [10,21]). Briefly [21], the reaction proceeds by an overall 4eþ process. The first step, a 2eþ oxidation of the C-8 to N-9 bond to give the substituted uric acid, is followed by an immediate 2eþ oxidation to the 4,5-diol analogue of uric acid, that rapidly fragments. Figure 1 allows comparing the voltammograms recorded at a bare GC electrode and at the modified electrode, using 0.1 M H 2 SO 4 as supporting electrolyte. As it can be seen, the NCE offers a higher peak height than that observed at a bare ).

Determination of Theophylline at a Cysteic Acid Modified Glassy Carbon Electrode

“…As previously reported [17], the oxidation system is characterized by an anodic peak in the positive-going step and no cathodic peak on the reverse scan, which indicates that the oxidation is irreversible following a well known mechanism [17,21,35]. Briefly [35], the reaction proceeds by an overall 4e…”

“…The test solution was 1.0 Â 10 À3 M theophylline in 0.05 M H 2 SO 4 . Such a supporting electrolyte was chosen on the basis of a previous study [21] aimed to the determination of caffeine at a Nafion-modified electrode. The results of the experiments demonstrated the reliability of the procedure against the alteration of the considered parameters in the reported ranges.…”

“…In fact, both species contain sulfonated groups and this explains why they show the same attractive ionexchange characteristics [26]. Based on this information and on a previous study about the determination of caffeine at a Nafion-modified electrode [21], this contribution deals with the determination of methylxanthines, with particular attention to theophylline, at a cysteic acid-modified glassy carbon electrode (CyME). After having assessed the best operational procedure, the sensor was applied for the quantification of theophylline in medication tablets.…”

Renewable New Copper Complex Bulk‐Modified Carbon Paste Electrode: Preparation, Electrochemistry, and Electrocatalysis

“…The interest is motivated by their extensive potential applications in catalysis, photo-and electrochromic devices and material science [2,3]. Up to now, there are several kinds of method to fabricate CMEs according to the way in which materials are attached into the substrates: i) Langmuir -Blodgett (LB) [4,5]; ii) chemical vapor deposition (CVD) [6 -8]; iii) electrochemically assisted covalent bond [9 -11]; iv) selfassembling (SA) [12,13]; v) drop-casting [14 -16]; vi) bulk modification [17,18]; vii) adsorption [19 -21]; viii) sol-gel [22,23]; ix) underpotential deposition (UPD) [24,25], and x) electropolymerization [26,27] etc.…”

Simultaneous Electrochemical Determination of Caffeine and Vanillin by Using Poly(Alizarin Red S) Modified Glassy Carbon Electrode

“…Electroanalytical methods have been commonly exploited as cheap, speedy, and simple options to modern separation and spectral methods. Over the years, to improve the sensitivity and long-term stability of the electrodes, the bare electrodes altered with several electrode modifiers such as nafion, conducting polymers, molecularly imprinted polymers, metal nanoparticles, carbon nanotubes (CNTs), and graphene were used for the individual determination of CAF (Švorc 2013;Brunetti et al 2007;Sun et al 2011a;Yang et al 2010;Sun et al 2011b;Amare and Admassie 2012;Wang et al 2004;Wei et al 2009;Xiong et al 2013;Lu et al 2012;Alizadeh et al 2010;Aklilu et al 2008;Ly et al 2008;Ly et al 2009;Habibi et al 2012;Martínez-Huitle et al 2010;Guo et al 2011;Yardım et al 2013) and VAN (Zheng et al 2010;Bettazzi et al 2006;Luque et al 2000;Peng et al 2012;Kong et al 2010;Deng et al 2015;Shang et al 2014;Luo and Liu 2012;Xinying 2014;Chethanaa et al 2012). It was noted that determination of CAF has predominantly been performed by voltammetric methods (Švorc 2013).…”