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 ).
