A new sensor was developed for the analysis of phenolic compounds by coating the surface of a glassy carbon electrode (GCE) with rhodium nanoparticles stabilized in 3-(1-tetradecyl-3-imidazolium) propanesulfonate (ImS3-14), a zwitterionic surfactant. The modifier was found to lower the electron transfer resistance by enhancing the electrical conductivity on the electrode surface. Some phenolic compounds were tested to determine the increase in the electroanalytical response when employing the proposed sensor in comparison to the bare GCE. p-Coumaric acid showed the greatest peak enlargement and, thus, it was selected as the target analyte. Chronocoulometry studies indicated that the increase in the peak is due to the adsorption of the p-coumaric acid on the sensor surface. The optimized analysis conditions were obtained in acetate buffer pH 4.0 employing differential pulse voltammetry, which provided a limit of detection of 472 nmol/L. The recoveries ranged from 98.3% to 104.4%, indicating satisfactory accuracy for the proposed method. The sensor exhibited good repeatability and reproducibility (both intraday and interday) and was applied successfully in the determination of p-coumaric acid in a cellulosic matrix.

Reagents and solutions All reagents, that is, paracetamol, starch, cellulose, chloroform, RhCl 2 , sodium acetate, sodium phosphate, acetic acid, phosphoric acid and sodium hydroxide, were analytical grade (Sigma-Aldrich) and used without further purification. The halloysite nanoclay was purchased from Sigma-Aldrich and contained nanotubes with 30-70nm of diameter and 1-3µm of length. All solutions were prepared in ultrapure water obtained from a Milli-Q System (Millipore, USA) with a resistivity of 18.2MΩ cm-1. The Rh-(ImS3-14) reverse micelle solution was synthesized according to a procedure previously described in the literature [19]. The paracetamol tablets were purchased at a local drugstore.

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