Selective electrochemical determination of caffeine at a gold-chitosan nanocomposite sensor: May little change on nanocomposites synthesis affect selectivity?

A simple and selective method for the determination of caffeine also in complex matrix has been developed at a gold electrode modified with gold nanoparticles (AuNPs) synthetized in a chitosan matrix in the presence of oxalic acid. The electrochemical behaviour of caffeine at both gold bare and gold electrode modifiedwith AuNPs with different morphology was carried out in acidic medium by cyclic voltammetry (CV), differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS). Electrochemical parameters were optimized in order to improve the electrochemical response to caffeine. The most satisfactory result, that means the higher electrochemical improvement, was obtained using a gold electrode modified with AuNPs synthetized in a chitosan matrix in the presence of oxalic acid, in aqueous solution containing HClO4 0.4 mol L-1 as supporting electrolyte. The performance of the sensor was then evaluated in terms of linearity range (2.0 × 10-6-5.0 × 10-2 mol L-1, R = 0.999), operational and storage stability, reproducibility (RSD = 3.7%), limit of detection (LOD = 1.0 × 10-6 mol L-1) and response to a series of interfering compounds as ascorbic acid, citric acid, gallic acid, caffeic acid, ferulic acid, chlorogenic acid, glucose, catechin and epicatechin. The sensor was then successfully applied to determine the caffeine content in commercial beverages and results were compared with those obtained with HPLC-PDA as an independent method and with those declared from manufacturers.