Paper-based electrochemical sensor for direct detection of caffeine in commercial beverage samples

Paper-based sensors offer a promising route toward portable and biodegradable electrochemical sensors for microvolume analysis; however, their application to the detection of caffeine remains limited. In this study, we developed a paper-based electrochemical sensor for direct detection of caffeine in commercial beverages without sample pretreatment. The sensor was fabricated by integrating multi-walled carbon nanotubes (MWCNTs) onto wax-modified paper supported by a cellulose nanofibre (CNF) film. Electrochemical investigations revealed an irreversible caffeine oxidation current at the MWCNTs-modified electrode, which was due to a diffusion-controlled reaction. The caffeine oxidation reaction was found to proceed through a two-proton, four-electron transfer mechanism, as determined by the potential peak shift and caffeine diffusion coefficient. The sensor enabled direct detection of caffeine in microvolume samples and was successfully applied to commercial beverages without any pretreatment. Square wave voltammetry demonstrated sensitive and selective caffeine detection with a linear response over a caffeine concentration range of 5-200 µM. Reliable caffeine quantification was achieved using only 20 µL of sample, with measured caffeine concentrations consistent with those obtained by liquid chromatography-mass spectrometry. These results demonstrate the potential of the developed paper-based sensor as a sustainable and practical platform for the direct electrochemical analysis of caffeine in beverages.