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Herein, a new, direct paper-based fluorimetric method is described for the quantitative determination of glutathione (GSH) molecules in nutritional supplements. Briefly, the proposed analytical method is based on the fluorescence emission resulting from the direct and selective chemical reaction of GSH molecules with the derivatization reagent that is o-phthalaldehyde (OPA) in acidic conditions at room temperature. The intensity of the emitted fluorescence on the surface of the analytical paper devices after irradiation with a lamp at 365 nm is proportional to the concentration of GSH and is measured using a smartphone as the detector. This methodology, which is suitable for measurements in laboratories with limited resources, does not require specialized instrumentation or trained personnel. The protocol governing the proposed method is simple and easily applicable. Essentially, the chemical analyst should adjust the value of pH on the surface of the paper by adding a minimal amount of buffer solution; then, after adding a few microliters of the derivatization reagent, wait for the surface of the paper to dry and, finally, add the analyte. Subsequently, the irradiation of the sensor and the measurement of the emitted fluorescence can be recorded with a mobile phone. In the present study, several parameters affecting the chemical reaction and the emitted fluorescence were optimized, the effect of interfering compounds that may be present in dietary supplements was examined, and the stability of these paper sensors under different storage conditions was evaluated. Additionally, the chemical stability of these paper devices in various maintenance conditions was studied, with satisfactory results. The detection limit calculated as 3.3 S/N was 20.5 μmol L -1 , while the precision of the method was satisfactory, ranging from 3.1% (intra-day) to 7.3% (inter-day). Finally, the method was successfully applied to three different samples of dietary supplements.

This study reports a new approach for the determination of copper ions in water samples that exploits the complexation reaction with diethyldithiocarbamate (DDTC) and uses widely available imaging devices (i.e., flatbed scanners or smartphones) as detectors. Specifically, the proposed approach is based on the ability of DDTC to bind to copper ions and form a stable Cu-DDTC complex with a distinctive yellow color detected with the camera of a smartphone in a 96-well plate. The color intensity of the formed complex is linearly proportional to the concentration of copper ions, resulting in its accurate colorimetric determination. The proposed analytical procedure for the determination of Cu 2+ was easy to perform, rapid, and applicable with inexpensive and commercially available materials and reagents. Many parameters related to such an analytical determination were optimized, and a study of interfering ions present in the water samples was also carried out. Additionally, even low copper levels could be noticed by the naked eye. The assay performed was successfully applied to the determination of Cu 2+ in river, tap, and bottled water samples with detection limits as low as 1.4 µM, good recoveries (89.0-109.6%), adequate reproducibility (0.6-6.1%), and high selectivity over other ions present in the water samples.