A ratiometric fluorescent, colorimetric, and paper sensor for sequential detection of Cu2+ and glutathione in food: AIEE and reversible piezofluorochromic activity

A new Schiff base 2 was rationally synthesized that exhibited aggregation induced emission enhancement (AIEE) and unique reversible piezofluorochromic properties. Probe 2 displayed a distinguished ratiometric fluorescent and colorimetric recognition of Cu2+ and further acted as a probe for the selective detection of glutathione (GSH) based on its 2–Cu2+ complex. Firstly, probe 2 showed a highly selective fluorescence quenching response towards Cu2+ over its competing ions that was attributed to the intermolecular charge transfer (ICT) to paramagnetic Cu2+. Based on ratiometric fluorescence titration, the limit of detection (LOD) of probe 2 for Cu2+ was calculated as 2.13 nM. The complexation mode of probe 2 for Cu2+ was confirmed through 1H NMR titration, Job plot, computational, and DLS studies. Subsequently, the addition of GSH to probe 2–Cu2+ complex immediately enhanced its fluorescence, and the emission of probe 2 was recovered which was ascribed to the displacement of Cu2+ by GSH and release of probe 2. The developed strategy for the identification of GSH was confirmed through 1H NMR titration experiments. Advantageously, probe 2 and probe 2–Cu2+ complex coated paper sensors were successfully developed for the nanoscale sensing of Cu2+ and GSH in food samples. The detection of Cu2+ and GSH in food and actual water samples was adequately accomplished through a very effective dip-coating method. Finally, a sequential detection strategy was used to design logic circuits.