Quinoline based reversible fluorescent probe for Pb²⁺; applications in milk, bioimaging and INHIBIT molecular logic gate

We report the modular design and synthesis of an amine dangled Schiff base quinoline–morpholine conjugate (QMC) for highly selective detection of Pb²⁺ ions via fluorimetry. The sensing strategy of QMC towards Pb²⁺ ion exhibits a large blue shift with fluorescent enhancement via the intramolecular charge transfer (ICT) process. At the same time, QMC coordination with Pb²⁺, the C-N single bond rotation between quinoline and morpholine rings and the C=N isomerization process were blocked. Best of our knowledge, this is the first blue shifted turn-on fluorescent chemosensor for Pb²⁺ ion via the ICT process. Furthermore, QMC selectively detects Pb²⁺ ion without any interference with alkali, alkaline earth, and transition metal ions, and limit of detection (LOD) downs to 13 μM, which is a permissible level of Pb²⁺ ion in drinking water reported by WHO. The 1:2 binding stoichiometry between QMC and Pb²⁺ was confirmed by fluorimetric, ¹H NMR titration, mass spectrometry, and theoretical studies. Finally, QMC was potentially applied for the sensing of Pb²⁺ ions in milk, red wine, live cells and an INHIBIT molecular logic function was constructed by using Pb²⁺ and EDTA as chemical inputs.