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 CN single bond rotation between quinoline and morpholine rings and the CN 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.