Multifunctional rhodamine Schiff base sensor for Pb2+: fluorogenic switching, reusability, and structural elucidation through spectroscopic and molecular simulation

Abstract

Lead ion (Pb²⁺) pollution is a major threat to public and environmental health. This paper reports the synthesis and characterization of a new rhodamine-derived Schiff base chemosensor (L) for selective and sensitive Pb²⁺ sensing. NMR, IR, mass spectrometry, and single-crystal X-ray diffraction were used for characterization. With Pb²⁺ binding in acetonitrile, L displayed a characteristic “turn-on” fluorescence and visible colour change. Binding experiments established a stoichiometry of 1 : 1 and a binding constant (K_a = 1.519 × 10⁴ M⁻¹) with a low detection level of 1.97 nM. DFT calculations corroborated the existence of a stable L–Pb²⁺ complex by coordination to two donor atoms. The sensor exhibited reversible switching behavior towards EDTA, acting as an INHIBIT logic gate. Molecular docking with the DNA-binding protein 5VBN indicated a binding energy of −5.9 kcal mol⁻¹, suggesting favourable interactions and possible biological importance. Its practical uses were confirmed by detection in real water samples, solid-state assays, and filter paper-based sensing, demonstrating its potential for environmental monitoring.