'AIE + ESIPT' Active 2-hydroxy-naphthalene Hydrazone for the Fluorescence Turn-on Sensing of Al³

The aggregation-induced emission (AIE) behaviour of an easy-to-prepare Schiff base 2-hydroxy-naphthalene hydrazone (L) was explored in mixed DMSO/HEPES medium by selecting DMSO as a good solvent, whereas HEPES buffer (H₂O, 10 mM, pH 7.4) as a poor solvent. The weakly fluorescent L in pure DMSO showed a fluorescence enhancement at 532 nm upon increasing the fraction of HEPES above 70% because of the self-aggregation of L and excited state intramolecular proton transfer (ESIPT) process. The AIE luminogen (AIEgen) L was applied for the sensing of metal ions in HEPES buffer (5% DMSO, 10 mM, pH 7.4). Among the fourteen different metal ions (Cu²⁺, Co²⁺, Ni²⁺, Mn²⁺, Mg²⁺, Fe³⁺, Fe²⁺, Zn²⁺, Cd²⁺, Hg²⁺, Pb²⁺, Al³⁺, Cr³⁺), AIEgen L showed a selective fluorescence enhancement at 435 nm in the presence of Al³⁺ without disturbing the fluorescence intensity at 532 nm due to the chelation-enhanced fluorescence effect (CHEF). The detection limit of 20 nM was estimated by performing the fluorescence titration of AIEgen L with Al³⁺. The reversibility of the Al³⁺ selective AIEgen L was demonstrated by adding a strong chelating agent EDTA. Finally, the practical utility of AIEgen L was validated by quantifying Al³⁺ in river and tap water samples.