A "turn-on" Fluorescent Probe with Ions-induced ESIPT for the Detection of Cd²⁺ and Zn²

Developing an Aggregation-Induced Emission Enhancement (AIEE) type probe with enhanced emission through metal ion-induced methods has been a significant challenge. In this work, we synthesized an AIEE fluorescent chemosensor P1 via a Schiff base reaction by 8-hydroxyquinoline-2-carbaldehyde and 2,4-difluorophenylhydrazine. Experimental results demonstrate that chemosensor P1 exhibits AIEE behavior in DMSO/glycerol mixed solvents. Significantly, this probe exhibits multifunctionality in ethanol solution: (1) specific recognition of Zn²⁺/Cd²⁺ ions against other cations; (2) colorimetric discrimination of concentration-dependent under white light; (3) metal ion-triggered red luminescence observed in the solution is similar to the characteristic emission signatures of the excited-state intramolecular proton transfer (ESIPT)-driven molecules in methanol. The system demonstrates good linear relationships between fluorescence intensity and ion concentration within specific ranges in ethanol: For Cd²⁺: 4.0 × 10⁻⁷-2.2 × 10⁻⁶ mol·L⁻¹ with a detection limit of 9.11 × 10⁻⁸ mol·L⁻¹; For Zn²⁺: 2.0 × 10⁻⁷-2.0 × 10⁻⁶ mol·L⁻¹ with a detection limit of 1.02 × 10⁻⁷ mol·L⁻¹. Thus, chemosensor P1 is capable of detecting both Cd²⁺ and Zn²⁺ with significant red fluorescence enhancement, which establishes a new strategy for designing ESIPT Schiff-base probes.