A dual fluorescence probe for Zn²⁺ and Al³⁺ through differentially response and bioimaging in living cells

A novel Schiff base fluorescent probe 7-Hydroxy-8-(((2-(hydroxymethyl)quinolin-8-yl)imino)methyl)-coumarin (XL) consist of formylcoumarin and aminoquinoline moieties was synthesized for dual detection of Zn²⁺ and Al³⁺ ions. Probe XL exhibited high selective and sensitive response towards Zn²⁺ and Al³⁺ ions through different color changes and significant fluorescence turn-on response (270 fold higher for Zn²⁺ and 230 fold higher for Al³⁺) in MeOH-H₂O (4/1, v/v) over other cations, with detection limits (LOD) as low as 3.75 × 10^-8 and 1.14 × 10^-8 M, respectively. Moreover, probe XL exhibited preferential selectivity for Al³⁺ through displacing Zn²⁺ from the XL-Zn²⁺ complex by ligand-to-ligand transfer process. The binding mechanism of intramolecular charge transfer (ICT) were proposed from fluorescence and UV-vis titrations, Job's plot, ¹H NMR titration, HRMS and DFT calculations. The probe was proven to be suitable for actual samples detection of Zn²⁺ and Al³⁺ ions. The complex XL-Zn²⁺ and XL-Al³⁺ exhibited dramatic fluorescent "turn-off" properties for PPi and PPi/F^- respectively through snatching metal ions and released free XL. Moreover, probe XL showed low biotoxicity and sequentially "off-on-off" fluorescent bio-imaging of Zn²⁺/Al³⁺ and PPi/F^- in PC12 cells.