Colorimetric chiral fluorescent sensors for Eu³⁺ and sequential enantioselective sensing of malate anion

Novel phenanthroline Schiff base fluorescent sensors L1, L2, and D1 were designed and synthesized. The sensing abilities of the compounds in the presence of metal cations (Li⁺ , Na⁺ , K⁺ , Ag⁺ , Mg²⁺ , Ba²⁺ , Ca²⁺ , Mn²⁺ , Pb²⁺ , Hg²⁺ , Ni²⁺ , Zn²⁺ , Cd²⁺ , Co²⁺ , Cu²⁺ , Cr³⁺ , Fe³⁺ , Fe²⁺ , Al³⁺ , and Eu³⁺ ) were studied by UV-vis and fluorescent spectroscopy. The compounds L1, L2, and D1 could act as Eu³⁺ ion turn-off fluorescent sensors based on ligand-to-metal binding mechanism in DMSO-H₂ O solution (v/v = 1:1, 10 mM Tris, pH = 7.4). Additionally, the L1-Eu³⁺ and D1-Eu³⁺ complexes could be applied as turn-on enantioselective sensors sensing of malate anion isomers with color changes. Furthermore, biological experiments using living PC-12 cells demonstrated that L1 and D1 had excellent membrane permeability and could be used as effective fluorescent sensors for detecting Eu³⁺ and malate anion in living cells.