A robust PET-based chiral Schiff base molecular probe for recognition of Hg2+ and Pb2+ and its application in live cell imaging

Abstract

A new salen-based chiral Schiff base, 6,6′-((1E,1′E)-((1R,2R)-cyclohexane-1,2-diylbis(azanylylidene))bis(methanylylidene))bis(2,4-dibromophenol) (H₂L) has been designed and synthesized. H₂L is a fluorescent chemical sensor that can rapidly and precisely detect harmful heavy metal ions such as Hg²⁺ and Pb²⁺ with well-separated excitation and emission wavelengths. Single-crystal X-ray diffraction analysis confirmed the structure of H₂L. The binding of Hg²⁺/Pb²⁺ causes a change in the emission behavior of H₂L from weakly fluorescent to highly fluorescent through the CHEF effect and inhibits the PET process. H₂L exhibited immense selectivity and tremendous sensitivity, with LOD values of 8.29 nM and 7.65 nM for Hg²⁺ and Pb²⁺, respectively. The 1 : 1 binding mode of H₂L·Hg²⁺ and H₂L·Pb²⁺ is validated by using Job's plot, ¹H NMR titration, and ESI-MS investigations. In addition, H₂L has been used to detect Hg²⁺/Pb²⁺ ions in real water samples and paper strips, and intracellularly in HeLa cells.