A ratiometric and selective fluorescent chemosensor for Ca(II) ions based on a novel water-soluble ionic Schiff-base

A new, simple and water-soluble chemosensor, based on the ionic Schiff-base (ISB), 1-(4-hydroxy-3-((2-mercaptophenylimino)methyl)-5-methoxybenzyl)-4-methoxypyridiniumchloride (3), was synthesized and applied to fluorescently sense Ca(II) ions. This fluorescent chemosensor demonstrates a high selectivity and sensitivity for Ca(II) ions over other physiological relevant alkali, alkaline-earth and transition metal ions. The mechanism of selectivity and sensitivity of ISB (3) to Ca(II) was suggested to be on the basis of internal charge transfer (ICT). Interestingly, chelation of Ca(II) to deprotonated ISB to form a complex with a 1:1 metal-ligand ratio can induce a decrease in the fluorescence emission peak centered at 545 nm and simultaneously enhance a new emission peak centered at 483 nm, thus affecting the ICT efficiency of the sensing system and providing ratiometric measurements. Furthermore, the same hypsochromic shift in the emission spectra was observed in both excitation states and absorption spectra. This newly developed sensor enables an ultrasensitive limit of detection (LOD) = 0.3 mu M and the rapid detection of Ca(II) in the physiological pH range (pH 7.2) and thus may offer a new promising strategy for detecting Ca(II) in environmental and biological systems.