Modulating the selectivity by switching sensing media: A bifunctional chemosensor selectivity for Cd²⁺ and Pb²⁺ in different aqueous solutions

A novel bifunctional fluorescent probe RI, based on rhodamine with isatin, was designed and synthesized. Switching of the selectivity of RI between various metal ions was achieved by judicious choice of sensing media. RI gives a turn-on fluorometric and colorimetric signal toward Cd²⁺ in 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) (10 mM, CH ₃OH-H₂O, 4:6, v/v, pH 7.6) buffer solution and a turn-on colorimetric signal toward Pb²⁺ in Tris-HCl (70 mM, CH ₃OH-H₂O, 6:4, v/v, pH 7.6) buffer solution. To the best of our knowledge, this is the first example of a chemosensor based on a small molecule that can selectively recognize both Cd²⁺ and Pb²⁺ in different sensing systems. Fluorescence imaging of Cd²⁺ in living cells was also obtained.