Biocompatible alkyne arms containing Schiff base fluorescence indicator for dual detection of CdII and PbII at physiological pH and its application to live cell imaging

An alkyne arms containing salen-type Schiff base ligand, 6,6 '-((1E,1 ' E)-(1,2-phenylenebis(azanylylidene))bis(ethan-1-yl-1-ylidene))bis(3-(prop-2-yn-1-yloxy)phenol) (H2L), is reported here as a dual chemosensor for Cd(II)and Pb(II)ions. The ligandH(2)Lwas characterized by various spectral methods. The ligandH(2)Lacts as a dual sensor for Cd(II)and Pb(II)in methanol/HEPES buffer (5 mM, pH 7.3; 1 : 9 v/v) at room temperature with well-separated excitation and emission wavelengths. The emission intensity at 553 nm ofH(2)Lis shifted and a new band appeared at 578 nm with the increase in the presence of Cd(II)when it is excited at 420 nm. With excitation at 410 nm, the emission intensity ofH(2)Lat 485 nm shifted and a new band appeared at 505 nm with the increase in the presence of Pb-II. The quantum yield ofH(2)Lincreases considerably in the presence of Cd(II)and Pb-II. The compoundH(2)Lis non-fluorescent; however, in the presence of Cd(II)and Pb-II, the compoundH(2)Lis highly fluorescent with well-separated excitation and emission wavelengths, indicating that the metal ion is coordinated through phenolic oxygen and imine nitrogen of the Schiff base blocking the PET (Photoinduced Electron Transfer) process and stimulating the CHEF (Chelation Enhanced Fluorescence) process, to increase the fluorescence intensity ofH(2)L. The detection limit values ofH(2)Lare in a nano-molar range for both metal ions, confirming very high sensitivity ofH(2)L. The L center dot M(II)binding mode and the recognition mechanism of the sensor were explored by Job's plot,H-1 NMR, FT-IR, pH and DFT calculations. Besides,H(2)Lwas successfully utilized in cell imaging studies for both metal ions in MCF 7 cells.