Biocompatible alkyne arms containing Schiff base fluorescence indicator for dual detection of Cd and Pb 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) (HL), is reported here as a dual chemosensor for Cd and Pb ions. The ligand HL was characterized by various spectral methods. The ligand HL acts as a dual sensor for Cd and Pb 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 of HL is shifted and a new band appeared at 578 nm with the increase in the presence of Cd when it is excited at 420 nm. With excitation at 410 nm, the emission intensity of HL at 485 nm shifted and a new band appeared at 505 nm with the increase in the presence of Pb. The quantum yield of HL increases considerably in the presence of Cd and Pb. The compound HL is non-fluorescent; however, in the presence of Cd and Pb, the compound HL is 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 of HL. The detection limit values of HL are in a nano-molar range for both metal ions, confirming very high sensitivity of HL. The L·M binding mode and the recognition mechanism of the sensor were explored by Job's plot, H NMR, FT-IR, pH and DFT calculations. Besides, HL was successfully utilized in cell imaging studies for both metal ions in MCF 7 cells.