Biocompatible alkyne arms containing Schiff base fluorescence indicator for dual detection of Cd^II and Pb^II at physiological pH and its application to live cell imaging

Abstract

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 ligand H₂L was characterized by various spectral methods. The ligand H₂L acts 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 of H₂L is 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 of H₂L at 485 nm shifted and a new band appeared at 505 nm with the increase in the presence of Pb^II. The quantum yield of H₂L increases considerably in the presence of Cd^II and Pb^II. The compound H₂L is non-fluorescent; however, in the presence of Cd^II and Pb^II, the compound H₂L 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 H₂L. The detection limit values of H₂L are in a nano-molar range for both metal ions, confirming very high sensitivity of H₂L. The L·M^II binding mode and the recognition mechanism of the sensor were explored by Job's plot, ¹H NMR, FT-IR, pH and DFT calculations. Besides, H₂L was successfully utilized in cell imaging studies for both metal ions in MCF 7 cells.