Methionine-pyrene hybrid based fluorescent probe for trace level detection and estimation of Hg(II) in aqueous environmental samples: Experimental and computational studies

A new fluorescent, Hg2+ selective chemosensor, 4-methylsulfanyl-2-[(pyren-4-ylmethylene)-amino] butyric acid methyl ester (L, MP) was synthesized by blending methionine with pyrene. It was well characterized by different analytical techniques, viz. 1H NMR, 13C NMR, QTOF mass spectra, elemental analysis, FTIR and UV-vis spectroscopy. The reaction of this ligand with Hg2+ was studied by steady state and time-resolved fluorescence spectroscopy. The Hg2+ complexation process was confirmed by comparing FTIR, UV-vis, thermal, QTOF mass spectra and 1H NMR data of the product with that of the free ligand values. The composition (Hg2+:L = 1:1) of the Hg2+ complex in solution was evaluated by fluorescence titration method. Based on the chelation assisted fluorescence quenching, a highly sensitive spectrofluorometric method was developed for the determination of trace amounts of Hg2+ in water. The ligand had an excitation and emission maxima at 360 nm and 455 nm, respectively. The fluorescence life times of the ligand and its Hg2+ complex were 1.54 ns and 0.72 ns respectively. The binding constant of the ligand, L with Hg2+ was calculated using Benesi-Hildebrand equation and was found to be 7.5630 × 104. The linear range of the method was from 0 to 16 μg L−1 with a detection limit of 0.056 μg L−1 for Hg2+. The quantum yields of the ligand and its Hg2+ complex were found to be 0.1206 and 0.0757 respectively. Both the ligand and its Hg2+ complex have been studied computationally (Ab-initio, Hartree Fock method) to get their optimized structure and other related physical parameters, including bond lengths, bond angles, dipole moments, orbital interactions etc. The binding sites of the ligand to the Hg2+ ion as obtained from the theoretical calculations were well supported by 1H NMR titration. The interference of foreign ions was negligible. This method has been successfully applied to the determination of mercury(II) in industrial waste water.