Hg(II) immobilization and detection using gel formation with tetra-(4-pyridylphenyl)ethylene and an aggregation-induced luminescence effect

Tetra-(4-pyridylphenyl)ethylene (TPPE), featuring an aggregation-induced luminescence effect (AIE), has been synthesized and used for selective detection of Hg2+ in DMF/H2O (3:7, v/v) binary solutions. There was a color change from colorless to yellow in the detection of the Hg2+ ions, in addition to an increased fluorescence emission. This shows that TPPE will function as an excellent "turn-on" fluorescence probe in the detection Hg2+. Moreover, the interference of Al3+, Ba2+, Mn2+, Ca2+, Fe3+, Cu2+, Ag+, Cd2+, Co2+, Ni2+, Mg2+, Pb2+, Zn2+, and Cr3+ ions was found to be negligible under optimized solvent conditions. Cysteine and EDTA were also found to form TPPE-based fluorescent switches with the Hg2+ ions. The practical use of the TPPE sensor was also demonstrated by using a specific test kit. Characterization using FT-IR, NMR titration, UV titration, EDS, and HR-MS techniques showed that Hg2+ will form a 1:1 complex with TPPE. Also, the observation of a Tyndall effect, in addition to UV absorption and fluorescence spectra, did clearly demonstrate the presence of an AIE. More noteworthy, TPPE and Hg2+ were found to form a metal-organic gel (MOG) in the DMF solution. The SEM, TEM, ICP, and Zeta potential analyses confirmed that the fluorescent MOG could further adsorb an excess of Hg2+ ions. The BET analyses revealed that the MOG showed a type IV-H3 hysteresis loop according to the International Union of Pure and Applied Chemistry classification. The results of the XRD analysis and of the spectroscopic titrations show that a π-π stacking may be the auxiliary driving force for the gel formation, after that a coordination has taken place. These results indicate that further research on structurally simple metal ion fluorescent probes, which are based on the AIE, is promising for the achievement of a simultaneous fluorescent detection and adsorption of heavy metal pollutants.