A Cyclam-Based Fluorescent Ligand as a Molecular Beacon for Cu2+ and H2S Detection

A pentadentate cage-like cyclam-based fluorescent ligand has been designed, synthesized, and characterized by 1D and 2D NMR spectroscopic analysis. To further characterize the title ligand, ESI-MS, UV/Vis spectroscopy, and fluorescence experiments have been also performed. Its suitability as a chemosensor for the detection of Cu2+ ions has been successfully explored with a variety of spectroscopic techniques (i.e., ESI-MS, UV/Vis, and fluorescence). A Job's plot experiment, together with ESI-MS experiments, indicate a 1:1 stoichiometry for the binding of Cu2+ to the cyclam-based ligand. Fluorescence experiments corroborate this hypothesis. The resulting system is selective in the recognition of copper and is stable for a week, as assessed by fluorescence spectroscopy. The CuII complex has been isolated and tested as a fluorescent probe for the detection of H2S. NMR spectroscopy and ESI-MS investigations have provided evidence that H2S recognition occurs through a copper-displacement mechanism, as reported in the literature for different copper complexes. The system functions with a selective response to HS–, harnessing a concentration-dependent “turn-on” of the initial fluorescence intensity.