Zn²⁺ -Ion Sensing by Fluorescent Schiff Base Calix[4]arene Macrocycles

A macrocyclic ligand (H₂ L) containing two o,o'-bis(iminomethyl)phenol and two calix[4]arene head units has been synthesized and its coordination chemistry towards divalent Ni and Zn investigated. The new macrocycle forms complexes of composition [ML] (M=Zn, M=Ni) and [ZnL(py)₂ ], which were characterized by elemental analysis; IR, UV/Vis, and NMR spectroscopy; electrospray ionization mass spectrometry (ESI-MS); and X-ray crystallography (for [ZnL(py)₂ ] and [NiL]). H₂ L allows the sensitive optical detection of Zn²⁺ among a series of biologically relevant metal ions by a dual fluorescence enhancement/quenching effect in solution. The fluorescence intensity of the macrocycle increases by a factor of ten in the presence of Zn²⁺ with a detection limit in the lower nanomolar region.