Supramolecular Assembly Mediated by Metal Ions in Aqueous Solution and Its Application in Their Analysis

A water-soluble fluorescent sensor based on the interaction of the twisted cucurbit[14]uril (tQ[14]) and thiazole orange (TO) in aqueous solution has been developed. Formation of the tQ[14]/TO complex gives rise to strong fluorescence in both of neutral and acidic aqueous solutions compared with that of free TO. Further experiments on the interaction of the tQ[14]/TO complex with metal ions revealed that from a series of nineteen selected common metal ions, 1) only Hg2+ ion can lead to the fluorescence enhancement and then quenching of the tQ[14]/TO complex (2:1) in neutral aqueous solution; 2) only Ba2+ can induce fluorescence quenching of the tQ[14]/TO (2:1) complex in aqueous HCl solution (pH 2). Furthermore, the tQ[14]/TO/Ba2+ systems exhibit reversible fluorescent intensity changes by successively adding SO42- and Ba2+; and 3) only Ba2+ or Pb2+ can induce fluorescence quenching of the tQ[14]/TO (15:1) complex in aqueous HCl solution (pH 2). Thus, the tQ[14]/TO complex may act as a supramolecular fluorescence-based sensor for Hg2+, Ba2+ and Pb2+ ion determination. The proposed fluorescence sensor exhibited high selectivity and sensitivity with a detection limit of 7.8×10−8 mol/L for Hg2+ ion, 4.6×10−7 mol/L for Ba2+ ion and 3.2×10−6 mol/L for Pb2+ ion respectively.