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 twisted cucurbit[14]uril (tQ[14]) and thiazole orange (TO) in aqueous solution was developed. Formation of the tQ[14]/TO complex gives rise to stronger fluorescence in both 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⁺ can lead to fluorescence enhancement and then quenching of the tQ[14]/TO (2:1) complex 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 changes in fluorescent intensity on successively adding SO₄^2- 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 can act as a supramolecular fluorescence-based sensor for the determination of Hg²⁺ , Ba²⁺ , and Pb²⁺ ions.