Cucurbit[7]uril-improved recognition by a fluorescent sensor for cadmium and zinc cations

The host-guest interaction of cucurbit[7]uril (Q[7]) with the new fluorescent sensor guest molecule (BIBPAH(+)), formed by alkylation of the secondary amine site in N,N-bis(2-pyridylmethyl)amine with 2-(bromomethyl)benzimidazole, was investigated by fluorescence and H-1 NMR spectroscopy. The results indicate the formation of an inclusion complex in a 1:1 ratio with an association constant of K-a=(2.9 +/- 0.7) x 10(5) L . mol(-1). There is minimal difference in the fluorescence intensities upon the introduction of each of Mn2+, Ni2+, Co2+, Cr3+, Ho3+ and Lu3+. However, the emission was slightly quenched on the addition of Cu2+ or Fe3+ whether Q[7] is present or not. Further investigation showed that encapsulation of BIBPAH(+) in Q[7] improves Cd2+ and Zn2+ recognition by enhancing the fluorescence intensity, leading to an improvement in the limits of detection. In contrast, comparative parallel experiments in which Cu2+, Mn2+, Ni2+, Co2+, Fe3+, Cr3+, Ho3+ or Lu3+ were substituted for Zn2+ or Cd2+ resulted in minimal changes in the fluorescence that occurs for the corresponding metal-free system. The natures of the encapsulated Zn2+ and Cd2+ complexes have been investigated using fluorescence and H-1 NMR spectroscopy as well as by quantum chemistry. The selectivity of a fluorescent sensor for Cd2+ and Zn2+ was improved by its encapsulation within the cavity of cucurbit[7] uril. [GRAPHICS] .