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

Abstract

<p>The host–guest interaction of cucurbit[7]uril (Q[7]) with the new fluorescent sensor guest molecule (<b>BIBPAH</b>^<b>+</b>), formed by alkylation of the secondary amine site in <i>N,N</i>-bis(2-pyridylmethyl)amine with 2-(bromomethyl)benzimidazole, was investigated by fluorescence and ¹H NMR spectroscopy. The results indicate the formation of an inclusion complex in a 1:1 ratio with an association constant of <i>K</i>_a = (2.9 ± 0.7) × 10⁵ 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 <b>BIBPAH</b>^<b>+</b> 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 ¹H NMR spectroscopy as well as by quantum chemistry.</p> <p>The selectivity of a fluorescent sensor for Cd²⁺ and Zn²⁺ was improved by its encapsulation within the cavity of cucurbit[7]uril.</p