Selective Recovery and Detection of Gold with Cucurbit[n]urils (n = 5-7)

Environmentally benign methods for gold recovery and detection are highly desirable for a sustainable future. Herein, we demonstrate a selective recovery and detection strategy of gold with cucurbit[n]urils (Q[n = 5-7]) by means of outer-surface interactions. Tetrachloroaurate anion ([AuCl₄]^-) is able to be rapidly precipitated with Q[n] in forms of supramolecular adducts. X-ray crystallography of four Q[n = 5-7]·[AuCl₄]^- complexes reveal that strong outer-surface interactions between Q[n = 5-7] and [AuCl₄]^- are the major driving force for the formation of Q[n = 5-7]·[AuCl₄]^- complexes. Impressively, each Q[6] macrocycle is surrounded by 12 [AuCl₄]^- anions. Each of these 12 [AuCl₄]^- anions is connected to four adjacent [AuCl₄]^- anions, leading to the formation of a three-dimensional supramolecular framework with tubular channels. In addition, we found an interesting inclusion complex [Au(OH₂)₄]³⁺⊂Q[7] in the Q[7]·[AuCl₄]^- complex, which is the first example of hydrated metal cation encapsulated inside the cavity of Q[n]. Spectroscopic data suggest that Q[n = 5-7] possess a high affinity and selectivity for [AuCl₄]^- even in the presence of other transition-metal ions. Q[n] modified electrodes are found to be an effective material for the detection of trace gold in dilute solutions.