Luminescent Metal–Organic Framework with Outstanding “Turn-On” Hg²⁺ Sensing Ability First Constructed by an AIE Ligand

Hg²⁺ is highly toxic and can cause serious harm to the environment and humans. Thus, it is vital to develop efficient Hg²⁺ sensors. In this work, a LMOF-based (LMOF = luminescent metal-organic framework) "turn-on" Hg²⁺ sensor (<b>1</b>) is first developed by an aggregation-induced emission (AIE) functional ligand. <b>1</b> with a formula of [Cd₃(<b>L</b>)₂(NDA)₃(H₂O)₂<b>·</b>(<b>L</b>)(MeOH)]_<i>n</i> (<b>L</b> = 4,4'-((2,7-di-<i>tert</i>-butyl-9<i>H</i>-fluoren-9-ylidene)-methylene)dipyridine; H₂NDA = 1,4-naphthalenedicarboxylic acid) exhibits a unique (3,4)-connected 2-D framework with 3-connected Cd1, 4-connected [Cd₂(COO)₃]⁺ cluster, and <b>L</b> and NDA^2- bridges. The fluorene rings of <b>L</b> in <b>1</b> stack in a back-to-back manner. <b>1</b> has excellent water, pH, and thermal stabilities and can "turn-on" detect Hg²⁺ with ultrahigh sensitivity, good selectivity, and recyclability in a water medium. Furthermore, <b>1</b> shows obvious naked-eye-visible emission change and an extremely fast sensing response toward Hg²⁺ (<1 S) and can maintain its enhanced intensity over 5 min. The test paper is developed for practical "turn-on" Hg²⁺ sensing application. The "turn-on" Hg²⁺ sensing mechanism of <b>1</b> is discussed in detail.