Nanocage-Based N-Rich Metal–Organic Framework for Luminescence Sensing toward Fe³⁺ and Cu²⁺ Ions

Abstract

Luminescent metal-organic frameworks (LMOFs) as sensors showing highly efficient detection toward toxic heavy-metal ions are in high demand for human health and environmental protection. A novel nanocage-based N-rich LMOF (<b>LCU-103</b>) has been constructed and characterized. It is a 2-fold interpenetrating structure built from N-rich {Zn₆(dttz)₄} nanocages extended by N-donor ligand Hdpa [H₃dttz = 4,5-di(1<i>H</i>-tetrazol-5-yl)-2<i>H</i>-1,2,3-triazole; Hdpa = 4,4'-dipyridylamine]. Notably, <b>LCU-103</b> contains abundant N functional sites anchoring on both the windows of nanocages and the inner channels of the framework that can interact with metal ions and then recognize them. As a result, it can serve as a luminescent sensing material for detecting trace amounts of Fe³⁺ and Cu²⁺ ions with low limits of detection (LODs) of 1.45 and 1.66 μM, respectively, through a luminescent quenching mechanism. Meanwhile, <b>LCU-103</b> as a LMOF sensor exhibits several advantages such as high sensitivity, appropriate selectivity (for Fe³⁺ in H₂O), recycling stability, and fast response times in <i>N</i>,<i>N</i>-dimethylformamide. Moreover, <b>LCU-103</b> also displays good luminescent quenching activity toward Fe³⁺ in H₂O and a simulated 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid biological system with low LODs of 1.51 and 1.52 μM, respectively. <b>LCU-103</b> test papers were further prepared to offer easy and real-time detection of Fe³⁺ and Cu²⁺ ions. Importantly, when density functional theory calculations and multiple experimental evidence, including X-ray photoelectron spectroscopy, UV-vis absorption, luminescence decay lifetimes, and quantum efficiencies, are combined, a preferred N-donor site and possible weak interaction sensing mechanism is also proposed to elucidate the quenching effect.