Zirconium metal organic cages: From phosphate selective sensing to derivate forming

Luminescent metal organic cages (MOCs) have attracted great interest as a unique class of sensing substrates. In this work, intrinsically fluorescent Zr-MOCs were successfully used as fluorescent probes for the sensitive and selective detection of phosphate anions in water and real samples. When the ligand and Zr ion clusters form a cage, the intrinsic fluorescence of the ligand was tuned from high to weak emission due to the ligand-to-metal charge transfer (LMCT) effect, and this weakened fluorescence can be restored by the addition of phosphate. The degree of fluorescence enhancement is positively correlated with the added phosphate concentration, and the efficacy of this strategy is demonstrated by a linear phosphate detection range of 5–500 µmol/L and a detection limit of 1.06 µmol/L. We discuss the interaction between phosphate and Zr in scattering spectrum and MS, respectively. In comparison to phosphate adsorption on Zr-metal organic frameworks (MOFs), where phosphate connects different numbers of cages, both blocking the LMCT effect and causing the cages to aggregate. We also found that the phosphate displaces the ligand from the cage when the phosphate concentration is further expanded, resulting in the formation of new derivatives. This derivative was shown to be useful as a Lewis acid catalyst and as a rare earth ion adsorbent.