Post-Synthetic Modification of Cd-MOF with Dual Emission as a Ratiometric Probe for Visual Detection

In contemporary society, heavy metal ions have posed significant threats to water systems and human health, for which timely and accurate detection is of much significance. In this work, a new metal-organic framework, CUST-1022, [Cd(H₂BPDC)(phen)]CH₃OH was synthesized via the solvothermal method. Due to the existence of uncoordinated carboxyl sites in CUST-1022, Eu³⁺ was introduced into CUST-1022 through the postsynthesis modification method to construct a ratiometric fluorescence sensing material, Eu³⁺@CUST-1022. Eu³⁺@CUST-1022 demonstrates exceptional performance in detecting Fe³⁺, CrO₄^2-, and Cr₂O₇^2-, with detection limits of 8.63 μM, 1.26 μM, and 1.91 μM, respectively, which represent a significant improvement (lower detection limits) over pristine CUST-1022 (15.72 μM for Fe³⁺, 5.46 μM for CrO₄^2-, and 2.86 μM for Cr₂O₇^2-). Moreover, the sensing process was accompanied by a distinct color change, enabling visual discrimination. A portable fluorescent sensor was fabricated by using cotton swabs, offering a novel strategy for the direct detection of heavy metal ions in aqueous solutions. The potential fluorescence quenching mechanism was found to be energy competition absorption and dynamic quenching. The ratio-type fluorescent sensor prepared by the postsynthesis modification method reduces the detection limit, which represents an effective strategy for constructing MOF-based sensors for ion detection in practical environmental applications.