Lanthanide Coordination Polymer-Based Composite Films for Selective and Highly Sensitive Detection of Cr₂O₇ ^2- in Aqueous Media

Due to the high carcinogenicity and bioaccumulation effects of dichromate ions in the human body, sensitive and rapid detection of Cr₂O₇ ^2- ions is necessary. Herein, two lanthanide coordination polymers based on a linear dicarboxylic acid ligand, named {Ln(cpon)(Hcpon)(H₂O)₃}_n [Ln = Tb, Tbcpon; Eu, Eucpon; H₂ cpon = 5-(4-carboxy-phenoxy)-nicotinic acid], have been successfully synthesized. These two isostructural compounds contain one-dimensional zigzag chains that consist of uncoordinated carboxyl groups and pyridine groups in the framework, and the one-dimensional chains can further form a three-dimensional supramolecular stacking structure by intermolecular interaction. Both Tbcpon and Eucpon show good luminescence performance and high stability. Tbcpon exhibits a good ability to sense Cr₂O₇ ^2- ions in aqueous solution. Moreover, the composite film material composed of Tbcpon and poly(methyl methacrylate) (PMMA) exhibits superior luminescence properties compared to those of pure Tbcpon. The Tbcpon-PMMA film exhibits an excellent ability to recognize Cr₂O₇ ^2- ions with high selectivity and a low detection limit of 5.6 ppb, which is much lower than the maximum contamination standard of 100 ppb in drinking water specified by the U.S. Environmental Protection Agency. Furthermore, the Tbcpon-PMMA film shows good recyclability for more than five cycles and anti-interference ability. After the introduction of the slightly soluble polymer poly(vinyl alcohol) (PVA), the Tbcpon-PVA composite film can effectively detect Cr₂O₇ ^2- ions in as little as 1 min. These composite films could be potentially used as test strips for trace detection and rapid detection of Cr₂O₇ ^2- ions in aqueous solution.