Aqueous-Phase Nanomolar Detection of Dichromate by a Recyclable Cd(II) Metal–Organic Framework

A new Cd(II)–organic framework with a quinoline-2,6-dicarboxylic acid (H2QDA) ligand having the formula {[Cd(QDA)]·DMF·0.5H2O}n (1) was synthesized by heating a mixture of Cd(ClO4)2·6H2O, H2QDA, concentrated HNO3, and DMF (N,N′-dimethylformamide) at 120 °C for 72 h. A single-crystal X-ray diffraction analysis confirmed that 1 has a rare ptr topology and Cd2+ attains highly distorted trigonal prismatic geometry with a CdO5N configuration. The as-synthesized compound 1 was activated by solvent exchange and drying at 100 °C for 24 h. By using analytical methods such as X-ray powder diffraction (XRPD), Fourier transform infrared (FT-IR), and thermogravimetric analysis (TGA), both 1 and its activated form (1′) were characterized. Compound 1′ exhibits good thermal stability, high water stability, and easy regeneration ability. The compound was used for the selective and sensitive detection of dichromate (Cr2O72–) in water via fluorescence quenching. The KSV and detection limit values for Cr2O72– in water for 1′ were found to be 9.65 × 104 M–1 and 81.6 nM (linear range: 0–1.2 μM), respectively. The compound also showed good recyclability and structural stability after the sensing experiment. Moreover, the quenching process was fast (30 s) in the presence of 3 mM aqueous Cr2O72– solution. The possible mechanism for the quenching effect of 1′ was also investigated. The compound is a promising fluorescent sensor by virtue of its great detection efficiency, reusability, and fast response rate.