Luminescent new zirconium-based coordination polymer for selective and sensitive detection of dichromate ions in aqueous media

Abstract

A new zirconium-based coordination polymer was synthesized through the coordination of zirconium(IV) ions with a newly synthesized π-conjugated amide-functionalized organic linker, namely N,N′-(1,2-phenylene)dioxamic acid. The ligand was prepared by reacting oxalic acid dihydrate (2 mmol) with o-phenylenediamine (1 mmol) in aqueous solution at 70 °C, forming a linker containing amide and carboxylate functionalities. The resulting linker solution was subsequently reacted with ZrOCl₂·8H₂O under hydrothermal conditions at 80 °C to form a robust and luminescent framework. The resulting compound 1 exhibits intense cyan photoluminescence under UV excitation at 365 nm and functions as a highly efficient optical sensor for hexavalent chromium (Cr(VI)) toxic ions in aqueous media. Upon exposure to Cr₂O₇²⁻, the fluorescence is significantly quenched due to a synergistic mechanism involving photoinduced electron transfer (PET) and inner filter effect (IFE). A Stern–Volmer quenching constant (K_SV) of 3.368 was obtained with a strong linear correlation (R² = 0.9525) up to 2.1 ppm. The calculated limit of detection (LOD) is 13 ppb, allowing for trace-level sensing well below regulatory limits. X-ray photoelectron spectroscopy confirmed the redox transformation of Cr(VI) to Cr(III), validating not only sensing but also treatment through reduction to less toxic Cr(III). The compound 1 sensor also exhibits high selectivity over interfering ions, demonstrating its practical potential for real-time environmental monitoring of toxic Cr(VI) species.