Dual-ligand lanthanide metal-organic framework probe for ratiometric fluorescence detection of mercury ions in wastewater

Abstract

By serving dipyridylic acid (DPA) and 2,5-dihydroxyterephthalic acid (DHTA) as the biligands, a novel lanthanide (Eu³⁺) metal-organic framework (MOF) namely Eu-DHTA/DPA was prepared for specific Hg²⁺ fluorescence determination. The dual-ligand approach can endows the resulting luminescent MOF with dual emission of ratiometric fluorescence and uniform size. Eu³⁺ produces intense red fluorescence when activated by the ligand DPA, while the other ligand DHTA produces yellow fluorescence. Under 273 nm excitation, the presence of Hg²⁺ in the monitoring environment causes an increase in the intensity of the DHTA fluorescence peak at 559 nm and a decrease in the intensity of the Eu³⁺ fluorescence peak at 616 nm. Hg²⁺ effectively quenches the fluorescence emission of the central metal Eu³⁺ in Eu-DHTA/DPA at 616 nm through a dynamic quenching effect. This recognition process occurs due to the coordination of Hg²⁺ with ligands such as benzene rings, carboxyl groups, and pyridine N in three-dimensional space. Hg²⁺ was detected by measuring the ratio between two fluorescence peaks (I_{559 nm}/I_{616 nm}) within the range 2-20 μM, achieving a remarkably low detection limit of 40 nM. The established ratiometric fluorescence method has been successfully applied to the determination of Hg²⁺ in industrial wastewater of complex composition. The method plays a crucial role in the rapid and sensitive monitoring of Hg²⁺ in real environmental samples. The recoveries ranged from 92.82% to 112.67% (n = 3) with relative standard deviations (RSD) below 4.8%. This study offers a convenient and effective method for constructing probes for Hg²⁺ monitoring, with practical applications in environmental monitoring.

Keywords: Dual-ligand; Mercury ions; Metal–organic framework; Ratiometric fluorescent probe; Wastewater.