Mixed-Ligand LMOF Fluorosensors for Detection of Cr(VI) Oxyanions and Fe3+/Pd2+ Cations in Aqueous Media.

Zn(II)/Cd(II)-based dual ligand Luminescent Metal-Organic Frameworks (LMOFs) {[M(ATA)(L)]}n·xH2O (1) and (2) were synthesized by versatile synthetic routes, viz., diffusion of precursor solutions, conventional reflux, and green mechanochemical (grinding) reactions from bipyridyl-based Schiff base, (E)-N'-(pyridin-4-ylmethylene)isonicotinohydrazide (L) and amino functionalized 2-aminoterephthalic acid (H2ATA) as linkers. Chemical and thermal stability, phase purity, and characterization of both LMOFs were established by various analytical methods. SXRD analysis revealed the 3D framework is composed of two-dimensional [M(ATA)]n nets doubly pillared by L through the terminal nitrogen atom. Selective and sensitive detection of chromate anions (CrO42-/Cr2O72-) and Fe3+/Pd2+ cations in the aqueous phase by fluorescent quenching of the LMOFs 1 and 2 has been established. Competitive experiments in the presence of interfering anions/cations with 1 and 2 revealed no major change in the quenching efficiency. The observed limits of detection (LOD) values by 1 for CrO42-/Cr2O72- were 0.25 μM (48 ppb)/0.43 μM (126 ppb) and for Fe3+/Pd2+ were 3.76 μM (0.61 ppm)/0.20 μM (35 ppb), whereas LOD values by 2 were 0.18 μM (35 ppb)/0.19 μM (55 ppb) and 1.77 μM (0.29 ppm)/0.10 μM (18 ppb), respectively. Simple fluorescent-based test paper strips have been developed for reliable and visual detection of the mentioned analytes in practical applications. The present investigation clearly demonstrates selective detection of CrO42-/Cr2O72- and Fe3+/Pd2+ in aqueous media, and the probable mechanism for the quenching phenomena based on structural aspects has also been discussed.