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(<b>ATA</b>)­(<b>L</b>)]}<i>_n</i>·<i>x</i>H₂O (<b>1</b>) and (<b>2</b>) were synthesized by versatile synthetic routes, viz., diffusion of precursor solutions, conventional reflux, and green mechanochemical (grinding) reactions from bipyridyl-based Schiff base, (<i>E</i>)-<i>N</i>′-(pyridin-4-yl­methyl­ene)­iso­nico­tino­hy­drazide (<b>L</b>) and amino functionalized 2-aminoterephthalic acid (H₂<b>ATA</b>) 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(<b>ATA</b>)]_<i>n</i> nets doubly pillared by <b>L</b> through the terminal nitrogen atom. Selective and sensitive detection of chromate anions (CrO₄^2–/Cr₂O₇^2–) and Fe³⁺/Pd²⁺ cations in the aqueous phase by fluorescent quenching of the LMOFs <b>1</b> and <b>2</b> has been established. Competitive experiments in the presence of interfering anions/cations with <b>1</b> and <b>2</b> revealed no major change in the quenching efficiency. The observed limits of detection (LOD) values by <b>1</b> for CrO₄^2–/Cr₂O₇^2– 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 <b>2</b> 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 CrO₄^2–/Cr₂O₇^2– and Fe³⁺/Pd²⁺ in aqueous media, and the probable mechanism for the quenching phenomena based on structural aspects has also been discussed