Sustainable Green Route to Synthesize Functional Nano-MOFs as Selective Sensing Probes for Cr^VI Oxoanions and as Specific Sequestering Agents for Cr₂O₇^2–

The efficient detection and segregation of Cr^VI oxoanions from wastewater remains a striking issue due to their carcinogenic and eco-unfriendly characteristics. Herein, we have demonstrated a single satisfactory resolution to address this problem. To this end, four new metal–organic frameworks (MOFs), namely, [{Ni­(5N₃-IPA)­(4,4′-azp)­(H₂O)}]_∞ (1), [{Zn₂(5N₃-IPA)₂(4,4′-azp)₂}­(H₂O)₈]_∞ (2), [{Cd­(5N₃-IPA)­(4,4′-azp)_0.5(H₂O)}­(H₂O)]_∞ (3), and [{Mn­(5N₃-IPA)­(4,4′-azp)}]_∞ (4), are derived from 5-azidoisophthalic acid (5N₃-H₂IPA) and 4,4′-azopyridine (4,4′-azp), followed by a mixed ligand approach exploiting solvothermal methods. Single-crystal X-ray diffraction (SXRD) analysis reveals that all of the MOFs (1–4) exhibit two-dimensional (2D) structures. Remarkably, MOFs 1, 2, and 3 exhibit excellent aqueous phase stability, which is confirmed by powder XRD (PXRD) measurements. Because of the insolubility of these MOFs in common organic solvents, a simple and convenient route is adapted to reduce the particle size of the as-synthesized materials to produce nanoscale MOFs (NMOFs). As suggested by the scanning electron microscopy (SEM) studies, nanospherical, nut-shaped, and nanorodlike morphologies are observed for NMOFs 1, 2, and 3, respectively, which supports the formation of nanostructures. When subjected to photoluminescence studies, aqueous suspensions of NMOFs 1, 2, and 3 are found to be highly luminescent in nature. Subsequently, detection of toxic Cr₂O₇^2– and CrO₄^2– is comprehensively tested using these NMOFs (1, 2, and 3) in water. Among the tested NMOFs, 2 exhibits exceptional capability for selective detection of both Cr₂O₇^2– and CrO₄^2– in the aqueous phase with very high quenching constants and very low detection limits (on ppm level). The mechanistic aspect of the detection phenomena is further assessed by time-resolved fluorescence measurements and UV–vis studies. To explain such an incredible affinity of the host framework toward Cr^VI oxoanions, we performed density functional theoretical (DFT) calculations. N₂ and CO₂ adsorption measurements establish the presence of intrinsic structural porosity in NMOF 2, as indicated by the SXRD data. Furthermore, efficient sorption capacity toward Cr₂O₇^2– (113.63 mg/g) and excellent reusability manifest the great potential of this material for use in real-life applications.