American Chemical Society
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Sustainable Green Route to Synthesize Functional Nano-MOFs as Selective Sensing Probes for CrVI Oxoanions and as Specific Sequestering Agents for Cr2O72–

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posted on 2020-01-07, 19:39 authored by Somali Mukherjee, Sumi Ganguly, Debabrata Samanta, Debasis Das
The efficient detection and segregation of CrVI 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 (14) 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 Cr2O72– and CrO42– 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 Cr2O72– and CrO42– 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 CrVI 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 Cr2O72– (113.63 mg/g) and excellent reusability manifest the great potential of this material for use in real-life applications.