Detection of nitrophenols with a fluorescent Zr(iv) metal-organic framework functionalized with benzylamino groups

Nitroaromatic compounds (NACs) are known explosives and environmental pollutants posing a risk for public health and national security. Thus, the development of efficient sensors for their rapid and efficient in-field detection is of high importance. Analytical methods based on fluorescence are gaining interest as current light detection technology allows the fabrication of miniaturized portable devices suitable for in-field use. Herein, we report the rational design and synthesis of Zr-1, a Zr(iv) based metal-organic framework (MOF) which is structurally analogous to UiO-66 with the assigned formula {Zr₆O₄(OH)₈(H₂O)₄(L-1)_4−x(NH₂bdc)_x}, (L-1 = 2-((benzyl)amino)-terephthalate; NH₂bdc²⁻ = 2-aminoterephthalate). Zr-1 incorporates a fluorescent ligand (L-1) with a pendant π-electron rich aromatic group and a basic secondary amine functionality, thereby targeting the selective detection of electron deficient and acidic NACs, 2,4,6-trinitrophenol (TNP) and 2,4-dinitrophenol (DNP). The stability of Zr-1 in an aqueous environment was confirmed by powder X-ray diffraction analysis on water treated samples. Fluorescence titration experiments on aqueous suspensions of acid activated Zr-1 (pZr-1) demonstrate that the material responds to small concentrations of TNP and DNP by displaying strong emission quenching, even in the presence of potentially competing compounds. The estimated limits of detection were found to be as low as 0.011 μM (2.5 ppb) for TNP and 0.026 μM (4.8 ppb) for DNP.