Fluorescent Zr(IV) Metal-Organic Frameworks for the Rapid and Sensitive Detection of Acidic Nitrophenols in Water

Nitroaromatic compounds (NACs) are widely used industrial chemicals, recognized as both environmental pollutants and explosive hazards. Optical sensors are gaining increasing research ground as they offer a platform for the rapid and efficient in-field detection of NACs. Herein, we report the rational design and synthesis of three new fluorescent metal-organic frameworks (MOFs) based on Zr(IV) and structurally analogous to UiO-66, with the assigned formula {Zr₆O₄(OH)₈(H₂O)₄(L)_1-x(NH₂bdc²⁻)_x}. The incorporation of strongly fluorescent ligands 2-((2-methoxybenzyl)amino)terephthalic acid (L-2), 2-((3-methoxybenzyl)amino)terephthalic acid (L-3), and 2-((4-methoxybenzyl) amino)terephthalic acid (L-4) resulted in the formation of MOFs Zr-2, Zr-3, and Zr-4, respectively. These materials feature pendant π-electron-rich aromatic groups and basic secondary amine functionalities, which are suitable for donor-acceptor interactions with the electron-deficient and acidic NACs 2,4,6-trinitrophenol (TNP) and 2,4-dinitrophenol (DNP). Fluorescence titration experiments on protonated samples (pZr-2, pZr-3, and pZr-4) demonstrate that our MOFs selectively respond to small concentrations of TNP and DNP in H₂O, by exhibiting strong emission quenching with detection limits estimated below 0.030 µM for TNP and 0.040 µM for DNP. Time-resolved fluorescence studies reveal very fast response times (∼1 s) for pZr-2 and pZr-3, making them promising candidates for real-time, in-field detection applications.