Three functionalized Zr(IV) MOFs detect nitrophenols in water by rapid and strong fluorescence quenching with detection limits below 0.040 µM, showing promise for real-time, in-field applications.

Abstract

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.