Dual-Emission Lanthanide-MOF Sensors Enabling Ratiometric Fluorescent Detection of 2,4,6-Trinitrotoluene with High Selectivity

Although designing dual-emission probes remains challenging, mixed-ligand metal-organic frameworks (MOFs) offer a promising platform for ratiometric fluorescent sensors. MOFs with multiple components of different metal centers and/or different ligands provide a great chance for the precise synthesis of ratiometric fluorescent sensors. Herein, <i>N</i>,<i>N</i>'-bis(5-isophthalic acid)-naphthalene diimide (NDI) with a large π-conjugate plane was selected to combine with 2,5-dihydroxyterephthalic acid (DHBDC), a typical molecule with an excited-state intramolecular proton transfer process, to form a mixed-ligand system. Their self-assembly with lanthanide metal ions (Ln³⁺) under solvothermal conditions led to a series of Ln-MOFs, named [Ln(DHBDC)_0.5(NDI)_0.5·solvents] (SNNU-380(Ln); Ln = Tb, Eu, Er, Dy, Gd, and Nd; solvents = <i>N</i>,<i>N</i>'-dimethylformamide (DMF) and/or <i>N</i>,<i>N</i>'-dimethylpropyleneurea (DMPU)). As expected, SNNU-380(Ln) exhibits controllable dual fluorescence emissions originating from two fluorescent ligands (420 nm emission from NDI and 550 nm emission from DHBDC) mildly regulated by lanthanide metal ions and shows unique fluorescence quenching for selective 2,4,6-trinitrotoluene (TNT) detection through a photoinduced electron transfer (PET) mechanism. Among them, SNNU-380(Eu) exhibits outstanding detection ability with the highest sensitivity (<i>K</i>_SV = 6.80 × 10³ M^-1), the lowest detection limit (0.18 ppm), a rapid response, exceptional selectivity among nitro-aromatic explosives (NAEs), and excellent recyclability, surpassing the performance of most MOF sensors.