On–Off Ratiometric Fluorescence Europium(III) Metal–Organic Framework for Quantitative Detection of the Inflammatory Marker Neopterin

Benefiting from the unique photoluminescence behavior of the lanthanide(III) ions and organic ligands, a lanthanide(III) metal–organic framework (Ln-MOF) material can simultaneously demonstrate photoluminescence of lanthanide(III) cations and organic molecules and endow its superior applications of fluorescence sensing behaviors. Herein, we present a europium(III) MOF material {[Eu₂(BPTA)·(CH₃COO)₂·3DMA]·0.5DMA·3H₂O}_n (1) (where H₄BPTA is 3,3′,5,5′-biphenyltetracarboxylic acid) for photoluminescence performance of quantitatively sensing the inflammatory marker neopterin (Neo). The obtained 1 comprises Eu₂(COO)₄ paddlewheel secondary building units, which could be bridged by BPTA^4– ligands to extend a 2D framework. The fluorescence titration indicates 1 can achieve simultaneous fluorescence behavior of Eu³⁺ ions and Neo via on–off ratiometric effects and thus could be exploited as the ratiometric fluorescence sensor matrix. Such a fluorescence phenomenon of 1 as a ratiometric sensor for quantitative detection of Neo via an on–off ratiometric effect is never observed in MOF chemistry. Moreover, naked-eye visible color variations of the fluorescence for 1 could be observed from red to blue with increasing concentrations of Neo, confirmed by fluorescent test strips as well as portable fluorescent hydrogels. And 1 also shows a low detection limit of 15.11 nM. A synergetic contribution of the competitive absorption, fluorescence resonance energy-transfer, and photoinduced electron-transfer mechanisms between Neo and the framework of 1 realizes the on–off ratiometric fluorescence behavior for Neo detection, supported by the UV–vis spectral overlap experiment and DFT calculations.