Dual-Emitting Mixed-Lanthanide Metal–Organic Framework for Ratiometric and Quantitative Visual Detection of 2,6-Pyridine Dicarboxylic Acid

Abstract

The detection of the major biomarker of <i>Bacillus anthracis</i>, 2,6-dipicolinic acid (DPA), has attracted great interest in recent years. In this work, mixed-lanthanide metal-organic frameworks (M'LnMOFs), Tb_<i>x</i>Eu_1-<i>x</i>-cppa (cppa = 5-(5-carboxypyridin-3-yl)isophthalic acid), with different Tb/Eu ratios, were solvothermally synthesized. The results reveal that ratiometric fluorescent probe [Tb_0.533Eu_0.467-(Hcppa)_1.5(H₂O)(DMF)]·3H₂O is water and acid-base stable and exhibits excellent sensitivity (LOD = 2.286 μM), high selectivity, and fast response (<2 min) for the detection of DPA. Due to the blocked energy transfer from Tb³⁺ to Eu³⁺ and the inner filter effect upon the addition of DPA, the fluorescent probe shows a distinct color change from orange-red to green. Furthermore, the visual detection of DPA was realized by identifying the RGB values of MOF-based agarose hydrogel films via a smartphone, highlighting the practical application of the fluorescent probe for DPA detection under aqueous solution conditions.