Regulating Ratiometric Fluorescence of Nanoporous\nDual-Centered Lanthanide Metal–Organic Frameworks for Quantitative\nand Visual Detection of Anthrax Biomarkers

Developing\na sensitive, rapid, and accurate method for the quantitative\nand visual detection of the anthrax biomarker dipicolinic acid (DPA)\nis of great significance to human health and public safety. Due to\nthe presence of the “antenna effect”, the sensitization\nof Eu<sup>3+</sup> ions and Tb<sup>3+</sup> ions can be facilely achieved\nby the organic ligands with different degrees and exhibits excellent\nluminescence behavior of Eu<sup>3+</sup> ions and Tb<sup>3+</sup> ions,\nenabling a dual-centered lanthanide metal–organic framework\n(Ln-MOF) to be a promising ratiometric fluorescence sensor for the\ntrace of DPA determination. Herein, we present a series of nanoporous\ndual-centered lanthanide metal–organic frameworks, denoted\nas {Eu<sub><i>x</i></sub>Tb<sub>1–<i>x</i></sub>(BPDC)(μ<sub>3</sub>-OH)(H<sub>2</sub>O)·H<sub>2</sub>O·DMF}<sub><i>n</i></sub> (<b>Eu<i><sub>x</sub></i>Tb</b><sub><b>1–<i>x</i></b></sub><b>-BPDC</b>) assembled with 2,2′-bipyridine-4,4′-dicarboxylic\nacid (H<sub>2</sub>BPDC), as ratiometric fluorescence sensors for\nDPA. Among these, <b>Eu</b><sub><b>0.5</b></sub><b>Tb</b><sub><b>0.5</b></sub><b>-BPDC</b> stands out\nas the optimal material, exhibiting superior fluorescence performance\nfor quantitative, visual, and ratiometric DPA detection. Moreover, <b>Eu</b><sub><b>0.5</b></sub><b>Tb</b><sub><b>0.5</b></sub><b>-BPDC</b> demonstrates excellent fluorescence stability\nin aqueous solution and superior fluorescence capability for ratiometric\nDPA detection with remarkable sensitivity (limit of detection (LOD)\n= 0.608 μM), superior selectivity (<i>K</i><sub>sv</sub> = 6.46 × 10<sup>6</sup> M<sup>–1</sup>), excellent anti-interference,\nand impressive reusability. Due to the contribution of blocked energy\ntransfer between the mixed-lanthanide ions and competitive absorption\nin the presence of DPA, <b>Eu</b><sub><b>0.5</b></sub><b>Tb</b><sub><b>0.5</b></sub><b>-BPDC</b> demonstrates\na visual color change observation from red to greenish-yellow under\nthe illumination of ultraviolet light. Most importantly, the colorimetric\ndetection of DPA can be achieved by fluorescence test strips and portable\nhydrogels, facilitating the practical application of <b>Eu</b><sub><b>0.5</b></sub><b>Tb</b><sub><b>0.5</b></sub><b>-BPDC</b> as ratiometric fluorescence sensors for DPA detection.