Coordination-Regulated Ratiometric Fluorescent Europium(III) Metal–Organic Framework for Smartphone-Integrated Visual Detection of Anthrax Biomarker in Real Samples

Regulating host–guest interactions between targeted analytes and lanthanide(III)-based metal–organic frameworks (Ln-MOFs) provides an effective strategy for achieving dynamic fluorescence behavior, enabling Ln-MOFs with attractive ratiometric sensing behavior. In this work, we demonstrate a 3D chain-based Eu-MOF {[Eu 4 (NBC) 6 ·(μ 2 -H 2 O) 2 ·(H 2 O) 4 ]·2H 2 O} n, constructed from 2,6-naphthalenedicarboxylic acid (H 2 NBC), which exhibits fluorescent dynamics behavior for the online and visual detection of anthrax biomarker 2,6-dipicolinic acid (DPA) via a coordination-induced strategy. Owing to photoinduced electron transfer (PET), competitive absorption, and coordination interactions, the Eu-MOF displays dual-emission signals from H 2 NBC ligands and Eu 3+ centers, endowing it with superior ratiometric fluorescence sensing performance, including exceptional selectivity and high sensitivity ( Ksv = 1.90 × 10 4 M –1 ), low LOD of 0.080 μM, and rapid response (60 s). Furthermore, a distinct colorimetric transition from red to blue under UV light enables naked eye detection of DPA, facilitating the development of portable Eu-MOF hydrogels for real-time monitoring. Additionally, a smartphone-integrated platform utilizing RGB analysis is developed for online DPA detection, while a Eu-MOF@PU film is designed for practical visual trace detection. Notably, the Eu-MOF demonstrates excellent recovery rates of 97.7–105.9% in tap water and 99.7–111.3% in Poyang Lake water for DPA detection, confirming its reliability for real-world applications.