Regulating the Energy Level of a Ratiometric Luminescent Europium(III) Metal–Organic Framework Sensor with Smartphone Assistance for Real-Time and Visual Detection of Carcinoid Biomarker

Regulating the energy level of a lanthanide (III) metal-organic framework (Ln-MOF) via an "antenna effect" is an effective strategy for achieving the ratiometric luminescent dynamics carcinoid biomarker sensing application. Herein, we present a ratiometric luminescent europium-based MOF material {[Eu(BPDC)(μ₃-OH)(H₂O)₂]·2.5H₂O}<i>_n</i> (<b>Eu-MOF</b>, H₂BPDC = 2,2'-bipyridine-4,4'-dicarboxylic acid and DMF = <i>N,N</i>-dimethylformamide), incorporating tetranuclear cubane-like [Eu₄(μ₃-OH)₄(COO)₄]^4- cluster cores, for carcinoid biomarker 5-hydroxyindole-3-acetic acid (5-HIAA) detection through the influence of Eu³⁺ ions sensitized by H₂BPDC ligands as well as 5-HIAA. Luminescence titration experiments indicate <b>Eu-MOF</b> can demonstrate simultaneous luminescence emission signals of Eu³⁺ cations and 5-HIAA, resulting in on-off ratiometric luminescent dynamics 5-HIAA detection in response to 5-HIAA concentrations and demonstrating superior luminescence stability and excellent luminescence emission performance such as low detection limit (LOD = 37.16 nM), high selectivity (<i>K</i>_sv = 2.39 × 10⁴ M^-1), fast response (≤1 min), and attractive recyclability (≥five times). Derived from the contribution of the competitive absorption and coordination interactions between Eu³⁺ cations of <b>Eu-MOF</b> and 5-HIAA, <b>Eu-MOF</b> exhibits a visually observable color change from red to purple by the naked eye illuminated by ultraviolet light, confirmed by the fabrication of portable luminescent hydrogels. More importantly, smartphone assistance based on RGB chromaticity analysis has been developed for the intelligent detection of 5-HIAA, and <b>Eu-MOF@PVA</b> and <b>Eu-MOF@PU</b> films have been proposed for real-time and visual detection of 5-HIAA, thereby enhancing the practical applicability of <b>Eu-MOF</b> in the quantitative trace detection of 5-HIAA.