A Portable Smartphone-Assisted Fluorescence Sensing Platform Based on Molecularly Imprinted Lanthanide MOF for Rapid Determination of Thiamethoxam in Vegetables

Thiamethoxam is a widely used neonicotinoid insecticide in agriculture, and its excessive use has raised growing concerns about food safety and environmental pollution. In this study, a portable fluorescence sensing platform was developed for the rapid detection of thiamethoxam by integrating a molecularly imprinted polymer-coated lanthanide metal–organic framework (Eu/Tb-MOF@MIP) with a smartphone-based imaging system. The synthesized Eu/Tb-MOF@MIP exhibited excellent stability in aqueous media and high thermal stability up to 270 °C, attributed to the protective imprinted polymer layer. Owing to the dual recognition mechanism arising from the ligand groups of the Ln-MOF and the specific imprinted cavities, the platform also displayed excellent selectivity toward thiamethoxam, with an imprinting factor of 3.06. Under optimized conditions, the platform enabled sensitive detection of thiamethoxam over a linear range of 0.01–9 mg·L−1, with a detection limit of 9.3 μg·L−1. When applied to spiked vegetable samples, the method achieved recoveries of 76.0–95.3% with relative standard deviations (RSDs) of 1.24–5.12%. Mechanistic investigation revealed that the fluorescence quenching was primarily attributable to the inner filter effect (IFE) and analyte-induced excited-state quenching, with the latter likely enhanced by MIP-mediated enrichment.