Bright and Selective: A Dicarboxylate‐Functionalized AIE Sensor for Th4+ in 100% Water with a Sub‐ppm Level Sensitivity

Thorium (Th), a naturally occurring radioactive element, poses long‐term health and environmental risks due to the radiotoxicity of its decay products and prolonged biological retention, necessitating its accurate detection in water systems. Existing fluorescence‐based sensors often suffer from poor water solubility, background emission, and matrix interferences, limiting their applicability in real aqueous environments. Herein, a novel turn‐on fluorescent sensor for Th 4+ in 100% aqueous media, based on a dicarboxylated tetraphenylethylene (TPE‐DCA) probe is reported. Th 4+ binding induces aggregation and results in a 180‐fold emission enhancement at 490 nm. The sensor operates optimally near neutral pH and achieves a detection limit of 11 n m (3 ppb), well below the WHO guideline for drinking water (245 ppb). It shows negligible interference from competing ions, including lanthanides and uranium, and performs robustly in tap, lake, and seawater samples. This work offers a simple, sensitive, and field‐relevant strategy for thorium monitoring in diverse environmental matrices.