A Rhodamine–Phenothiazine-Based Dual Colorimetric and Fluorometric Probe for Highly Selective Detection of Hg²⁺ Ions

The selective and sensitive detection of mercury(II) ions (Hg²⁺) remains a significant challenge because of their high toxicity, bioaccumulation, and severe environmental and health impacts. Herein, we report the design and synthesis of a novel rhodamine–phenothiazine conjugate as a highly selective fluorescent and colorimetric probe for the rapid detection of Hg²⁺ ions. The probe integrates the excellent fluorescence switching behavior of the rhodamine spirolactam framework with the electron-rich phenothiazine donor, enabling efficient signal transduction upon metal-ion recognition. In the absence of Hg²⁺, the probe remains in the nonfluorescent, colorless spirolactam form. Upon coordination with Hg²⁺, selective ring opening of the spirolactam generates a pronounced color change from colorless to pink along with a significant fluorescence enhancement, allowing both naked-eye and fluorometric detection. The probe exhibits remarkable selectivity toward Hg²⁺ over a wide range of competing metal ions, excellent sensitivity with a low detection limit, and a rapid response under physiological conditions.  Moreover, the probe displays excellent reversibility in the presence of suitable chelating agents and demonstrates practical applicability for the detection of Hg²⁺ in environmental water samples with satisfactory recovery. Owing to its high selectivity, rapid response, and dual colorimetric–fluorometric output, this rhodamine–phenothiazine probe represents a promising platform for the facile monitoring of mercury contamination in environmental and biological systems.