Perylene‐Rhodamine Nanoparticles With FRET‐Driven “Off–On” Fluorescence for Selective and Sensitive Detection of Hg ²⁺ in Water

Mercury contamination remains a critical environmental challenge, demanding fast, selective, and portable detection systems. Herein, we report the fabrication of perylene‐rhodamine organic nanoparticles ( PerRho‐ONPs ) as efficient FRET‐based molecular probes for the selective detection of Hg 2+ ions in aqueous media. The hybrid fluorophore was synthesized via amidation between a perylene derivative and rhodamine ethylenediamine, followed by nanoprecipitation to generate uniform organic nanoparticles. Comprehensive characterization using UV–Vis, fluorescence spectroscopy, NMR, FTIR, TEM, and MALDI‐TOF MS confirmed their structural integrity and nanoscale morphology. PerRho‐ONPs displayed a pronounced “off–on” fluorescence switching behavior in response to Hg 2+ among 18 competing metal ions, accompanied by a distinct colorimetric change. The system exhibited high selectivity and sensitivity with a detection limit of 0.36 µM and a 1:2 binding stoichiometry confirmed by Job's plot analysis. Density functional theory (DFT) calculations revealed strong coordination between Hg 2+ and the oxygen and nitrogen donor sites of the rhodamine moiety, leading to spirolactam ring opening and efficient intramolecular energy transfer. The combined experimental and theoretical results highlight PerRho‐ONPs as a simple, robust, and water‐dispersible nanosensor for mercury monitoring in environmental and Mercury Sensor, Perylene, Rhodamine, nanoaggregate, organic nanoparticlesbiological systems.