Nanozyme-driven ratiometric monitoring of uranyl ions in aquatic systems: Mechanisms, environmental validation, and bioaccumulation assessment.

Uranium contamination in aquatic ecosystems poses significant ecological and public health risks due to its chemical toxicity and radiological persistence, highlighting an urgent need for reliable on-site monitoring tools. We present a self-calibrating dual-mode nanozyme platform through a coordination-driven assembly mechanism. Iron-doped carbon dots (Fe-CDs) act as a dual-functional probe, combining intrinsic fluorescence with peroxidase-like activity to catalyze oxidation of o-phenylenediamine (OPD) into yellow-emitting 2,3-diaminophenazine (DAP). Notably, an inner filter effect (IFE) occurs due to spectral overlap between DAP absorption and Fe-CDs emission, quenching the blue fluorescence of Fe-CDs and activating yellow emission from DAP. DFT calculations confirm that UO22 + preferentially coordinates with the phenazine N in DAP, triggering supramolecular assembly. This process induces a spectroscopic shift that disrupts the IFE-quenching the yellow fluorescence of DAP while restoring the blue fluorescence of Fe-CDs. This self-calibration significantly enhances reliability by minimizing false signals under complex conditions. The platform was successfully applied to real marine samples with excellent recoveries (97.0-102.0 %), and visualize neurotropic accumulation of UO22+ in zebrafish tissues (head > viscera > muscle). This work provides a robust effective tool for accurate uranium monitoring in environmental matrices and offers valuable insights into its biogeochemical behavior and ecological impact.