<p>Mercury ions (Hg²⁺) are categorized as environmental pollutants, which distributed in water, soil, and food systems due to environmental contamination. Hence, designing a sensitive assay for the convenient determination of Hg²⁺ is of great importance. Herein, S and O-doped graphite phase nitrogenized carbon quantum dots (S, O-C₃N₄QDs) was encapsulated within a europium -based metal-organic framework (Eu-MOF) to construct a novel ratiometric fluorescent nanoprobe for the quantitative detection of Hg²⁺. The native emission of S, O-C₃N₄QDs at 445&#xa0;nm is used as a response signal, while Eu-MOF with fluorescence offers a reference signal at 619&#xa0;nm. Hg²⁺ exhibits high affinity for the surface functional groups of S/O-C₃N₄ QDs, forming non-fluorescent chelation complexes that induce static quenching. This results in significant attenuation of the fluorescence intensity at 445&#xa0;nm, while the emission at 619&#xa0;nm remains invariant. A ratiometric fluorescence sensing platform was established based on the intensity ratio (F₄₄₅/F₆₁₉) for the selective detection of Hg²⁺. The linear range of S, O-C₃N₄QDs/Eu-MOF of Hg²⁺ was 0.25–35 µM and with a detection limit of 4.3 nM. The satisfying results demonstrate the effectiveness of the developed S, O-C₃N₄QDs/Eu-MOF-based fluorescence probe for Hg²⁺ detection, highlighting its promising potential for environmental monitoring applications.</p>