<p>A simple, fast and sensitive method based on the fluorescence quenching of carbon dots was developed for the determination of Mg²⁺ in environmental and biological sample. Carbon dots (CDs) were synthesized from arugula seeds by hydrothermal method. The precursor is cheap, easily available and environment friendly. CDs were characterized by selected techniques. They showed a bright blue fluorescent under UV light while under day light the synthesized CDs showed brown colour. The CDs were highly fluorescent. The interaction of CDs with various selected cations such as Ni²⁺, Sr²⁺, Cd²⁺, Mg²⁺, Cu²⁺, Al³⁺, Co²⁺, K⁺, Ca²⁺, Pb²⁺, Cr³⁺, Na⁺, Zn²⁺, Fe²⁺, Cu⁺, Mn²⁺, Mo⁶⁺, Sb³⁺, Sn⁴⁺, Cr⁶⁺, Ba²⁺, Li⁺ and Fe³⁺ was studied to check the probable potential application for the studied ions. All the studied ions did not show any effect on the intensity of CDs except Mg²⁺ which quenched the intensity of the CDs. A linear relation was found between the quenching of CDs intensity and Mg²⁺ concentration. The quenching was explained with the Stern-Volmer equation that produced the Stern-Volmer constant (<InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\:{K}_{sv}\)</EquationSource> </InlineEquation>). <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(\:{K}_{sv}\)</EquationSource> </InlineEquation>, detection limit (DL) and quantification limit (QL) were observed in the order 2.85 × 10⁵ mol L^− 1, 32.6 n mol L^− 1 and 108 n mol L^− 1 correspondingly. Method based on the fluorescence quenching of CDs is reproducible and useful for the Mg²⁺ determination in environmental and biological samples.</p> Graphical Abstract <p></p>