Abstract The fluorescence quenching of carboxyl-rich g-C₃N₄ nanoparticles was found to be selective to Ag⁺ and Ce³⁺ with a limit of detection as low as 30 pM for Ag⁺ ions. A solid-state thermal polycondensation reaction was used to produce g-C₃N₄ nanoparticles with distinct green fluorescence and high water solubility. Dynamic light scattering indicated an average nanoparticle size of 95 nm. The photoluminescence absorption and emission maxima were centered at 405 nm and 540 nm respectively which resulted in a large Stokes shift. Among different metal ion species, the carboxyl-rich g-C₃N₄ nanoparticles were selective to Ag⁺ and Ce³⁺ ions, as indicated by strong fluorescence quenching and a change in the fluorescence lifetime. The PL sensing of heavy metal ions followed modified Stern–Volmer kinetics, and CNNPs in the presence of Ag⁺/Ce³⁺ resulted in a higher value of K _app (8.9 × 10⁴ M⁻¹) indicating a more efficient quenching process and stronger interaction between CNNP and mixed ions. Sensing was also demonstrated using commercial filter paper functionalized with g-C₃N₄ nanoparticles, enabling practical on-site applications.