In this work, several g-C₃N₄ samples have been synthesized by thermal co-polymerization of melamine and urea precursors, analyzing the impact and advantages of the combination of both precursors, and finally choosing the sample with the highest fluorescent emission to study the detection of Ag⁺, Fe³⁺, and Cr(VI) species. The g-C₃N₄ samples were characterized by several techniques such as XRD, FTIR, DRS, SEM, and PL where the excitation wavelength (λ_exc) was set at 250 nm for monitoring the quenching of the emission band centered at 371 nm. In this sense, the 10U-g-C₃N₄ sample was examined for the detection of metal species in aqueous media, showing excellent correlation coefficients (R² > 0.988) and limits of detection (LOD) of 7.2, 8.4, and 1.9 μmol/L for Fe³⁺, Ag⁺, and Cr(VI), respectively. The sensing of Ag⁺ and Fe³⁺ ions can be related to a photoinduced electron transfer (PET) mechanism, while the detection of Cr(VI) species is associated with an inner filter effect (IFE). The results show that the graphitic carbon nitride obtained by a co-polymerization route yields better results than g-C₃N₄ obtained by the calcination of single precursors. This approach enhances the fluorescence sensing of different metal species with a simple synthetic procedure and avoids complex, harmful, or expensive processes.