Off - on fluorescent probe using sulfur-doped graphitic carbon nitride nanosheets for detection of copper ion and cysteine

• A novel off–on fluorescent probe was developed using S-g-C 3 N 4 NSs. • The probe detects Cu 2+ via fluorescence quenching through a PET mechanism. • Cysteine restores fluorescence by displacing Cu 2+ from the nanosheet surface. • Exhibits excellent sensitivity with LODs of 0.033 µM for Cu 2+ and 0.019 μM for Cys. • Successfully applied for Cu 2+ and Cys determination in real water and tablet samples. A sensitive off-on fluorescent probe was demonstrated using sulfur-doped graphitic carbon nitride nanosheets (S-g-C 3 N 4 NSs) for the sequential determination of copper ions (Cu²⁺) and cysteine (Cys). The S-g-C 3 N 4 NSs, synthesized via simple thermal polymerization method, exhibited strong fluorescence, which was selectively quenched by Cu²⁺ via a photoinduced electron transfer (PET) mechanism. The resulting S-g-C 3 N 4 /Cu 2+ complex then served as an effective platform for sensing Cys, which restored the fluorescence by competitively binding Cu²⁺. The probe demonstrated excellent sensitivity with low limit of detection (LOD) of 0.033 µM for Cu 2+ and 19 nM for Cys, high selectivity over potential interferents, and good stability. The method was successfully applied for the determination of Cu 2+ in tap water and Cys in multivitamin tablets, achieving satisfactory recoveries, which confirms its potential for practical environmental and biological monitoring.