<p>Nitrogen and sulfur co-doped graphene quantum dots (N,S-GQDs) have attracted considerable attention owing to their large surface area, high conductivity, low toxicity, excellent photostability, biocompatibility, and abundant surface functional groups. Developing mild, low-cost, and environmentally sustainable synthesis routes using renewable precursors remains a major challenge. Here, a green and feasible strategy is demonstrated for synthesizing N,S-GQDs via a mild H₂O₂-assisted oxidative cleavage process. Nitrogen-sulfur co-doped porous carbon was prepared using orange-peel biomass waste as the carbon source and methylene blue as the dual N/S dopant. By adjusting the ratio of orange peel to methylene blue, the microstructure of the porous carbon was modulated, enabling the synthesis of N,S-GQDs with tunable fluorescence from green to yellow. The as-synthesized N,S-GQDs exhibited high dispersibility, excellent photostability, abundant nitrogen-sulfur functional groups, and stronger fluorescence intensity in the green-emissive sample. A turn-off fluorescence sensor constructed using the green N,S-GQDs achieved selective dopamine detection with a broad linear range and demonstrated applicability in urine sample analysis. This study provides a sustainable approach for converting biomass waste into fluorescent N,S-GQDs, offering prospects for renewable carbon utilization and biosensing applications.</p> Graphical Abstract <p></p>