Facile sulfur-doping by self-photosensitization of graphene quantum dots in DMSO for improved sensing

A photosensitization-based synthesis strategy is established enabling the room-temperature preparation of sulfur-doped graphene quantum dots (S-GQDs). Under 365-nm UV irradiation, GQDs produce singlet oxygen (¹O₂), catalyzing the oxidation of dimethyl sulfoxide (DMSO) into sulfur-containing intermediates, which subsequently react with surface functional groups on GQDs to achieve precise sulfur doping. The introduced defect states significantly enhanced radiative recombination, raising the quantum yield to 29.5%. The S-GQDs showed outstanding stability, including superior photobleaching resistance (minimal fluorescence change after 3.5 h UV exposure), high thermal stability (minimal variation from 20 to 80 °C), excellent pH tolerance (< 10% fluctuation within pH 3-12), and stable fluorescence in high-salinity and long-term storage conditions. Utilizing photoinduced electron transfer (PET) and fluorescence resonance energy transfer (FRET) mechanisms, the S-GQDs exhibited sensitivity and selectivity toward picric acid (PA), with a linear response between 1 and 60 μM and a detection limit of 0.56 μM. Practical tests confirmed reliable detection of PA on human finger surfaces, and the results were in good  agreement with those obtained by high-performance liquid chromatography (HPLC). This photosensitization-based oxidation strategy provides a green, scalable route for preparation of high-performance S-GQDs that have broad potential applications in environmental monitoring, bioanalysis, etc.