This study presents a novel photodynamic therapy (PDT) agent based on graphene quantum dots (GQDs) that exhibits high singlet oxygen (¹O₂) generation efficiency, making it a promising candidate for cancer treatment. The GQDs, synthesized using a hydrothermal method with polythiophene derivatives, display broad absorption in the UV-visible range and strong deep-red emission. They demonstrate excellent biocompatibility, high ¹O₂ quantum yield (~1.3), and superior photostability and pH resistance, surpassing conventional PDT agents. In vitro and in vivo studies show that GQDs can be used for simultaneous imaging and efficient cancer therapy. The GQDs generate ¹O₂ through a multistate sensitization process, which involves energy transfer from both the singlet and triplet states of the GQDs to ground-state oxygen, leading to high ¹O₂ quantum yield. The GQDs also show strong photostability and good biocompatibility, making them suitable for biomedical applications. The study highlights the potential of GQDs as a new generation of carbon-based nanomaterials for PDT, with superior performance in terms of ¹O₂ generation, water dispersibility, photo- and pH-stability, and biocompatibility. The results suggest that GQDs could be used for efficient, environment-friendly, and visible-light-responsive photocatalytic degradation of persistent organic pollutants and microorganisms.This study presents a novel photodynamic therapy (PDT) agent based on graphene quantum dots (GQDs) that exhibits high singlet oxygen (¹O₂) generation efficiency, making it a promising candidate for cancer treatment. The GQDs, synthesized using a hydrothermal method with polythiophene derivatives, display broad absorption in the UV-visible range and strong deep-red emission. They demonstrate excellent biocompatibility, high ¹O₂ quantum yield (~1.3), and superior photostability and pH resistance, surpassing conventional PDT agents. In vitro and in vivo studies show that GQDs can be used for simultaneous imaging and efficient cancer therapy. The GQDs generate ¹O₂ through a multistate sensitization process, which involves energy transfer from both the singlet and triplet states of the GQDs to ground-state oxygen, leading to high ¹O₂ quantum yield. The GQDs also show strong photostability and good biocompatibility, making them suitable for biomedical applications. The study highlights the potential of GQDs as a new generation of carbon-based nanomaterials for PDT, with superior performance in terms of ¹O₂ generation, water dispersibility, photo- and pH-stability, and biocompatibility. The results suggest that GQDs could be used for efficient, environment-friendly, and visible-light-responsive photocatalytic degradation of persistent organic pollutants and microorganisms.