Fluorine-containing graphene quantum dots with a high singlet oxygen generation applied for photodynamic therapy
Recently, graphene quantum dots (GQDs) have been extensively studied in biomedical areas such as bio-imaging, bio-sensing and photothermal therapy due to their superior optical and physiochemical properties compared to traditional organic biomarkers. Application of GQDs in photodynamic therapy (PDT) has been explored since 2014, but currently the main challenges are inadequate singlet oxygen (1O2) quantum yield (QY), poor solubility and biocompatibility. Herein, we report on the synthesis of a new kind of fluorine-containing GQDs (F-GQDs) by an oxidative cutting method using fluorinated graphite as the raw material. The as-synthesized F-GQDs sample demonstrates an average particle size of 2.1 nm with the fluorine doping content of 1.43%. The F-GQDs have a more excellent water solubility and biocompatibility than the GQDs, and emit strong green fluorescence at 365 nm excitation with the relative fluorescence QY of 13.72%. Moreover, the fluorescence imaging effect as well as photodynamic activity successfully tested in both in vitro HepG2 cell line model and a 3D multicellular spheroids model that mimics the tumour microenvironment. Further studies using UV-visible spectroscopy through the degradation of water-soluble 9,10-anthracenediyl-bis(methylene) dimalonic acid(ABDA) demonstrate that the F-GQDs sample generate 1O2 efficiently (QY = 0.49) under a visible light irradiation. Compared to non-fluorinated GQDs, the as-reported F-GQDs manifest to be a more promising photosensitizer for image-guided PDT.