▎ 摘　　要

Owing to the superior photoluminescence property, low toxicity and good biocompatibility, nitrogen-doped graphene quantum dots (NGQDs) have been regarded as promising nanomaterials for biological applications such as bioimaging. However, many of the preparation methods are complicated, high cost, eco-unfriendly, and with a low product yield. Here, we demonstrate a novel top-down approach for NGQDs preparation, in which the low cost graphite was used as a precursor, ammonium persulfate as an oxidative molecule and nitrogen source, and H2O2 as an oxidative agent, N-methyl-2-pyrrolidone as a solvent and potential functionalizer. Meanwhile, the solvent extraction was applied for the first time to purify NGQDs. The separated NGQDs display green and blue fluorescence, deriving from the difference sizes and nitrogen doped types. The total product yield of NGQDs calculated to be about 52%, containing 88% of green-emissive NGQDs and 12% of blue-emissive NGQDs. Meanwhile, our NGQDs own low cytotoxicity, and display a good bioimaging performance in the in vitro and in vivo investigation. The synthesis idea in our work might be also applicable to obtain other kinds of quantum dots from the readily obtainable bulk materials.