▎ 摘　　要

Background: The preparation and biological applications of ultra-small graphene quantum dots (GQDs) with accurate-controlled size are of great significance. Methods: Here in, we report a novel procedure involving pyrolysis of trisodium citrate and subsequent ultrafiltration for fabricating monolayer GQDs with ultra-small lateral size (1.3 +/- 0.5 nm). Results: The GQDs exhibit blue photoluminescence with peak position independent of excitation wavelength. The quantum yield of GQDs is measured to be 3.6%, and the average fluorescence lifetime is 2.78 ns. Conclusion: Because of high stability and low toxicity, GQDs are demonstrated to be excellent bioimaging agents. The ultra-small GQDs can not only distribute in the cytoplasm but also penetrate into the nuclei. We ensure that this work will add a new dimension to the application of graphene materials for nanomedicine.