In summary, the results manifested that when modified with different chemical groups, GQDs still possessed excellent biocompatibility and low cytotoxicity to cells, which may make them more promising in bioimaging and other biomedical applications. Authors’ information XY, MJ, and XW are master’s degree candidates. ZL is a researcher assistant, and YJ is an associate researcher. ZG is a deputy director and professor. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 61275187, No. 61378089, and No. 61335011), Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20114407110001 and No. 200805740003), RAD001 and

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