Nitrogen (N) doped CDs (N-CDs) were prepared using citric acid and hexamethylenetetramine by a one-step hydrothermal method, characterized by transmission electron microscopy (TEM), Powder X-ray Diffractometer (PXRD), Nuclear Magnetic Resonance (13C NMR), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR), and ultraviolet–visible (UV–vis) absorption spectra. The doping mechanism and drug-loading characteristics of N-CDs were also studied. The results showed that N-CDs had both down- and up-conversion fluorescence properties and the average particle size was about 4.28 nm. N-CDs could be used as a drug carrier for doxorubicin (DOX) (N-CDs-DOX) via the formation of the NC bond, and the drug-loading rate of N-CDs-DOX was 35.88%. The N-CDs-DOX had pH-targeted properties and low cytotoxicity. In vitro release of N-CDs-DOX belonged to the Weibull model and Fick diffusion. The results showed that N-CDs could deliver DOX successfully, and collocate with DOX in the SH-SY5Y cells through two-photon imaging. The N-CDs-DOX system had sustained-release and pH-targeted properties. N-CDs were expected to deliver a type of drugs containing CO group and be used in cancer treatment.
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