This paper is devoted to synthesizing a new type of CDs (carbon dots) with excellent NIR (near-infrared) emission in a biological water environment synthesized from small molecules. Citric acid was adopted as the precursor and treated by one-pot hydrothermal process in DMF solution with the assistance of a microwave. Urea (MH) and ammonium fluoride (MF) were adopted as nitrogen sources to synthesize two types of CDs, respectively. These conditions contributed to generate nanostructured carbon with a higher content of Pyrrolic-N, enrich the functional groups, and exfoliate the ordered layerstacking structure, which finally contributed to the higher NIR absorption band at 808 nm. The physicochemical properties and photothermal conversion ability were fully evaluated by UV–Vis-NIR (ultraviolet–visible light-NIR) absorption and photothermal experiments. MF possessed stronger absorption property and temperature-rising effect in the NIR region than MH, but both exhibited desirable photothermal stability. Next, the in vitro and in vivo experiments demonstrated that both MF and MH exhibited no significant toxicity for cells. NIR irradiation on CDs solution displayed an excellent killing effect on HeLa (breast cancer) and MCF7 (cervical cancer) cells but strongly depended on the concentration of CDs. MH had a weaker killing effect on MCF7 cells compared with MF in the same concentration. But HeLa cells suffered death from lower concentration of MH under NIR irradiation. Both MH and MF exhibited excellent therapy effects and no obvious tissue damage for these major organs of nude mice and BALB/C mice. Above all, both MF and MH with excellent photothermal effect under NIR irradiation had desirable NIR-triggered therapeutic effect on MCF7 and HeLa cells, while they also exhibited good biocompatibility without NIR irradiation.

Facile microwave hydrothermal synthesis of citric acid-derived carbon dots for photothermal therapy of cancers under NIR irradiation
    Abstract
    1 Introduction
    2 Materials and experimental methods
        2.1 Synthesis of CDs.
        2.2 Chemical and Optical Characterizations of CDs.
        2.3 Photothermal Performance Measurement.
        2.4 In Vitro Cytotoxicity Assay and Photothermal Ablation.
        2.5 In Vivo Experiments.
    3 Results and discussion
    4 Conclusions
    Acknowledgements 
    References

• Yingying Jin(Tianjin Medical University Cancer Institute and Hospital, National Clinical Research Center for Cancer, Tianjin’s Clinical Research Center for Cancer, Tianjin Key Laboratory of Digestive Cancer, Key Laboratory of Cancer Prevention and Therapy, Tianjin 300060, People’s Republic of China)
• Huanhuan Qiao(Tianjin Key Laboratory of Brain Science and Neural Engineering, Academy of Medical Engineering and Translational Medicine, Tianjin University, Tianjin 300072, People’s Republic of China)
• Yichi Zhang(School of Environmental Science and Engineering, Tianjin Key Lab of Biomass/Wastes Utilization, Tianjin University, Tianjin 300072, People’s Republic of China)
• Yujia He(School of Environmental Science and Engineering, Tianjin Key Lab of Biomass/Wastes Utilization, Tianjin University, Tianjin 300072, People’s Republic of China)
• Shuangning Xie(School of Environmental Science and Engineering, Tianjin Key Lab of Biomass/Wastes Utilization, Tianjin University, Tianjin 300072, People’s Republic of China)
• Yiwen Gu(School of Environmental Science and Engineering, Tianjin Key Lab of Biomass/Wastes Utilization, Tianjin University, Tianjin 300072, People’s Republic of China)
• Fawei Lin(School of Environmental Science and Engineering, Tianjin Key Lab of Biomass/Wastes Utilization, Tianjin University, Tianjin 300072, People’s Republic of China) Corresponding author