Novel ionic liquid-functionalized carbon quantum dots (IL-CDs) were prepared by hydrothermal method, and characterized with FT-IR, TEM and XPS. The IL-CDs exhibited narrower particle size distribution with more uniform dispersion and the surface potential changes from negative to positive due to the function of IL. IL-CDs could be quenched (“turned off”) after adding ascorbic acid (AA), and as an “on–off”, fluorescent probe could be established for direct analysis AA. The linear range of AA was 0.34–30.00 μg/mL and the LOD was 0.11 μg/mL. The method was successfully applied to the determination of AA in real samples with satisfactory results.

    Abstract
    1 Introduction
    2 Experimental
        2.1 Reagents and instruments
        2.2 Synthesis of IL-CDs
        2.3 Fluorescence determination
        2.4 Measurement of fluorescence quantum yield
        2.5 Sample pretreatment
    3 Results and discussion
        3.1 Characterization of the IL-CDs
        3.2 Optical properties of IL-CDs
        3.3 Analytical application
        3.4 Mechanism of fluorescence
    4 Conclusions
    Acknowledgements 
    References

• Ouwen Xu(College of Guangling, College of Chemistry and Chemical Engineering, Yangzhou University)
• Shuyu Wan(College of Guangling, College of Chemistry and Chemical Engineering, Yangzhou University)
• Jing Yang(College of Guangling, College of Chemistry and Chemical Engineering, Yangzhou University)
• Hanyang Song(College of Guangling, College of Chemistry and Chemical Engineering, Yangzhou University)
• Ji Xia(College of Guangling, College of Chemistry and Chemical Engineering, Yangzhou University)
• Luzheng Dong(College of Guangling, College of Chemistry and Chemical Engineering, Yangzhou University)
• Xiashi Zhu(College of Guangling, College of Chemistry and Chemical Engineering, Yangzhou University)