Carbon dots (CDs) with tunable fluorescence emissions have been developed from a wide range of small organic molecules with various bottom-up syntheses. However, most of them were prepared under high temperatures and high pressures with long reaction times and tedious purification processes. In addition, previously reported carbon dots frequently displayed excitation-dependent emissions, which restrict their further applications. Herein, we present a simple and rapid microwaveassisted solvothermal synthesis of multicolour carbon dots with excitation-independent emissions. In ethylene glycol, the green (G)-CDs emitting at 537 nm with a quantum efficiency (QY) of 15% were obtained by using a single precursor of phloroglucinol, and blue (B)- and yellow (Y)-CDs emitting at 436 nm and 557 nm with QYs of 55% and 28% were derived with additives of o- and m-phenylenediamine, respectively. Analyses of their chemical structures and optical processes suggest that highly polymeric carbon dots were uniformly formed from the small molecules and their fluorescences were predominantly originated from rapid direct recombination. Furthermore, emissions at different wavelengths were mainly attributed to different degrees of oxidation (13.9%, 15.2% and 16.4% oxygen in B-, G- and Y-CDs, respectively) and different proportions of pyrrolic nitrogen (10.4% and 1.40% in B- and Y-CDs, respectively). To demonstrate the application feasibility, the obtained carbon dots were utilized for ion detection and anti-counterfeiting. Based on static quenching of the carbon dots’ fluorescence, micro amounts of ferric ion in water samples were detected selectively and reproducibly. Moreover, the anti-counterfeiting pattern constructed by the carbon dots emitted fluorescence under ultraviolet illumination, but concealed perfectly under daylight. This achievement is of great potential for developing multicolour carbon dots of high qualities.

    Abstract
        Graphical abstract
    1 Introduction
    2 Experimental
        2.1 Chemicals
        2.2 Synthesis of CDs
        2.3 Characterizations
        2.4 Fe3+ ion detection and anti-counterfeiting
    3 Results and discussions
        3.1 Synthesis and structural analysis of carbon dots
        3.2 Optical properties and mechanisms of carbon dots
        3.3 Applications for Fe3+ ion detection and anti-counterfeiting
    4 Conclusion
    Acknowledgements 
    References

• Xiao‑Lan Wei(Chongqing Key Laboratory of Catalysis and Functional Organic Molecule, College of Environment and Resources, Chongqing Technology and Business University)
• Qin‑Ling Shi(Chongqing Key Laboratory of Catalysis and Functional Organic Molecule, College of Environment and Resources, Chongqing Technology and Business University)
• Lan Jiang(Chongqing Key Laboratory of Catalysis and Functional Organic Molecule, College of Environment and Resources, Chongqing Technology and Business University)
• Yu Qin(Chongqing Key Laboratory of Catalysis and Functional Organic Molecule, College of Environment and Resources, Chongqing Technology and Business University)