Nitrogen-doped carbon dots (CDts) with tunable fluorescence properties in aqueous media were synthesized hydrothermally. The excitation wavelength variation to obtain the maximum emission produced a blue shift in the emission peaks upon dilution in an aqueous solution. The shift can be explained by a re-absorption phenomenon in a concentrated solution. The interparticle interaction within was responsible to show dilution-dependent optical behavior. The as-synthesized solution of CDts did not show any prominent absorption peak over a wide range. However, upon dilution, two peaks became predominant. The concentration-dependent behavior was observed during the interaction with metal cations. Cationic salts of Co(II) and Hg(II) caused quenching at different dilutions of CDts. This might be explained by the exposure of different surface functional groups during dilution and metal-ion–CDts charge transfer. The quenched fluorescence of CDts was rescued using ascorbic acid. Therefore, the one-pot detection of Co(II)/Hg(II) and ascorbic acid was designed through a ‘Turn Off/On’ phenomenon.

    Abstract
    1 Introduction
    2 Experimental section
        2.1 Synthesis of nitrogen-doped carbon dots (CDts)
        2.2 Designing the sensing platform
        2.3 Real sample application
    3 Results and discussion
        3.1 Synthesis and characterization of as-synthesized CDts
        3.2 Effect of solvent on CDts
        3.3 Sensing of metal ions
        3.4 Detection of Co(II)
        3.5 Hg(II)AA detection
        3.6 Importance of the work
    4 Conclusion
    Acknowledgements 
    References

• Jayasmita Jana(School of Chemical Engineering, University of Ulsan)
• Seung Hyun Hur(School of Chemical Engineering, University of Ulsan)