Heavy metal ions pollution has become of worldwide critical concern, thus, it is particularly important to monitor it in the environment and food for ensuring human health. In this study, p-phenylenediamine and 2-mercaptothiazoline were used to prepare nitrogen (N) and sulfur (S) co-doped carbon dots (N/SCDs) for fluorescent and colorimetric detection of Cu2+. The fabricated N/SCDs with bright green fluorescence showed excellent optical characteristics and favorable water solubility. In an aqueous system, a significant fluorescence quenching of N/SCDs at 512 nm is obtained in the presence of Cu2+. It also caused a significant colorimetric response with the color of prepared N/SCDs solution changed from colorless to yellow. Under optimal conditions, the analytical results showed that the linear range spanning from 5 to 400 μM, with a detection limit of 0.215 μM in fluorescence and 0.225 μM in colorimetric detection. In addition, N/SCDs displayed high selectivity toward Cu2+. No obvious interference was observed over other metal ions. Furthermore, we have also used N/SCDs to monitor Cu2+ in tap and lake water. The recovery of Cu2+ ranged between 89.6% and 113.1%. Exhibiting remarkable sensitivity and selectivity, the designed sensor offers a promising detection method for Cu2+ detection in the real sample.

The fluorescent and colorimetric dual-response sensor based on carbon dots doped with nitrogen and sulfur for detecting copper ions
    Abstract
    1 Introduction
    2 Experimental section
        2.1 Chemical reagents
        2.2 Characterizations
        2.3 Synthesis procedure of NSCDs
        2.4 Stability of CDs
        2.5 Fluorescence and absorbance analysis of Cu2+
        2.6 Selective detection of Cu2+
        2.7 Actual sample testing
    3 Results and discussion
        3.1 Characterization of NSCDs
        3.2 Optical characterization of the NSCDs
        3.3 Fluorometric detection of Cu2+
        3.4 UV–vis detection of Cu2+
        3.5 Selectivity analysis of NSCDs-based sensor
        3.6 Measurement of Cu2+ in real sample
    4 Conclusion
    Acknowledgements 
    References

• Mengxue Zhang(Institute of Business Scientific, Henan Academy of Sciences, Zhengzhou 450002, Henan, China)
• Hongna Yin(Institute of Business Scientific, Henan Academy of Sciences, Zhengzhou 450002, Henan, China)
• Libin Wan(Institute of Business Scientific, Henan Academy of Sciences, Zhengzhou 450002, Henan, China)
• Haidong Gao(Institute of Business Scientific, Henan Academy of Sciences, Zhengzhou 450002, Henan, China)
• Song Liu(Institute of Chemical Biology and Nanomedicine (ICBN), College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China)
• Yeru Liu(Institute of Business Scientific, Henan Academy of Sciences, Zhengzhou 450002, Henan, China)