Nitrogen, sulfur, phosphorus, and chlorine co-doped carbon nanodots as an "off-on" fluorescent probe for sequential detection of curcumin and europium ion and luxuriant applications.

Nitrogen, sulfur, phosphorus, and chlorine co-doped carbon nanodots (NSPCl-CNDs) were fabricated by acid-base neutralization and exothermic carbonization of glucose. The obtained NSPCl-CNDs possess excellent fluorescence properties and good biocompatibility. Curcumin (Cur) can dramatically quench the fluorescence of NSPCl-CNDs based on a synergistic effect of electrostatic interaction, inner filter effect, and static quenching, so a "turn-off" fluorescent probe for Cur detection was constructed with linear ranges of 0.24-13.16 μM and 13.62-57.79 μM. The LOD and LOQ of this fluorescent probe for Cur are 8.71 nM and 29.03 nM, respectively. More importantly, the fluorescence of the NSPCl-CNDs-Cur system can be recovered by europium ion (Eu3+), so a "turn-on" fluorescent probe for Eu3+ determination was established. The linear range, LOD, and LOQ for the detection of Eu3+ were 2.36-32.91 μΜ, 73.29 nM, and 244.30 nM, respectively. The proposed fluorescence methods were successfully utilized for Cur and Eu3+ determination in real samples with recoveries in the range 95.64-104.13% and 97.06-98.70%, respectively. Furthermore, the qualitative analysis of Cur can be realized by reagent strips with satisfying results. Finally, the as-constructed "off-on" fluorescent probe was successfully used to sequentially analyze Cur and Eu3+ at the cellular level. This method is simple and easy to implement, manifesting that NSPCl-CNDs have potential application value in fluorescent probing, food and drug testing, environmental monitoring, and cellular labeling. Graphical abstract.