**4. Conclusions**

In summary, MPA-capped MPA-InP/ZnS QDs were synthesized by a solvothermal method; they exhibited bright yellow fluorescence and excellent monodispersity in aqueous solution. A highly sensitive and highly selective fluorescent probe for the detection of trace Cu2+ was developed using the synthesized MPA-InP/ZnS QDs. The fluorescence intensity of the MPA-InP/ZnS QDs can be quenched significantly in the presence of Cu2+ due to the SQE, which showed an excellent linear relationship with Cu2+ in the concentration range of 0–1000 nM, with a detection limit of 0.22 nM. Furthermore, the probe was applied to the detection of Cu2+ in environmental water and drink samples, indicating that MPA-InP/ZnS QDs could be used as a fluorescence probe in the application of highly sensitive and rapid detection of trace Cu2+ in real water.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/foods10112777/s1, Figure S1: The response curves of InP/ZnS QDs solutions with concentrations of 6 nM, 10 nM, 14 nM and 18 nM to different concentrations of Cu2+, Figure S2: The data comparison image of the detection results of the two methods, Figure S3: The results comparison image of this work and literature.

**Author Contributions:** Conceptualization, Z.X. and J.Z.; experimental design, Z.X.; data curation, Z.X.; formal analysis, Z.X.; investigation, Z.X.; methodology, Z.X. and J.Z.; writing—original draft, Z.X.; conceptualization, J.Z.; writing—review and editing, J.Z.; formal analysis, C.S.; supervision, C.S. and Y.W.; funding acquisition, X.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Research and Development Projects in Key Areas of Guangdong Province (grant number 2019B020225001) and Young Beijing Scholar.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
