**4. Conclusions**

In this work, by using sulfur powder as a precursor and polyethylene glycol as a stabilizer, SQDs were prepared successfully through a microwave-assisted method. The synthesis time of the SQDs was greatly shortened to only 90 min. The synthesized SQDs had good water solubility and good fluorescence stability. A sensitive and selective fluorescence assay based on IFE for detection of ALP was developed by using synthesized SQDs. In the presence of Cr (VI), the fluorescence of SQDs was quenched, and then recovered by the addition of AAP and ALP. The SQDs as fluorescence probes can find a wide range of applications in different sensing areas.

**Supplementary Materials:** The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/nano12162787/s1, Figure S1: Fluorescence emission spectra of SQDs under different excitation wavelength, Figure S2: Selectivity of the SQDs-Cr (VI) system for AA.

**Author Contributions:** Conceptualization, M.Y., X.C. and J.Z.; investigation, F.M. and Q.Z.; methodology, F.M. and Q.Z.; formal analysis, F.M. and Q.Z.; resources, M.Y., X.C.; software, F.M. and Q.Z.; supervision, M.Y., X.C. and J.Z.; writing—original draft, F.M.; writing—review and editing, M.Y., X.C. and J.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors are thankful for the support of this work by the National Natural Science Foundation of China (Grant No. 22174163), the Hunan Provincial Science and Technology Plan Project, China (No. 2019TP1001), and the Project of Intelligent Management Software for Multimodal Medical Big Data for New Generation Information Technology, Ministry of Industry and Information Technology of People's Republic of China (TC210804V).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data is contained within the article.

**Conflicts of Interest:** The authors declare no conflict of interest.
