**4. Conclusions**

In this study, CQDs were produced from HMF, furfural, and microcrystalline cellulose via HTC at 220 ◦C for 30 min. The optical and morphological properties of the produced CQDs were investigated. It was observed that CQDs showed green luminescence under short-wavelength UV light (254 nm). Additionally, they can absorb light in a broad range. The variation of the absorption range was justified from the FTIR spectra where CQDs from different precursors showed different chemical bonds (e.g., alcohol, ketone, etc.). Although all the studied CQDs were within two to nine nm in diameter, the mean diameter varied with the precursor. Depending on the particle size, the CQDs showed different emission capacities. Finally, this study provides an insight into the effect of precursor on the CQD properties. So, the precursors for the CQDs should be selected based on the targeted applications.

**Author Contributions:** Conceptualization, M.R.H., N.S., M.T.R.; methodology, M.R.H., N.S.; formal analysis, M.R.H., N.S., T.Q.; investigation, M.R.H., N.S., M.T.R.; writing—original draft preparation, M.R.H., N.S., T.Q.; writing—review and editing, M.R.H., N.S., T.Q., M.T.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** The research is partially funded by the United States Department of Agriculture (USDA) grant (2019-67019-2928) through M.T.R.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors acknowledge Travis A. White from the Department of Chemistry and Biochemistry at Ohio University for allowing to use emission spectrofluorometer. The authors are also thankful to Kyle McGaughy from the biofuels lab at the Florida Institute of Technology for his laboratory efforts in this project.

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