**4. Conclusions**

In summary, we successfully prepared RTP AA-CPDs using acrylic acid and ammonium oxalate as raw materials. AA-CPDs have the characteristics of producing green RTP under the dual-mode excitation of visible/ultraviolet light. Under environmental conditions, the phosphorescence lifetime of the AA-CPDs solid is 412.03 ms, and the afterglow of visible/ultraviolet light excitation is 5 s and 10 s for naked eye observation, respectively. The carboxyl and amino groups on the surface of AA-CPDs are connected by intermolecular hydrogen bonds. These hydrogen bonds may provide space protection (isolation of oxygen) around the AA-CPDs phosphor, which can stably excite the triplet state. In addition, through comparative experiments, we also verified that N doping promotes the generation of triplet excitons in AA-CPDs. More importantly, the prepared AA-CPDs can be used as a potential new type of smart hidden security material, which can be used in fields such as advanced anti-counterfeiting and the hiding of complex graphics. This research provides a method for simple and rapid preparation of metal-free RTP materials.

**Author Contributions:** H.W.: Experimental design, synthesis, and characterization; H.Y.: Data analysis, supervision, and funding; X.Z.: Characterization and funding; S.W.: Measurements and analysis; W.C. and H.Y.: Conceptualization, Supervision, and validation; H.W., H.Y., A.A.-Z., W.C., and G.N.: Writing and editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** Financial supports from the XingLiao Talent Project Grants of Liaoning Province, China (No. XLYC1902076), the University of Science and Technology Liaoning Talent Project Grants (No. 601010302) and the Key Laboratory of Chemical Auxiliaries Synthesis and Separation of Liaoning Province, China (No. ZJNK2002, No. ZJNK2005) are highly appreciated. Nie GC would like to acknowledge the Guangxi Innovation Driven Development Major Project of Guangxi Province, China (No. Guike AA2030201) and Nanning Scientific Research and Technology Development Plan Project of Guangxi Province, China (No. RC20200001) for support. W.C. would like to thank the support from Solgro Inc. and UT Arlington for a distinguished award.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

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