**4. Conclusions**

In this paper, we prepared Ag@Au nanoparticles via a seed-mediated growth method. By adjusting the amount of silver seeds, we realized the transformation of Ag@Au nanoparticles from a spherical to a sea-urchin-like structure, and proposed the possible growth mechanism. We further explored the SERS enhancement performance of the prepared nanoparticles, using 4-ATP as the test molecule. The results show that the order of SERS enhancement effects, from high to low, is spherical Ag@Au nanoparticles, sea-urchinshaped Ag@Au nanoparticles, and mesoporous gold nanoparticles. Finally, with Ag@Au spherical nanoparticles as the enhancement material, we achieved the highly sensitive SERS detection of fentanyl, with the lowest detection concentration reaching 10−<sup>7</sup> .

**Author Contributions:** Methodology, Y.Q.; validation, W.Y., B.W. and Y.Q.; resources, W.Y., J.W. and X.Z.; data curation, J.W.; writing—original draft preparation, Y.Q.; writing–review and editing, Y.W., B.W. and Y.Q.; supervision, X.Z.; project administration, J.W. and W.Y.; funding acquisition, W.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by "the National Key Research and Development Program of Zhejiang Province, grant number 2021C03135", "the National Key Research and Development Program of China Public Safety Risk Prevention and Control, grant number 2018YFC0807201", Zhejiang Police College scientific research project, grant number 2019XJY001 and "Zhejiang Provincial Natural Science Foundation of China under grant number, LGF20C090001" The APC was funded by the National Key Research and Development Program of Zhejiang Province.

**Institutional Review Board Statement:** The Institutional Review Committee has informed and agreed to the publication of this article.

**Informed Consent Statement:** All authors know and agree to the publication of this article.

**Data Availability Statement:** The data presented in this study are available within the article.

**Acknowledgments:** We would like to express our heartfelt thanks to Mao Zhanpeng for his assistance in the electron microscope test and the assistance provided by XFNANO with the experimental materials.

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