**4. Discussion**

In this study, we developed a dye-sensitized heterogeneous lanthanide nanoparticle to regulate the energy transfer pathway for UV enhancement by 808 nm excitation. We systematically studied the influence of dye concentration, excitation wavelength, and distance between the dye and the sensitizer Nd3+ on upconversion emission, especially in the UV spectral region. Dye loading can improve the absorption of excitation light and thus improve the efficiency of energy-transfer-mediated upconversion. Moreover, our experimental results demonstrated a strengthened multiphoton upconversion process, which can be ascribed to the dominant effect of ligand loading on upconversion emission from high-lying energy states. The fundamentals gained from our investigations may provide insights into promoting the multiphoton upconversion process and the future design of organic–inorganic hybrid luminescent nanoparticles for applications in photocatalysis, biomedicine, environmental science, and more.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/ 10.3390/nano11113114/s1. Figure S1: TEM and size distribution of Gd-CSYS2S3 nanoparticles; Figure S2: XRD of Gd-CSYS2S3 nanoparticles; Figure S3: EDX of Gd-CSYS2S3 nanoparticles; Figure S4: EDX lining analysis of Gd-CSYS2S3 nanoparticles; Figure S5: luminescence emission of NaGdF4:18%Yb, 2%Er@NaYF4:20%Yb@NaGdF4:10%Yb, 50%Nd@NaGdF4 with and without IR-806 loading; Figure S6: luminescence emission of Gd-CSYS2S3 nanoparticles with different Nd3+ doping before and after IR-806 loading; Figure S7: normalized intensity of luminescence spectra of Gd-CSYS2S3 with various contents of IR-806; Figure S8: TEM images of as-synthesized nanoparticles with different structures for distance effect studies; Figure S9: schematic illustration of five types of core–multishell structures including Gd-CSYS2S3, Y-CS1SYS3, Y-CS1SYS3S4, Gd-CS1SYS3, and Gd-CS1SYS3S4; Figure S10: luminescence spectra of as-synthesized nanoparticles with different structures for distance effect studies; Figure S11: normalized intensities of luminescence spectra of corresponding nanoparticles for distance effect studies; Figure S12: the lifetime decay of Tm3+ at 650 nm in Gd-CSYS2S3 and Gd-CSYS2S3@IR-806 nanoparticles under 808 nm excitation.

**Author Contributions:** Conceptualization, Q.S. and C.H; methodology, Q.S. and M.W.; validation, Q.S., C.H. and M.W.; investigation, M.W., Q.S., H.W. and S.W.; resources, Q.S.; data curation, M.W.; writing—original draft preparation, M.W.; writing—review and editing, Q.S. and C.H; visualization, M.W.; supervision, Q.S. and C.H.; project administration, Q.S.; funding acquisition, C.H. and Q.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (Nos. 82002893 and 21701109).

**Data Availability Statement:** All of the relevant data are available from the correspondence authors upon reasonable request. Source data are provided with this paper.

**Acknowledgments:** The authors acknowledge Han for helpful discussions. The authors thank Jin and Guan for their help with the HAADF-STEM measurements.

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