**3. Results**

#### *3.1. Synthesis of Core–Multishell Upconversion Nanoparticles*

We previously designed a heterogeneous core–multishell nanoparticle with enhanced UV upconversion emission, involving six- and five-photon upconversion processes [30]. The optimum doping concentration and nanoparticle design were determined according to our previous reports [36]. From our previous photoluminescence results, the optimized nanostructure was determined to be NaGdF4:49%Yb/1%Tm@NaGdF4:20%Yb@ NaGdF4:10%Yb/50%Nd@NaGdF4. Recently, we found that when the NaGdF4:20%Yb was replaced with NaYF4:20%Yb, UV emission was significantly enhanced due to the effective suppression of energy consumption induced by interior energy traps. Herein, we chose this heterogeneous nanostructure as an experimental model to further enhance upconversion emission in the UV range. We first synthesized the core–multishell NaGdF4:49%Yb,1%Tm@NaYF4:20%Yb@NaGdF4:10%Yb,50%Nd@NaGdF4 (Gd-CSYS2S3) nanoparticles using a layer epitaxial growth method. Transmission electron microscopy (TEM) images showed that the nanoparticles had a uniform size of about 28 nm and the thickness of each layer was ~2 nm (Figure S1). The as-prepared nanoparticles were identified as the hexagonal phase by X-ray powder diffraction (XRD, JCPDS file number 27-0699, Figure S2). In addition, the constitution of the heterogeneous core–multishell nanostructures was confirmed by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), elemental mapping images and energy dispersive X-ray (EDX) spectra (Figure 1b,c, Figures S3 and S4), where brighter regions correspond to heavier elements (Gd, Yb, and Nd) and lighter regions correspond to lighter elements (Y).

## *3.2. Remarkable UV Enhancement*

To enhance upconversion emission in the UV range, we chose a near-infrared (NIR) fluorescent dye (IR-806) to sensitize upconversion nanoparticles, due to its intense absorption in the NIR range [33]. As shown in Figure 2a, the fluorescence spectrum of IR-806 has considerable overlap with the absorbance of Gd-CSYS2S3 nanoparticles with Nd3+ (4F3/2→4I9/2) sensitizer, ensuring an effective energy transfer from IR-806 to the nanoparticles. We then utilized a modified Hummelen's method to load the IR-806 onto the surface of Gd-CSYS2S3 nanoparticles [32]. In addition, free IR-806 has an absorption band at 1708 cm<sup>−</sup><sup>1</sup> in the FTIR spectrum, corresponding to the stretching mode of –COOH. The absorption band at 1708 cm<sup>−</sup><sup>1</sup> disappeared when IR-806 was bound on to the surface of the nanoparticles. Nevertheless, absorption bands at 1560 and 1450 cm<sup>−</sup><sup>1</sup> were observed on Gd-CSYS2S3@IR-806, corresponding to the antisymmetric and symmetric vibration modes of the –COO− group. This indicates that the IR-806 carboxylic acid group was bound onto the surface of Gd-CSYS2S3, since the carboxylic region changed [33]. The successful preparation of IR-806-loaded Gd-CSYS2S3 nanoparticles was demonstrated by Fourier-transform infrared spectroscopy (FTIR) analysis (Figure 2b), which is consistent with the previous report [32]. In addition, we also compared the absorption spectra of Gd-CSYS2S3, IR-806, and Gd-CSYS2S3@IR-806. As shown in Figure 2c, Gd-CSYS2S3@IR-806 nanoparticles showed an intense absorbance band peaking at ~800 nm, further proving the successful loading of IR-806. Consequently, we observed more than 70-fold enhancements in Tm3+ emission over the whole wavelength range from 240–700 nm by Gd-CSYS2S3@IR-806 compared with Gd-CSYS2S3 nanoparticles, owing to the fact that the absorption cross section of Gd-CSYS2S3 was significantly enhanced after IR-806 loading. Furthermore, we also observed more than 600-fold, 300-fold, 150-fold, and 30-fold enhancements in UVC (240–280 nm), UVB (280–320 nm), UVA (320–400 nm), and visible (400–700 nm) regions, respectively (Figure 2e). Similarly, we synthesized the NaGdF4:18%Yb,2%Er@NaYF4:20%Yb@NaGdF4:10%Yb, 50%Nd@NaGdF4 nanoparticles with IR-806 loading. The emission intensity in the UV spectral region increased by more than 60 times, while the intensity in the visible region increased by only 30 times (Figure S5). Taken together, these results demonstrated that the overall enhancements were dominated by increased emission in the UV spectral regions, which is consistent with the dominant effect of ligand coordination on multiphoton upconversion [37]. Notably, the enhancement factors in the UV spectral region are remarkably larger than those in the visible region, offering enticing prospects for NIR light-mediated UV upconversion nanoparticles.

**Figure 1.** Schematic illustration and characterization of Gd-CSYS2S3 heterogeneous nanoparticles. (**a**) Diagrammatic representation of Gd-CSYS2S3 nanostructure. (**b**) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of Gd-CSYS2S3 nanoparticles. Inset: high-resolution TEM of as-prepared Gd-CSYS2S3 nanoparticle. (**c**) HAADF-STEM image and elemental mapping image of Gd-CSYS2S3 nanoparticles, revealing the spatial distribution of the Y, Nd, Gd, and Yb elements in the heterogeneous nanoparticles.

#### *3.3. Optimum Weight Ratio between IR-806 and Nanoparticles*

We determined the optimum weight ratio of Gd-CSYS2S3:IR-806 by setting a series of weight gradients from 120:1 to 180:1 (mNPs:mIR-806). As shown in Figure 3a, the optimum weight ratio was determined to be 160:1. The optimized number of dye molecules on the surface of Gd-CSYS2S3 nanoparticles was calculated to be 395 [32]. Note that the absorbance of Gd-CSYS2S3@IR-806 increased as IR-806 increased. However, when the weight ratio of Gd-CSYS2S3: IR-806 was smaller than 160:1, the emission intensity decreased due to fluorescence quenching caused by dye self-quenching. Due to the critical role of the Nd3+ sensitizers in mediating energy transfer from the dye to the upconversion nanoparticles, we verified that the optimum doping concentration of Nd3+ was 50 mol% (Figure S6). We then quantified the energy transfer efficiency of IR-806 to Gd-CSYS2S3 by measuring the lifetime of the IR-806 in a pair of Gd-CSYS2S3 samples with and without Nd3+ nanoparticles. Due to energy trapping by Nd3+, the lifetime is shortened from 1.20 ns to 1.13 ns for Gd-CSYS2S3@IR-806. However, the lifetime of IR-806 was essentially unchanged after loading on Gd-CSYS90%Y,10%YbS3@IR-806, due to the absence of Nd3+ dopants. The energy transfer efficiency was calculated to be 5.8% according to the following equation [38]:

> τD

(1)

E = 1 − τDA

**Figure 2.** Preparation and characterization of Gd-CSYS2S3@IR-806. (**a**) IR-806 emission spectrum and Gd-CSYS2S3 nanoparticles absorption spectrum. (**b**) FTIR of Gd-CSYS2S3@IR-806 and IR-806. (**c**) The absorption spectra of Gd-CSYS2S3, IR-806, and Gd-CSYS2S3@IR-806. (**d**) Emission spectra of Gd-CSYS2S3 with and without IR-806 loading under 808 nm CW diode laser at a power density of 10 W/cm2. (**e**) The enhancement factors of upconversion emission were obtained by comparing the results for samples with and without IR-806 loading. The emission intensities were calculated by integrating the spectral intensities in the UVC (240–280 nm), UVB (280–320 nm), UVA (320–400 nm), and visible (400–650 nm) ranges.

#### *3.4. The Effect of Excitation Wavelength on UV Upconversion Emission*

To investigate the enhancement effect on upconversion emission under 793, 808, and 980 nm excitation, we measured two series of Gd-CSYS2S3 nanoparticles with different amounts of IR-806 loading. As shown in Figure 3d, the emission intensities of Gd-CSYS2S3 were slightly improved after IR-806 loading under 793 nm excitation. In contrast, their emission intensities decreased under 980 nm excitation (Figure 3e). These results can be ascribed to poor matching between the excitation wavelengths (793 nm and 980 nm) and the absorption of IR-806. We then normalized the luminescence spectra of Gd-CSYS2S3 nanoparticles under three different excitation wavelengths. We found that the ratio was unchanged for UVC, UVB, UVA, and visible spectral regions under 793 nm and 980 nm excitation. In contrast, the normalized intensity of the UVC spectral region clearly increased (Figure S7), indicating effective energy transfer from IR-806 to the nanoparticles under 808 nm excitation.

**Figure 3.** Optimizing the weight ratio of Gd-CSYS2S3 to IR-806 and calculating the energy transfer efficiency. (**a**) The emission spectrum of Gd-CSYS2S3 (4 mL in CHCl3, 0.375 mg/mL) after adding various masses of IR-806 dye under 808 nm excitation. (**b**) The absorption spectrum of Gd-CSYS2S3 (4 mL in CHCl3, 0.375 mg/mL) with various masses IR-806 dye. (**c**) The decay curves of Gd-CSYS2S3, Gd-CSYS2(90%, 10%Yb)S3@IR-806, and Gd-CSYS2S3@IR-806. (**d**,**<sup>e</sup>**) The emission spectra of Gd-CSYS2S3 (4 mL in CHCl3, 0.375 mg/mL) after adding various masses of IR-806 dye under 793 nm and 980 nm excitation, respectively.

#### *3.5. The Effect of IR-806 Sensitizer Distance on UV Upconversion*

To study the effect of the distance between Nd3+ and IR-806 on UV upconversion emission, we synthesized a pair of nanoparticles: Gd-CSYS2S3 and Gd-CSYS2 (without the third shell protection) shown in Figure 4a. Comparing the emission intensities of Gd-CSYS2S3 and Gd-CSYS2 with and without IR-806, the emission intensities of the Gd-CSYS2 nanoparticles without shell protection increased by more than 230 times overall, while only 70-fold enhancement was observed in Gd-CSYS2S3, which has 2 nm thickness shell protection. Furthermore, UV and visible emission intensities increased more than 500-fold and 130-fold, respectively, for the nanoparticles without shell protection (Figure 4b). Notably, the transfer efficiency decreased as 1/R<sup>6</sup> [39]. Therefore, the enhancement factor decreased as the distance between the dye and the sensitizer increased.

Similarly, we synthesized two pairs of nanoparticles: NaGdF4@ NaGdF4:49%Yb,1%Tm @NaYF4:20%Yb@NaGdF4:10%Yb,50%Nd@NaGdF4 (Gd-CS1SYS3S4) vs. NaGdF4@NaGdF4: 49%Yb,1%Tm@NaYF4:20%Yb@NaGdF4:10%Yb,50%Nd (Gd-CS1SYS3) and NaYF4@NaGdF4: 49%Yb,1%Tm@NaYF4:20%Yb@NaGdF4:10%Yb,50%Nd@ NaGdF4 (Y-CS1SYS3S4) vs. NaYF4 @NaGdF4:49%Yb,1%Tm@NaYF4:20%Yb@NaGdF4:10%Yb, 50%Nd (Y-CS1SYS3) (Figure S8). The core–multishell structures are illustrated in Figure S9. To study the effect of different structures on emission enhancement, NaGdF4 and NaYF4 without any dopants were used

as a core to shorten the distance between the NaGdF4:49%Yb,1%Tm emissive layer and IR-806. The emission intensities of IR-806 grafted on Gd-CS1SYS3 and Gd-CS1SYS3S4 increased 99 and 20 times, respectively, while the luminescence intensity in the UV region increased by 118 and 25 times and that in the visible region increased by 82 and 16 times, respectively. Moreover, the emission intensities of Y-CS1SYS3 and Y-CS1SYS3S4 improved by 72 and 18 times after IR-806 loading. We also observed 81-fold and 22-fold enhancements in the UV spectral region and 63-fold and 14-fold enhancements in the visible region (Figure S10). These results are also consistent with our luminescence analysis, in that a significant enhancement in the UV luminescence of Gd-CSYS2S3 nanoparticles was observed compared to the visible range (Figure S11).

**Figure 4.** The effect of the distance between IR-806 and sensitizer Nd3+ on upconversion emission. (**a**) Schematic illustration of the nanostructural design to study the distance effect on upconversion emission. (**b**) The emission spectra of Gd-CSYS2S3, Gd-CSYS2S3@IR-806, Gd-CSYS2, Gd-CSYS2@IR-806 under 808 nm excitation.

## *3.6. Energy Transfer Mechanism*

As shown in Scheme 2, IR-806 effectively absorbs the laser energy due to the absorption cross section under 808 nm excitation. To generate an efficient dye sensitization process, Nd3+ plays a critical role in bridging the energy transfer from the dye to the upconversion nanoparticles. Nd3+ ions trap the energy from the 808 nm laser and IR-806 mainly via the fluorescence–resonance energy transfer process and then gather photons at the 4F5/2 energy state. Subsequently, relaxing to the 4F3/2 energy state, Nd3+ transfers the energy to Yb3+ by an efficient energy transfer process. As an energy migrator, the excited Yb3+ populates the energy states of Tm3+ and gives rise to emission at 475 nm (1G4→3H6), 450 nm (1D2→3F4), 360 nm(1D2→3H6), 345 nm(1I6→3H5), and 290 nm(1I6→3H6). Apart from emitting, Tm3+ serves as an energy donor donating energy to the Gd3+ ions via a five-photon process. Meanwhile, the six-photon upconversion process of 253 nm (6D9/2→8S7/2) and the five-photon upconversion processes of 273 nm (6IJ→8S7/2), 276 nm (6IJ→8S7/2), 279 nm (6IJ→8S7/2), 306 nm (6P5/2→8S7/2), and 310 nm (6P7/2→8S7/2) are observed with the assistance of the appropriate energy matching of the following transition of 2F5/2→2F7/2 (9750 cm<sup>−</sup>1, Yb3+): 6PJ→6DJ (∼8750 cm<sup>−</sup>1, Gd3+). Notably, the utilization of an optically inert NaYF4 host lattice with Yb3+ dopants as the interlayer plays a decisive role in protecting the energy by cooperative dye and Nd3+ sensitization from interior lattice defects, making it possible to effectively further increase UV via dye sensitizing.

#### *3.7. Back Energy Transfer from Nanoparticles to IR-806*

As well as increasing the luminescence intensity, a back energy transfer process from IR-806 to Gd-CSYS2S3 occurred. As depicted in Figure 5 and Figure S12, the lifetime of Gd3+ at 253, 276, and 310 nm, and Tm3+ at 290, 345, 475, and 650 nm slightly decreased after IR-806 loading, which can be ascribed to the nonradiative energy transfer from Gd3+ and Tm3+ to IR-806 [40–42].

**Scheme 2.** Schematic illustration of the mechanism for cascade energy transfer in Gd-CSYS2S3@IR-806. Upon 808 nm laser excitation, IR-806 first absorbs excitation energy and transfers it to Nd3+. Next, Yb3+ accepts the energy from Nd3+, contributing to populating photons in the 3P2 state of Tm3+ through a continuous five-photon energy transfer process and then relaxing to the 1I6 state of Tm3+. Trapping the energy from both five-photon upconversion from Tm3+ and one-photon upconversion from Yb3+, six-photon and five-photon upconversion luminescence from 6DJ, 6IJ, and 6PJ state of Gd3+ is observed.

**Figure 5.** The decreased lifetime of Tm3+ and Gd3+ for Gd-CSYS2S3@IR-806. (**<sup>a</sup>**–**f**) The Tm3+ and Gd3+ lifetime decay curves of Gd-CSYS2S3 and Gd-CSYS2S3@IR-806 at 253, 276, 290, 310, 360, and 475 nm under 808 nm excitation, respectively.
