Fig. 1. Schematic diagram illustrating the construction of the surface-RE-rich NaREF4@NaxREFx+3 nanoparticles with superior loading capacity enabled by heterovalent cation exchange (HCE).

Fig. 2. (a-b) TEM images of the as-prepared UCNPs (a) before and (b) after treatment with Lu(CF3COO)3 at 300 °C for 2 h. (c) Corresponding XRD patterns of (a) and (b). (d) HAADF-STEM image and (e) elemental mapping analysis of (b). (f) Corresponding FFT pattern of (e).

Fig. 3. (a) Schematic illustration of the HCE process between Lu3+ and Na+ when β-NaYF4:Yb,Er reacting with Lu(CF3COO)3 at 300 °C. (b) Additional HAADF-STEM image and corresponding elemental mapping analysis of the same sample presented in Fig. 2(b).

Fig. 4. (a) Aberration-corrected HAADF-STEM image of the resulting core@NaxLuFx+3 UCNPs after treatment with Lu(CF3COO)3 projected from the [0001] direction. (b) Enlarged view of the selected area in (a). Note: the bright atomic columns represent RE atoms, while Na atoms are invisible. (c) Intensity profile along the red line in (b). (d) Atomic arrangement of β-NaYF4 in a unit cell. (e) Schematic diagram showing the formation of the surface-RE-rich structure induced by the HCE between RE3+ and Na+.

Fig. 5. (a-b) TEM images of the as-prepared (a) core and (b) pure core@NaxLuFx+3 without NaLuF4 nanoparticles. (c-d) Corresponding (c) TGA curves and (d) densities of surface ligands (OA/cm2) of (a) and (b). Inset of (d): their corresponding photographs in cyclohexane solution. (e) Zeta potentials of (a) and (b) measured in different surface status including ligand-free and modified with different surface ligands (see Fig. S8 for details)..

Fig. 6. (a) UC emission spectra of the as-prepared core@shell and core@shell@NaxLuFx+3 under 808 nm excitation, and UV-vis absorption spectra of the as-prepared core@shell-RB and core@shell@NaxLuFx+3-RB. (b) UC emission spectra of core@shell-RB and core@shell@NaxLuFx+3-RB under 808 nm excitation in DMF. Inset: their corresponding luminescence photographs. (c) Corresponding luminescence decay curves of Er3+ emission at 542 nm. (d) Corresponding cytotoxicity test on the viability of HeLa cells after incubation with different concentrations of the above four samples for 48 h, as measured by MTT assay. (e) Relative viability of HeLa cells treated with the above four samples at the concentration of 200·μg/mL under 808·nm excitation at a power density of 2·W/cm2 for 10 min. (f) Quantified ROS generation in HeLa cells using H2DCFDA probe as indicator.