Fig. 1. Schematic design of core-shell structured upconversion nanoparticles synthesized with the hot injection method.

Fig. 2. (a), (b) TEM images and (c) the corresponding size distribution of the of NaYF₄: Yb, Tm.

Fig. 3. (a)−(d) The schematic core-shell structure, TEM images and size distribution of NaYF₄:Yb, Tm@NaYF₄ (about 43.8 nm); (e)−(h) the schematic core-shell structure, TEM images and size distribution of NaYF₄:Yb, Tm@NaYF₄ (about 63.2 nm); (i)−(l) the schematic core-shell structure, TEM images and size distribution of NaYF₄:Yb, Tm@NaYF₄ (about 80.8 nm); (m)−(p) the schematic core-shell structure, TEM images and size distribution of NaYF₄:Yb, Tm@NaYF₄ (about 100.7 nm).

Fig. 4. (a) The energy transfer mechanisms of NaYF₄:Yb, Tm@NaYF₄ UCNPs; (b) the upconversion luminescent spectra and (c) the relative luminescent intensity of core-shell structured UCNPs with different shell thicknesses.

Fig. 5. (a) Schematic structure of UCNP@PEG; (b) TEM image of UCNP@PEG; (c) cell viabilities of HeLa cells after incubation with various concentrations of UCNP@PEG

Fig. 6. Upconversion luminescent images of HeLa cells after culturing with UCNP@PEG for 0.5, 3.0 and 6.0 h.