Fig. 2. FRET-enhanced upconversion of core–shell–shell UCNPs with 980 nm irradiation. (a) Schematic structure of core–shell–shell (CSS) UCNPs (NaYbF₄@NaYb_0.92Er_0.08F₄@NaLu_1−xYb_xF₄) (b) TEM image and size distribution of the as-prepared CSS NaYbF₄@NaYb_0.92Er_0.08F₄@NaLuF₄ UCNPs. (c) Integrated UCL intensity of sulfo-RB-UCNPs (NaYbF₄@NaYb_0.92Er_0.08F₄@NaLu_1−xYb_xF₄) (x = 0, 0.25, 0.5, 0.75 and 1) as a function of sulfo-rhodamine B concentration (λ_ex = 980 nm, 63 W cm⁻²); integrated intensities were normalized to the integrated intensity of UCNPs without sulfo-RB. (d) Integrated emission enhancement of sulfo-RB-CSS²⁵-NaLu_1−xYb_xF₄ with Yb³⁺ contents in the outermost shell varied from 0 to 1; the enhancements were obtained by calculating the ratio of integrated UCL intensity of the sulfo-RB-UCNPs versus the corresponding UCNPs.