Fig. 1. FRET-enhanced upconversion of core–shell UCNPs with 980 nm irradiation. (a) Schematic of active-shelled core–shell UCNP (NaYb_0.92Er_0.08F₄@NaLu_1−xYb_xF₄) emission enhancement through FRET to a dye. (b) The Jablonski diagram of the corresponding FRET-enhanced upconversion mechanism. ET: energy transfer; EM: energy migration. (c–h) TEM images and size distributions of the as-prepared core NaYb_0.92Er_0.08F₄ and core–shell NaYb_0.92Er_0.08F₄@NaLu_1-xYb_xF₄ (x = 0, 0.25, 0.5, 0.75 and 1) UCNPs with various Yb³⁺ doping contents in the outermost shell. (i) Integrated emission enhancement of NaYb_0.92Er_0.08F₄@NaLu_1−xYb_xF₄ (x = 0, 0.25, 0.5, 0.75 and 1) sulfo-Cy3-UCNPs; the enhancements were obtained by calculating the ratio of UCL intensity of the sulfo-Cy3-UCNPs versus UCNPs. (j) Normalized upconversion emission spectra of NaYb_0.92Er_0.08F₄@NaLu_0.25Yb_0.75F₄ UCNPs with and without sulfo-Cy3 at a dye concentration of 4.00 μM (λ_ex = 980 nm, 63 W cm⁻²). (k) Normalized upconversion emission spectra of NaYb_0.92Er_0.08F₄@NaLuF₄ UCNPs with and without sulfo-Cy3 at a dye concentration of 4.00 μM (λ_ex = 980 nm, 63W cm⁻²). The emission intensity at 576 nm of sulfo-Cy3-UCNPs (the emission peak of sulfo-Cy3) in (i) and (j) was normalized to unity, respectively.