Fig. 2.

Selected milestones overcoming concentration quenching in homogeneously doped upconversion nanocrystals. a Integrated upconversion luminescence intensity as a function of excitation irradiance for a series of Tm³⁺-doped (0.2–8%) nanocrystals. Adapted from ref. ¹⁷. b Upconversion luminescence intensity of single 8 nm UCNPs with 20 and 2% Er³⁺, each with 20% Yb³⁺, plotted as a function of excitation intensity. Adapted from ref. ¹⁸. c Experimental results (black circle and red square) and theoretical modelling (black and red curves) of integrated upconversion luminescence intensities of a set of NaYF₄:Nd³⁺ UCNPs with and without indocyanine green (ICG) sensitization. Adapted with permission from ref. ⁴⁵ Copyright (2016) American Chemical Society. d Luminescence spectra of colloidal dispersion of NaYF₄:x%Er@NaLuF₄ nanocrystals (x = 5, 25, 50, 100); Inset: luminescence images of NaYF₄:x%Er@NaLuF₄ in cyclohexane excited with a 980 nm laser. Adapted with permission from ref. ²⁵ Copyright (2017) American Chemical Society. e Integrated upconversion luminescence intensity of α-NaY_0.98-xYb_xF₄:2%Er@CaF₂ (x = 0.2, 0.4, 0.6, 0.8, 0.98). Adapted with permission from ref. ⁴⁷ Copyright (2017) Royal Society of Chemistry. f Schematic of the experimental configuration for capturing upconversion luminescence of NaYF₄:Yb³⁺,Tm³⁺ nanocrystals using a suspended-core microstructured optical-fibre dip sensor. Adapted from ref. ⁵³. g Upon irradiation by a NIR laser at the high-power density, the reflection wavelength of the photonic superstructure red-shifted, whereas its reverse process occurs upon irradiation by the same laser but with the lower-power density. Adapted with permission from ref. ⁵⁵ Copyright (2014) American Chemical Society. h Photographs of a microresonator with and without optical excitation. Adapted from ref. ⁵⁶. i UCNPs functionalized with a nanobody recognizing enhanced green fluorescent protein (EGFP) could rapidly and specifically target to EGFP-tagged fusion proteins in the mitochondrial outer membrane, and this protein interaction process could be detected by lanthanide resonance energy transfer (LRET) in living cells. Scale bar: 10 μm. Reproduced with permission from Drees et al.⁵⁷ copyright John Wiley and Sons. j Schematic of channelrhodopsin-2 activated in HeLa cells by strong blue upconversion luminescence from NaYbF₄:Tm³⁺@NaYF₄ core@shell structure. Adapted with permission from ref. ⁵⁸ Copyright (2017) American Chemical Society