Fig. 4.

Cross-relaxation-enabled nanotechnology using highly doped upconversion nanocrystals. a Upconversion mechanisms of Er³⁺ at low- and high-doping levels and the fluorescence photographs of NaYbF₄:Er³⁺ UCNPs with different concentrations of the activator, the cross-relaxation effect induces pure red emission with increasing Er³⁺ concentration. Adapted with permission from ref. ⁸⁹ Copyright (2014) American Chemical Society. b Application of single-band upconversion nanoprobes for multiplexed in situ molecular mapping of cancer biomarkers. Adapted from ref. ⁹³. c Core@shell design and energy-looping mechanism in highly Tm³⁺-doped NaYF₄, and their application for deep-tissue brain imaging. Reproduced with permission from ref. ⁷³ Copyright (2016) American Chemical Society. d Design and mechanism of NaYF₄-based core@shell nanocrystals capable of emitting tunable colours through the combined use of a continuous wave 808 nm laser and a 980 nm laser with short or long pulse. Adapted from ref. ⁶³. e Its application in 3D and full-colour display systems with high-spatial resolution and locally addressable colour gamut, scale bars represent 1 cm. Adapted from ref. ⁶³. f Simplified energy-level diagrams depicting the energy transfer between Yb³⁺, Er³⁺, and Ce³⁺ ions, downshifting luminescence spectra of the NaYF₄:Yb³⁺,Er³⁺ UCNPs with and without Ce³⁺ doping and their application for cerebral vascular image through the second NIR window. Adapted from ref. ⁹¹. g Lifetime tuning scheme and time-resolved confocal images of NaYF₄:Yb³⁺,Tm³⁺ UCNPs with increasing concentration of Tm³⁺, scale bar represents 1 μm. Adapted from refs. ^90,101. h Their application in lifetime-encoded document security, scale bars represent 5 mm. Adapted from ref. ⁹⁰. i Energy-level diagrams of highly Tm³⁺-doped UCNPs under 980 nm and/or 808 nm illumination. Adapted from ref. ¹. j Confocal versus STED super-resolution images of the 40 nm 8% Tm³⁺-doped UCNPs, scale bar represents 500 nm. Adapted from ref. ¹. k Confocal versus STED super-resolution images of cellular cytoskeleton labelled with antibody-conjugated 11.8 nm NaGdF₄:18%Yb³⁺,10%Tm³⁺ nanocrystals, scale bar represents 1 μm. Adapted from ref. ⁹²