Fig. 1. Principal upconversion mechanisms of lanthanide-doped nanomaterials. (a) Excited-state absorption; (b) energy-transfer upconversion; (c) photon avalanche; (d) cooperative energy transfer; (e) energy migration-mediated upconversion (red arrows stand for direct excitation processes, blue arrows represent radiative emission processes, dashed arrows represent energy transfer processes, A means activator ion, S means sensitizer ion, M means migratory ion, and L means ladder ion)

Fig. 2. Principle of super-resolution microscopes. (a) Stimulated emission depletion microscope^[50]; (b) fluorescence emission difference microscope^[50]; (c) super-linear excitation-emission microscope^[54]; (d) structure illumination microscopy^[54]

Fig. 3. Applications of UCNP in various super-resolution microscopy. (a) Super-resolution of low-power STED microscopy^[21]; (b) immunofluorescence labeling of cellular cytoskeleton protein desmin with antibody-conjugated UCNPs and super-resolution imaging^[45]; (c) non-bleached FED microscopy^[48]; (d) Fourier-domain heterochromatic fusion^[50]; (e) deep tissue imaging by near-infrared emission saturation microscopy^[49]; (f) super-linear excitation-emission conventional confocal for three-dimensional subdiffraction imaging^[52]; (g) super-resolution uSEE-STED imaging of UCNPs endocytosed by a neuron phenotype cell^[53]; (h) nonlinear SIM microscopy super-resolution^[56]

Fig. 4. Applications of UCNP in Multiplexed super-resolution microscopy. (a) Deep learning assisted decoding of multi-channel single τ²-dots in super-resolution mode^[57]; (b) super-resolved multiplexing of mixed UCNPs^[58]; (c) time-domain anticounterfeiting by using three types of Nd-Yb-Tm series τ²-dots security inks with different rising-decay fingerprints; (d) multiplexed single molecule digital assays using five types of Nd-Yb-Er series τ²-dots probes to quantify the five kinds of target pathogen single-stranded DNAs^[13]