Fig. 1. Schematic illustration of upconversion luminescence and mechanism. (a) Photographs of the upconversion luminescence in 1% mass fraction colloidal solutions of nanocrystals in dimethyl sulfoxide (DMSO) excited at 10270 cm^-1 (~975 nm)^[28]. A1 denotes total upconversion luminescence of the NaYF₄∶20%Yb³⁺, 2%Er³⁺ sample, A2, A3 show the same luminescence through red and green color filters, respectively, and

Fig. 2. Principle of bilayer formation by coating the oleate-capped UCNP with an amphiphile possessing a hydrophilic or ionic end group, thus converting the hydrophobic particles to hydrophilic particles

Fig. 3. Core/shell nanoparticles (NaYF₄∶20%Yb³⁺,2% Er³⁺/NaYF₄-PMAO-BHMT) dispersed in water at different pH from 3 to 13 and serum-supplemented cell growth medium and respective images under 980 nm excitation (bottom)^[38]

Fig. 4. Schematic illustration of the synthesis of silane-modified NaYF₄∶Yb³⁺, Er³⁺ and inversion spectra under different excitation wavelengths^[39]. (a) Schematic illustration of C18 silane-modified NaYF₄∶Yb³⁺, Er³⁺ loaded with the probe Eu(TTA)₃(TPPO)₂; (b) excitation spectra at 980 nm, the inset shows the temperature-dependent intensity ratio value of the two upcon

Fig. 5. Schematic representation of the silica shell formed on oleate-capped UCNP. After the initially hydrophobic particles are converted to hydrophilic particles, the process is accompanied by large changes in the ζ potential

Fig. 6. Illustration of the procedure for preparing UCNP coated with heparin and basic fibroblast growth factor (bFGF)^[59]

Fig. 7. UCNP-based probes with different energy acceptors to detect different inorganic ions, reactive oxygen species, gas molecules, protein, DNA, and RNA

Fig. 8. LRET-based detection strategies

Fig. 9. Schematic illustration of upconversion materials used in photothermal therapy^[79]. (a) Schematic illustrations of NaLuF₄∶Yb,Er@NaLuF₄@C for accurate PTT at facile temperature;(b) hematoxylin and eosin (H&E) histologic section of the border of tumor (Tu) and the adipocytes (Ad) in normal fat tissue. After photothermal treatment (middle), the tumor region became loose and fragile, while the adipocytes in normal fat tissue are

Fig. 10. Schematic illustration for detection of Ag^+[81]

Fig. 11. Schematic illustration for detection of Ca^2+[83]

Fig. 12. IFE-based detection of Fe³⁺ in wastewater samples by UCNP with N, N-diethyl-p-phenylenediamine (EPA) acceptor^[85]

Fig. 13. Schematic illustration for detection of Zn^2+[86]

Fig. 14. Schematic illustration for detection of Cu²⁺ in mice model with Alzheimer's disease^[89]

Fig. 15. O₂ concentration monitoring diagram. (a)(b) Schematic illustrations of oxygen-sensitive mechanism with downconversion channel and upconversion channel and synthesis of core-shell UCNP@mSiO₂ -Ir^[92]; (c)(d) reversible monitoring of O₂ concentration using a ruthenium complex as an oxygen indicator^[93]

Fig. 16. Principle of upconversion nanoprobe for the detection of hydroxyl radical^[98]

Fig. 17. ONNO-detection mechanism based on upconversion fluorescence^[100]. (a) The design of chromophore-assembled UCNP for nitrosative hepatotoxicity in vivo detection; (b) UV/Vis spectra of chromophore measured with and without the presence of ONOO^-and the upconversion emission spectrum of UCNP; (c)proposed reaction turn-on luminescence by which the energy acceptor Cy7 (marked with green star) degrades after oxidation by ONOO^-

Fig. 18. Schematic illustration of LRET-based upconversion immunosensor^[106]

Fig. 19. Schematic illustration of aptamer-based upconversion fluorescence detection. (a) UV light-activatable ATP sensing mechanism of the aptamer-based probe^[110]; (b) design of DNA nanodevices based on the integration of the aptamer probe with upconversion nanotransducer for NIR-activated intracellular ATP sensing^[111]