*3.5. Combustion Method*

Different from the solvothermal method that uses lower temperature and longer reaction time, the combustion method provides a time-saving (a few minutes) method for the preparation of rare earth doped nanoparticles, and the reaction temperature is generally 500–3000 ◦C. This process takes place in the form of a combustion wave under a controlled explosion. The combustion method is based on a highly exothermic reaction. A variety of oxides and oxysulfide-containing single nucleotide chain nucleotides were synthesized by this method. For example, Zhang et al., used the glycine-nitrate process to synthesize monoclinic phase Gd2O3:Er3+ upconversion nanoparticles [122]. Dissolve the Gd2O3 and Er2O3 upconversion nanoparticles in diluted nitric acid, evaporate and heat. Rapid self-sufficient combustion will produce fluffy powder, which is heated to 600 ◦C for 1 h to remove nitrates and organic residues. The combustion method has the advantages of saving time and energy, but at the same time there is inevitably the phenomenon of aggregation of synthetic materials.

It is also worth mentioning that flame synthesis is another fast method for preparing upconversion nanoparticles. Ju et al., synthesized Y2O3:Yb,Er (or Tm,Ho) nanoparticles with a gas-phase flame one-step method, with an average size of less than 30 nm [123]. The results show that temperature has a strong influence on particle size, morphology, and photoluminescence intensity.

#### **4. Surface Modification of UCNPs**

The surface of UCNP obtained by various methods usually contains water-transporting organic ligands (amine oleate, octadecene, oleic acid, etc.). This makes UCNPs difficult to dissolve in water, which affects their biomedical applications to a certain extent. Therefore, it is necessary to modify the surface of UCNPs. So far, various surface modification methods have been reported, including silica coating, ligand exchange, ligand oxidation, ligand attraction, and layer-by-layer assembly [124–133], as shown in Figure 3.

**Figure 3.** (**a**) Schematic flow diagram of different surface modification methods (silica coating, ligand exchange, ligand oxidation, ligand attraction, and layer–by–layer assembly). (**b**) Chemical reaction formula for preparing silica coating with tetraethyl orthosilicate. (**c**) Basic reaction formula of ligand oxidation process.
