*4.3. Ligand Oxidation*

Ligand oxidation is another effective method to obtain water-soluble UCNP based on the selective oxidation of surface carbon-carbon double bonds (R−CH = CH−R'). This method requires the presence of unsaturated bonds in the original ligand. For example, the OA ligand on the surface of UCNPs can be oxidized to azelaic acid (HOOC(CH2)7 COOH), thereby making UCNP a hydrophilic material. Li's group uses m-chloroperoxybenzoic acid as an epoxidizing reagen<sup>t</sup> to oxidize the oleic acid ligand with carbon double bonds on the surface to a ternary epoxy compound, and then interact with organic molecules containing active functional groups (such as MPEG oh) to carry out a ring-opening reaction. Hydrophilic molecules are grafted onto oleic acid molecules to form the water-soluble conversion of luminescent nanoparticles [133]. Yan et al. used a clean and easily available strong oxidant ozone to oxidize the OA on the surface of UCNPs to azelaaldehyde or azelaic acid through ozone decomposition under certain conditions [138]. However, the ligand oxidation method has disadvantages such as long time and low efficiency, which is not conducive to actual production.
