*4.2. Ligand Exchange*

Ligand exchange is a surface modification method that replaces the original hydrophobic ligands with some hydrophilic ligands without significantly affecting the chemical and optical properties of UCNP itself. Chow's group prepared oil-soluble upconversion luminescent nanocrystals by pyrolysis, which are wrapped by oleylamine molecules. Then they exchanged ligands with dicarboxylic acid polyethylene glycol polymers and oleylamine molecules on the surface. Hydrophilic dicarboxylic acid PEG molecules can not only convert nanocrystals into water-soluble form, but also further couple the surface carboxyl groups with biomolecules [136]. Murray and colleagues reported the use of nitrosotetrafluoroborate (NOBF4) to replace the OA and OM ligands attached to the surface of nanoparticles, so that the nanoparticles can exist stably in a variety of polar media for a long time without producing aggregation or precipitation [137].
