*4.4. Ligand Attraction*

The amphiphilic substance with both hydrophilic and hydrophobic functional groups acts on the surface of the nanoparticles. The hydrophobic functional groups in the substance are adsorbed by the oleic acid ligands on the surface of UCNPs through hydrophobic interaction, while the hydrophilic functional groups act as surface modification. Yi et al., first synthesized NaYF4:Yb/Er@NaYF4 upconversion luminescent nanoparticles with surface oleylamine modified by thermal decomposition method, using polyacrylic acid embedded with octylamine and isopropylamine as modifiers, using the hydrophobicity of polymer molecules. The hydrophobic interaction between the octyl and isopropyl groups of the UCNPs and the hydrophobic hydrocarbon groups on the surface of the UCNPs makes the

polymer coat the surface of the UCNPs. The carboxyl groups in the polymer molecules make the hydrophobic UCNPs hydrophilic, realizing the resistance to UCNPs.
