*4.1. Silica Coating*

Surface silanization (or coated silica coating) is an inorganic surface treatment strategy that can make nanoparticles have water solubility and biocompatibility. It is known that silica is highly stable, biocompatible, and optically transparent. When used as a coating material, the method of surface silanization can flexibly provide abundant functional groups (such as -COOH, -NH2, -SH, etc.) to meet the various needs of binding with biomolecules. Wang's group reported that the surface of UCNP is coated with polyhedral oligomeric silsesquioxane (POSS) to make the particles highly hydrophobic. They also described the preparation of liquid marbles based on optically and magnetically active dualfunctional UCNPs, and their use as microreactors for the study of photodynamic therapy of cancer cells [134]. Veggel et al., reported the synthesis of SiO2-coated LaF3:Yb3+/Er3+ UCNPs using the Stöber method, and the thickness of the silica shell was controlled below 15 nm [135].
