PEG-Phosphate is used to PEGylate metal oxide particles and surfaces to form stable monolayer protection. The strong binding properties of organophosphorus molecules to metal oxides result from the creation of a stable Mâ€“Oâ€“P structure via phosphateâ€“metal coordinative interactions. Phosphate-based coupling agents form self-assembled monolayer on the surface of metal oxide nanoparticles such as iron oxide, superparamagnetic particlesÂ and upconverting and down-conversion nanoparticles to form thermodynamically stable dispersion. Phosphoric acid is symmetrically bonded through its two or three oxygen atoms via bidentate or tridentate Mâ€“Oâ€“P coordination bonds, resulting in a stronger bridging geometry than monodentate bonding. Bifunctional PEG phosphate reagents are provide via custom synthesis service.