Novel Phosphonate-Functional Poly(ethylene oxide)-Magnetite Nanoparticles Form Stable Colloidal Dispersions in Phosphate-Buffered Saline

Many studies have concerned the colloidal stability of magnetite nanoparticles coated with poly(ethylene oxide) (PEO), but their long-term stability when such complexes are exposed to physiological media is still not well understood. This paper describes effects of different functional anchor groups, including one carboxylate, one ammonium, one phosphonate zwitterion, and three phosphonate zwitterions, on the structural stabilities of PEO−magnetite nanoparticle complexes in water and phosphate buffered saline (PBS). While PEO oligomers bound to the magnetite through any of these functional groups remain stably bound in water, only the complexes with polymers bound through the zwitterionic phosphonates were stable in PBS. The stabilities of the PEO-magnetite complexes with these phosphonate zwitterions in PBS allowed for investigating colloidal properties of their dispersions as functions of the number of phosphonates per chain and the chain densities and molecular weights. In contrast to all of the other complexes studied, PEO−magnetite nanoparticles bound through three phosphonate zwitterions on one end of the PEO exhibited no significant change in size for over 24 h even when they were dispersed in PBS. The colloidal properties of all of these dispersions are discussed in light of their compositions and structures.