Hydrophilization of Magnetic Nanoparticles with Modified Alternating Copolymers. Part 1: The Influence of the Grafting

Iron oxide nanoparticles (NPs) with a diameter of 21.6 nm were coated with poly(maleic acid-alt-1-octadecene) (PMAcOD) modified with grafted 5000 Da poly(ethyelene glycol) (PEG) or short ethylene glycol (EG) tails. The coating procedure utilizes hydrophobic interactions of octadecene and oleic acid tails, while the hydrolysis of maleic anhydride moieties as well as the presence of hydrophilic PEG (EG) tails allows the NP hydrophilicity. The success of the NP coating was found to be independent of the degree of grafting, which was varied between 20 and 80% of the -MacOD- units, but depended on the length of the grafted tail. The NP coating and hydrophilization did not occur when the modified copolymer contained 750 Da PEG tails independent of the grafting degree. To explain this phenomenon, the micellization of the modified PMAcOD copolymers in water was analyzed by small-angle X-ray scattering. The PMAcOD molecules with the grafted 750 Da PEG tails form compact noninteracting disklike micelles, whose stability apparently allows for no interactions with the NP hydrophobic shells. The PMAcOD containing the 5000 Da PEG and EG tails form much larger structures capable of an efficient coating of the NPs. The coated NPs were characterized using transmission electron microscopy, dynamic light scattering, zeta-potential measurements, and thermal gravimetry analysis. The latter method demonstrated that the presence of long PEG tails in modified PMAcOD allows the attachment of fewer macromolecules (by a factor of similar to 20) compared with the case of nonmodified or EG-modified PMAcOD, emphasizing the importance of PEG tails in NP hydrophilization. The NPs coated with PMAcOD modified with 60% (toward all -MAcOD- units) of the 5000 PEG tails bear a significant negative charge and display good stability in buffers. Such NPs can be useful as magnetic cores for virus-like particle formation.