Ferrofluid based on polyethylene glycol-coated iron oxide nanoparticles: Characterization and properties

We report here the development of stable aqueous suspensions of superparamagnetic iron oxide nanoparticles stabilized with unmodified polyethylene glycol (PEG) at four molecular weights (2000, 4000, 6000 and 10,000 Da) and several PEG/iron ratios. The obtained ferrofluid was an opaque dispersion of pH ≈ 5.0 with an iron content ranging from 15 mg mL−1 to 17 mg mL−1 (depending on the molecular weight of the PEG used), and a ζ-potential of 20 ± 2 mV. The diameter of uncoated particles determined by X-ray diffraction was ≈12 nm; a value that ensures superparamagnetic properties and suitability for use in hyperthermic therapy. When coated and determined by dynamic light scattering, the hydrodynamic diameter of the particles was 56 nm for the lowest PEG/iron ratio. Thermogravimetric analysis showed that the content of PEG in the ferrofluid ranged from 6.5% to 23.8%, depending on the different molecular weights and amount of PEG used. The interaction of PEG with magnetite is probably due to dipole–cation binding between the ether group of the polymer and the positive charge of magnetite. As regards physical stability, the ferrofluid was stable in the absence of salt over a period of more than two years.