A GENERALIZED METHOD FOR MAGNETITE NANOPARTICLE STERIC STABILIZATION UTILIZING BLOCK COPOLYMERS CONTAINING CARBOXYLIC ACIDS

INTRODUCTION: Magnetic nanoparticles that display high saturation magnetization and high magnetic susceptibility are of great interest for medical applications. Nanomagnetite is particularly desirable because it displays strong ferrimagnetic behavior, and is less sensitive to oxidation than magnetic transition metals such as cobalt, iron, and nickel. For in-vivo applications, it is important that well-defined organic coatings surround the nanomagnetite particles. It is rationalized that this will prevent any aggregation of the nanoparticles in-vivo, and may also enable efficient excretion and protection of the body from toxicity. Magnetite nanoparticles can be prepared by coprecipitating iron (II) and iron (III) chloride salts in the presence of ammonium hydroxide at pH 9-10. Oleic acid is known to effectively stabilize dispersions of nanomagnetite in nonpolar solvents [1,2]. Stabilization occurs because the carboxylic acid group covalently reacts with the surface of the magnetite and the aliphatic chain extends out into the nonpolar solvent, preventing aggregation of the particles by a steric (entropic) mechanism. One goal of this work has been to develop a generalized methodology for stabilizing nanomagnetite dispersions using well-defined, non-toxic, block copolymers, so that the resultant magnetite-polymer complexes can be used in a range of biomedical materials. Our objectives have included: (1) Understanding what types of polymer structures bind irreversibly to magnetite at the physiological pH and what block lengths are desirable, (2) Tailoring polymer block lengths to maximize the concentration of bound magnetite, yet preserve good dispersion and (3) Designing copolymers with both hydrophilic and hydrophobic tail blocks to enable dispersion in different types of carrier fluids.