Hydrophilic Magnetite Nanoparticles Complexed with Poly(ethylene oxide) Copolymers – Synthesis and Physical Behavior

J.S. Riffle and A.Y. Carmichael, Virginia Tech

U.O. Häfeli and J. Unnithan, The Cleveland Clinic Foundation and D. Bardenstein, Case Western Reserve University, Cleveland, OH

T.G. St. Pierre and L.A. Harris, University of Western Australia, Perth, AU

Magnetic nanoparticles that display high saturation magnetization and high magnetic susceptibility are of great interest for medical applications. Magnetite nanoparticles display strong ferrimagnetic behavior, and are less sensitive to oxidation than magnetic transition metal nanoparticles such as cobalt, iron and nickel. For in-vivo applications, well-defined organic coatings are needed to surround the magnetite nanoparticles and prevent any aggregation. One goal has been to develop complexes of magnetite nanoparticles coated with well-defined hydrophilic copolymers, so that the complexes can be dispersed in aqueous fluids. Focal points of the synthetic work include: (1) Investigations of polymer systems which bind irreversibly to magnetite at the physiological pH, (2) The design of block copolymers with anchor and tail blocks to enable dispersion in biological fluids, and (3) Investigations of copolymer block lengths to maximize the concentration of bound magnetite and to understand effects on cell interactions.

Hydrophilic triblock copolymers with controlled concentrations of pendent carboxylic acid binding groups were designed as steric stabilizers for magnetite nanoparticles.¹ These copolymers were comprised of controlled molecular weight poly(ethylene oxide) tail blocks and a central, polyurethane anchor block containing carboxylic acids. Stoichiometric aqueous solutions of FeCl₂ and FeCl₃ were condensed by reaction with NH₄OH to form magnetite nanoparticles, then a dichloromethane solution of the block copolymer was added to adsorb the copolymer onto the magnetite surfaces. Stable magnetite dispersions were prepared with all of the triblock copolymers.

The polymer-nanomagnetite conjugates had a maximum saturation magnetization of 34 emu g^-1. Magnetization curves showed minimal hysteresis. Powder X-ray diffraction (XRD), selected area electron diffraction (SAED) and high resolution electron microscopy (HREM) confirmed the magnetite crystal structure. Transmission electron microscopy (TEM) showed that the dispersions contained magnetite particles coated with the polymers with a mean diameter of 8.8 ± S.D. 2.7 nm. The in vitro biocompatibility and viability as measured with the MTT assay - a test for proper functioning of the cells' mitochondrial processes - in both prostate cancer cells (C4-2) and human retinal pigment epithelial cells showed that the polymer-nanomagnetite conjugates with the longer tail blocks (5K, 15K) did not influence cell growth. Shorter tail blocks (0.75K) on the polymer-nanomagnetic complexes, however, reduced the viability of the cells.

1. L. A. Harris, J. D. Goff, A. Y. Carmichael, J. S. Riffle, J. J. Harburn, T. G. St. Pierre and M. Saunders, Magnetite Nanoparticle Dispersions Stabilized with Triblock Copolymers, Chemistry of Materials, 15(6), 1367-1377 (2003).