(519b) The Design of Well-Defined Polymer-Magnetite Complexes for Biomedical Applications

Magnetite nanoparticles have many uses, in particular as contrast agents for magnetic resonance imaging (MRI) and for incorporation into magnetic ferrofluids.^1,2 Magnetite particles have relatively low cytotoxicities, and their surfaces are amenable to the adsorption of biocompatible macromolecules.³ Thus, specific functionalities can be attached to magnetite nanoparticles in order to optimize a specific property for appropriate applications.⁴ Rapid improvement in biomaterials would be greatly enhanced by starting with a well-defined system with a controlled, narrow particle size distribution rather than using systems where aggregation is not controlled well and the particle size distribution is poorly defined, as is the case with some commercial products. Thus, the goal of our work has been to develop a model to distinguish between slight aggregates and single polymer-particle systems and to allow for design of complexes with controlled size and colloidal stability.

In this work, we examined two distinctly different polymer-magnetite systems. The first consisted of poly(dimethylsiloxane) (PDMS) stabilized magnetite nanoparticles. The magnetite was synthesized using two different methods. The first method was the coprecipitation of iron chlorides with ammonium hydroxide. Through adaptation of a density distribution model and comparison to dynamic light scattering (DLS) results, it was determined that there were slight aggregates formed from this method. Following extensive magnetic separation, a single polymer-particle system could be obtained. However, approximately 80 percent of the material must be separated to achieve this well-defined system, and so a better synthetic route was needed. Thus, a second magnetite synthesis was adapted from Pinna et al.⁵, which consists of the high temperature decomposition of iron acetylacetonate at high temperature in benzyl alcohol. Again, through comparison of the model to DLS, it was determined that the complexes were not aggregated. The model matched the experimentally obtained numbers to approximately seven percent, which is in line with experimental error.

The second polymer-magnetite system utilized the high temperature magnetite synthesis, but used a polyether as the polymer stabilizer. The polyether stabilizers used were poly(ethylene oxide) (PEO) homopolymers and amphiphilic poly(propylene oxide)-b-poly(ethylene oxide) (PPO-b-PEO) block copolymers. The same methodology was used to model the complexes as was used for the PDMS system. Again, the model matched within eight percent of experimental measurements obtained from DLS, indicating a well-defined particle size distribution. The extended Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was used to predict the colloidal stability of these complexes. The model was able to predict when the onset of aggregation would occur, thus allowing us to determine when a single-particle system began to cluster.

Thus, we now have a tested tool for designing the size of polymer-magnetite complexes to within eight percent of a desired value. This tool also allows for the determination of the onset of aggregation, allowing the optimization of the polymer loading. Finally, because there are no adjustable parameters in the model, this method should be applicable to various polymer-particle systems, and allow for the rapid development of well-defined systems for biomedical applications.

1. Mefford, et al. Chemistry of Materials, 2008, 20(6), 2184-2191.

2. Huffstetler, P. P, et al. Polymer Preprint, 2008, 49(2), 1103-1104.

3. Harris, L., et al. Chemistry of Materials 2003, 15(6), 1367-1377.

4. Miles, W.C., et al. Langmuir 2009, 25(2), 803-813.

5. Pinna, N., et al. Chemistry of Materials 2005, 17, 3044-3049.