Magnetic properties of iron oxide nanoparticles made them attractive for extensive studies and applications in biology, medicine and material sciences. There is a growing interest in application of these nanoparticles as MRI contrast agents. Due to their high magnetic moment, they enhance proton relaxation via outer-sphere mechanism and act as negative (T2) contrast agents.  In so-called positive (T1) contrast agents, the inner-sphere relaxation mechanism is utilized, due to interaction of protons with the high-spin d5 transition metal ions such as MnII and FeIII, or more commonly, the f7 GdIII ions.  The best currently known blood pool MRI agents are based on iron oxides and considered non-toxic.  It is documented that reducing the particle size below 5 nm lowers their magnetic moment, and therefore the outer-sphere relaxivity r2.  At the same time larger surface-to-volume ratio of these small particles causes the greater involvement of iron atoms in the spin-lattice relaxation process.   Consequently particle size reduction can be the way to obtain better T1 contrast agent.  We developed a synthesis of a highly pure magnetite by a high-temperature hydrolysis of iron(II) and iron(III) chelated alkoxide complexes in solutions of parent alcohol diethylene glycol (H2DEG).  [Fe(HDEG)2] + 2[Fe(HDEG)2Cl] + 2H2O + 2OH- ? Fe3O4 + 5H2DEG + 2Cl-    Magnetite nanoparticles were obtained as colloids stabilized by electrical double-layers of solvated ions and containing no capping ligands or surfactants, except for H2DEG. Current work is a systematic study of the effects of particle size, oxidation state and surface complexation on relaxivity properties. In order to study the redox properties, several oxidation reactions were carried out on prepared colloids using dry oxygen or bromine. 2Fe3O4(H2DEG) + Br2 ? 2Fe3O4(HDEG) + 2HBr 4Fe3O4(H2DEG) + O2 ?  4Fe3O4(HDEG) + 2H2O   While bromination occurs faster compared to the oxygenation, the by-product HBr can reduce the nanoparticle size or decompose them. Reaction with oxygen does not affect the nanoparticle size significantly.  In a post-synthesis step several capping ligands, such as citric or oleic acid, were used to solubilize the particles in different media.  Isolated nanoscale solids were characterized by X-Ray diffractomety, IR, TGA, TEM, and elemental analysis. All of the obtained colloids were characterized by Dynamic light scattering, and zeta-potential measurements. Relaxivity NMR experiments were performed in pure H2DEG and in water.  It was found that proton relaxation time is affected by both, the particle size and the iron oxidation state.  Details of these studies will be presented.

Superparamagnetic iron oxide nanoparticles with variable size and oxidation state as prospective labeling and imaging agents

244th National Meeting (2012)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.