Magnetic nanoparticles have long been recognized as T2-contrast agents for MRI. We report colloidally-stable magnetite clusters with hydrophobic and hydrophilic cores, and controlled sizes and compositions. Magnetite nanoparticles (~ 8 nm) were synthesized by the reduction of ferric acetylacetonate in benzyl alcohol. Hydrophobic-core clusters were prepared by rapid nanoprecipitation using an amphiphilic poly(D,L-lactide)-poly(ethylene oxide) diblock copolymer stabilizer. For the hydrophilic-core clusters, magnetite-poly(ethylene oxide-b-acrylate) complexes were synthesized, followed by crosslinking the amine tips of the H2N-PEO corona with PEO diacrylate. The transverse relaxivities of the two classes of clusters correlated with particle sizes and compositions. Interestingly, the hydrophilic-core clusters had r2s approaching the static dephasing limit of ~ 600 s-1.mM Fe-1 and were more than twice that of the hydrophobic-core clusters at similar sizes and compositions. We posit that the high magnetic field gradients accessible to water protons by diffusion within the core of the hydrophilic clusters lead to the higher relaxivities.

Structure-relaxivity relationships of well-defined magnetite clusters for sensitive magnetic resonance imaging

245th National Meeting (2013)

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