Controllable Assembly of Hydrophobic Superparamagnetic Iron Oxide Nanoparticle with mPEG-PLA Copolymer and Its Effect on MR Transverse Relaxation Rate

Assembly of individual superparamagnetic iron oxide nanoparticles (SPION) into cluster is an effective way to prepare MRI contrast agent with high relaxivity. In this study, we fabricated SPION clusters with different sizes and configurations by assembly of amphiphilic mPEG-PLA copolymer with hydrophobic SPION in aqueous solution. The evolution of cluster size and configuration with the amount of copolymer and the effect of cluster size on the transverse relaxivity was studied. T2 relaxation rates of clusters with different sizes at iron concentration of 0.1mM were compared with the theoretical predictions. We found that the relative amount of copolymer/SPION was crucial for the formation of SPION cluster. The transverse relaxivity of the condense SPION clusters (CSC) was size-dependent. The experimentally measured T2 relaxation rates of the clusters were lower than the theoretical predictions. In motional average regime (MAR) region, T2 relaxation rates were more consistent with the theoretical values when transmission electron microscope (TEM) evaluated size was used. Therefore, for fabrication of SPION clusters with assembly of mPEG-PLA and hydrophobic SPION, delicate balance between the amount of copolymer and SPION should be pursued, and for comparison of experimental T2 relaxation rate with theoretical predictions, TEM evaluated size was more suitable.