Toward Design of Magnetic Nanoparticle Clusters Stabilized by Biocompatible Diblock Copolymers for T 2‑Weighted MRI Contrast

We report the fabrication of magnetic particles comprised of clusters of iron oxide nanoparticles, 7.4 nm mean diameter, stabilized by a biocompatible, amphiphilic diblock copolymer, poly(ethylene oxide-b-d,l-lactide). Particles with quantitative incorporation of up to 40 wt % iron oxide and hydrodynamic sizes in the range of 80–170 nm were prepared. The particles consist of hydrophobically modified iron oxide nanoparticles within the core-forming polylactide block with the poly(ethylene oxide) forming a corona to afford aqueous dispersibility. The transverse relaxivities (r 2) increased with average particle size and exceeded 200 s–1 mM Fe–1 at 1.4 T and 37 °C for iron oxide loadings above 30 wt %. These experimental relaxivities typically agreed to within 15% with the values predicted using analytical models of transverse relaxivity and cluster (particle core) size distributions derived from cryo-TEM measurements. Our results show that the theoretical models can be used for the rational design of biocompatible MRI contrast agents with tailored compositions and size distributions.