(81c) Tunable Clustering of Magnetic Nanoparticles Using Poly(amino acid) Corrals for Improved r2 Relaxation

Superparamagnetic iron oxide nanoparticle (SPION) clusters produce localized magnetic field inhomogeneities which could dramatically enhance T₂ relaxivity for magnetic resonance (MRI) and magnetic particle imaging (MPI). In this work, we report high and tunable T₂ relaxivity in hydrophobic SPION clusters self-assembled with amphiphilic polyethylene glycol-b-poly(L-Leucine) block copolymers, PEG-b-p(L-Leu)_m where the PEG chains were terminated by methoxy (CH₃-O-PEG₁₁₃) or hydroxyl groups (HO-PEG₇₇). SPION clusters ranged in size up to 200 nm with the cluster diameter being dependent on the chain length of the hydrophobic p(L-Leu)_m block. The r₂ relaxivity, determined from the SPIONs concentration dependence of T₂, of the assembled SPIONs increased with cluster diameter consistent with greater magnetic dipole-dipole interactions within larger clusters and the chemical exchange theory. Terminating the PEG segment with a hydroxyl group that has a high affinity for water resulted in SPION clusters formed with HO-PEG₇₇-b-p(L-Leu)_m having a near 10-fold higher r₂ relaxivity compared to mPEG₁₁₃-b-p(L-Leu)_m block copolymers, as well as commercially available SPION-based MRI contrast agents.