Controlled loading of superparamagnetic nanoparticles in fluorescent nanogels as effective T2-weighted MRI contrast agents

Spherical superparamagnetic iron oxide nanoclusters (IONCs) with well-controlled shape and size were fabricated. The formation of IONCs was induced by a solvent template-assisted organization of nanoparticles in a polymeric nanogel. An amphiphilic brush copolymer was chosen as the nanogel material because it had a high density of alkyl side-chains that interdigitate through hydrophobic interactions in water to form a stable nanogel matrix. Additionally, the hydrophilic backbone of the copolymer mixed into the nanogel matrix conferred both colloidal stability and important water swelling properties. The hydrodynamic size of IONCs was well-controlled to <200 nm using appropriate emulsion process conditions and displayed excellent long-term dispersibility in water and phosphate buffer saline. The IONCs acted as effective centers of magnetism and MRI measurements clearly showed substantial improvement as the packing density of the magnetic cores increased. A method of estimating intra-particle magnetic interaction distance was established based on calculations from SPION/IONC size and SPION loading. Further functionality was readily introduced by modifying the nanogel with fluorescein for optical tagging. This work offers a robust and versatile platform for the development of waterborne nanoprobes with tunable magnetic properties and versatile chemical functionalities for bio-applications.