Molecular Stereocomplexation for Enhancing the Stability of Nanoparticles Encapsulated in Polymeric Micelles for Magnetic Resonance Imaging

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A generalizable approach for improving the stability of polylactide-based (PLA-based) micelles for encapsulating nanoparticles (NPs) is demonstrated, using stereocomplexation between a pair of poly (ethylene glycol)- b -poly( d -lactide)/poly(ethylene glycol)- b -poly( l -lactide) block copolymer blends. Three different superparamagnetic ferrite-based NPs with distinct nanostructures are first prepared by the high-temperature pyrolysis method, including spherical MnFe 2 O 4, cubic MnFe 2 O 4, and core–shell MnFe 2 O 4 @Fe 3 O 4 . The diameters of these NPs are approximately 7–10 nm as revealed by transmission electron microscopy. These hydrophobic NPs can be encapsulated within self-assembled, stereocomplexed PLA (sc-PLA) micelles. All sc-PLA micelle systems loaded with three different NPs exhibit enhanced stability at elevated temperatures (20–60 °C) and with extended storage time (∼96 h) compared with analogous samples without stereocomplex formation, confirmed by dynamic light scattering measurements. The magnetic NP-loaded micelles with mean diameters of approximately 150 nm show both biocompatibility and superparamagnetic property. Under a 1.5 T magnetic field, cubic MnFe 2 O 4 (c-MnFe 2 O 4 )-loaded micelles exhibit an excellent negative contrast enhancement of MR signals (373 mM –1 ·s –1 ), while core–shell MnFe 2 O 4 @Fe 3 O 4 -loaded micelles show a slightly lower signal for MR imaging (275 mM –1 ·s –1 ). These results suggest the potential of using sc-PLA-based polymer micelles as universal carriers for magnetic resonance imaging contrast agents with improved stability for different applications such as cancer diagnosis.