Heavily superparamagnetic magnetite-loaded polymeric worm-like micelles that have an ultrahigh T2 relaxivity

Abstract

Composite worm-like micelles (CWMs) that encapsulate a large number of superparamagnetic magnetite nanoparticles (SMNPs) within the core while still having a small width should have a high magnetic resonance sensitivity and simultaneously a desirable circulation behaviour in biological systems. However, the preparation of such CWMs is still a challenge. Herein we report the fabrication of CWMs via two-stage self-assembly of SMNPs guided by a block copolymer and DNA, respectively. First, poly(ethylene glycol)-b-poly(4-vinylpyridine) co-assembled with SMNPs to form spherical composite micelles, guiding the clustering of multiple SMNPs within the core of the micelles. Then, at the second stage, DNA co-assembled with and guided the linear aggregation and fusion of the micelles, resulting in the CWM encapsulating a large number of SMNPs in the linear core. As a proof of concept, CWMs with a T₂ relaxivity as high as 704 mM⁻¹ s⁻¹ and a low MRI detection limit of 2.2 μg mL⁻¹ (∼0.02 nM CWMs) were prepared; it is significant that the T₂ relaxivity of the SMNPs assembled in the CWMs is around 20 times that of the same SMNPs in the same amount but individually dispersed in the suspension. Additionally, although heavily loaded with SMNPs, the CWMs have a width as small as 20.5 nm and a core–shell structure with PEG as the shell. These features should make the CWMs very promising in their use as ultra-sensitive magnetic resonance imaging probes.