¹⁹F MRI of Polymer Nanogels Aided by Improved Segmental Mobility of Embedded Fluorine Moieties

Using fluorinated probes for ¹⁹F MRI imaging is an emerging field with potential utility in cellular imaging and cell tracking in vivo, which complements conventional ¹H MRI. An attractive feature of ¹⁹F-based imaging is that this is a bio-orthogonal nucleus and the naturally abundant isotope is NMR active. A significant hurdle however in the ¹⁹F MRI arises from the tendency of organic macromolecules, with multiple fluorocarbon substitutions, to aggregate in the aqueous phase. This aggregation results in significant loss of sensitivity, because the T₂ relaxation times of these aggregated ¹⁹F species tend to be significantly lower. In this report, we have developed a strategy to covalently trap nanoscopic states with an optimal degree of ¹⁹F substitutions, followed by significant enhancement in T₂ relaxation times through increased segmental mobility of the side chain substituents facilitated by the stimulus-responsive elements in the polymeric nanogel. In addition to NMR relaxation time based evaluations, the ability to obtain such signals are also evaluated in mouse models. The propensity of these nanoscale assemblies to encapsulate hydrophobic drug molecules and the availability of surfaces for convenient introduction of fluorescent labels suggest the potential of these nanoscale architectures for use in multimodal imaging and therapeutic applications.