Direct correlation of PNIPAM thermal transition and magnetic resonance relaxation of iron oxide nanoparticles

Poly(N-isopropylacrylamide) (PNIPAM), which undergoes a temperature dependent transition from hydrophilic to hydrophobic, has played a crucial role in the development of stimuli-responsive multifunctional nanoparticles. In particular, iron oxide nanoparticles coated with PNIPAM have been effectively developed to enable stimuli responsive drug delivery and imaging agents. However, the PNIPAM transition from hydrophilic to hydrophobic at physiologically relevant temperatures renders colloidal nanoparticles unstable resulting in aggregation and precipitation from solution. Consequently, a direct correlative analysis of the effect of the thermally induced phase transition of PNIPAM on the magnetic resonance properties of nanoparticles has not been possible as the changes in proton relaxivity have been dominated by the colloidal agglomeration of the nanoparticles. Herein, we report colloidally stable thermoresponsive PNIPAM-grafted-PGMA coated magnetite core/shell nanoparticles (PNIPAM-PGMA-NPs) that enable the direct analysis of the effect of PNIPAM phase changes in solution on the overall magnetic resonance relaxivity of nanoparticles in suspension.