Poly(N-isopropylacrylamide)-Coated Superparamagnetic Iron Oxide Nanoparticles: Relaxometric and Fluorescence Behavior Correlate to Temperature-Dependent Aggregation

Magnetic nanoparticles coated with polymers have existing and further potential for applications in medicine, including magnetic resonance imaging (MRI) imaging for diagnostics and assessing biodistribution, and their capacity to remotely generate heat in response to alternating current magnetic fields to elicit biological responses. Advances in such domains rely on developing better understanding of how such materials respond to magnetic fields. This paper reports thermosensitive properties of poly(N-isopropylacrylamide-co-Nile Red)-coated magnetic nanostructures upon passing through the lower critical solution temperature (LCST) of the polymer brush, and correlation of this behavior with formation of aggregates. Discrete magnetite nanoparticles coated with brush layers comprised of terminally attached poly(N-isopropylacrylamide-co-Nile Red) afforded highly water-dispersible nanoparticles (intensity average diameter of 38 nm) with good colloidal stability in phosphate buffers. The nanoparticles displayed enhanced transverse (T-2) NMR relaxometric behavior as the temperature was increased through the LCST. Moreover, incorporation of the environmentally sensitive Nile Red dye into the copolymer leads to significant changes in fluorescence emission intensity upon aggregation that present new possibilities for the remote fluorescence detection of aggregated structures.