Nitroxide Radicals@US-Tubes: New Spin Labels for Biomedical Applications

The encapsulation of nitroxide radicals within ultrashort (ca. 50 nm) singlewalled carbon nanotubes (US-tubes) is achieved. Tempo- and Iodo-Tempo@ US-tubes are characterized by thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Electron paramagnetic resonance (EPR) spectra display characteristic signals due to the detection of the spin probes within the US-tubes. Longitudinal proton relaxivities ( r 1 ) of both nitroxide@US-tubes samples are 7 to 13 times greater than the free nitroxide radicals in solution, giving relaxivities comparable to the clinical contrast agent (CA) Magnevist. In addition, transverse proton relaxivities ( r 2 ) show unprecedented proton relaxation enhancement in comparison to any other reported nitroxide radical-based system or the clinically approved T 2 CA, Resovist, under the same conditions. T 2 -weighted magnetic resonance imaging (MRI) phantom images show that the encapsulation of nitroxide radicals within the US-tubes produces good contrast enhancement due to their high r 2 relaxivities. The nitroxide radicals@US-tube agents are a new promising class of spin probes for MRI and electronic paramagnetic resonance imaging (EPRI) labeling, tracking, and diagnosis.