Superoxide overproduction is known to occur in
multiple disease states requiring critical care yet non-invasive detection of
superoxide in deep tissue remains a challenge. Herein, we report a metal-free
magnetic resonance imaging (MRI) and electron paramagnetic resonance (EPR)
active contrast agent prepared by “click conjugating” paramagnetic organic
radical contrast agents (ORCAs) to the surface of tobacco mosaic virus (TMV).
While ORCAs are known to be reduced <i>in vivo</i>
to an MRI/EPR silent state, their oxidation is facilitated specifically by
reactive oxygen species—in particular superoxide—and are largely unaffected by
peroxides and molecular oxygen. Unfortunately, single molecule ORCAs typically
offer weak MRI contrast. In contrast, our data confirm that the macromolecular
ORCA-TMV conjugates show marked enhancement for <i>T<sub>1</sub></i> contrast at low field (<3.0 T), and <i>T<sub>2</sub></i> contrast at high field
(9.4 T). Additionally, we demonstrated that the unique topology of TMV allows
for “quenchless fluorescent” bimodal probe for concurrent fluorescence and
MRI/EPR imaging, which was made possible by exploiting the unique inner and
outer surface of the TMV nanoparticle. <a>Finally, we show
TMV-ORCAs do not respond to normal cellular respiration, minimizing the
likelihood for background, yet still respond to enzymatically produced
superoxide in complicated biological fluids like serum.</a>