Hyperpolarization of Silicon Nanoparticles with TEMPO Radicals

Silicon-based particles can be hyperpolarized via dynamic nuclear polarization to enhance ²⁹Si magnetic resonance signals. Application of this technique to nanoscale silicon particles has been limited because of the low signal enhancements achieved; it is hypothesized that this is due to the low number of endogenous electronic defects inherent to the particles. We introduce a method of incorporating exogenous radicals into silicon nanoparticle suspensions in order to improve the hyperpolarization of ²⁹Si nuclear spins to levels sufficient for in vivo MR imaging. Calibration of radical concentrations and polarization times are reported for a variety of silicon particle sizes (30-200 nm in diameter), with optimal radical concentrations of 30-60 mM. Addition of the radical slightly affects the T₁ relaxation of the nanoparticles; however, these losses in T₁ are overcome by the overall improvement in ²⁹Si magnetization. With optimal amounts of the added radical, ²⁹Si T₁ times are ∼20 min, and MR images in phantoms can be achieved over an hour after hyperpolarization. Co-registered ¹H/²⁹Si MR imaging of nanoparticles administered to a mouse model is also presented.