Dynamic Nuclear Polarization of Silicon-Based Nanoparticle Magnetic Resonance Imaging Agents

Introduction Silicon-based nanoparticles offer promise as biologically targeted magnetic resonance imaging (MRI) agents based on their exceptional NMR properties [1], lack of background signal and diverse functionalization chemistry [2]. Specifically, the long room-temperature nuclear relaxation (T1) times makes them suitable candidates as ex-vivo polarized imaging agents, as they can be transported and administered on practical time scales without significant loss of polarization. Low temperature Dynamic Nuclear Polarization (DNP) has been shown to be powerful technique for enhancing the nuclear spin polarization of silicon particles many orders of magnitude above the equilibrium value [3]. This technique employs microwave irradiation of paramagnetic defects that exist at the silicon-silicon dioxide interface, which in turn polarize nearby nuclear spins. Polarization of the crystalline core of the nanoparticle is then achieved via nuclear spin diffusion through dipole-dipole interactions from nuclei near the surface. In this study we present results from DNP experiments on silicon-based nanoparticles of a variety of sizes, morphologies and fabrication methods. We will also discuss requirements for transporting prepolarized particles and mechanisms for imaging the hyperpolarized nanoparticles in-vivo.