Silicon nanoparticles combine room-temperature spin–lattice (T1) relaxation times exceeding 15 min with bio-compatibility but often require more than 10 h of dynamic nuclear polarization (DNP) to achieve 2–4% silicon polarization. In this work, we show that it is possible to hyperpolarize 20 nm silicon nanoparticles with room-temperature T1 ≥ 40 min via cross-polarization (CP) from solvent protons using exogenous radicals and achieve more than 7% polarization in around 1 h. This effectively doubles the steady-state polarization in around one-tenth of the time compared to direct DNP with endogenous defects under the same experimental conditions. Silicon polarization levels with CP appear limited by proton polarization levels (around 9%), making future improvements with better sample formulations likely.
Witte et al. (Wed,) studied this question.