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Si nanoparticles (NPs) have been actively developed as a hyperpolarizedmagnetic resonance imaging (MRI) agent with an imaging window of more than onehour. However, the progress in the development of NPs has been hampered by theincomplete understanding of their structural properties that correspond toefficient hyperpolarization build up and long polarization decays. In this workwe study dynamic nuclear polarization (DNP) of single crystal porous Si (PSi)NPs with defined doping densities ranging from nominally undoped to highlydoped with boron or phosphorus. To develop such PSi NPs we perform low-loadmetal-assisted catalytic etching for electronic grade Si powder followed bythermal oxidation to form the dangling bonds in the $\text{Si/SiO}_2$interface, the $P_b$ centers, which are the endogenous source of the unpairedelectron spins necessary for DNP. The controlled fabrication and oxidationprocedures allow us to thoroughly investigate the impact of the magnetic field,temperature and doping on the DNP process, as well as to identify the ratelimiting step for the polarization buildup and decay. We argue that the buildupand decay rate constants are limited by the polarization transfer across thenuclear spin diffusion barrier determined by the large hyperfine shift of thecentral \textsuperscript{29}Si nuclei of the $P_b$ centers. Finally, we findthe room temperature relaxation of low boron doped PSi NPs reaching $75 \pm 3$minutes and nuclear polarization levels exceeding $\sim 6$ % when polarized at6.7 T and 1.4 K. Our study thus establishes solid grounds for furtherdevelopment of Si NPs.

Controlled synthesis and characterization of porous silicon nanoparticles for dynamic nuclear polarization