Proton Hyperpolarization Relay from Nanocrystals to Liquid Water

Dynamic nuclear polarization (DNP) using transient electron spin polarization generated by photoexcitation can improve nuclear magnetic resonance (NMR) sensitivity far beyond the thermal equilibrium limit for analysis in life science and drug discovery. However, DNP of liquid water at room temperature remains an important challenge. In previous studies, polarization has been transferred directly from the electron spins in the solid to the nuclear spins of the target, and this has been limited to near-surface solid or highly-viscous targets. Here, we propose a new method called hyperpolarization relay, in which the polarization of electron spins is transferred to proton spins in the nanocrystals and then to proton spins in bulk water by the nuclear Overhauser effect (NOE). Molecular nanocrystals doped with a polarizing agent that generates a highly-polarized photoexcited triplet were synthesized by a reprecipitation method while controlling the size of the nanocrystals. As the size of the nanocrystals decreases, the efficiency of polarization transfer from nanocrystals to water was improved due to the increase in the surface area.