Room temperature hyperpolarization of nuclear spins in bulk

Abstract

Dynamic nuclear polarization (DNP), a means of transferring spin polarization from electrons to nuclei, can enhance the nuclear spin polarization (hence the NMR sensitivity) in bulk materials at most 660 times for 1 H spins, using electron spins in thermal equilibrium as polarizing agents. By using electron spins in photo-excited triplet states instead, DNP can overcome the above limit. We demonstrate a 1 H spin polarization of 34%, which gives an enhancement factor of 250,000 in 0.40 T, while maintaining a bulk sample (∼0.6 mg, ∼0.7 × 0.7 × 1 mm 3 ) containing >10 19 1 H spins at room temperature. Room temperature hyperpolarization achieved with DNP using photo-excited triplet electrons has potentials to be applied to a wide range of fields, including NMR spectroscopy and MRI as well as fundamental physics.N uclear spin is a useful probe for noninvasive analysis of bulk materials such as chemical compounds, industrial products, biological samples, and human bodies. The signal from a spin ensemble is proportional to the polarization P. In thermal equilibrium in a magnetic field B at temperature T, P for spin-1/2 particles is given bywhere Z is the Planck constant, k is the Boltzmann constant, and γ is the gyromagnetic ratio. In a magnetic field of several teslas at room temperature, the nuclear spin energy ZγB=2 is much smaller than the thermal energy kT, so nuclear spins are only slightly polarized. This is the major reason why the sensitivity of NMR spectroscopy and MRI is so low. Dynamic nuclear polarization (DNP) is a means of transferring spin polarization from electrons to nuclei. As a method to enhance the bulk nuclear polarization, DNP has been successfully applied to areas ranging from fundamental physics (1-3) to materials science (4), biology (5-7), and medical science (8), since it was discovered 60 y ago (9, 10). As long as electron spins in thermal equilibrium are used as polarizing agents, the upper limit of the polarization enhancement is 660 for 1 H spins and cryogenic temperatures of around 4.2 K are required for hyperpolarization in the order of 10% even in the strong magnetic fields used for NMR. Hyperpolarization at room temperature will simplify instrumentation and expand the sample variety to materials that prefer ambient temperatures. Other techniques such as optical pumping in semiconductors (11) and the Haupt effect (12) also require cryogenic temperature for increasing bulk polarization beyond 10%.One solution for overcoming the upper limit of the enhancement factor of the conventional DNP, γ e =γ n , is to use nonthermalized electron spins as polarizing agents. A number of organic molecules have photo-excited triplet states where, due to the selection rule in the intersystem crossing from the excited singlet state to the triplet state, the population distribution is highly biased. DNP using electron spins in the photo-excited triplet state can achieve hyperpolarization independent of the magnetic field strength and temperature (13)(14)(15)(16). In this work, we have achieved a bulk hy...

All-optical hyperpolarization of electron and nuclear spins in diamond

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Spins in exotic nuclei: RI beam experiments with polarized targets

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Time domain DNP with the NOVEL sequence

“…NOVEL was soon replaced by ISE which gives a much higher efficiency due to the adiabatic sweep of the magnetic field. 34,[38][39][40] Note that both of these samples were especially chosen to demonstrate the NOVEL or ISE effect. Thus, one of our goals in the results reported here was to investigate the potential of the NOVEL sequence in samples used in current applications of DNP in NMR, i.e., samples doped with a few tens of mM of stable free radicals such as the narrow-line species 1,3-bisdiphenylene-2-phenylallyl (BDPA) or nitroxides, both of which are used in contemporary CW DNP experiments.…”