High-Field Liquid-State Dynamic Nuclear Polarization in Microliter Samples

Abstract

Nuclear hyperpolarization in the liquid state by dynamic nuclear polarization (DNP) has been of great interest because of its potential use in NMR spectroscopy of small samples of biological and chemical compounds in aqueous media. Liquid state DNP generally requires microwave resonators in order to generate an alternating magnetic field strong enough to saturate electron spins in the solution. As a consequence, the sample size is limited to dimensions of the order of the wavelength, and this restricts the sample volume to less than 100 nL for DNP at 9 T (∼260 GHz). We show here a new approach that overcomes this sample size limitation. Large saturation of electron spins was obtained with a high-power (∼150 W) gyrotron without microwave resonators. Since high power microwaves can cause serious dielectric heating in polar solutions, we designed a planar probe which effectively alleviates dielectric heating. A thin liquid sample of 100 μm of thickness is placed on a block of high thermal conductivity aluminum nitride, with a gold coating that serves both as a ground plane and as a heat sink. A meander or a coil were used for NMR. We performed H DNP at 9.2 T (∼260 GHz) and at room temperature with 10 μL of water, a volume that is more than 100× larger than reported so far. TheH NMR signal is enhanced by a factor of about -10 with 70 W of microwave power. We also demonstrated the liquid state of P DNP in fluorobenzene containing triphenylphosphine and obtained an enhancement of ∼200.

Influence of Rotational Motion of Nitroxides on Overhauser Dynamic Nuclear Polarization: A Systematic Study at High Magnetic Fields

“…Unfortunately, in this case, the size of the mw resonator severely limits the size of the sample to below 1 μL. Successful Overhauser DNP experiments on liquids have also been performed with oversized resonance structures or without such mw resonators, achieving larger sample volumes. , Additionally, similar to the polarization transfer mechanism active in solids, all of these efficiencies decrease rapidly at higher magnetic fields. For Overhauser DNP in liquids at high magnetic fields, this is due to the lack of the necessary fast translational and rotational correlation times on a sub-ps time scale…”

Spin density localization and accessibility of organic radicals affect liquid-state DNP efficiency

“…At high magnetic fields, the choice of an optimal PA would help the application of the method in analytics and high resolution NMR spectroscopy. [11][12][13][14][15] Furthermore, higher NMR enhancements could boost the applications of OE-DNP at low fields (o2 T), which include in-flow hyperpolarization for magnetic resonance imaging 16,17 or chromatography, 18,19 NMR relaxometry of low-g nuclei, 20 hydration dynamic studies, 21,22 and DNP-NMR spectroscopy. [23][24][25] In the experimental practice, nitroxide derivatives (NODs) have been established as optimal PAs for OE-DNP in the liquid state at room temperature and ambient pressure.…”

Synergies between Hyperpolarized NMR and Microfluidics: A Review

“…Despite the gain in polarization level in the improved device, the maximum polarization (∼7%) is still an order of magnitude lower than the 129 Xe polarization levels achievable in large-scale polarizers [13].…”

“…Hyperpolarized 129 Xe has been used to study porous materials, and to extract information about the structure of the nano-and micro-scale pores [159][160][161]. In this type of work microfluidic control is not exerted over the hyperpolarized species, and the specific advantages of microfluidics are not utilized, and hence this work is outside the scope of this review.…”