Bulk Nuclear Hyperpolarization of Inorganic Solids by Relay from the Surface

Abstract

NMR is a method of choice to determine structural and electronic features in inorganic materials, and has been widely used in the past, but its application is severely limited by its low relative sensitivity. We show how the bulk of proton-free inorganic solids can be hyperpolarized with a general strategy using impregnation dynamic nuclear polarization through homonuclear spin diffusion between low-γ nuclei. This is achieved either through direct hyperpolarization or with a pulse cooling cross-polarization method, transferring hyperpolarization from protons to heteronuclei at particle surfaces. We demonstrate a factor of 50 gain in overall sensitivity for the Sn spectrum of powdered SnO, corresponding to an acceleration of a factor >2500 in acquisition times. The method is also shown for P spectra of GaP,Cd spectra of CdTe, and Si spectra of α-quartz.

High-Field Magic Angle Spinning Dynamic Nuclear Polarization Using Radicals Created by γ-Irradiation

“…Quantitative 13 C CPMAS experiments show that 30 kGy glucose has 81% of the signal intensity of non-irradiated glucose, confirming that the effects of paramagnetic broadening/depolarization are minimal ( Figure S9). Since glucose is a microcrystalline organic solid, the traditional IWI method was also used to obtain a DNP-enhanced 13 C CPMAS spectrum. 10 Ground glucose impregnated with a 16 mM TEKPol tetrachloroethane gave eC CP of (with a s recycle delay), comparable to eC CP measured for the γ-irradiated glucose at optimal field position ( Figure S8).…”

“…[8][9][10][11][12] Recently, it was demonstrated that spin-1/2 nuclei with moderate natural isotopic abundances and gyromagnetic ratios ( Si, Cd, Sn) can also relay the hyperpolarization into the bulk of inorganic solids. [13][14] In these cases, however, the spin diffusion rate is greatly reduced compared to 1 H and, as a consequence, these experiments typically require very long polarization delays/spin diffusion periods and provide reduced DNP enhancements (εH ≈ 10-85, Scheme 1B). [13][14] Radicals may also be introduced during synthesis/precipitation of solid materials.…”

“…[13][14] In these cases, however, the spin diffusion rate is greatly reduced compared to 1 H and, as a consequence, these experiments typically require very long polarization delays/spin diffusion periods and provide reduced DNP enhancements (εH ≈ 10-85, Scheme 1B). [13][14] Radicals may also be introduced during synthesis/precipitation of solid materials. For example, biradical polarizing agents have been added during precipitation/crystallization of organic solids from solution or been directly incorporated into polymers by swelling them with radical solution or precipitating them from radical solution.…”

Probing the Surface Structure of Semiconductor Nanoparticles by DNP SENS with Dielectric Support Materials

“…The theoretical maximum DNP signal enhancement (ε) is proportional to the gyromagnetic ratio of the electrons and a given nucleus (maximum εH of 658 for protons). 59,61 DNP solid-state NMR has previously been applied to a variety of systems including biomolecules, [62][63] pharmaceuticals, 64 polymers, 65 heterogeneous catalysts, [66][67][68] bulk inorganic materials, [69][70][71] battery materials, 72 and nanoparticles. 51-52, 55, 66, 73-80 DNP surface enhanced NMR spectroscopy (SENS) has been demonstrated as a general method to enhance NMR signals from interfaces/surfaces of inorganic materials and heterogeneous catalysts.…”

Improved sensitivity and quantification for 29Si NMR experiments on solids using UDEFT (Uniform Driven Equilibrium Fourier Transform)

“…the microwave-driven transfer of polarization from unpaired electrons to 29 Si nuclei. This approach has been demonstrated first at low static magnetic field B0 = 1.4 T [38], and more recently at B0 ≥ 9.4 T [39][40][41]. DNP can be combined with either 1 H 29 Si CPMAS [39], its multiple contact version [41], or CPMG [42].…”