Paramagnetic Metal‐Ion Dopants as Polarization Agents for Dynamic Nuclear Polarization NMR Spectroscopy in Inorganic Solids

Abstract

Dynamic nuclear polarization (DNP), a technique in which the high electron spin polarization is transferred to surrounding nuclei via microwave irradiation, equips solid-state NMR spectroscopy with unprecedented sensitivity. The most commonly used polarization agents for DNP are nitroxide radicals. However, their applicability to inorganic materials is mostly limited to surface detection. Paramagnetic metal ions were recently introduced as alternatives for nitroxides. Doping inorganic solids with paramagnetic ions can be used to tune material properties and introduces endogenous DNP agents that can potentially provide sensitivity in the particles' bulk and surface. Here we demonstrate the approach by doping Li Ti O (LTO), an anode material for lithium ion batteries, with paramagnetic ions. By incorporating Gd(III) and Mn(II) in LTO we gain up to 14 fold increase in signal intensity in static Li DNP-NMR experiments. These results suggest that doping with paramagnetic ions provides an efficient route for sensitivity enhancement in the bulk of micron size particles.

Recent developments in MAS DNP-NMR of materials

“…In hot-melt extrusion, the solid constituents are mechanically mixed at elevated temperatures. Paramagnetic metal ions have been incorporated into the inorganic solids using co-crystallization [71] or via solid-state reactions [72,73].…”

“…Direct DNP complements the indirect version since it allows for the observation of heteronuclei more distant from protons. For instance, direct DNP has been used to observe subsurface or bulk sites in materials, for which protons are absent from the bulk (see Figure 14b and Figure 25) [45,72,73,119,195] as well as non-protonated surface 17 O sites (see Figure 14d) [140]. Furthermore, as the dipolar couplings between heteronuclei are smaller than among protons, spin diffusion among heteronuclei is much slower than among protons.…”

“…DNP has also been applied to enhance the NMR signals of the core region of inorganic microparticles. Such enhancement is possible, when the core of the microparticles contains PAs [71][72][73]. DNP has particularly been used to detect the distinct 17 O sites of crystalline inorganic microparticles doped with Mn(II) ions, including battery anode materials Li4Ti5O12 and Li2ZnTi3O8, as well as the materials NaCaPO4 and MgAl2O4 (see Figure 25) [73].…”

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

“…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. [15][16][17][18] Metal centers added as dopants to organic solids [19][20] or that are intrinsic to inorganic materials [21][22] have also been used to hyperpolarize nuclei in the bulk. Alternatively, several early DNP studies used neutron beam irradiation to induce formation of radicals suitable for DNP.…”

Large-scale ab initio simulations of MAS DNP enhancements using a Monte Carlo optimization strategy

“…Numerous procedures for the incorporation of the unpaired electrons into the samples have been proposed, each of which is applicable to a discrete class of materials: for instance, frozen solutions for molecular species and proteins, 5,6 impregnation methods for surfaces, [7][8][9][10] physical incorporation for pharmaceutical formulations, 11 and incorporation of metal ion dopants for inorganic materials. 12 These methods all attempt to optimize the contact between the radical polarization source and the target nuclei of interest. Additionally, DNP enhancement factors can be greatly improved by tailoring the quality of the polarizing agent.…”