In recent years magic angle spinning - dynamic nuclear polarization (MAS-DNP) developed as an excellent approach for boosting the sensitivity of solid state NMR (ssNMR) spectroscopy, thereby enabling the characterization of challenging systems in biology and chemistry. Most commonly, MAS-DNP is based on the use of nitroxide biradicals as polarizing agents. In materials science, since the use of nitroxides often limits the signal enhancement to the materials’ surface and subsurface layers, there is need for hyperpolarization approaches which will provide sensitivity in the bulk of micron sized particles. Furthermore, for many functional materials, e.g. materials used for energy storage and conversion, the use of exogenous polarization agents is limited due to chemical reactivity at the materials’ interface.
Here I will discuss the utilization of paramagnetic metal ion dopants as endogenous DNP agents for sensitivity enhancement in inorganic solids. By introducing the dopants at low concentrations, we obtain NMR signal enhancements of more than two orders of magnitude, thereby enabling the detection of structurally revealing nuclei such as 17O at natural abundance (<0.04%). I will describe the conditions for achieving sensitivity enhancement from a range of paramagnetic dopants and discuss their suitability for probing local vs. remote environments of the dopant. Finally, I will address some of the challenges in implementing this approach in functional solids such as electrode materials for batteries.
The approach offers an alternative route for efficiently detecting reactive surface species and opens the way for structural studies based on high sensitivity NMR of challenging nuclei in the bulk of inorganic solids.