Solid-state NMR can be used to obtain atomic level structure and dynamics of inorganic materials, but its application is sometimes limited by its relatively low sensitivity. Impregnation dynamic nuclear polarization (DNP) is a strategy that can be used to improve sensitivity in solid-state NMR experiments of powders, provided that hyperpolarization that is generated close to the particle surface can be relayed towards the bulk by spontaneus ¹H spin diffusion.

Here we show how the relayed DNP method can be extended to proton-free inorganic materials, by using a combination of impregnation DNP, cross-polarization, and slow spin diffusion between weakly magnetic nuclei such as ¹¹⁹Sn and ³¹P.¹ The method is demonstrated to provide a sensitivity gain of a factor 50 for the ¹¹⁹Sn spectra of SnO₂, with an additional factor of 3.5 when the magic-angle-spinning rate is modulated during the experiment.² This correponds to acceleration of up to a factor of 30 000 in acquisition times, allowing access to materials that were previously unfeasible. We also show how the pathways of spin diffusion can be probed with multi-dimensional experiments.

Figure 1. A) Impregnation DNP of a powdered solid. B) Spontaneus ¹¹⁹Sn-¹¹⁹Sn spin diffusion relays hyperpolarization from surface to bulk in the ¹¹⁹Sn spectrum of SnO₂.