Sensitivity Enhanced NMR Spectroscopy for Probing Bulk Material Properties and the Surface of Inorganic Solids

The sensitivity boost gained by dynamic nuclear polarization (DNP), a process in which the large electron spin polarization is transferred to surrounding nuclei, expands the applicability of solid state NMR spectroscopy to challenging systems ranging from membrane and amyloid proteins in biology to the surfaces of porous catalysts and nanoparticles in materials science. Typically, DNP is performed by adding a solution of nitroxide radicals to a sample which is diamagnetic. While very efficient, the use of organic radicals introduces several constraints on the approach: (i) sensitivity is limited to the surface and sub surface layers of the material, (ii) the approach is not suitable for reactive samples, and (iii) in the case of non-porous materials the addition of the radical solution limits the amount of sample that can be studied.

Here we present an alternative approach which equips NMR with enhanced sensitivity in the bulk of micron sized inorganic solids and does not constrain the sample amount and reactivity. Utilizing paramagnetic metal ion dopants at low concentrations we obtain DNP enhancement 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 conditions for achieving sensitivity enhancement as well as selectivity for probing the local coordination environment of the dopant. The approach opens the way for correlating the structural changes in local order due to doping with the effect of the dopants on materials’ properties. Furthermore it offers an alternative route for sensitizing the detection of reactive surface species.