Nuclear magnetic resonance (NMR) is a powerful technique for studying the molecular-level detail of complex structures, such as advanced materials with technological uses, such as energy storage or catalysis. However, even with the high magnetic fields available today, NMR suffers from low sensitivity due to the small nuclear spin polarizations involved, so that long acquisitions or large samples are required. This problem is overwhelming for dilute species and limits the ability of NMR to draw conclusions about technologically important issues such as the nature of binding sites or the adhesion of adsorbates to surfaces. However, weak NMR signals can be enhanced at low temperatures (100 K) by dynamic nuclear polarization (DNP) in which the large electron spin polarization from an implanted radical is transferred to nearby nuclei. Recent progress with high-power microwave sources known as gyrotrons has made DNP possible at the high magnetic fields found in modern NMR instruments (up to 18.8 T), and signal enhancements > 300-fold have been achieved for frozen biomolecules, corresponding to a reduction by a factor of 100,000 in experiment time. More recently pioneering experiments on mesoporous silica with the radicals for DNP implanted from a solution impregnated into the pores obtained DNP-enhanced NMR signals for functionalizing groups at the pore surface. The sensitivity enhancement achieved with DNP means that previously unobtainable structural details can be quickly obtained by solid-state NMR, even for surfaces.
New products and devices for catalysis, energy storage or drug delivery cannot be developed without knowledge of the relationships between the structure and properties of their component materials. Molecular-level characterization is key to the rational design of new materials for technological applications with improved properties. DNP-enhanced solid-state NMR is a transformative technology offering a step-change in capability which completely overcomes the sensitivity limitations of NMR. Conventional methods for the high-resolution analysis of the surfaces of materials involve high-energy electrons or X-rays interacting with well-defined and relatively clean surfaces in a low-pressure environment. Therefore, the surface conditions during measurement differ from those prevailing in real-life applications of materials in for example catalysis or drug delivery. DNP-enhanced NMR does not suffer from these limitations, and the approach allows the power of solid-state NMR to be brought to bear for the first time on real-life technologically useful materials. The objectives of the project were the improved sample preparation protocols, new experimental approaches and proof of principle studies necessary to make DNP-enhanced solid-state NMR the method of choice for the molecular-level characterization of the surfaces of materials, including for example the catalytic converters used on vehicle exhausts.
