Probing the atomic-level structure of nanomaterials and nanocatalysts and using solid-state NMR enhanced by Dynamic Nuclear Polarization

Nanostructured materials, including micro- or meso-porous solids as well as nanoparticles, are promising systems for many important applications, including catalysis, gas storage, drug delivery, medical imaging and the capture of radioactive compounds. The rational design of these nanostructured materials requires a clear understanding of structure–property relationships and hence calls for characterization methods endowed with atomic resolution. As a local and non-destructive technique, solid-state nuclear magnetic resonance (NMR) provides precious insight into the atomic-scale structure and dynamics of nanomaterials. Nevertheless, the low sensitivity of NMR often precludes the observation of surface species or defects, particularly when the observed nuclei have low gyromagnetic ratios, low natural abundances or long longitudinal relaxation times (T₁).

In this context, we have recently demonstrated that Dynamic Nuclear Polarization (DNP) can enhance the sensitivity of solid-state NMR experiments and hence provides new insights into the functional properties of nanostructured materials, including mesoporous silica^1,2, mesoporous alumina³, microporous metal-organic frameworks⁴ and nanoparticles.⁵ Sensitivity enhancements of 1-2 orders of magnitude compared to conventional NMR has enabled an easy detection of surface and defect sites.

DNP-NMR is notably a powerful tool to probe the grafting mode of organic moieties on silica surface.² More recently we have shown in collaboration with V. Polshettiwar from TIFR that DNP-NMR provides crucial insights to understand the catalytic activity of nitridated fibrous silica nanoparticles. We have also used the high sensitivity of DNP-NMR to probe proximities between Al sites near the surface of mesoporous alumina.³