The recent developments of high field dynamic nuclear polarization (DNP) in combination with magic angle spinning (MAS) have greatly expanded the capabilities of solid-state NMR by increasing the sensitivity of experiments by a few orders of magnitude. Through the pioneering work of Griffin and coworkers1 in developing the technique and the commercialization of the hardware by Bruker2, it is now fairly routine to perform in situ MAS-DNP experiments on solid samples containing radicals at temperatures around 100 K, while using a gyrotron to irradiate the radicals for DNP. The initial success of these experiments was largely demonstrated on biological systems such as membrane proteins3, nanocrystalline proteins4, and amyloid fibrils5 using a cryoprotecting glass-forming matrix, typically a mixture of 60:30:10 d6-glycerol: D22O:H2O and TOTAPOL6 as a polarizing agent.
Our recent efforts at the EPFL7,8 have extended this approach to non-biological systems and other nuclei with an emphasis on finding useful sample preparation techniques for a broader range of systems. One such class of materials is nanoporous silica with organic functional groups at the surface, which can be used as a scaffold for catalysts. In these materials, one can use a mixture of 90:10 D2O:H2O solvent with ~10-20 mM of TOTAPOL as a polarizing solvent, which is then added to the nanoporous material by incipient wetness impregnation.
Since the solvent is contained in the pores and not as a bulk solution, it remains glassy at 100 K and allows effective DNP enhancements. This allows for a fast acquisition of 2D 1H-13C and 1H-29Si correlation spectra (in a few hours) and characterization of the bonding patterns and orientation with respect to the surface of organic functionalities. These experiments are often difficult or impossible to do otherwise due to the low concentration of functional groups on the surface. The procedure is also effective for characterizing the surfaces of granular materials like γ-alumina. Cross-polarization from the hyperpolarized protons of the solvent to the surface of γ-alumina allows for selection of 27Al sites near the surface while suppressing the bulk of the 27Al spins.
Finally, we have developed a model for solvent-less DNP in polymer systems where the polarizing agent TOTAPOL is covalently attached to the polymer and then diluted with unlabeled polymer. The enhancements are modest, but considering the lack of dilution due to added solvent and reduced recycle times, the reduction in experimental time can still be a factor of 100 or more.
Figure 1. (adapted from ref. 7) The structure of nanoporous silica functionalized with phenol groups is depicted in the schematic left. The DNP solvent solution containing TOTAPOL is absorbed by the pores resulting in DNP enhancements of 30 or more as shown by the 13C cross-polarization spectra on the right.
Figure 2. (from ref. 8 ) Further characterization of functional groups on silica surfaces can be seen through 1H-29Si correlations. With routine DNP enhancements fast 2D spectra can be acquired to determine the interactions of the functional group with the surface or lack thereof.
