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Analytical Chemistry

Boosting Solid-State NMR

Surface Science: Signal-enhancing technique makes NMR a powerful probe of surfaces

by Jyllian N. Kemsley
September 20, 2010 | A version of this story appeared in Volume 88, Issue 38

Credit: Adapted from J. Am. Chem. Soc. View Enlarged Image
Credit: Adapted from J. Am. Chem. Soc. View Enlarged Image

A technique to amplify nuclear magnetic resonance signals 50-fold or more can be applied to solid-state samples, reports an international team of researchers (J. Am. Chem. Soc., DOI: 10.1021/ja104771z).

The approach could represent a breakthrough for studying systems, such as heterogeneous catalysts and sensors, that involve surface chemistry on bulk solids, says Malcolm Levitt, a chemistry professor at the University of Southampton, in England, who was not involved in the work. “Although NMR experiments on surfaces have been performed for a while, such experiments are usually gravely limited by lack of signal,” Levitt says. The new approach “may well initiate a new and powerful strand of NMR applications.”

The NMR technique used in the new work is called dynamic nuclear polarization (DNP). It involves adding to a sample a stable radical compound, then irradiating the solution with high-frequency microwaves. The irradiation transfers the polarization of the radical’s electron spin to the nuclear spins of the species of interest, increasing the NMR signals. The technique has been used successfully to study a variety of solution and frozen systems.

But those studies all involved homogeneous samples, and it wasn’t clear that DNP-enhanced NMR could be adapted to study surfaces. Lyndon Emsley, scientific director of the European Center for High Field NMR, in Lyon, France, and colleagues succeeded by taking a porous silica substrate functionalized with phenol or imidazolium groups and wetting the dry samples with a solution containing the organic radicals TEMPO or TOTAPOL. The researchers then packed the material, which they describe as a “translucent slush,” into NMR sample tubes.

In the resulting NMR spectra, the surface groups’ signals were enhanced up to about 50-fold. In one case, the boosted signals allowed the researchers to collect in 35 minutes a 13C NMR spectrum that would otherwise take at least 70 days, the authors note. The DNP signal enhancement will also make multidimensional NMR spectra a possibility for solid-state samples.

Having obtained NMR spectra of the functional groups tethered to the surface, Emsley and coworkers are now turning their attention to studying nuclei embedded in the surfaces themselves, such as the silicon atoms that make the links to the functional groups, Emsley says. “Knowing the nature of the surface at the active site is one of the key challenges in catalysis,” he says, and DNP-enhanced NMR should provide a powerful new tool for researchers to study such systems.



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