Turning up the NMR - Amplification technique boosts surface spectra

Amplifying NMR

A technique to amplify nuclear magnetic resonance signals 50-fold or more can be applied to the surfaces of solid-state samples, according to research published in the Journal of the American Chemical Society.

Lyndon Emsley of the University of Lyon, working with colleagues there and at the University of Montpellier, and the Ecole Normale Superieure, in Paris, France, and at the Ecole Polytechnique Federale de Lausanne (EPFL), in Lausanne, Switzerland, have demonstrated surface enhanced NMR spectroscopy using dynamic nuclear polarization for surface species covalently incorporated into a silica framework. The polarization is transferred from the solvent protons to the rare nuclei (carbon-13 in this case) at the surface giving at least a fifty times stronger signal than would otherwise be obtained.

Solid-state NMR spectroscopy can characterize both inorganic and hybrid materials effectively and so has potential in the direct investigation of both bulk surface characteristics for the industrially important substrates of silica and alumina and their adsorbates, including grafted molecules and organic fragments. The technique could thus offer detailed insights into surface chemistry of catalysts and sensors where the development of new and improved systems is driven almost entirely by an understanding of the relationship between structure and activity.

Insensitivity

Unfortunately, NMR is intrinsically an insensitive technique and acquiring detailed spectra for some systems in which the concentrations of NMR-active nuclei is very low can take several hours, if not days. Correlation spectra are often precluded from the surface scientist's repertoire of analytical techniques. The enhancement offered by Emsley and colleagues could change all that.

"In the past few years, DNP has made great progress, and the technique, which was originally developed for low magnetic fields, has been shown to be applicable in high magnetic fields and notably to frozen solutions," the team explains. The researchers point out that, "The nature of the polarizing agent, the composition of the solvent mixture, and the capacity of the solvent to form a homogeneous glass at low temperature appear to be critical ingredients in a successful DNP experiment." This presents yet another obstacle to the would-be surface spectroscopists: how to adapt DNP so that it circumvents the heterogeneous nature of the system and the lack of solvent.

Experimental exploits

The researchers have now developed the very experimental conditions needed to allow DNP to be exploited in the NMR of surfaces and have demonstrated their solution using an organic-inorganic mesostructured material obtained by sol-gel processing with a template. "Sample preparation is key to obtaining DNP enhancements in these materials," the team explains. "Here we carefully wetted the dry samples by incipient wetness impregnation with a solution of the organic radical species before filling the rotor with the wet solid." DNP is then obtained by adding a relatively stable radical, either TEMPO or TOTAPOL, to the sample and blasting it with high-energy microwave radiation.

Emsley and colleagues point out that without their technique, the same carbon-13 NMR experiment would take ten weeks, whereas they obtained a spectrum in about half an hour.

"The implications of this new method for a broad range of surface science problems are enormous. This approach is in principle general and could be applied even to the investigation of adsorbed substrates or, by finding appropriate experimental conditions (solvent, radical concentration, etc.), to the investigation of surface catalytic processes," the team concludes.

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