DNP-Enhanced Solid-state NMR: New Sample Preparation Approaches and Applications

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Solid-state nuclear magnetic resonance (NMR) is a powerful technique for studying the molecular-level structure of complex and heterogeneous materials. However, even with the high magnetic fields available today, solid-state NMR suffers from low sensitivity, because of 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 usefulness of NMR studies of e.g. surfaces, adsorbates or rare isotopes. Fortunately, weak NMR signals can be enhanced at low temperatures (~100 K) by dynamic nuclear polarisation (DNP) where the large electron spin polarisation from an implanted radical is transferred to nearby nuclei. Progress with high-power microwave sources has made DNP possible at the high fields found in modern NMR spectrometers (up to 21 T). Large signal enhancements up to 300-fold (at 9.4 T) have been achieved for frozen biomolecules, corresponding to a reduction by a factor of 100,000 in experiment time. DNP is therefore a transformative technology which will result in a significant increase in the sensitivity of solid-state NMR. The potential step-change in capability it offers will eventually allow the power of solid-state NMR to be brought to bear on many real-life materials for the first time. The information gained will inform progress in the design of new materials by research scientists and hence support the commercial development of new technologies by the industrial sector.However, despite the substantial signal gains obtained with DNP for the favourable cases described in the literature, reliability and reproducibility remain major issues, and in our experience some 50% of DNP-enhanced solid-state NMR studies of materials attempted at the Nottingham DNP MAS NMR Facility result in unworkably low enhancements (< ~5). One critical aspect of DNP is sample preparation (incorporation of the radical), with many factors currently requiring empirical optimization to maximize signal enhancement, and yet systematic studies are rarely carried out, mainly because DNP instrument time is limited. Surfaces, porous materials and nanoscale particulates are usually polarised after wetness impregnation of the free volume by a radical solution, and many factors (radical concentration, solvent volume, sample morphology etc.) require empirical optimization to maximize sensitivity. As a result, most DNP studies of materials rely on published protocols which often do not result in the expected signal enhancements. These issues of reliability and reproducibility within the context of sample preparation are a major obstacle to DNP ever achieving its full potential for the molecular-level characterization of materials.The proposed research aims to overcome these problems, in order to realise the potential impact of DNP, by developing new approaches to sample preparation. The research will make use of the state-of-the-art DNP-enhanced solid-state NMR instrumentation at the Nottingham DNP MAS NMR Facility (see Track Record) purchased with the aid of a £2.4M EPSRC Strategic Equipment grant. The main items of funding sought in this proposal comprise the access charges required to cover the use of the instrument and the salary costs for a postdoctoral researcher to carry out the programme. Success with these new approaches to sample preparation will make novel high-impact applications of DNP to materials possible. This aspect of the proposed research will inform progress in the design of new materials by our research collaborators from within Nottingham and support the commercial development of new technologies by our partners from the industrial sector.