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

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.

New products and devices for e.g. catalysis, energy storage or drug delivery cannot be developed without knowledge of the relationships between the structure and properties of their component materials. Molecular-level characterization is therefore key to the rational design of new materials for technological applications with improved properties. For example, the atomic-level characterization which solid-state NMR provides is a pre-requisite for intelligent molecular design of new materials. Over the last decades the impact of solid-state NMR on diverse areas of science, technology and industry has been considerable. For example, the method has provided crucial insight into the role of defects in electrode materials in cycling of rechargeable batteries, into the effect of polymorphism on pharmaceutical formulations and into the structure of ion channels in cell membranes targeted by new drugs. However, despite these successes, solid-state NMR suffers from low sensitivity, because of the low nuclear spin polarization involved even at high magnetic field.

The research described here will help to overcome the limitations of solid-state NMR by exploiting new developments in DNP, available at Nottingham thanks to a £2.5M award from EPSRC's Strategic Equipment Fund. DNP provides substantial gains in signal for solid-state NMR and has the potential to transform studies of e.g. low concentration sites and functionalizing molecules at surfaces, and of dilute or low sensitivity NMR isotopes. However, there are problems with the reliability and reproducibility of the signal enhancements obtained. If these problems can be overcome with the new approaches to sample preparation proposed here, then the power of solid-state NMR can be brought to bear for the first time on the characterization of real-life and technologically-useful materials. The potential to gain useful information from DNP-enhanced solid-state NMR is evidenced by the support for the research by project partners from the commercial sector, which include Johnson Matthey and L'Oréal. New insights into materials will have an impact on technological advances in areas such as catalysis, gas storage, drug delivery, therapeutics, and functional or biocompatible materials which will, in turn, help to drive the future economic success of the UK.

The research proposed here aims to make DNP-enhanced solid-state NMR the method of choice for studies of materials and their surfaces. If successful, the information which can be gained from DNP-enhanced solid-state NMR will transform many EPSRC research areas, such as biomaterials and tissue engineering, catalysis, functional ceramics and inorganics, carbon nanotechnology and polymers, as well as several indicated as "grow" in the recent "balancing capability" exercise, such as soft matter, energy storage and materials for energy applications. In addition, DNP-enhanced solid-state NMR will support two of the "great technologies" listed in the UK government's industrial strategy, specifically advanced materials, and energy, as well as grand challenges, including the directed assembly of extended structures and the nanoscale design of functional materials. Finally, the research is an excellent fit with all three objectives of the recently published UKRI-EPSRC 2019 Delivery Plan, delivering economic impact, realising the potential of research and enabling the UK engineering and physical sciences to deliver. For example, one strategy for catalysing growth is investment in "new opportunities in materials research" and one key aim is to "future-proof state-of-the-art research infrastructure", by "effective delivery of strategic equipment".