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

固体核磁共振是研究复杂和非均相材料分子级结构的有力工具。然而，即使在今天的高磁场下，固态核磁共振也存在灵敏度低的问题，因为涉及到小的核自旋极化，因此需要长时间的采集或大样本。这个问题对于稀薄物种来说是无法克服的，并限制了对表面、吸附物或稀有同位素的核磁共振研究的有用性。幸运的是，在低温(~100K)下，弱的核磁共振信号可以通过动态核极化(DNP)来增强，动态核极化是指注入自由基产生的大电子自旋极化转移到附近的原子核。随着高功率微波源的进步，在现代核磁共振光谱仪(高达21T)中发现的高场强的DNP成为可能。对于冷冻的生物分子，已经实现了高达300倍(9.4T)的大信号增强，相应地实验时间减少了100,000倍。因此，DNP是一种变革性的技术，它将显著提高固态核磁共振的灵敏度。它提供的潜在能力阶梯式变化最终将使固态核磁共振的能力首次应用于许多现实生活中的材料。所获得的信息将为研究科学家在新材料设计方面的进展提供信息，从而支持工业部门新技术的商业开发。然而，尽管在文献中描述的有利情况下，使用DNP获得了大量的信号增益，但可靠性和重复性仍然是主要问题，根据我们的经验，在诺丁汉DNP MAS核磁共振设施尝试的材料的DNP增强固体核磁共振研究中，约50%的增强结果导致不可行的低增强(&lt；~5)。DNP的一个关键方面是样品制备(加入自由基)，目前许多因素需要经验优化以最大限度地提高信号增强，但系统研究很少进行，主要是因为DNP仪器时间有限。表面、多孔材料和纳米颗粒通常在自由体积被自由基溶液浸湿后极化，并受到多种因素(自由基浓度、溶剂体积、样品形态等)的影响。需要经验优化以最大限度地提高灵敏度。因此，大多数对DNP材料的研究依赖于已发表的方案，而这些方案往往不会产生预期的信号增强。这些在样品制备方面的可靠性和重现性问题是DNP充分发挥其在分子水平表征材料的潜力的主要障碍。拟议的研究旨在克服这些问题，以便通过开发新的样品制备方法来实现DNP的潜在影响。这项研究将利用诺丁汉DNP MAS核磁共振设施(见记录)的最先进的DNP增强型固态核磁共振仪器，该设备是在GB 240万EPSRC战略设备赠款的帮助下购买的。本提案所要求的主要资金项目包括支付使用该仪器所需的使用费和一名博士后研究人员执行该方案的工资费用。这些新的样品制备方法的成功将使DNP在材料上的新的高影响应用成为可能。拟议研究的这一方面将使我们来自诺丁汉的研究合作者在设计新材料方面取得进展，并支持我们来自工业部门的合作伙伴对新技术的商业开发。

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.