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

固体核磁共振（NMR）是研究复杂和非均相材料分子水平结构的有力技术。然而，即使有今天可用的高磁场，固态核磁共振也存在低灵敏度的问题，因为涉及的核自旋极化很小，因此需要长时间采集或大样本。这个问题对于稀释的物质来说是压倒性的，并且限制了核磁共振研究的有用性，例如表面，吸附剂或稀有同位素。幸运的是，弱核磁共振信号可以在低温（~100 K）下通过动态核极化（DNP）增强，其中植入自由基的大电子自旋极化转移到附近的原子核。高功率微波源的进步使得DNP在现代核磁共振光谱仪（高达21 T）中发现的高场下成为可能。对于冷冻生物分子，已经实现了高达300倍（9.4 T）的大信号增强，相当于实验时间减少了10万倍。因此，DNP是一项变革性技术，它将显著提高固态核磁共振的灵敏度。它所提供的能力的潜在变化最终将使固态核磁共振的力量首次在许多现实生活中的材料上发挥作用。所获得的信息将为研究科学家在设计新材料方面的进展提供信息，从而支持工业部门对新技术的商业开发。然而，尽管DNP在文献中描述的有利情况下获得了大量的信号增益，但可靠性和可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DNP增强固体核磁共振材料研究中，约有50%的DNP增强固体核磁共振结果是不可用的低增强（< ~5）。DNP的一个关键方面是样品制备（自由基的加入），目前有许多因素需要经验优化以最大限度地增强信号，但系统的研究很少进行，主要是因为DNP仪器时间有限。表面，多孔材料和纳米级颗粒通常在自由基溶液浸渍自由体积后发生极化，并且许多因素（自由基浓度，溶剂体积，样品形貌等）需要经验优化以最大化灵敏度。因此，大多数材料的DNP研究依赖于已发表的协议，这些协议通常不会产生预期的信号增强。在样品制备的背景下，这些可靠性和可重复性的问题是DNP在材料的分子水平表征中实现其全部潜力的主要障碍。提出的研究旨在克服这些问题，以实现DNP的潜在影响，通过开发新的方法来制备样品。该研究将使用诺丁汉DNP MAS核磁共振设施（见跟踪记录）中最先进的DNP增强固态核磁共振仪器，该仪器是在EPSRC战略设备补助金240万英镑的帮助下购买的。本提案寻求的主要资金项目包括支付仪器使用所需的使用费和博士后研究员执行方案的薪金费用。这些样品制备新方法的成功将使DNP在材料上的新型高影响应用成为可能。拟议研究的这一方面将为我们在诺丁汉的研究合作者在新材料设计方面的进展提供信息，并支持我们在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.