SINGLET-DIFFUSION-NMR TO PROBE TRANSLATIONAL DYNAMICS IN POROUS MEDIA

扩散-核磁共振是一项强大的技术，应用范围从材料科学到医学。利用核磁共振的非侵入性和无害性，该技术可以推断分子在各种介质中扩散的信息。扩散核磁共振的应用范围广泛，从分析科学到医学，它可以用来根据不同的扩散系数来筛选复杂的分子混合物，在医学上，它被用来获得分子在其中具有不同扩散性质的生物组织之间的对比。多孔介质在自然界中普遍存在，例如岩石、骨骼、木材等，可能是最适合通过扩散-核磁共振来表征的体系。事实上，科学文献中包含了许多此类研究的例子。然而，由于传统的核磁共振信号通常只持续几秒钟，扩散-核磁共振研究具有一定的局限性。扩散的测量是通过记录分子在溶液中扩散时核磁共振信号强度的变化在微观水平上进行的。扩散时间越长，分子扩散越远，记录的信号变化越剧烈，测量越准确。分子扩散受扩散材料微观结构的影响，因此扩散-核磁共振是一种非常灵敏的微结构探测工具，因此在多孔介质研究中具有很大的实用价值。然而，它对维度的敏感性与所探索的时间尺度直接相关，即与可用的扩散时间有关。因此，由于常规核磁共振信号的寿命，扩散时间的限制将该技术限制在100微米以内的几何形状。由于许多有趣的多孔结构的孔大于100微米，因此该技术无法探测这些系统中的孔连通性，因此无法提供在许多领域(包括石油工程和电池开发)具有仪器重要性的曲折程度。在过去的10年里，我一直在研究长寿命自旋态的主题，这是一种特殊的核自旋态配置，显示出非常长的寿命，在某些情况下甚至可以达到一小时长。这种寿命延长可用于扩散核磁共振，以延长扩散时间，在扩散测量中获得更好的准确性，并有可能获得关于孔连通性的信息，从而测量曲折性。这项建议涉及利用长寿命状态来延长扩散-核磁共振实验中的可达扩散时间的方法的发展和评估，从而使测量多孔介质中的曲折性、宏观分区和扩散各向异性成为可能。这项研究的主要成果是：1.支持长寿命状态的扩散分子探针，以获得非常长的扩散时间2。通过编码长寿命自旋态的位置信息来测量扩散的核磁共振方法3.模拟多孔系统上的复杂核磁共振实验的模拟程序4。对多孔性介质中的曲折性、各向异性扩散和宏观结构的测量所提出的方法有望使实验室和对表征多孔性材料和/或开发新材料感兴趣的行业受益(例如，锂电池电极在电化学循环过程中的曲折性增加被认为是观察到的性能下降的部分原因)。扩散各向异性在磁共振成像中特别重要，它被用于扩散张量成像，这是一种使用扩散各向异性来绘制体内纤维方向的技术：本项目中开发的方法和程序也有可能影响这一领域。

Diffusion-NMR is a powerful technique with applications that span from material science to medicine. With the characteristic non-invasiveness and non-harmfulness of Nuclear Magnetic Resonance (NMR), the technique can infer information on molecular diffusion in various media. Diffusion-NMR has applications that range from analytic sciences where it can be used for example to sort out complex molecular mixtures according to different diffusion coefficients up to medicine where it is used to obtain contrast between biological tissues within which molecules have different diffusion properties.Porous media, which are ubiquitous in nature with examples including rocks, bones, wood etc. are perhaps the most suitable systems to be characterised through diffusion-NMR. And indeed scientific literature contains numerous examples of such investigations. However, because conventional NMR signals last typically for only up to a few seconds, diffusion-NMR studies have some limitations. The measurements of diffusion are done on a microscopic level by registering the changes in the intensity of an NMR signal as molecules diffuse in solution. The longer the diffusion time the farther the molecules diffuse, so that the the registered change in signal is more dramatic and the more accurate the measurement.Molecular diffusion is affected by the microscopic structure of the material a molecule diffuses within, therefore diffusion-NMR is a very sensitive tool to probe micro-structures, hence its great utility in porous media investigations. However its sensitivity to dimensions is directly linked to the timescale explored i.e. to the available diffusion time. Limitations to diffusion time due to the lifetime of conventional NMR signals therefore restrict the technique to geometries within 100 micrometers. Since many interesting porous structures have pore larger than 100 micrometers the technique cannot probe pore connectivity in those systems hence cannot provide a measure of tortuosity which is of instrumental importance in many areas including oil engineering and battery development. In the past 10 years I have been investigating the topic of long-lived spin states which are particular configurations of nuclear spin states displaying very long lifetimes that can reach in some cases even an hour length. This lifetime extension can be used in diffusion-NMR to prolong the diffusion time and obtain a better accuracy in diffusion measurement plus the possibility to access information on pore connectivity and hence measure tortuosity. This proposal deals with the development and assessment of methodology that exploit long-lived states to expand the accessible diffusion time in diffusion-NMR experiments thus giving access to measurement of tortuosity, macroscopic compartmentation and diffusion anisotropy in porous media. The main outcomes of this research are:1. molecular probes of diffusion that support long-lived states to give access to very long diffusion times2. NMR methodology to measure diffusion by encoding positional information on long-lived spin states 3. a simulation procedure for simulation of complex NMR experiments on porous systems4. measurements of tortuosity, anisotropic diffusion and macrostructures in porous media The proposed methodology is expected to benefit laboratories and industries with interests in characterising porous material and/or developing new materials (an increase in the tortuosity of lithium batteries' electrodes during electrochemical cycling is thought to be partially responsible for the observed reduction of performances, for example). Diffusion anisotropy is of particular interest in MRI where it is exploited in diffusion-tensor-imaging, a technique that uses diffusion anisotropy to map the direction of fibres in the body: methods and procedures developed in this project have the potential to impact this area too.