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。利用核磁共振技术的无创、无害特性，可以推测分子在各种介质中的扩散信息。扩散核磁共振的应用范围很广，从分析科学，它可以根据不同的扩散系数来分类复杂的分子混合物，到医学，它被用来获得分子具有不同扩散特性的生物组织之间的对比。多孔介质在自然界中无处不在，例如岩石，骨骼，木材等，可能是最适合通过扩散核磁共振表征的系统。事实上，科学文献中包含了许多这样的研究例子。然而，由于传统的核磁共振信号通常只能持续几秒钟，扩散核磁共振研究有一些局限性。扩散的测量是在微观水平上完成的，通过记录分子在溶液中扩散时核磁共振信号强度的变化。扩散时间越长，分子扩散距离越远，因此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敏感的探测微观结构的工具，因此在多孔介质研究中具有很大的实用性。然而，它对维度的敏感性直接与所探索的时间尺度有关，即与可用的扩散时间有关。由于传统核磁共振信号寿命的限制，扩散时间因此限制了该技术的几何形状在100微米以内。由于许多有趣的多孔结构的孔隙大于100微米，该技术无法探测这些系统中的孔隙连通性，因此无法提供扭曲度的测量，而扭曲度在包括石油工程和电池开发在内的许多领域都具有重要意义。在过去的十年里，我一直在研究长寿命自旋态的主题，长寿命自旋态是核自旋态的特殊构型，它的寿命非常长，在某些情况下甚至可以达到一个小时的长度。这种寿命延长可以用于扩散核磁共振，延长扩散时间，获得更好的扩散测量精度，并有可能获得孔隙连通性信息，从而测量扭曲度。本提案涉及开发和评估方法，利用长寿命状态来扩大扩散-核磁共振实验中的可达扩散时间，从而可以测量多孔介质中的扭曲度，宏观区隔和扩散各向异性。本研究的主要成果有：1。支持长寿命状态的分子扩散探针可以获得很长的扩散时间。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。在多孔系统上模拟复杂核磁共振实验的模拟程序。所提出的方法有望使对表征多孔材料和/或开发新材料感兴趣的实验室和行业受益（例如，在电化学循环过程中，锂电池电极扭曲度的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MRI中特别有趣，它被用于扩散张量成像，这是一种利用扩散各向异性来绘制人体纤维方向的技术：本项目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.