Singlet-Diffusion-NMR to probe translational dynamics in porous media

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 times

2. 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 systems

4. 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.