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The University of Southampton

Singlet-Diffusion NMR

Diffusion-NMR is a powerful technique with applications that span from material science to medicine. When combined with long-lived spin states it can provide important translational dynamics information such as tortuosity and compartmentation in macrostructures

MRI images of macroscopic diffusion

With the characteristic non-invasiveness and non-harmfulness of Nuclear Magnetic Resonance (NMR),  diffusion-NMR techniques can infer information on molecular diffusion in various media. Diffusion-NMR has applications that range from analytical 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 based on differences in molecular diffusion.

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 further 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. 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 pores 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.

Long-lived spin states are particular configurations of nuclear spin states displaying very long lifetimes that can reach in some cases one hour. 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.

We are developing 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.

Our efforts in this research area are focusing on:


  1. developing molecular probes of diffusion that support long-lived states to give access to very long diffusion times
  2. developing NMR methodology to measure diffusion by encoding positional information on long-lived spin states
  3. developing a simulation procedure for simulation of complex NMR experiments in porous systems
  4. measuring  tortuosity, anisotropic diffusion and macrostructures in porous media
  5. developing low-field hardware to facilitate these experiments
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