Reports: G9
45861-G9 Studying Flow Dispersion through Porous Media by Novel Molecular Contrast NMR Imaging
My research group’s driving force is the fascination that significant nuclear magnetic resonance (NMR) signal enhancement provides exciting new opportunities. The technology of my choice to seek new opportunities is dynamic nuclear polarization (DNP) because it is most generally applicable to amplify NMR signals of many nuclei of a wide range of molecules through polarization transfer from unpaired electron spins of stable radicals to the (dipolar or scalar) coupled nuclear spins. My group’s unique DNP approach provided new opportunities for detecting dynamic intermediate states, mapping out interfacial phenomena at the molecular length scales and capturing other dilute signatures that escape conventional NMR analysis without dedicated signal amplification schemes. With these general goals and approaches in mind, we utilized the ACS PRF support to focus on the development of hyperpolarized water as a unique and authentic magnetic resonance imaging (MRI) contrast agent to highlight the perfusion and dispersion of water through water-saturated media. During the course of the grant period, we also developed an alternative approach to measure equilibrium diffusive water dynamics in and around porous media, as describe in the subsection 3.
Our focus on capturing dynamic processes led to our method of choice to be the Overhauser DNP mechanism. The Overhauser effect is the oldest DNP effect reported on and extensively discussed as early as in the sixties, yet, my group reinvented the use of the Overhauser DNP effect in following three significant and novel ways.
1. Development of Portable Overhauser DNP Setup
My group developed solid state component-based hardware that allows for optimal Overhauser DNP performance at 0.35 Tesla. The wide frequency-tunable (8-12 GHz) YIG source allows for field dependent DNP measurements and can be operated in continuous wave as well as pulsed mode, the home-built amplifier driver provides high (>25 Watt) microwave (mw) power and frequency bandwidth, the home-built mw cavity allows for frequency tuning across the frequency band with high Q (~1000-3000) and the setup is relatively inexpensive and fully portable.
The ACS PRF funds were instrumental in allowing the development of this portable and versatile DNP setup that is critical for field measurements as well as collaborative efforts. The ACS PRF funds are acknowledged in the 2008 publication in the Journal of Magnetic Resonance that describes the development of this portable system.
2. Hyperpolarized Water as an Authentic MRI Contrast Agent
My group invented an original concept of employing pure water in a highly 1H spin polarized state (i.e. hyperpolarized water) as an MRI contrast agent to authentically visualize water’s macroscopic evolution in aqueous media. Our technique, remotely enhanced liquids for image contrast (RELIC), utilizes 1H NMR signal of water that is enhanced outside the sample in continuous-flow mode and immediately delivered to the sample to obtain maximum contrast between entering and bulk fluids. Hyperpolarization suggests an ideal contrast mechanism to highlight the ubiquitous and specific function of water in physiology, biology, and materials because the physiological, chemical, and macroscopic function of water is not altered by the degree of magnetization. Overhauser DNP is capable of enhancing the 1H NMR signal of water at 0.35 T by up to 2 orders of magnitude in continuous flow and under ambient conditions by using highly spin-polarized unpaired electrons that are covalently immobilized onto a porous, water-saturated, gel matrix.
We published our development of utilizing spin labeled gel materials in the Journal of Magnetic Resonance in 2008, and the ACS PRF is acknowledged for support.
We successfully demonstrated hyperpolarized water as an image contrast agent to distinctively visualize vortices in model reactors and dispersion patterns through porous media with a setup placed in an electromagnet as well as with the portable DNP setup built using ACS PRF funds.
This work was presented at numerous conferences, and is published in an invited book chapter of Wiley-VCH in 2008. While no student salaries were supported with this grant, one of my PhD student who played a key role in this project (Mark Lingwood) was able to present the here mentioned results at the Experimental Nuclear Magnetic Resonance conference in 2007 using travel support from this grant.
3. Probing Interfacial Hydration Water Dynamics In and Around Porous Media
My group introduced the idea of site-directed Overhauser spectroscopy by employing specifically tethered spin labels to study surface and interfacial hydration dynamics. This site-specific localization of spin label is only meaningful and powerful because the Overhauser effect scales with r-3, and thus is a very local effect with >80% of the dipolar relaxation coming from water approaching the spin label within 5 Å. This property enabled us to address the local hydration dynamics in porous media and soft matter with site-specificity. The significant signal amplification provided us with unprecedented sensitivity to detect surface and interfacial hydration dynamics of spin labeled materials.
This represents an alternative and novel approach to experimentally measure equilibrium water dynamics within pores and on the surface of porous media, as opposed to measuring macroscopic flow dispersion. This is an approach that was not proposed in the grant proposal. However, the hardware development supported through the ACS PRF enabled key aspects of these developments. Thus, the ACS PRF support is acknowledged in our 2008 publication in the journal Langmuir.
