New NMR technique offers damage limitation for fragile materials

Scientists at the US Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a way for nuclear magnetic resonance (NMR) spectroscopy to analyze molecular interactions in viscous solutions and fragile materials such as liquid crystals.

In a first, they have developed a technique that allows hyperpolarized xenon gas to be dissolved into minute samples of substances without disrupting their molecular order. This technique brings the analytic power of hyperpolarized-gas NMR to materials that are too fragile to accept xenon gas through bubbling or shaking, which are the conventional delivery methods. It could help scientists to learn more about advanced polymers, liquid-crystal displays, and filters and catalysts for industrial processes, to name just a few applications.

The research was performed in the lab of NMR pioneer Alexander Pines, a senior faculty scientist with Berkeley Lab's Materials Sciences Division and a professor of chemistry at the University of California, Berkeley (UC Berkeley). Ashley Truxal and Clancy Slack, who are UC Berkeley graduate students and members of Berkeley Lab's Materials Sciences Division, conducted the research with several other scientists. Their work is published in a paper in Angewandte Chemie.

"Our device provides a new, robust way of introducing hyperpolarized xenon gas into a sample without perturbing the order of its molecules," says Pines. "It will allow us to use NMR to study new types of viscous and fragile materials, as well as materials that hierarchically aggregate into more complex structures, such as synthetic membranes and biological cells."

NMR spectroscopy, like the related technique magnetic resonance imaging (MRI), uses superconducting magnets to polarize the alignment of the spins of the atomic nuclei in a sample. Applying a radio frequency pulse to the sample causes the spins of the nuclei to flip and then relax back to alignment, producing a characteristic frequency of their own. This frequency is converted by NMR detectors into a spectral readout that reveals information about the type, distribution and reaction state of the molecules in the material.

Often, however, only a small percentage of the nuclear spins in a sample are polarized, which significantly limits NMR's sensitivity. One way to boost the strength and sensitivity of NMR signals is to hyperpolarize the nuclear spins, meaning the nuclei are polarized far beyond their thermal equilibrium conditions.

The isotope xenon-129 is relatively easy to hyperpolarize and gives a large NMR signal in response to small changes in its surroundings. As a consequence, it is often bubbled into a material that scientists want to analyze with NMR, allowing the spin of the xenon nuclei to reveal information about the material. But hyperpolarized xenon gas has one big limitation: when it's bubbled into a viscous solution or a molecularly-aligned material, the bubbles disrupt the sample, sometimes to the point of destroying it.

Berkeley Lab scientists have now overcome this limitation, by finding a way to dissolve hyperpolarized xenon gas into fragile samples without wreaking havoc on their molecular order. Their approach involves placing the sample to be studied inside hollow silicone membrane fibers, or columns, and flowing xenon through the columns while the NMR signal is acquired. The xenon gas then diffuses out of the columns, to be replaced by new gas.

"Our system essentially breathes xenon in and out of the columns, so the signal source is constantly replenishing," explains Truxal. "In addition to being non-disruptive to the sample, the approach requires a very small amount of sample, so the NMR analysis is very efficient."

The scientists have demonstrated their non-disruptive approach on two materials that can't be probed by hyperpolarized xenon gas using conventional techniques. In one experiment, they used the approach to track phase changes in MBBA, an organic liquid crystal. "Understanding precisely when and why a liquid crystal undergoes a phase change can help us take advantage of the properties, perhaps leading to better electronic displays for example," says Truxal. They also used the device to analyze a bacteriophage with liquid crystalline properties, indicating that the technique can be applied to a wide range of biological materials.

This story is adapted from material from Lawrence Berkeley National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.