First widely used in the mid-20th century, magnetic resonance imaging (MRI) has since become an indispensable technique for studying materials to their atoms, revealing molecular structure and other details without interfering with the material.
“The problem with MRI, however, is that because it’s such a low-energy technique, it’s not very sensitive,” Hahn said. “It’s very detailed, but you don’t get much signal.” As a result, large amounts of sample material may be needed compared to other techniques, and the general weakness of the signals makes MRI less than ideal for studying complex chemical processes.
One way to remedy this situation lies in dynamic nuclear polarization (DNP), a popular technique in which energy is “taken up” by nearby electrons to amplify the signal emitted by the nuclei.
“Electrons have much higher energy than nuclei,” Hahn explained. Embedded in specially designed “radical” molecules, the polarization of these unpaired electrons is transferred to the nuclei to improve their signal.
As hot a topic as DNP has become in the last decade, however, Hahn thinks we’re still just scratching the surface.
“Although DNP radically changed the MRI landscape, at the end of the day only a handful of design polarizing agents were used,” Hahn said. “A polarizing agent has been used to polarize hydrogen nuclei, but the power of DNP is greater than that. In general, many other sources of electron spin can polarize many other types of nuclear spins.”
In an article published in the magazine Chem, Khan and colleagues expanded the boundaries of MRI with the first demonstration of dynamic nuclear polarization using the transition metal vanadium (IV). According to Hahn, their new approach – called “ultrafine DNP spectroscopy” – offers a look at the typically obscure local chemistry around transition metals, which are important for processes such as catalysis and reduction-oxidation reactions.
“We may now be able to use endogenous metals that are present in catalysts and many other important materials,” Hahn said, without having to add polarizing agents – these radical molecules – to produce a stronger NMR signal.
The irony of transition metals such as vanadium and copper, Hahn explained, is that these atoms tend to be functional centers – places where important chemistry takes place.
“Both these centers of action and functional centers are very difficult to analyze (with NMR) because they tend to become invisible,” she said. Electron spirons in the transition metal tend to shorten the life of the NMR signal, she explained, causing them to disappear before they can be detected.
This would not be the first time chemistry has been observed around transition metals, Hahn said, citing studies looking at the chemical environment around gadolinium and manganese. However, the commercially available tool used in these studies offers a “very narrow view”.
“But there are many more metals that are much more important to chemistry,” she said. “So we have developed and optimized tools that improve the frequency range from a very narrow range of commercial instruments to a much wider range.”
With their ultrafine DNP spectroscopy, the researchers also found that the signal did indeed erase in a certain area around the metal, called the spin diffusion barrier, but if the nuclei were located outside that zone, the signal became visible.
“There are ways to lighten this environment, but you need to know how and why,” Khan said, adding that co-authors Sheetal Kumar Jain of UC Santa Barbara and Chung-Jui Yu of Northwestern University will continue to research and implement this new method while pursuing their academic and research careers.
The new computational model aims to make magnetic resonance imaging an even more powerful tool for researchers
Sheetal Kumar Jain et al. Dynamic nuclear polarization with vanadium (IV) metal centers, Chem (2020). DOI: 10.1016 / j.chempr.2020.10.021
Provided by the University of California, Santa Barbara
Quote: The new analytical approach improves the detection of magnetic resonance imaging in previously “invisible” regions (2020, November 16), retrieved on November 16, 2020 from https://phys.org/news/2020-11-analytic – Approach-nuclear-magnetic-resonance .html
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