Diamonds: The NMR spectroscopist's best friend?

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  • Published: Jan 7, 2016
  • Author: David Bradley
  • Channels: NMR Knowledge Base
thumbnail image: Diamonds: The NMR spectroscopist's best friend?

Shiny

The shiniest of carbon allotropes, diamond, could turn out to be the NMR spectroscopist's and the MRI radiologist's best friend, thanks to work undertaken by scientists at the the Lawrence Berkeley National Laboratory and the University of California (UC) Berkeley, California, USA. (Courtesy of Berkeley Lab)

The shiniest of carbon allotropes, diamond, could turn out to be the NMR spectroscopist's and the MRI radiologist's best friend, thanks to work undertaken by scientists at the Lawrence Berkeley National Laboratory and the University of California (UC) Berkeley, California, USA.

In a study led by chemist Alexander Pines measurements of the first bulk room-temperature NMR hyperpolarization of carbon-13 nuclei in diamond in situ at arbitrary magnetic fields and crystal orientations have been made. The team reports a signal enhancement of several orders of magnitude above what is normally seen in these techniques with conventional magnets at ambient temperature. Moreover, the hyperpolarization was achieved with microwaves, rather than relying on precise magnetic fields for hyperpolarization transfer. The work was carried out by Pines' group members Jonathan King, Keunhong Jeong, Christophoros Vassiliou, Chang Shin, Ralph Page, Claudia Avalos and Hai-Jing Wang.

The research suggests that it is possible to boost the bulk nuclear spin polarization by six percent, which is equivalent to an NMR signal enhancement of about 170000 times over thermal equilibrium. The hyperpolarized spin can be detected in situ with a conventional NMR probe and without the need for sample shuttling or precise crystal orientation. The team suggests that their new approach to hyperpolarization could make new NMR studies in both solids and liquids that were previously not possible under ambient conditions.

Enhancing

"Our results in this study represent an NMR signal enhancement equivalent to that achieved in the pioneering experiments of Lucio Frydman and co-workers at the Weizmann Institute of Science, in Israel, but using microwave-induced dynamic nuclear hyperpolarization in diamonds without the need for precise control over magnetic field and crystal alignment," Pines explains. "Room-temperature hyperpolarized diamonds open the possibility of NMR/MRI polarization transfer to arbitrary samples from an inert, non-toxic and easily separated source, a long sought-after goal of contemporary NMR/MRI technologies."

NMR and MRI are indispensible techniques in modern chemical science, medicine and biomedical research because of their chemical specificity and non-destructive nature. However, a lack of sensitivity has always been a problem with good polarization being the limiting factor on signal strength. Of course, Pines and his researchers have worked on many solutions over the years, but during the last couple of years diamond crystals have been their focus and specifically an impurity called a nitrogen-vacancy (NV) centre within the lattice. NV centres allow optical and spin degrees of freedom are coupled, which is key to the NMR boost they have observed.

"An NV centre is created when two adjacent carbon atoms in the lattice of a pure diamond crystal are removed from the lattice leaving two gaps, one of which is filled with a nitrogen atom, and one of which remains vacant," Pines explains. "This leaves unbound electrons in the centre between the nitrogen atom and a vacancy that give rise to unique and well-defined electron spin polarization states."

Ambient solution

The team had already shown that a low-strength magnetic field could be used to transfer NV centre electron spin polarization to neighbouring carbon-13 nuclei, resulting in hyperpolarized nuclei through dynamic nuclear polarization. But, it always requires a high-strength magnetic field and cryogenic temperatures. The researchers have now eliminated those two constraints by placing a permanent magnet near the diamond.

"In our new study we're using microwaves to match the energy between electrons and carbon-13 nuclei rather than a magnetic field, which removes some difficult restrictions on the strength and alignment of the magnetic field and makes our technique easier to use," explains King. "Also, in our previous studies, we inferred the presence of nuclear polarization indirectly through optical measurements because we weren't able to test if the bulk sample was polarized or just the nuclei that were very close to the NV centres. By eliminating the need for even a weak magnetic field, we're now able to make direct measurements of the bulk sample with NMR."

Hyperpolarized diamonds could be easily integrated into existing fabrication techniques to create high surface area diamond devices for NMR and MRI machines. "We envision highly enhanced NMR of liquids and solids using existing polarization transfer techniques, such as cross-polarization in solids and cross-relaxation in liquids, or direct dynamic nuclear polarization to outside nuclei from NV centres," King adds. He points out that previously, the team demonstrated such transfer of polarization to solid surface and liquids using laser polarized xenon-129. "Our hyperpolarization technique based on optically polarized NV centres is far more robust and efficient and should be applicable to arbitrary target molecules, including biological systems that must be maintained at near ambient conditions."

Related Links

Nature Commun 2015, online: "Room-temperature in situ nuclear spin hyperpolarization from optically pumped nitrogen vacancy centres in diamond"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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