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Diamond dust gets hyperpolarized carbon nuclei

By Melissae Fellet July 26, 2018
Diamond dust
Envisioned nanodiamond polarizer. NV is nitrogen vacancy; MW is microwave. Credit: Science Advances

A simple system for nuclear magnetic resonance (NMR) spectroscopy has been shown to enhance signals from carbon-13 nuclei in diamond dust. The result is increased sensitivity that could bring NMR spectrometers out of specialized facilities and onto every chemist’s benchtop, the researchers say.

“Magnetic resonance is a notoriously insensitive technique,” says Lucio Frydman, at the Weizmann Institute of Science, who was not involved with the current work. “However, the same reasons [for its insensitivity also] make it noninvasive and safe to use on babies.”

NMR spectroscopy, or magnetic resonance imaging (MRI) as it is called for medical imaging, uses a pulse of radio waves to polarize, or flip, a very small percentage of spins in particular atomic nuclei. A strong magnetic field applied around a sample aligns the flipped spins, and a radio frequency receiver detects energy lost as the polarized spins return to their original state. This detected energy has frequencies unique to specific atomic nuclei and their surrounding electronic environments, providing identifiable spectral signatures that chemists use to obtain molecular structural information.

Current NMR spectrometers rely on large superconducting magnets to maximize the number of polarized spins and increase the amount of detectable signals. Another way that researchers increase the number of polarized nuclei is with a technique called dynamic nuclear polarization. This approach relies on materials, such as diamond containing atom-like defects, which transfer spin polarization from electrons to nuclei.

The defects in this type of diamond are impurities called nitrogen vacancy centers (NV centers). In this impurity, nitrogen replaces one carbon atom in the diamond, leaving an adjacent vacant spot in the crystal lattice. Nitrogen vacancies illuminated by light generate hyperpolarized 13C nuclei in nearby atoms. However, an expensive single crystal of diamond, often cooled to cryogenic temperatures and specifically oriented in a high magnetic field, is needed to get the sharpest signal from the hyperpolarized nuclei.

Ashok Ajoy, at the University of California-Berkeley, and his colleagues wanted to generate hyperpolarized carbon nuclei using diamond powder, which is more affordable than the single crystals used previously. The researchers built a system that continuously illuminates the powder’s diamond nanoparticles with bright laser light. Then they transferred the optically induced electronic polarization to the carbon nuclei using repeated continuous sweeps of microwaves, ranging from 1.9 GHz to 3.9 GHz, produced in a weak magnetic field. The signal from hyperpolarized 13C nuclei in a NMR spectrometer with 7 T field was 300 to 400 times greater than from nuclei that had not been hyperpolarized.

“No one succeeded in hyperpolarizing diamond nanoparticles before because most approaches used higher magnetic fields [to transfer polarization],” Ajoy says. Using a lower field—about 1 mT to 30 mT, roughly the field strength produced by a refrigerator magnet—is the key to this work, he says.

“This work is an accessible and user-friendly way to generate hyperpolarization from powdered diamond, which is crucial to access more applications,” says Meghan Halse, at the University of York, who was not involved with the study. The increased sensitivity provided by hyperpolarized nuclei could help benchtop NMR spectrometers with low field magnets generate detailed spectra, she says. Ajoy says the team is now incorporating their homemade optical polarizer into an affordable benchtop spectrometer.

The next challenge—for this and all other NMR techniques with hyperpolarized diamond—is to transfer energy from hyperpolarized nanoparticles to other molecules, Frydman says. Those molecules could be dissolved in liquid surrounding the dust, like samples in conventional solution-based NMR methods. The diamond nanoparticles could also be covered with biomolecules that bind to small molecules or tumor cells, creating probes for biosensing in laboratory or clinical applications.

Read the article in Science Advances.