A research team from the University of California Berkeley has employed a new type of shiny biological tracer: microdiamonds. Microdiamonds have had some of their carbon atoms kicked out and replaced by nitrogen, leaving behind empty spots in the crystal—nitrogen vacancies—that fluoresce when hit by laser light.
The researchers exploited an isotope of carbon—carbon-13 (C-13)—that occurs naturally in the diamond particles at about 1% concentration, but also could be enriched further by replacing many of the dominant carbon atoms, carbon-12. The polarized C-13 nuclei yield a stronger signal for nuclear magnetic resonance (NMR)—the technique at the heart of MRI.
In his latest experiments, chemistry professor Ashok Ajoy employed a magnetic field equivalent to that of a cheap refrigerator magnet and an inexpensive green laser to hyperpolarize the carbon-13 atoms in the crystal lattice of the microdiamonds.
"It turns out that if you shine light on these particles, you can align their spins to a very, very high degree—about three to four orders of magnitude higher than the alignment of spins in an MRI machine," Ajoy said. "Compared to conventional hospital MRIs, which use a magnetic field of 1.5 teslas, the carbons are polarized effectively like they were in a 1,000-tesla magnetic field."
When the diamonds are targeted to specific sites in cells or tissue—by antibodies, for example, which are often used with fluorescent tracers—they can be detected both by NMR imaging of the hyperpolarized C-13 and the fluorescence of the nitrogen vacancy centers in the diamond.
In the future, Ajoy’s developments could allow for an inexpensive NMR imaging machine on every chemist's benchtop.
Photo: The microdiamonds used as biological tracers are about 200 microns across, less than one-hundredth of an inch. They fluoresce red but can also be hyperpolarized, allowing them to be detected both optically and by radio-frequency NMR imaging, boosting the power of both techniques. Credit: Ashok Ajoy, UC Berkeley