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| A visualization of the nano-NMR experiment, with the sample positioned on the diamond surface. |
MRI scanners fill entire hospital rooms. NMR spectrometers are the size of refrigerators. But the physics underpinning both techniques — the behavior of atomic nuclei in magnetic fields — doesn't have to operate at that scale. Two independent research groups have now demonstrated that MRI and NMR can be shrunk down to the nanoscale, potentially opening the door to imaging and chemical analysis of individual molecules.
Both teams achieved this using the same remarkable tool: nitrogen-vacancy (NV) centers in diamond. An NV center is a point defect in the diamond crystal where a nitrogen atom replaces a carbon atom next to a vacancy in the lattice. These defects behave like tiny, exquisitely sensitive magnetic field sensors that can be interrogated with laser light and microwave pulses. Because they operate at room temperature and can be placed extremely close to a sample, they're ideal for detecting the tiny magnetic signals produced by nuclear spins in nearby molecules.
How Nano-MRI Works
In conventional MRI and NMR, large superconducting magnets generate strong, uniform fields that align the nuclear spins in a sample, and then radio waves are used to detect how those spins precess and relax. The signal is averaged over billions of nuclei, which is why the technique works well on macroscopic samples but falls apart at the nanoscale — there simply aren't enough nuclei to produce a detectable signal with conventional equipment.
The NV-center approach sidesteps this problem. By positioning a diamond with a near-surface NV center just nanometers away from a sample, researchers can detect the magnetic noise generated by even a handful of nuclear spins. The NV center's quantum state is sensitive enough to pick up these tiny fluctuations, and optical readout makes the measurement straightforward.
The Road to Molecular Imaging
Both groups describe their results as a first step toward true three-dimensional molecular-scale MRI — the ability to image individual proteins, DNA strands, or other biomolecules atom by atom. That would be a revolutionary capability for structural biology and drug development. The challenge now is improving sensitivity, spatial resolution, and the ability to work with biologically relevant samples.
It's a long way from these proof-of-concept demonstrations to a practical molecular imager, but the physics is there. The question is whether the engineering can catch up.
Source: Physics World
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