Magnetic detector identifies single protons

MRI-like technique could eventually image individual biological molecules

Editor’s note: The study described in this article was retracted in the Jan. 9, 2015 Science. IBM physicist Daniel Rugar says that when he and his team attempted to replicate the experiment, they realized that the signal for a particular atom in the diamond, carbon-13, could be mistaken for that of a proton. Based on this finding, the authors of the study concluded that they did not have sufficient evidence to claim the detection of a single proton.

A small MRI-like device made of diamond has performed a scan of a single proton, an achievement that’s a big step toward using magnetic imaging to look in-depth at viruses, proteins and other nanosized biological objects.

“It’s a really nice milestone,” says Daniel Rugar, a physicist at the IBM Almaden Research Center in San Jose, Calif.

The new device works on the same principle as the monstrous magnetic resonance imaging machines in hospitals: Strong magnetic fields orient the spins of hydrogen nuclei, each of which consist of a single proton. Then the machine emits radio waves. The hydrogen nuclei absorb the radio radiation and re-emit it at a different frequency, which the scanner can detect and use to identify the location of the hydrogen.

MRIs work because human tissues include trillions upon trillions of water molecules, and thus hydrogen nuclei. Plus, the response of hydrogen nuclei to a magnetic field varies slightly depending on their surroundings, allowing technicians to distinguish between bone, fat and tissue.

For the last two decades, physicists have been working to shrink MRI down to visualize the nanoscale. “We want to apply MRI tricks to studying viruses, cells and individual molecules,” says Christian Degen, a solid state physicist at ETH Zurich. In 2009 he was part of a team that used a magnetic sensor to image a virus made up of, among other components, about 10,000 hydrogen atoms.

To zoom in even farther, Degen and his team built a device made of diamond. Pure diamond is a rigid lattice of carbon atoms. The researchers extracted two adjacent carbon atoms near the diamond’s surface and replaced one with an atom of nitrogen. When the researchers shine green light on it, this simple impurity, called a nitrogen-vacancy center, emits red light.

The red light serves as a nanometer-sized flashlight whose brightness depends on the surrounding magnetic field. “It’s really an atomic-sized magnetic field sensor,” Rugar says. “We’re still discovering all the things that can be done with it.”

Then the researchers placed a thin film of a material called ammonium hexafluorophosphate atop the diamond, which deposited hydrogen nuclei just above the diamond’s surface. Slight changes in the brightness of the emitted light from the nitrogen-vacancy center indicated the presence of a single proton less than a nanometer away, Degen’s team reports October 16 in Science.

The result is an important advance toward using magnetic scanning to probe the structural intricacies of individual biological molecules, says Jörg Wrachtrup, a quantum physicist at the University of Stuttgart in Germany. Scientists use X-ray crystallography and nuclear magnetic resonance spectroscopy for that purpose, but those techniques provide information about a composite of many, many molecules. A detector with the sensitivity of Degen’s device would be able to probe molecules one by one, perhaps helping scientists better understand exotic proteins such as prions, Wrachtrup says.

Building a protein-scanning detector is still an ambitious goal. Degen’s device has a detection range of about a nanometer, but proteins have diameters of about 10 nanometers. Researchers will have to figure out a way to probe molecules of that size without sacrificing sensitivity. “That’s where a lot of work is needed,” Wrachtrup says. “But I’m confident that this is doable.”

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