Nanoscale nuclear magnetic resonance

Nuclear magnetic resonance (NMR) and x-ray crystallography, the standard tools for determining a molecule’s three-dimensional structure, both typically require macroscopic samples of many identical molecules either arrayed in a crystal or dissolved in solution. But for many proteins and other biomolecules, those requirements have proven prohibitive. Now two teams—one led by Daniel Rugar (IBM Research Division, San Jose, California) and David Awschalom (University of California, Santa Barbara), and the other led by Friedemann Reinhard and Jörg Wrachtrup (University of Stuttgart, Germany)—have taken a step toward single-molecule structure determination by performing NMR on nanoscale volumes. Both groups used nitrogen–vacancy (NV) centers: point defects in diamond that comprise a nitrogen atom adjacent to a lattice vacancy. NV centers are well studied for their spin characteristics: Their spin-1 ground state is isolated from its environment, has a long coherence time, and can be manipulated and measured optically. Those properties, which make NV centers promising qubit candidates, also make them sensitive magnetometers. An NV center a few nanometers beneath the diamond surface responds to the magnetic field of the spin-½ hydrogen-1 nuclei in an organic sample just above the surface, as shown in the figure. The detection volume to which the NV center is sensitive—currently about the size of a large protein molecule—could potentially be made even smaller by moving the defect closer to the surface. The researchers anticipate that by embedding the NV center in a scanning probe tip, they will eventually be able to obtain nanoscale magnetic resonance images. (H. J. Mamin et al., Science 339, 557, 2013; T. Staudacher et al., Science 339, 561, 2013.)—Johanna Miller