Aug 26, 2010
Taking MRI to the nanoscale by force
Over the last 20 years, researchers have been making steady progress improving the sensitivity of force-detected magnetic resonance. Sensitivity has doubled roughly every 8 months and presently surpasses the sensitivity of conventional, inductive nuclear magnetic resonance detectors by 8 orders of magnitude. In 2009, researchers at the IBM Almaden Research Center and the Stanford Center for Probing the Nanoscale demonstrated the promise of these developments by using magnetic resonance force microscopy (MRFM) to capture 3D images of individual virus particles.
Conventional MRI techniques employ pick-up coils to detect the small changes in magnetic field induced by flipping nuclear moments contained in a sample. These magnetic signals are so weak that conventional instruments cannot resolve objects smaller than several microns – about the size of a small cell. MRFM improves on this sensitivity by mechanically detecting the magnetic forces produced by nuclear moments.
A microfabricated cantilever is used to sense the forces arising between nuclear moments in a sample and a nearby nano-magnet. These are forces familiar to everyone: they are the same forces felt when one brings two refrigerator magnets close together. In MRFM, radio frequency pulses cause nuclear moments in a sample to periodically flip, generating an oscillating force on the cantilever. It is just as if one were to alternately flip the orientation of one of two refrigerator magnets, switching between attractive and repulsive forces. These alternating forces, in turn, drive the cantilever to oscillate and this motion is detected by an optical interferometer.
Force-detected MRI is more sensitive to nanoscale samples than conventional techniques because much smaller detectors can be made. For the inductive technique to be sensitive, the size of the pick-up coil must be similar to the size of the sample. For nanoscale samples, this is practically impossible. On the other hand, high-quality cantilevers can now be fabricated with dimensions far below a micron such that the sample mass is significant compared to the bare resonator mass.
MRFM has the unique capability to image the interior of nanoscale objects non-invasively and with intrinsic chemical selectivity. Despite the tremendous improvements made in the last few years, important obstacles must still be overcome in order for the technique to become a useful tool for biologists and materials scientists. The possibility of extending MRFM to atomic resolution – whereby molecules could be imaged atom-by-atom in 3D – appears within reach, though it remains a technically very challenging prospect.
A full review of technology can be found in the journal Nanotechnology.
About the author
Prof. Martino Poggio is an assistant professor in the physics department at the University of Basel, Switzerland, where he leads a group focusing on ultra-sensitive micro- and nano-mechanical resonators and their coupling to quantum states. He continues to work on problems related to force-detected magnetic resonance.