“Another important result of our research is that we have demonstrated a new magnetic resonance protocol that allows us to apply NMR techniques to encode so-called spin noise,” explained team leader Raffi Budakian. “That is, we encode information in the statistical fluctuations of all the nuclear spins in a sample rather than in their thermal spin polarization – as is usually the case.”

“In nanometre-scale volumes, the signal generated by nuclear spin fluctuations is generally much larger than that produced by the thermal spin polarization,” he added. “It is, however, difficult to work with these spin fluctuations because both the sign and magnitude of the signal fluctuate. Our technique provides a new way to implement NMR pulse sequences that would work with such a randomly fluctuating signal.”

The researchers begin by attaching the sample to be analysed to the tip of a silicon nanowire mechanical resonator – which is a tiny plank of silicon roughly 15 µm long and 50 nm wide. They then place this nanowire over a metal constriction 240 nm wide and 100 nm thick. By passing high-frequency electric currents through the constriction, they are able to generate intense magnetic fields on the sample and perform magnetic resonance.

The team then oscillates this electric current through the constriction to generate a magnetic field gradient that alternates at the same frequency as the nanowire vibrates. The force of interaction between the spins in the sample and the alternating inhomogeneous magnetic field produces tiny, angstrom-scale vibrations of the nanowire that can then be measured using an optical interferometer included in the set-up.

Well established methods in MRI

“Our technique in fact uses well established methods in MRI,” Budakian told nanotechweb.org. “Fourier-transform imaging is routinely used in MRI and is a very efficient sample-imaging technique, but the main difference in our new method is that we encode information in the spin noise rather than in the thermal polarization.”

The Illinois team was able to successfully image 1H spins in a polystyrene sample using its method and obtained a 2D projection of the proton density in the material with a spatial resolution as small as 10 nm.

According to the researchers, the technique could come in handy for imaging biological samples. “Our near-term goal is to achieve even higher spatial resolution and begin imaging virus particles,” added Budakian. “We would ideally like to tomographically image virus particles in 3D and, with sufficient improvement, might even be able to image macromolecules such as proteins in the future.”

The current work is reported in Physical Review X doi: 10.1103/PhysRevX.3.031016.

Further reading

Diamond downsizes classical MRI and NMR (Feb 2013)
MRI resolution reaches 90 nm (Apr 2007)
Taking MRI to the nanoscale by force (Aug 2010)