Technique could be sensitive enough to detect structure of a single protein.
Diamond-based quantum devices can now make nuclear magnetic resonance measurements on the molecular scale. Work by two independent groups will make it easier to find out the structure of single biological molecules such as proteins without destroying or freezing them.
Nuclear magnetic resonance (NMR) and its close cousin magnetic resonance imaging (MRI) give information about a sample’s structure by detecting the weak magnetic forces in certain atomic nuclei, such as hydrogen. They work by detecting how molecules collectively resonate — like guitar strings that vibrate together — with electromagnetic waves of specific wavelengths. The techniques provide information about the structure of samples without damaging them — which is particularly important if the sample is a human body.
But to some researchers, whole bodies are less interesting than the molecules that they are made up of. “I want to push NMR and MRI to the molecular level,” says Friedemann Reinhard, a physicist at the University of Stuttgart in Germany. His team is one of two that have used NMR to detect hydrogen atoms in samples measuring just a few nanometers across.
Probing single molecules a few nanometres wide has been a major frustration in NMR. The detectors need to be a similar size to the sample, and the magnetic coils usually used cannot easily be made smaller than a few micrometers.
NMR and MRI measurements on the nanoscale have been done using powerful nanomagnets in a technique called magnetic resonance force microscopy — but that only worked with very cold samples.
Rugar and Reinhard took a different approach. Both teams made diamonds with defects in their crystal structure — a single nitrogen atom next to a missing carbon atom, a few nanometres below the surface. This gives the diamond a red fluorescent glow, which can be bright or dull depending on which way the nitrogen’s electrons are spinning.