Nitrogen vacancy (NV) defects occur when two neighbouring carbon atoms in diamond are replaced by a nitrogen atom and an empty lattice site. NV nanodiamonds are ideal as biological probes because they are non-toxic and photostable. More importantly, they are capable of detecting the very weak magnetic fields that come from the electronic or nuclear spins in a sample. In contrast to conventional magnetic resonance imaging techniques in biology in which millions of spins are required to produce a measurable signal, the NV defects can detect individual target spins.

The team, led by Lloyd Hollenberg of the University of Melbourne, succeeded in detecting gadolinium "spin labels" attached to some of the molecules that form the lipid bilayer of an artificial cell membrane. Gd is a common magnetic resonance imaging contrast agent and the electrons in a Gd atom, each of which have a spin of 1/2, give the atom a relatively large total spin of 7/2 that acts as a fluctuating magnetic field source. The NV sensor in the nanodiamonds acts as a single quantum spin that is very sensitive to these fluctuating magnetic fields.

Scrambling the NV centre quantum state

"We control the quantum state of our NV probe using a laser and read it out after a pre-defined delay by detecting the photons emitted by the NV," explained Hollenberg. "In fact, the quantum state of the NV centre becomes ‘scrambled’ in the presence of the Gd spin targets, and we can measure this effect."

The technique is all about using the quantum properties of the NV centre to detect magnetic fields at the nanoscale, he told "There are many situations in biology where being able to do this might be important – for instance, for detecting free radicals or ion channels in situ, and in real-time high-resolution imaging of neuron function."

The team, which includes researchers from the Japan Atomic Energy Agency and the University of Stuttgart in Germany, says that it is now working towards using smaller nanodiamonds that are even more sensitive to fluctuating magnetic fields and looking at how they may be used to detect fundamental spins in different biological systems.

The present work is detailed in PNAS.