Fluorescing diamonds inside living cells

The point defect in diamond known as the NV center—a nitrogen atom substituted for a carbon atom and adjacent lattice vacancy—has become a promising ingredient in recent efforts to develop atomic-scale sensors. When optically excited, the defect exhibits stable fluorescence, even in a crystal as small as 5 nm. And its ground state is magnetically sensitive—the spin 0 level is separated from degenerate spin ±1 levels by a microwave transition of 2.9 GHz. That sensitivity allows one to detect weak magnetic fields by observing the quantum spin state, which can be manipulated by microwave pulses and then read out optically by monitoring the fluorescence intensity. (The intensity depends on which of the three spin states is populated.) Researchers led by the University of Melbourne’s Lloyd Hollenberg have now performed such magnetic resonance experiments on individual nanodiamonds placed inside human cells. The quantum spin levels of the defects acted both as local magnetometers and as fingerprints to spectrally distinguish each nanodiamond in the complicated cell environment. Using a confocal microscope, the researchers were able to identify and track at nanometer precision individual NV centers in the cells from the optical emission (red). They were also able to measure the coherence time of the spin states; that work sets the stage for sensing the cells’ local magnetic fluctuations in response to, for example, the transport of charge through cell membrane ion channels, which are important drug targets. (L. P. McGuinness et al., Nat. Nanotechnol., in press, doi:10.1038/nnano.2011.64.)—R. Mark Wilson