I wrote in 2013 about work that opens the possibility of single-cell (or single-molecule) NMR and other techniques, using nitrogen-vacancy (NV) diamond surfaces. I’m glad to report that there’s enough going on in this field to be the subject of a minireview in Angewandte Chemie.
You make these things by synthesizing diamond particles with some deliberate nitrogen impurites, to work some of those atoms into the lattice, and then irradiating them. That produces vacancies next to the nitrogen atoms, which can be monitored by the onset of fluorescence in the diamonds themselves (and a change in their visible-light color). Handling them and treating them to work with biomolecules and the like is not a trivial job, though. The review goes into the various schemes that people have tried to functionalize the diamond surface or coat it for greater stability. The hope is that you could produce nanodiamond particles with these defects, small enough to be taken up into cells, but that’s apparently quite a challenge. Diamond particles on that scale tend to aggregate, it seems, and the high surface-area/volume ratio of the nanoparticles, while a feature in many ways, can also be a problem.
The techniques that seem very promising with these things are based on their magnetic sensitivity and their fluorescent properties. They don’t photobleach, so imaging techniques like STED and other assay methods such as FRET could work very well indeed. On the magnetic end, single-molecule NMR and detection of magnetic atoms (such as the iron in various proteins) is also doable -and in fact, this is the only technique now known to do such things. You can also hyperpolarize these nitrogen-vacancy centers and get even more sensitivity that way.
I would have to assume that a lot of nanotechnology money is flowing into this area – if not, it probably should be. If we could fabricate these things precisely enough, all sorts of unusual applications might result. The interactions between these centers would be another layer of possibility for new devices – you could imagine whole circuits incorporating NV nodes, and although I’m no engineer, there would have to be a lot of currently-impossible things that could be done with them, what with their ability to sense magnetic fields at the level of individual atoms. This is the sort of thing that makes me think I’m living in the 21st century as described in some of the science-fiction stories I read while growing up – and not, for example, as described in some of the others, no matter how strongly current events might suggest the latter.