Nitrogen–vacancy (NV) centers, a type of point defect in diamond, are prized for their long coherence times and the easy readability of their spin states. The defects have potential not only in quantum information but as tiny magnetometers, the basis for nanoscale NMR. NV centers implanted a few nanometers below a diamond surface can detect the presence—or even the chemical identity—of molecules a few nanometers above it.

But what about molecules on the surface? Many systems, from batteries to biosensors, involve chemical processes at liquid–solid interfaces, but most tools for studying surface chemistry work only under an ultrahigh vacuum, so they require expensive machinery and can’t directly probe ambient conditions.

NV-center NMR has the sensitivity to study surface reactions. But surface processes of interest almost always involve surfaces of materials other than diamond. It’s therefore necessary to interface an NV-laden diamond with a chemically relevant surface material without disrupting either the delicate NV-center spins or the surface properties.

Now Kristina Liu (shown in the photo), her PhD adviser Dominik Bucher, and their colleagues at the Technical University of Munich have demonstrated surface-sensitive NV-center NMR. With the help of collaborators in materials science, they used atomic-layer deposition to coat the diamond with a few nanometers of aluminum oxide, a common support material in surface science. The coating compromised the NV centers’ coherence, but only a little, and they were still capable of a sensitive NMR readout.

When the researchers exposed the coated diamond to a solution of a fluorinated organic molecule, they observed the real-time growth of a ¹⁹F NMR signal, shown in the figure, as the molecules attached themselves to the surface and organized into a self-assembled monolayer. (The relatively unchanging hydrogen-1 peak in the figure probably comes from H atoms in the diamond itself, not from molecules on the surface.)

The spectral resolution, at a few kilohertz, is a step backward from that of previous NV-center NMR experiments (see Physics Today, May 2018, page 21), which achieved peaks as narrow as a few hertz—fine enough to distinguish atoms not only by their elemental type but also by subtle differences in their chemical environment. In an anisotropic environment such as a solid or interface, such resolution is harder to come by. There are tricks for making it work (see the article by Clare Grey and Robert Tycko, Physics Today, September 2009, page 44), but their implementation would complicate an otherwise simple and inexpensive experiment.

Meanwhile, in related work, Peter Maurer (University of Chicago), Nathalie de Leon (Princeton University), and colleagues have shown that even biomolecules can be anchored to an Al₂O₃-coated diamond without ruining either the biomolecular structure or the NV-center coherence. Those experiments haven’t yet reached the stage of NMR measurements, but combining their techniques with Bucher and colleagues’ work could bring NV-center NMR into the realm of single-molecule biophysics. (K. S. Liu et al., Proc. Natl. Acad. Sci. USA 119, e2111607119, 2022; M. Xie et al., Proc. Natl. Acad. Sci. USA 119, e2114186119, 2022.)