  Conventional nuclear magnetic resonance (NMR) spectroscopy relies on
acquiring signal from a macroscopic ensemble of molecules to gain information
about molecular structure and dynamics. Transferring this technique to
nanoscale sample sizes would enable molecular analysis without the effects of
averaging over spatial and temporal inhomogeneities and without the need for
macroscopic volumes of analyte, both inherent to large ensemble measurements.
Nanoscale NMR based on nitrogen vacancy (NV) centers inside bulk diamond chips
achieves single nuclear spin sensitivity and the resolution required to
determine chemical structure, but their detection volume is limited to a few
nanometers above the diamond surface for the most sensitive devices. This
precludes them from use for nuclear spin sensing with nanoscale resolution
inside thicker structures, such as cells. Here, we demonstrate the detection of
NMR signals from multiple nuclear species in a (19 nm)3 volume using versatile
NV-NMR devices inside nanodiamonds that have a typical 30 nm diameter. The
devices detect a signal generated by a small number of analyte molecules on the
order of 1000. To use these devices in situ, the detected signal must be
corrected for the unknown geometry of each nanodiamond device. We show that
such a calibration could be performed by exploiting the signal from a thin
layer of nuclei on the diamond surface. These results, combined with the low
toxicity of nanodiamonds and their amenability to surface functionalization,
indicate that nanodiamond NV-NMR devices could become a useful tool for
nanoscale NMR-based sensing inside living cells.
