  Nuclear magnetic resonance (NMR) spectroscopy is a widely used tool for
chemical analysis and molecular structure identification. Because it typically
relies on the weak magnetic fields produced by a small thermal nuclear spin
polarization, NMR suffers from poor molecule-number sensitivity compared to
other analytical techniques. Recently, a new class of NMR sensors based on
optically-probed nitrogen-vacancy (NV) quantum defects in diamond have allowed
molecular spectroscopy from sample volumes several orders of magnitude smaller
than the most sensitive inductive detectors. To date, however, NV-NMR
spectrometers have only been able to observe signals from pure, highly
concentrated samples. To overcome this limitation, we introduce a technique
that combines picoliter-scale NV-NMR with fully integrated Overhauser dynamic
nuclear polarization (DNP) to perform high-resolution spectroscopy on a variety
of small molecules in dilute solution, with femtomole sensitivity. Our
technique advances mass-limited NMR spectroscopy for drug and natural product
discovery, catalysis research, and single cell studies.
