  Atomic-scale magnetic field sensors based on nitrogen vacancy (NV) defects in
diamonds are an exciting platform for nanoscale nuclear magnetic resonance
(NMR) spectroscopy. The detection of NMR signals from a few zeptoliters to
single molecules or even single nuclear spins has been demonstrated using
NV-centers close to the diamond surface. However, fast molecular diffusion of
sample molecules in and out of nanoscale detection volumes impedes their
detection and limits current experiments to solid-state or highly viscous
samples. Here, we show that restricting diffusion by confinement enables
nanoscale NMR spectroscopy of liquid samples. Our approach uses metal-organic
frameworks (MOF) with angstrom-sized pores on a diamond chip to trap sample
molecules near the NV-centers. This enables the detection of NMR signals from a
liquid sample, which would not be detectable without confinement. These results
set the route for nanoscale liquid-phase NMR with high spectral resolution.
