Detection of magnetic dipolar coupling of water molecules at the nanoscale using quantum magnetometry

It is a crucial issue to study interactions among water molecules and hydrophobic interfacial water at the nanoscale. Here we succeed in measuring the nuclear magnetic resonance spectrum of a diamond-water interfacial ice with a detection volume of about $2.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}22}$ L. More importantly, the magnetic dipolar coupling between the two protons of a water molecule is resolved by measuring the signal contributed from about 7000 water molecules at the nanoscale. The resolved intramolecule magnetic dipolar interactions are about 15 and 33 kHz with spectral resolution of 5 kHz. This work provides a platform for hydrophobic interfacial water study under ambient conditions, with further applications in more general nanoscale structural analysis.