  Single-cell analysis can unravel functional heterogeneity within cell
populations otherwise obscured by ensemble measurements. However, non-invasive
techniques that probe chemical entities and their dynamics are still lacking.
This challenge could be overcome by novel sensors based on nitrogen-vacancy
(NV) centers in diamond, which enable nuclear magnetic resonance (NMR)
spectroscopy on unprecedented sample volumes. In this perspective, we briefly
introduce NV-based quantum sensing and review the progress made in microscale
NV-NMR spectroscopy. Lastly, we discuss approaches to enhance the sensitivity
of NV ensemble magnetometers to detect biologically relevant concentrations and
provide a roadmap towards their application in single-cell analysis.
