Electron spin resonance (ESR) has emerged as a powerful spectroscopic technique to study the properties of metal nanoparticles (NPs) protected by a self-assembled monolayer (SAM) of organic molecules. This technique has been employed to explore the capacity of homoligand monolayers to bind to a hydrophobic probe or to "sense" the hydrophobicity of mixed-ligand monolayers. Moreover, spin labels anchored to the metal surface enable the investigation of the dynamic of the ligands that form the monolayer. Here we review these applications with the aim of unravelling the many features of monolayer-protected metal NPs.
