The Surface of Colloidal Nanoparticles Revealed by Vibrational Sum Frequency Scattering Spectroscopy

Solvation shells strongly influence the interfacial chemistry of colloidal systems, from the activity of proteins to the colloidal stability and catalysis of nanoparticles. Despite their fundamental and practical importance, solvation shells remain largely undetected by spectroscopy. Here, we apply for the first-time vibrational sum frequency scattering spectroscopy (VSFSS)—an interface-specific technique—to colloidal nanocrystals, with porous metal–organic frameworks (MOFs) as a case study. Due to the porous nature of the solvent-particle boundary, MOF particles challenge conventional models of colloidal and interfacial chemistry. Spectra indicate the presence of ordered solvation shells and spontaneous ordering within the MOF. VSFSS also applies to colloidal TiO2 and other common nanoparticles, revealing solvation shells with distinct spectral features that arise from the specific surface chemistry of each material. These findings explain the unexpected colloidal stability of MOF colloids, while providing a roadmap for applying VSFSS to wide-ranging colloidal nanocrystals in general.