Poly(ethylene glycol) ligands for high-resolution nanoparticle mass spectrometry

Measurements of the core and ligand monolayer compositions of small gold nanoparticles (NPs) using electrospray ionization (ESI) mass spectrometry were performed by incorporating ionization tags, methoxy penta(ethylene glycol) thiolate ligands (-S-PEG), into the ligand monolayers via ligand exchange. During ESI, alkali metal ions (M+) coordinate to the -S-PEG ligands and give the NPs positive charge. Atomically precise, high-resolution measurements show unequivocally that the NP composition is Au-25(ligand)(18). The predominant ions, M4Au25(ligand)(18)(3+) and M5Au25(ligand)(18)(4+), have 1- charge on the core. Because ligand exchange is a statistical process, there is a distribution of mixed-monolayer exchange products, which is reflected in the mass spectra.