Charging of Ligand-Protected Gold Nanoparticles in Electrospray Ionization: Probing Ligand Bond Strength by Charge Detection Mass Spectrometry

The ionization of large, megadalton (MDa) and gigadalton (GDa), nanoparticles leading to their mass spectrometric analysis has been an interesting challenge in the past decades. In this work, the charging behavior of 20 nm sized gold nanoparticles (AuNPs) protected with 11-mercaptoundecyl-<i>N</i>,<i>N</i>,<i>N</i>-trimethylammonium bromide (MUTAB) ligands of ∼50 MDa was studied by using Charge Detection Mass Spectrometry (CDMS) interfaced with electrospray ionization. The findings indicate that these nanoparticles can achieve a charging state higher than the Rayleigh limit for a 20 nm solvent droplet. However, citrate-protected 60 nm sized AuNPs of ∼1.3 GDa charge only ∼0.45 times the Rayleigh limit, under identical electrospray conditions. The higher efficiency of charging in Au-MUTAB nanoparticles may be correlated to the higher ligand binding energy of the gold-sulfur interface compared to the gold-citrate interface. This work showcases the impact of ligand selection on the electrostatic behavior, stability, and charging of nanoparticles in electrospray ionization mass spectrometric applications.