Electrostatic Titrations Reveal Surface Compositions of Mixed, On-Nanoparticle Monolayers Comprising Positively and Negatively Charged Ligands

A general-purpose procedure based on electrostatic titrations was developed to determine the compositions of mixed self-assembled monolayers (m-SAMs) of oppositely charged ligands on metal nanoparticles (NPs). In this procedure, nanoparticles of unknown m-SAM compositions and charges are titrated with the positively charged NP “standards”. These compositions and the differences in the free energies of adsorption of the [+] and [−] are then determined from the points of precipitation at which the overall charge on all NPs is neutralized. The results of the titrations agree well with traditional core-etching/NMR studies. However, the titration-based approach is nondestructive and requires significantly less material than NMR.