Structure and Stability of PEG- and Mixed PEG-Layer-Coated Nanoparticles at High Particle Concentrations Studied In Situ by Small-Angle X-Ray Scattering

Ligand-layer structure and stability of gold nanoparticles (AuNP) coated with α-methoxypoly(ethylene glycol)-ω-(11-mercaptoundecanoate) (PEGMUA) layers and mixed layers of PEGMUA and 11-mercaptoundecanoic acid (MUA) at high AuNP concentrations are studied in situ by small-angle X-ray scattering (SAXS). The thickness of the ligand layer is modified by the molecular weight of the PEG-ligands (2 and 5 kDa), and the PEG-grafting density is decreased by coadsorption of MUA. The response of the conjugates to a pressure of up to 4 kbar is probed. The results indicate strongly hydrated PEG layers at high grafting densities. The stability of the mixed ligand-layer conjugates is lower. This is most probably due to enhanced interparticle PEG–PEG interactions at lower grafting densities. The presented study demonstrates that a detailed structural characterization of polymer ligand layers in situ and in response to external stimuli is possible with SAXS