Characterization of Poly(ethylene oxide)-Capped Gold Nanoparticles in Water by Means of Transmission Electron Microscopy, Thermogravimetric Analysis, Mass Density, and Small Angle Scattering

Abstract

A combined analysis of experiments, small-angle neutron and X-ray scattering, transmission electron microscopy (TEM), mass density, and thermogravimetric measurements (TGA), was applied to gold nanoparticles (NP) grafted with thiol-terminated poly(ethylene glycol). TEM micrographs indicate that the gold core diameter is ca. 20% polydisperse. The associated polydispersion in volume dictates that the nanoparticle extensive properties thus determined, such as the number of gold atoms and polymer chains, have similar errors (ca. 30%). On the other hand, by combining the results of mass density and TGA the following intensive properties of the nanoparticle can be determined with a precision of 1%: (i) the volume fraction occupied by the gold core, (ii) the density of the nanoparticle, and (iii) the polymer terminations at the interface. An accurate characterization of the structure and hydration of NP is then obtained by fitting the scattering data and use of the density and TGA data to impose constraints on the fits. The scattering data can be fitted with a core−two shell model. The inner core contains only gold atoms and the two shells represent the interfacial region containing the polymer and the hydrating water molecules. The two shells differ in the local solvent−polymer composition: the inner shell is composed almost exclusively of polymer, whereas the outer is hydrated (about 60% of the volume occupied by polymers). Interesting differences have been observed with isotopic substitution in the solvent (H2O/D2O ratio).