A Study on Colloidal Interactions between Gold Nanoparticles by using Cryogenic Electron Tomography

Over the course of the last decades, nanoparticles have been objects of many researches because of their remarkable properties derived by the dimensions within the nanoscale. The study of their interactions is crucial for their understanding and their implementation in many significant applications such as drug delivery systems, biosensing, clinical and biomedical diagnostics. Cryogenic Electron Tomography (Cryo-ET) is used to evaluate the long-range colloidal interactions of gold nanoparticles within the DLVO (Boris Derjaguin, Lev Landau, Evert Verwey and Theodoor Overbeek) theoretical framework. Cryo-ET is based on the 3D reconstruction of a vitrified nanoparticle dispersion, allowing to derive the 3D radial distribution function of the nanoparticles from which the mean interaction potential and the structure factor can be calculated. The data obtained are validated by small angle X-ray scattering (SAXS) for the same samples prior to the vitrification process. This study is carried out on 11-mercapto-1-undecane sulfonate (MUS)-coated gold nanoparticles as a function of different parameters such as nanoparticle concentration and ionic strength. With known characteristics of the nanoparticles such as purity, core size, surface charge, and ligand density, the DLVO theory is applied to calculate the theoretical structure factors of the nanoparticles under the same experimental conditions, so as to compare them directly with the structure factors experimentally determined. It is concluded that DLVO theory can only partially explain certain aspects of nanoparticle dispersions such as the concentration dependence with some corrections. The effect of temperature on nanoparticle interactions is studied through 2D cryogenic transmission electron microscopy (Cryo-TEM) using gold nanoparticles coated with the thermo-responsive polymer poly(N-isopropylacrylamide) (PNIPAM) as a model. It is shown that the addition of salt lowers the temperature at which the volume phase transition from a hydrophilic state to a hydrophobic aggregated state takes place. At temperatures higher than the phase transition point, Cryo-TEM shows globular aggregate states in presence of isolated nanoparticles, suggesting a micellar-like reversible aggregation mechanism.