Impact of surface curvature, grafting density and solvent type on the PEGylation of titanium dioxide nanoparticles.

TiO2 nanoparticles (NPs) are attracting materials for biomedical applications, provided that they are coated with polymers to improve solubility, dispersion and biocompatibility. Conformation, coverage density and solvent effects largely influence their functionality and stability. In this work, we use atomistic molecular dynamics simulations to study polyethylene glycol (PEG) grafting to highly curved TiO2 NPs (2-3 nm) in different solvents. We compare the coating polymer conformations on NPs with those on (1 0 1) flat surfaces. In water, the transition from mushroom to brush conformation starts only at high density (σ = 2.25 chains/nm2). In dichloromethane (DCM), at low-medium coverage (σ < 1.35 chains/nm2), several interactions between the PEG chains backbone and undercoordinated Ti atoms are established, whereas at σ = 2.25 chains/nm2 the conformation clearly becomes brush-like. Finally, we demonstrate that these spherical brushes, when immersed in water, but not in DCM, follow the Daoud-Cotton (DC) classical scaling model for the polymer volume fraction dependence with the distance from the center of star-shaped systems.