Interaction Between Soft Nanoparticles and Phospholipid Membranes: Effect of the Polymer-Grafting Density on Nanoparticle Adsorption

Nanoparticles (NPs) have large potential for biological applications as typically they exhibit strongly size-dependent properties. Specifically, the interaction of NPs with phospholipid membranes is of significant relevance to nanomedicine and the related field of nanotoxicology. Therefore, the investigation of NP interactions with model membranes is not only of fundamental importance but also of practical value to understand NP interactions with more complex cell membranes. Supported lipid bilayers (SLBs) provide a powerful platform to study such interactions. Here, we report on the interaction of SiO2–NPs, covered with cationic polymer (PDMAEMA) of different grafting density but approximately constant polymer layer thickness, with SLBs of differing charge density. We studied binding of the NPs to the SLBs by quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM). A key result of the study is that at low solution pH and in the presence of electrostatic attraction, the amount of adsorbed NPs drastically decreases with increasing polymer brush grafting density beyond a critical value. However, upon increasing the solution pH (thereby lowering the apparent electrostatic attraction) even NPs with high polymer grafting density adsorb. In this transitional range, NP adsorption depends strongly on NP concentration becoming reduced at higher concentration. The experimental observations were interpreted by simple models taking into account electrostatic and van der Waals interactions that allow to gain some insights into the prevailing conditions.