Polymer brush grafted anionic SiO 2 @PMAA (poly(methacrylic acid)) and cationic SiO 2 @PDMAEMA (poly(2-(dimethylamino)ethyl methacrylate)) inorganic/polymer hybrid nanoparticles with different core radii (d NP = 50− 140 nm) and different amounts of attached polymer were synthesized via surface-initiated atomic transfer radical polymerization (ATRP). To avoid irreversible aggregation, a three-step surface modification had to be employed, thereby keeping the nanoparticles always dispersed. For SiO 2 @PMAA the shell thickness changes with the monomer concentration, while for SiO 2 @PDMAEMA the grafting density was changed by monomer concentration and the shell thickness remained constant. We assume that the control over the grafting density relies on the nature of the complexation potential of the PDMAEMA. The structural characterization of the polymer grafted SiO 2 -NPs was done in detail by different scattering methods combined with thermogravimetric analysis, and details of the brush characteristics are obtained by small-angle neutron scattering (SANS). With this approach we were able to produce silica nanoparticles with anionic and cationic polymer shells, where the softness of the NPs can be controlled by the amount of polymer, which are pH-responsive and colloidally stable over a large pH range.

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.

As silica nanoparticles (SiO 2 NP) gain increasing interest for medical applications it is important to understand their potential adverse effects for humans. Here we prepared well-defined core-shell fluorescently labelled SiO 2 NP of 15, 60 and 200 nm diameter and analyzed their cytotoxicity in THP-1 derived macrophages, A549 epithelial cells, HaCaT keratinocytes and NRK-52E kidney cells. We observed a size-dependent cytotoxicity in all cell types in serumfree conditions. HaCaT cells were least and macrophages or lung derived A549 cells were highly sensitive towards SiO 2 NP treatment. Differences in cytotoxicity could be correlated with different uptake rates. By using flow cytometry and confocal microscopy we quantified the uptake. Furthermore we used specific inhibitors for clathrin-and caveolinmediated endocytosis to elucidate the uptake mechanisms, which were found to be dependent on the NP size and the cell type. Clathrin-mediated endocytosis was involved in the uptake of SiO 2 NP of all sizes and was the major pathway for 60 nm or 200 nm SiO 2 NP. Caveolin-mediated endocytosis contributed to the uptake of 60 and 200 nm SiO 2 NP in THP-1 macrophages but only to uptake of 200 nm SiO 2 NP in A549. However, in the presence of serum all SiO 2 NP were non-toxic. The presence of serum furthermore could alter the uptake mechanism. In summary, this study demonstrates size-and cell type dependent differences in SiO 2 NP uptake and toxicity.