Membra cel

Commercially available fumed (pyrolytic) SiO₂-NPs (Aerosil^® 200, 98% SiO₂, specific surface area of 200 m² g⁻¹) were purchased from Evonik (former Degussa). Fumed SiO₂-NPs are produced by continuous flame hydrolysis, are reported to be non-porous by the manufacturer and Mebert and co-workers^{8 (link)}, and are less hydroxylated than colloidal SiO₂-NPs^{37 (link)}. All chemicals used were per analysis grade unless it is stated otherwise. Water was pre-purified by a Milli-Q system (18.2 MΩ.cm arium 611DI, Sartorius Stedim Biotech, Germany). Dialysis membranes were purchased from Roth (Membra-Cel™, 14 kDa cut-off).
Both cell culture medium and food matrices are relevant chemically complex matrices that reportedly pose significant analytical challenges for NP analytics^{43 (link),44} . We selected three representative complex matrices according to the following criteria: (1) the cell culture media DMEM is widely used in in vitro NP-cell interaction studies^{45 (link)}; (2) tomato sauce is a typical food matrix containing with <61 mg kg⁻¹ comparatively little SiO₂^{39 (link)}; and (3) potato seasoning is a foodstuff where E551, i.e. food grade SiO₂, was listed on the packaging as an anti-caking ingredient. The potato seasoning (Qualité & Prix Country Potato Seasoning Blend, Germany) and the tomato sauce (Cirio Rustic Tomato Purée, Italy) were purchased from a local supermarket.

10 mg 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC, Avanti Polar Lipids) dissolved in chloroform were dried under a stream of nitrogen in a 10 mL round bottom flask. For fluorescently labelled proteoliposomes 0.028 mg or 0.112 mg (0.25 mol% or 1 mol%) 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-(7-nitro-2-1,3-benzoxadiazol-4-yl) ammonium salt (NBD-PE, Avanti Polar Lipids) dissolved in chloroform were added before drying. Remaining chloroform traces were removed under vacuum overnight in a desiccator. The lipid was hydrated with 2 mL potassium phosphate buffer (20 mM KP_i pH 7.2, 100 mM KCl, 1 mM TCEP) on a Thermomixer (Thermomixer, Eppendorf) at RT and 800 rpm for 15 min. The liposomes were destabilized by adding OG to 0.75% (w/v) and further shaking at RT and 800 rpm for 15 min. 400 µg purified PR were added (final OG concentration 0.8% (w/v)) to the destabilized liposomes. The suspension was extruded (Mini-Extruder, Avanti Polar Lipids) by 19 passages through a 200 nm pore size polycarbonate membrane. The suspension was dialyzed in 14 kDa molecular weight cut-off dialysis tubing (Membra-cel, Carl Roth) against 2 L of 20 mM Tris-HCl pH 7.5 with magnetic stirring overnight at RT.