Milli q system

High performance liquid chromatography (HPLC)-grade methanol and acetonitrile were purchased from Friendemann Schmidt (Parkwood, Western Australia). Ethanol, isopropanol and 1-propanol were obtained from QReC™ (Auckland, New Zealand). Decanoic acid was purchased from Merck (Darmstadt, Germany), and MNPs (Iron (II, III) oxide nanoparticles, (Fe₃O₄)) were supplied by Bendosen Laboratory Chemicals (Bendosen, Norway). CAP standard was purchased from Dr Ehrenstorfer GmbH (Augsburg, Germany). Sodium chloride (NaCl) and sodium hydroxide (NaOH) were purchased from R & M Chemicals (Essex, UK). All chemicals were of at least analytical reagent grade and were used without further purification. Ultrapure deionized water (resistivity, 18.2 MΩ cm^–1) was generated by a model Sartorius Milli-Q system (Göttingen, Germany). Stock solution of CAP (1000 mg l^–1) was prepared by dissolving an appropriate amount of CAP in acetonitrile. Working standard solutions were freshly prepared by diluting the stock solution with water.

Standard samples of three paraben compounds, namely, propyl paraben (PP), butyl paraben (BP), and benzyl paraben (ArP), 1-butyl-3-methylimidazolium chloride (BMIM-Cl), toluene-2,4-diisocyanate (TDI), and anhydrous dimethyl sulfoxide-d₆ (DMSO) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Iron(ii) chloride tetrahydrate (FeCl₂·4H₂O) and iron(iii) chloride hexahydrate (FeCl₃·6H₂O) were purchased from R&M Chemicals (Essex, UK), whereas β-cyclodextrin (βCD, 99%) was commercially available and was purchased from Acros (Hungary). Moreover, acetonitrile (ACN), methanol (MeOH) (HPLC grade, 99.7%), acetone (technical grade), and ammonia solution (25%) were supplied by Friendemann Schmidt (Parkwood, Australia), whereas anhydrous N,N-dimethylformamide (DMF) was purchased from Merck (Kenilworth, NJ, USA). Analytical-grade absolute ethanol (denatured, 99.7%) was purchased from J. Kollin Chemicals (Midlothian, UK), whereas deionised (DI) water (18.2 MΩ cm) was provided by a Sartorius Milli-Q system (Aubagne, France).

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.