, 2012 and Kumarathasan et al., 2014). Single-wall CNTs see more and multi-wall CNTs were surface-modified by an oxidation process following our previously reported procedure (Kumarathasan et al., 2012). In brief, the oxidized materials had 30–80% lower content of metal species (e.g. Ni, Fe, Co, Mo), contained polar surface –COOH groups, had shortened length and a decreased specific surface area (Kumarathasan et al., 2012), as well as showed lower tendency to flocculate and had smaller hydrodynamic diameter, than their pristine CNT counterparts (Kumarathasan et al., 2014).

From here on, pristine single-wall CNTs, oxidized single-wall CNTs, pristine multi-wall CNTs and oxidized multi-wall CNTs will be referred to as CNT-1, CNT-2, CNT-3 and CNT-4, respectively. Amorphous

silica nanoparticles; SiNP-1 (10–20 nm, cat # 637238) and SiNP-2 (12 nm, cat # 718483) were obtained from Sigma–Aldrich Canada Co. (Oakville, ON, Canada). Briefly, from the Sigma–Aldrich product specification sheets and certificates of analysis, SiNP-1 was determined to be an amorphous nanopowder, with 20 nm average size particles (SAXS) and 30 ppm trace metals content (ICP), while SiNP-2 was determined to be an amorphous nanopowder, with 12 nm primary particle size (TEM), 210 m2/g surface area (SBET) and 30.7 ppm trace metals content (ICP). Standard Reference Materials (SRMs); SiO2 (respirable cristobalite, SRM-1879a), TiO2 (titanium dioxide, SRM-154b) were from the National Institute of Standards and Technology (Gaithersburg, MD, USA). CTB (resazurin) reagent was purchased from Promega (Fitchburg, WI, USA). Cell culture media and supplements ZVADFMK were obtained from

Hyclone (Logan, UT, USA). All other reagents were purchased from Thermo-Fisher (Nepean, ON, Canada). To prepare stocks, CNTs and all additional particles were weighed and re-suspended in sterile particle preparation buffer (Tween-80, 25 μg/mL; NaCl, 0.19% w/v) to final concentration of 3 mg/mL and 10 mg/mL, as required, using a Dounce glass homogenizer Sucrase (Nadeau et al., 1996). The particle suspensions were sonicated in ice-cold water for 20 min using a Branson 1510 water bath sonicator (Branson, Shelton, CT, USA) and homogenized with 25 strokes of the homogenizer piston. The particle suspensions were aliquoted into sterile centrifuge tubes with O-ring seals and sterilized in an Isotemp water bath (Thermo-Fisher) at 56 °C for 30 min (Vincent et al., 1997). For experiments, particle suspensions were diluted with the appropriate serum-free culture medium and particle preparation buffer to make final particle concentrations to be applied to the cell cultures, which were sonicated for additional 10 min prior to dosing. Note, that TiO2 particles were washed three times with methanol and three times with phosphate buffered saline prior to the preparation of the stocks (Vincent et al., 1997). A549, human alveolar type II epithelial cells and J774A.