Cell viability assay was performed to assess the cytotoxicity of SPIONs. Cell culture media and reagents were purchased from Thermo Fisher Scientific (USA) or Sigma-Aldrich (USA). Caco-2 cells (originally obtained from the American Type Culture Collection), passage 95-105, were maintained in Dulbecco's modified Eagle's medium, containing 10% (v/v) fetal bovine serum and 1% (v/v) nonessential amino acids. The cells were cultured in a humidified incubator (at 37 °C, 10% CO2) in 75 cm 2 tissue culture flasks. Stock suspensions of the SPIONs in Milli Q water were prepared at 10 mg mL -1 for γ-Fe2O3 and Mn0.5Fe2.5O4 and at 2 mg mL -1 for Zn0.5Fe2.5O4. The suspensions were prepared with a cup horn ultrasonicator (Sonics, USA) until no change in hydrodynamic diameter was observed. The hydrodynamic diameter was measured by dynamic light scattering (Litesizer 500, AntonPaar, Austria). The stock suspensions were diluted further with cell culture medium to achieve concentrations of SPIONs at 100, 150, and 200 μg mL -1 .