Caco-2 cell viability tests. Caco-2 cells were grown in 96-well plates and differentiated for 21 days. Cells were then exposed for 24 h or 72 h to concentrations of CeO2 NPs ranging from 21.25 to 170 μg/mL. Left side) ATP tests: cell viability was determined by reading the level of bioluminescence (CellTiter-Glo luminescent cell viability assay, Promega). Right side) XTT tests: cell viability was determined by mitochondrial enzyme activity via XTT reagent (In Vitro toxicology assay kit XTT based, Sigma-Aldrich). An experimental positive control was obtained by exposing cells to H2O2 in both tests. Cell viability was not altered for concentrations up to 170 mg/L.