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Histograms showing cell viability measured by MTT assay in A549 cells after 24 h and 48 h exposure to pristine multiwalled CNTs: MWCNT; laboratory-made functionalized nanotubes: MW-COOH (carboxyl functionalized) and MW-NH2 (amino-functionalized); carbon black: CB; silica: SiO2. Data represent the mean ± SD over the mean experimental values of each of the three independent experiments.*P < 0.05 compared to control (100% cell viability).
