Fig. 4. PEG nanotubes only show toxicity when loaded with doxorubicin. C6 glioma cell viability analysis (PrestoBlue assay) after incubation with empty nanotubes or doxorubicin loaded nanotubes (drug to nanotube weight ratio of 0.08 : 1) for one day (a) or three days (b) showing that the PEG nanotubes only reduce C6 glioma viability when loaded with doxorubicin. MWCNTs show dose dependent toxicity which is augmented by doxorubicin loading (n = 4, error bars represent ± standard deviation, positive control = 4 μg mL⁻¹ doxorubicin, * represents statistical significant difference to empty PEG nanotubes (two way ANOVA with Tukey's multiple comparison test (P ≤ 0.05))). (c) Brightfield microscope images of C6 glioma cells after three days of culture showing impaired cell viability in the presence of MWCNTs (empty and loaded) and doxorubicin loaded PEG nanotubes (nanotube concentration = 15 μg mL⁻¹, scale bars represent 100 μm, images of the positive control are in Fig. S7†).