Viability and proliferation of endothelial cells after 48 h exposure to AuNPs. The endothelial cells hCMEC/D3 (A + C) and HDMEC (B + D) were exposed for 48 hours to different amounts of gold nanoparticles ranging from 50 μM up to 1000 μM. Cell viability was measured by the MTS assay (A + B) and compared to untreated cells (Ctrl), whose viability was set at 100%. The proliferation was detected by measuring the proliferation factor Ki-67 (C + D). Each result represents the mean ± standard deviation of four independent experiments, each of these was performed at least in triplicates (ONEway ANOVA with Dunnetts t-test: * P < 0,5; ** P < 0,05; *** P < 0.001).