Figure 4: Representative images of randomly selected microscopic fields of A549 cells marked with caspase 3 antibody after 24 h incubation with: Cd-SiO2NPs ((a)–(d)), CdCl2 ((e)–(h)), SiO2NPs ((i)–(l)) at the doses indicated in the figure. Activation of caspase 3 is indicated by green spots already at the lowest tested dose of 1 μg/mL in both Cd-SiO2NP and CdCl2 treatment groups, while there was no positivity in SiO2NPs treated cells at all the tested doses similarly to the unexposed cells (control). (Scale bar: 100 μm).