Fig. 2. In vitro biocompatibility assays: cytotoxicity by LDH released into the medium after 3 days of incubation with different concentrations of the (a) ZnO and (c) ZnO–NH₂ NCs. Proliferation results in terms of mitochondrial activity (MTT) after 3 days of incubation and with different concentrations of (b) ZnO and (d) ZnO–NH₂ NCs (p < 0.05, significant differences compared to the control are denoted by an asterisk (*)).