OBJECTIVETo investigate the cytotoxicity and its mechanism of ZnO nanoparticles on human leukemic monocyte lymphoma cell line U937.

METHODSFour different size ZnO (10, 30, 60, 500 nm) were carefully characterized. The survival rate and viability were measured by trypan blue assay and MTT assay for each size ZnO particles at different concentrations (12, 120, 240, 600, 1200 µmol/L). The zinc probe, Fluozin-3, was used to detect the intracellular free zinc. Transmission electron microscopy was adopted to observe the cellular ultrastructure and the uptake of ZnO.

RESULTSAll four kinds of ZnO were rod shape, with a purity of > 99.9 wt%, and they were classified as zincite phase crystal and their surface areas were in accordance with the sizes. The viability (ZnO-n10: (97 ± 19)%, (91 ± 4)%, (24 ± 4)%, (15 ± 2)%; ZnO-n30: (111 ± 4)%, (81 ± 3)%, (24 ± 2)%, (27 ± 8)%; ZnO-n60: (105 ± 11)%, (73 ± 20)%, (43 ± 11)%, (28 ± 14)%; ZnO-µm: (88 ± 16)%, (62 ± 7)%, (22 ± 4)%, (13 ± 5)%) of cells exposed to ZnO decreased with the increasing of the concentration of ZnO from 12 to 600 µmol/L (r values were 0.965, 0.979, 0.998, 0.992, and the t values were 19.8, 25.3, 76.3, 40.9, respectively, P < 0.05). The liability (ZnO-n10: (98 ± 1)%, (67 ± 2)%, (59 ± 7)%, (13 ± 13)%, (5 ± 4)%; ZnO-n30: (98 ± 1)%, (97 ± 2)%, (50 ± 3)%, (20 ± 14)%, (7 ± 2)%; ZnO-n60: (97 ± 2)%, (88 ± 5)%, (48 ± 10)%, (12 ± 5)%, (4 ± 1)%; ZnO-µm: (96 ± 1)%, (76 ± 3)%, (58 ± 3)%, (19 ± 5)%, (20 ± 10)%) of cells exposed to ZnO decreased with the increasing of the concentration of ZnO from 12 to 600 µmol/L (r valued at 0.982, 0.956, 0.972, 0.980, and the t valued at 19.3, 12.1, 15.6, 18.5, respectively, P < 0.05). The increase of the zinc concentration showed by the zinc fluorescence probe was 121 ± 11, which was similar to the fluorescence of cells treated with ZnAc(2) (132 ± 14, F = 0.6, P > 0.05) at the Zn-equivalent concentration. There was no statistic difference for the percents of high zinc content cells in total cells exposed to ZnO-n30 (87.6 ± 2.6)% and these exposed to ZnAc(2) (86.9 ± 3.2)% (F = 1.5, P > 0.05).

CONCLUSIONZnO nanoparticles are highly cytotoxic to U937 cells and the solubilization of ZnO is the main toxicological mechanism.