pH-Dependent Toxicity of High Aspect Ratio ZnO Nanowires in Macrophages Due to Intracellular Dissolution

Abstract

High-aspect ratio ZnO nanowires have become one of the most promising products in the nanosciences within the past few years with a multitude of applications at the interface of optics and electronics. The interaction of zinc with cells and organisms is complex, with both deficiency and excess causing severe effects. The emerging significance of zinc for many cellular processes makes it imperative to investigate the biological safety of ZnO nanowires in order to guarantee their safe economic exploitation. In this study, ZnO nanowires were found to be toxic to human monocyte macrophages (HMMs) at similar concentrations as ZnCl(2). Confocal microscopy on live cells confirmed a rise in intracellular Zn(2+) concentrations prior to cell death. In vitro, ZnO nanowires dissolved very rapidly in a simulated body fluid of lysosomal pH, whereas they were comparatively stable at extracellular pH. Bright-field transmission electron microscopy (TEM) showed a rapid macrophage uptake of ZnO nanowire aggregates by phagocytosis. Nanowire dissolution occurred within membrane-bound compartments, triggered by the acidic pH of the lysosomes. ZnO nanowire dissolution was confirmed by scanning electron microscopy/energy-dispersive X-ray spectrometry. Deposition of electron-dense material throughout the ZnO nanowire structures observed by TEM could indicate adsorption of cellular components onto the wires or localized zinc-induced protein precipitation. Our study demonstrates that ZnO nanowire toxicity in HMMs is due to pH-triggered, intracellular release of ionic Zn(2+) rather than the high-aspect nature of the wires. Cell death had features of necrosis as well as apoptosis, with mitochondria displaying severe structural changes. The implications of these findings for the application of ZnO nanowires are discussed.

Zinc oxide nanoparticles induce toxic responses in human neuroblastoma SHSY5Y cells in a size-dependent manner

“…[28][29][30] However, in this study, we believe that shape was not the main factor in toxicity discrepancy when comparing these two nanomaterials with low-aspect-ratio particles. Müller et al 33 attributed the toxic effects of ZnO nanowires to released zinc ions, rather than to the high aspect ratio, implying that the rapid dissolution of ZnO may have reduced the effect of the NP shape. The average size of the ZnO a NPs was 47.1±5.1 nm, with width 27.9±1.8 nm, and the average size of the ZnO b NPs was 18.5±1.2 nm, with width 6.8±0.5 nm, which was smaller than that of the ZnO a NPs.…”

A review of mammalian toxicity of ZnO nanoparticles

“…26 Furthermore, Müller et al showed that pH-triggered intracellular release of ionic Zn 2+ is responsible for the toxicity of ZnO nanowires. 27 The toxic effects of ZnO NPs are due to their solubility, resulting in increased intracellular [Zn 2+ ]. This results in cytotoxicity, oxidative stress, and mitochondrial dysfunction.…”

“…ZnO nanowires were taken up by macrophages through phagocytosis and dissolved in lysosomes. 27 Again, the effects of ZnO NPs are likely caused by Zn 2+ .…”

Zinc Oxide Nanoparticles: Opportunities and Challenges in Veterinary Sciences

“…ZnO NPs exhibit protein adsorption property which might be responsible for their cytotoxicity [142]. The pH-triggered (acidic) intracellular release of Zn 2+ is mainly responsible for the toxicity of ZnO nanowires [143].…”