Systematic Investigation of the Physicochemical Factors That Contribute to the Toxicity of ZnO Nanoparticles

Abstract

ZnO nanoparticles (NPs) are prone to dissolution, and uncertainty remains whether biological/cellular responses to ZnO NPs are solely due to the release of Zn(2+) or whether the NPs themselves have additional toxic effects. We address this by establishing ZnO NP solubility in dispersion media (Dulbecco's modified Eagle's medium, DMEM) held under conditions identical to those employed for cell culture (37 °C, 5% CO2, and pH 7.68) and by systematic comparison of cell-NP interaction for three different ZnO NP preparations. For NPs at concentrations up to 5.5 μg ZnO/mL, dissolution is complete (with the majority of the soluble zinc complexed to dissolved ligands in the medium), taking ca. 1 h for uncoated and ca. 6 h for polymer coated ones. Above 5.5 μg/mL, the results are consistent with the formation of zinc carbonate, keeping the solubilized zinc fixed to 67 μM of which only 0.45 μM is as free Zn(2+), i.e., not complexed to dissolved ligands. At these relatively high concentrations, NPs with an aliphatic polyether-coating show slower dissolution (i.e., slower free Zn(2+) release) and reprecipitation kinetics compared to those of uncoated NPs, requiring more than 48 h to reach thermodynamic equilibrium. Cytotoxicity (MTT) and DNA damage (Comet) assay dose-response curves for three epithelial cell lines suggest that dissolution and reprecipitation dominate for uncoated ZnO NPs. Transmission electron microscopy combined with the monitoring of intracellular Zn(2+) concentrations and ZnO-NP interactions with model lipid membranes indicate that an aliphatic polyether coat on ZnO NPs increases cellular uptake, enhancing toxicity by enabling intracellular dissolution and release of Zn(2+). Similarly, we demonstrate that needle-like NP morphologies enhance toxicity by apparently frustrating cellular uptake. To limit toxicity, ZnO NPs with nonacicular morphologies and coatings that only weakly interact with cellular membranes are recommended.

ZnO nanoparticles (ZnO-NPs) and their antifungal activity against coffee fungus Erythricium salmonicolor

“…Taking account of conventional criteria used to classify antifungal compounds, an important aspect to consider is the ''action site'' of the nanoparticles (on the cell membrane, cell wall, DNA or RNA), as well as its surface characteristics. Therefore, in putting together a possible mechanism to account for the antifungal capacity of ZnO NPs, the wall of the fungus might be viewed as a target, also taking into account the surface physicochemical characteristics of ZnO (Altunbek et al 2014;Wöll 2007), as well as its other physicochemical characteristics-size and shape, for examplethat can also affect the toxicity of this oxide (Mu et al 2014). The fungal wall, in controlling cell permeability, is the part of the cell that interacts with the external environment, and thus with the ZnO NPs present in the fungal culture of interest in this work.…”

Safety Studies of Metal Oxide Nanoparticles Used in Food Industry

“…A recent study showed that primary particles of ZnO having sizes of 30 ± 20 nm, 40 ± 20 nm, and 1024 ± 496 nm may form aggregates/agglomerates of sizes which hydrodynamic diameters vary from 70 to 150 nm, 90-160 nm, and 250-540 nm when suspended in water forming suspensions having zeta potential values of − 14 ± 6 mV, 5 mV, and 19 ± 5 ± 5 mV, respectively (Mu et al 2014). …”

“…In conjunction with the instability of the ZnO NP, the degree of solubility allows the oxidation and release of Zn +2 into the medium. It has been observed that the NP showing high solubility in a cell culture, such as ZnO NP exhibited higher toxicity in mammalian cell lines than those having low solubility, such as TiO 2 NP (Mu et al 2014).…”

Nanomaterials

“…In parallel, Zhang et al have shown that the amorphous silica NM interaction with a lipid membrane is related to NM size where small NM (18 nm) cause a 'freezing effect' of otherwise fluid phospholipid bilayers that can result in membrane fracture while large particles (>78 nm) promote membrane wrapping and internalization. 73 The dispersion stability of ZnO NM in water and cell culture media has been shown to be dependent on particle shape and coating, 74 and on the additional presence of serum proteins. 70 As might be expected, specific polymer coatings designed to improve the dispersion of ZnO NM such as coating with an aliphatic polyether do produce a stable dispersion by DLS in both water and cell culture media, while uncoated ZnO NM are significantly more agglomerated and less stable regardless of size.…”

“…70 As might be expected, specific polymer coatings designed to improve the dispersion of ZnO NM such as coating with an aliphatic polyether do produce a stable dispersion by DLS in both water and cell culture media, while uncoated ZnO NM are significantly more agglomerated and less stable regardless of size. 74 A characterization protocol for ZnO NM has been described which identifies the presence of zinc carbonate and hydroxide impurities on the surface of uncoated ZnO. 75 These impurities probably impact on the dispersion stability and may form from atmospheric contamination 75 or because of the partial solubility and reprecipitation of ZnO in water 76 or certain cell culture media such as Dulbecco's Modified Eagle Medium (DMEM) ( Figure 6).…”