Implications of the stability behavior of zinc oxide nanoparticles for toxicological studies

Abstract

The increasing use of zinc oxide (ZnO) nanoparticles in sunscreens and other cosmetic products demands a risk assessment that has to be done in toxicological studies. Such investigations require profound knowledge of the behavior of ZnO in cell culture media. The current study was performed to get well-dispersed suspensions of a hydrophilic (ZnO-hydro) and a lipophilic coated (ZnO-lipo) ZnO nanomaterial for use in in vitro tests. Therefore, systematic tests were carried out with common dispersants (phosphate, lecithin, proteins) to elucidate chemical and physical changes of ZnO nanoparticles in water and physiological solutions (PBS, DMEM). Nonphysiological stock suspensions were prepared using ultrasonication. Time-dependent changes of pH, conductivity, zeta potential, particle size and dissolution were recorded. Secondly, the stock suspensions were added to physiological media with or without albumin (BSA) or serum (FBS), to examine characteristics such as agglomeration and dissolution. Stable stock suspensions were obtained using phosphate as natural and physiological electrostatic stabilizing agent. Lecithin proved to be an effective wetting agent for ZnO-lipo. Although the particle size remained constant, the suspension changed over time. The pH increased as a result of ZnO dissolution and formation of zinc phosphate complexes. The behavior of ZnO in physiological media was found to depend strongly on the additives used. Applying only phosphate as additive, ZnOhydro agglomerated within minutes. In the presence of lecithin or BSA/serum, agglomeration was inhibited. ZnO dissolution was higher under physiological conditions than in the stock suspension. Serum especially promoted this process. Using body-related dispersants (phosphate, lecithin) non-agglomerating stock suspensions of hydrophilic and lipophilic ZnO were prepared as a prerequisite to perform meaningful toxicological investigation. Both nanomaterials showed a non-negligible dissolution behavior that strongly depended on the surrounding conditions. Agglomeration of ZnO particles in physiological media is a complex function of particle coating, used dispersants and serum proteins if supplemented. The present study gives a clear guideline how to prepare and handle suspensions with ZnO for in vitro testing and allows the correlation between the chemical-physical particles behavior with findings from toxicological tests.

A combined proteomic and targeted analysis unravels new toxic mechanisms for zinc oxide nanoparticles in macrophages

“…The particles formed aggregates of 218 ± 90 nm upon dilution in the medium, and the size of these aggregates decreased to 140 ± 16 nm after 24 h at 37°C in the serum-containing culture medium. This decrease in size has been previously observed in the same complete culture medium [52] and is consistent with the well-known dissolution of zinc oxide nanoparticles in culture media [52].…”

ZnO nanocrystals derived from organometallic approach: Delineating the role of organic ligand shell on physicochemical properties and nano-specific toxicity

“…As listed in Table 1, the surface charges of ZnO-MAA, ZnO-MEAA and ZnO-MEEAA NCs dispersed in water were 36.2 ± 1.9 mV, 34 ± 1.1 mV and 30.8 ± 2.9 mV. The resulting ZnO-AAA NCs exhibit a mildly positive zeta potential greater than the critical ± 30 mV 30 (note that the magnitude of the zeta potential provides information about NCs stability and NCs with zeta potentials more positive than +30 mV or more negative than −30 mV are considered colloidally stable) and thus the NCs are fairly stable in aqueous environment likely due to both the mini-PEG-type coating and the presence of sufficient repulsive forces between individual NCs. Moreover, the observed relatively high surface positive charges could be also associated with low Zn 2+ ions dissolution from the inorganic core 17,58 .…”

“…Another important parameter that determines cellular contact and particle uptake, and has a significant effect on nanomaterials toxicity is the surface charge of NCs, typically expressed as the zeta potential. Some recent scientific reports indicate that positively charged NCs usually penetrate cells more readily 28–30 or induce higher endothelial cells leakiness 31,32 . Nevertheless, both systematic studies on toxicological profiles of surface-modified ZnO NCs and the nano-specific toxicity defined by multiparametric structure-property relationship remain highly challenging 33 .…”

Influence of surface chemical properties on the toxicity of engineered zinc oxide nanoparticles to embryonic zebrafish

“…Bare ZnO NPs (lacking surface ligands) are known to cause delayed embryo hatching, developmental abnormalities [12] through dissolution and release of ionic zinc [13–14] as well as induction of DNA damage through generation of reactive oxidative species (ROS) [12,15]. ZnO NPs are often coated with a variety of capping agents or surface ligands with differing chemical properties to functionalize the surface and improve stability against agglomeration and dispersibility in a given medium [16]. These surface alterations have the potential to alter their toxicity as a result of differences in the release of Zn 2+ ions and ROS production compared to bare ZnO NPs [17–18].…”