Abstract
Zinc oxide nanoparticle (ZnO NP)-based sunscreens are generally considered safe because the ZnO NPs do not penetrate through the outermost layer of the skin, the stratum corneum (SC). However, cytotoxicity of zinc ions in the viable epidermis (VE) after dissolution from ZnO NP and penetration into the VE is ill-defined. We therefore quantified the relative concentrations of endogenous and exogenous Zn using a rare stable zinc-67 isotope (67Zn) ZnO NP sunscreen applied to excised human skin and the cytotoxicity of human keratinocytes (HaCaT) using multiphoton microscopy, zinc-selective fluorescent sensing, and a laser-ablation inductively coupled plasma–mass spectrometry (LA-ICP-MS) methodology. Multiphoton microscopy with second harmonic generation imaging showed that 67ZnO NPs were retained on the surface or within the superficial layers of the SC. Zn fluorescence sensing revealed higher levels of labile and intracellular zinc in both the SC and VE relative to untreated skin, confirming that dissolved zinc species permeated across the SC into the VE as ionic Zn and significantly not as ZnO NPs. Importantly, the LA-ICP-MS estimated exogenous 67Zn concentrations in the VE of 1.0 ± 0.3 μg/mL are much lower than that estimated for endogenous VE zinc of 4.3 ± 0.7 μg/mL. Furthermore, their combined total zinc concentrations in the VE are much lower than the exogenous zinc concentration of 21 to 31 μg/mL causing VE cytotoxicity, as defined by the half-maximal inhibitory concentration of exogenous 67Zn found in human keratinocytes (HaCaT). This speaks strongly for the safety of ZnO NP sunscreens applied to intact human skin and the associated recent US FDA guidance.
Highlights
Advances in the rapidly growing field of nanotechnology have generated several new and effective products being used in cosmetic formulations [1–3], in dermatology [4,5], and in nanomedicine [6–8]
Whilst there has been concern about the general safety of zinc oxide nanoparticle sunscreens in topical products, work to date has focussed on whether nanoparticles can be found in the viable epidermis after permeation through the stratum corneum (SC) and via furrows and the follicles, including by lateral diffusion [75]
These zinc oxide nanoparticles, in turn, release zinc ions when the skin pH is either neutral or acidic and high zinc ion concentrations have been shown to be toxic in isolated keratinocytes [41]
Summary
Advances in the rapidly growing field of nanotechnology have generated several new and effective products being used in cosmetic formulations [1–3], in dermatology [4,5], and in nanomedicine [6–8]. Due to the widespread application of nanotechnology products in industry, medicine and daily life, concerns have been raised as to whether they lead to adverse effects to humans and environment [9–11], including previously unknown toxicities [12]. This aspect has led to the field of nanotoxicology, defined as a toxicological assessment of engineered nanomaterials [13]. ZnO NPs with a diameter between 20 and 100 nm are incorporated into sunscreen formulations [16], which is the size range selected for the stable isotope engineered ZnO NPs designed and used in this study
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