Cobalt Oxide Nanoparticles: Behavior towards Intact and Impaired Human Skin and Keratinocytes Toxicity

Skin absorption and toxicity on keratinocytes of cobalt oxide nanoparticles (Co 3 O 4 NPs) have been investigated. Co 3 O 4 NPs are commonly used in industrial products and biomedicine. There is evidence that these nanoparticles can cause membrane damage and genotoxicity in vitro , but no data are available on their skin absorption and cytotoxicity on keratinocytes. Two independent 24 h in vitro experiments were performed using Franz diffusion cells, using intact (experiment 1) and needle-abraded human skin (experiment 2). Co 3 O 4 NPs at a concentration of 1000 mg/L in physiological solution were used as donor phase. Cobalt content was evaluated by Inductively Coupled–Mass Spectroscopy. Co permeation through the skin was demonstrated after 24 h only when damaged skin protocol was used (57 ± 38 ng·cm −2 ), while no significant differences were shown between blank cells (0.92 ± 0.03 ng cm −2 ) and those with intact skin (1.08 ± 0.20 ng·cm −2 ). To further investigate Co 3 O 4 NPs toxicity, human-derived HaCaT keratinocytes were exposed to Co 3 O 4 NPs and cytotoxicity evaluated by MTT, Alamarblue ® and propidium iodide (PI) uptake assays. The results indicate that a long exposure time ( i.e. , seven days) was necessary to induce a concentration-dependent cell viability reduction (EC 50 values: 1.3 × 10 −4 M, 95% CL = 0.8–1.9 × 10 −4 M, MTT essay; 3.7 × 10 −5 M, 95% CI = 2.2–6.1 × 10 −5 M, AlamarBlue ® assay) that seems to be associated to necrotic events (EC 50 value: 1.3 × 10 −4 M, 95% CL = 0.9–1.9 × 10 −4 M, PI assay). This study demonstrated that Co 3 O 4 NPs can penetrate only damaged skin and is cytotoxic for HaCat cells after long term exposure.