Zinc-metal-organic frameworks with tunable UV diffuse-reflectance as sunscreens

Abstract

Background: UV exposure continues to induce many health issues, though commercial sunscreens are available. Novel UV filters with high safety and efficacy are urgently needed. Metal-organic frameworks (MOFs) could be a suitable platform for UV filter development, due to their tunable optical, electrical, and photoelectric properties by precise controlled synthesis.

Results: Herein, four zinc-based MOFs with various bandgap energies were chose to investigate their optical behaviors and evaluate their possibility as sunscreens. Zeolitic imidazolate framework-8 (ZIF-8) was found to possess the highest and widest UV reflectance, thereby protecting against sunburn and DNA damage on mouse skin and even achieving a comparable or higher anti-UV efficacy relative to the commercially available UV filters, TiO₂ or ZnO, on pig skin, a model that correlates well with human skin. Also, ZIF-8 exerted appealing characteristics for topical skin use with low radical production, low skin penetration, low toxicity, high transparency, and high stability.

Conclusion: These results confirmed ZIF-8 could potentially be a safe and effective sunscreen surrogate for human, and MOFs could be a novel source to develop more effective and safe UV filters.

Keywords: Diffuse-reflectance; Metal–organic frameworks; Sunscreens; UV; ZIF-8.

Fig. 1

Physical and chemical characterizations of zinc-based MOFs. A TEM images of TiO₂, ZnO and ZIF-8. B Digital photographs of TiO₂, ZnO and ZIF-8. ZIF-8 suspension was more transparent relative to TiO₂ and ZnO. C Diffuse reflection spectra and D calculated UV–Vis absorbance spectra for TiO₂, ZnO and zinc-based MOFs. ZIF-8 showed the widest and highest UVA and UVB scattering with lowest λ_max. E ZIF-8 degradation in artificial sweat (pH 6.5, 32 °C). F EPR spectra of POBN-OH spin abduct signal produced by TiO₂, ZnO and ZIF-8 suspensions (800 µg mL⁻¹ in ethanol). ZIF-8 induced less EPR signal compared to TiO₂ after UV exposure. G Fluorescence spectra of TiO₂, ZnO and ZIF-8 after excitation with UV light (308 nm). ZIF-8 showed an emission at 621 nm. H Thermographs and I quantitative analysis of glycerol, TiO₂, ZnO and ZIF-8 after UV exposure for 2 h. (RT: room temperature) 2–3 °C temperature increases were observed for all four groups compared to room temperature

Fig. 2

Toxicity on HaCaTs and HEKas. A Cytotoxicity of TiO₂, ZnO and ZIF-8 at various concentrations toward HaCaTs and HEKas using MTT assay. B Digital images of HaCaTs and HEKas after treatment with TiO₂, ZnO or ZIF-8. C Nucleus images with DAPI staining after cells were treated with TiO₂, ZnO or ZIF-8. Obvious chromatin condensation was observed for ZnO-treated cells. D Flow cytometry analysis of cell apoptosis induced by TiO₂, ZnO and ZIF-8

Fig. 3

Photoprotective effects on HaCaTs and HEKas. A Representative fluorescence images of cells with DNA tail after pre-protection and subsequent UV exposure. B, C Quantitative analysis of (B) HaCaTs and (C) HEKas with DNA tail. D, E) Cell viability of (D) HaCaTs and E HEKas with pre-protection and subsequent UV exposure

Fig. 4

Photoprotective effects on mouse skin. A Representative digital graphs of mouse dorsal skin three days after pre-protection and subsequent UV exposure. ZIF-8 group showed less erythema compared to no protection or glycerol groups. B Microscope photographs of skin with H&E staining. C Microscope photographs of skin with CPD immunohistochemistry. D, E Quantitative analysis of (D) epidermal thickness and E CPD positive cells in the skin after pre-protection and subsequent UV exposure. ZIF-8 inhibited epidermal hyperplasia and CPD formation

Fig. 5

Photoprotective effects on pig skin. A Representative digital images of pig skin after pre-protection and subsequent UV exposure. Fewer erythema was observed for ZIF-8 group, compared to no protection group. B Photographs of skin with H&E staining after pre-protection and subsequent UV irradiation. (Red arrows point to parakeratosis.) C γ-H₂AX immunofluorescence images of skin sections after pre-protection and subsequent UV exposure. (Nucleus: blue; γ-H₂AX: red; White arrows point to γ-H₂AX⁺ cells.) D, E Quantitative analysis of (D) epidermal thickness and (E) γ-H₂AX positive cells on pig skin with pre-protection and subsequent UV exposure. ZIF-8 inhibited epidermal hyperplasia and γ-H₂AX formation

Fig. 6

Long term toxicity and penetration into mouse or pig skins. After repeated dosing for total six times in half a month, the skin was collected and subjected for H&E staining. A Representative H&E staining images of mouse skin. B Quantitative analysis of epidermal thickness. No obvious epidermal thickness was increased after long-term application of ZIF-8. C–F Penetrations of (C, E) ZIF-8, ZnO, and (D, F) TiO₂ into (C, D) mouse and E, F pig skin. Skin was treated with glycerol, ZIF-8, ZnO, and TiO₂ for 6 h, then the Zn²⁺ and Ti⁴⁺ in skin were detected by ICP-MS. ZnO easily penetrated mouse and pig skin, but not for ZIF-8 and TiO₂