Two-Dimensional TiO2 Ultraviolet Filters for Sunscreens

Abstract

Titanium dioxide (TiO₂) has been an important protective ingredient in mineral-based sunscreens since the 1990s. However, traditional TiO₂ nanoparticle formulations have seen little improvement over the past decades and continue to face persistent challenges related to light transmission, biosafety, and visual appearance. Here, we report the discovery of two-dimensional (2D) TiO₂, characterized by a micro-sized lateral dimension (~1.6 μm) and atomic-scale thickness, which fundamentally resolves these long-standing issues. The 2D structure enables exceptional light management, achieving 80% visible light transparency-rendering it nearly invisible on the skin-while maintaining UV-blocking performance comparable to unmodified rutile TiO₂ nanoparticles. Its larger lateral size results in a two-orders-of-magnitude reduction in skin penetration (0.96 w/w%), significantly enhancing biosafety. Moreover, the unique layered architecture inherently suppresses the generation of reactive oxygen species (ROS) under sunlight exposure, reducing the ROS generation rate by 50-fold compared to traditional TiO₂ nanoparticles. Through precise metal element modulation, we further developed the first customizable sunscreen material capable of tuning UV protection ranges and automatically matching diverse skin tones. The 2D TiO₂ offers a potentially transformative approach to modern sunscreen formulation, combining superior UV protection, enhanced safety and a natural appearance.

Keywords: Biosafety; Sunscreen; Titanium dioxide; Two-dimensional.

Fig. 1

Scheme of impact of 2D TiO₂ (left side) accompanied with natural skin color and no security risk, and 0D TiO₂ (right side) accompanied with whitening phenomenon, free radical production and penetration on the appearance of skin surface

Fig. 2

Optical characterization of 2D TiO₂. A UV–visible transmittance spectra of a 0.8 g L⁻¹ aqueous 2D TiO₂ solution and 0D TiO₂ solution. B NAF for 0.2, 0.4, and 0.8 g L⁻¹ aqueous 2D TiO₂ solutions and 0D TiO₂ solutions. C Changing in saturation (∆C) and lightness (∆L) relative to the original leather color when coating with emulsions with a 4 w/w% solid content on synthetic leather, where the spherical shape represents the application on black leather, while the square shape represents the application on khaki leather. Smaller absolute values of ∆C and ∆L indicate less color alteration after emulsion coating, consistent with the requirement of a natural appearance. D Photographs of top views of emulsions with 4 w/w% solid content coated on synthetic leather, corresponding to C

Fig. 3

Mechanisms of the high NAF of 2D TiO₂. A AFM image and B thickness distribution of 2D TiO₂ under different preparation conditions, where ①, ②, and ③ are samples with different thicknesses. C UV–visible spectra of 2D TiO₂ with different thicknesses and of 0D TiO₂. D Simulation of the optical transmittance of 2D TiO₂ with thicknesses of 1 to 10 nm

Fig. 4

Phototoxicity of 2D TiO₂. A Radical scavenging test of TiO₂ for DPPH, showing the DPPH absorption peaks before and after UV irradiation. With equal initial concentrations of DPPH, differences in the absorption peaks under dark conditions possibly arise from the adsorption of DPPH by TiO₂. B EPR analysis of reactive oxygen species generated by TiO₂ under UV irradiation. C Lifetime of transient photo-generated excitons in 2D TiO₂ and 0D TiO₂ from photoluminescence spectra. The insets show the lifetime of excitons as a function of intensity. D Fluorescence images of HSF cells before and after UV irradiation. Scale bar 200 μm. E Effect of different types of TiO₂ on the ROS growth rate of HSF under UV irradiation

Fig. 5

Skin penetration of 2D TiO₂. A Curves depicting the mass percent of 2D and 0D TiO₂ penetrating into a receiving pool over time in the Franz diffusion cell test, with testing time ranging from 2 to 24 h. B Environmental SEM image of the cross section of pig skin coated by the emulsion containing 2D TiO₂. C Gray value as a function of distance, representing the fluorescence intensity and distance mapping of the dashed region corresponding to the fluorescence microscopy image in D. D Fluorescence images of labeled 2D TiO₂ (upper ones) and 0D TiO₂ (lower ones) on pig skin after 4 h of application

Fig. 6

A conceptual prototype of sunscreens with 2D TiO₂ and doped 2D TiO₂. A and B Statistics on epidermal thickening in mouse skin and histological sections stained with H&E of sunscreens with 2D TiO₂ and 0D TiO₂, pure emulsion, and unprotected controls. C and D Statistics of the keratin increase in mouse skin and sections with Masson’s trichrome staining of the aforementioned sunscreens. All scale bars are 100 μm. E Transmittance spectra of 2D TiO₂ doped with iron. F Radar graph of integrated performance of three types of TiO₂