Ultraviolet (UV) radiation, particularly UVB, poses severe threats to human health, contributing to photoaging, epidermal DNA damage, and an elevated risk of skin cancer. Consequently, daily sun protection has become a fundamental focus in cosmetic formulation development. Physical sunscreens like titanium dioxide (TiO₂) are highly favored UVB filters. However, traditional TiO₂ faces critical challenges: micronized or nano-sized particles aggregate and cause an unnatural white cast (whitening) on the skin, its high photocatalytic activity under solar exposure generates skin-damaging reactive oxygen species (ROS), and concerns remain regarding cytocompatibility and potential skin penetration or inhalation safety.
To resolve these limitations, researchers have developed a novel composite by loading titanium dioxide onto dendritic mesoporous silica (MSN). Mesoporous silica was selected because its center-radial pore structure, large pore volume, and massive internal surface area offer highly accessible, uniform anchoring sites that prevent TiO₂ agglomeration. Furthermore, silica’s low refractive index (~1.47), amorphous structure, and wide bandgap provide exceptional visible light transparency, while its favorable cytocompatibility profile offers a safe, non-toxic carrier matrix. By confining the active sunscreen agent inside the protective channels of MSN, researchers aimed to reduce overall photoactivity and skin irritation while maintaining high UV protection.
Methods
To synthesize the composite, dendritic mesoporous silica (MSN) pretreated with silane coupling agents was loaded with titanium dioxide via an in situ deposition method and calcined at 450 °C. Structural and morphological characteristics of MSN@TiO₂ were analyzed using SEM, TEM, XRD, and XPS, while hydrodynamic particle sizes were measured by dynamic light scattering. Sunscreen performance, including optical transparency and theoretical SPF, was evaluated through UV-Vis spectroscopy and PMMA plate-based transmittance measurements. Finally, photocatalytic activity was assessed via DPPH radical scavenging under simulated solar light, and in vitro cytocompatibility on L929 fibroblasts was verified using CCK-8 and Calcein-AM/PI live-dead staining assays.
Key Findings
- Significantly Improved Transparency and Low Whitening: Aqueous dispersions of MSN@TiO₂ showed a visible light transmittance of 32% to 91% (in the 400–700 nm region) compared to only 15% to 60% for commercial TiO₂. When applied to dark yellow and black artificial leather models, the composite yielded exceptionally low brightness change values (ΔL = 3.27 ± 1.15 and 3.89 ± 1.25, respectively), which are only about one-third of the whitening caused by commercial TiO₂ (ΔL = 11.60 ± 1.33 and 8.96 ± 1.13).
- Highly Suppressed Photocatalytic Activity: The net photocatalytic DPPH radical scavenging rate of MSN@TiO₂ was drastically reduced to just 7.13% ± 4.07%, which is less than one-seventh of the activity exhibited by commercial TiO₂ (55.66% ± 4.54%). A simple physical mixture of MSN and TiO₂ still retained a higher photocatalytic rate of 17.60% ± 2.83%, proving that the structural composite architecture is essential for safety.
- Underlying Electronic Band Structure Modulation: High-resolution XPS and diffuse reflectance UV-Vis spectra revealed that loading onto MSN shifts the valence band maximum upward (from 2.91 V to 2.84 V vs. NHE). This shift weakens the thermodynamic driving force of photogenerated holes to oxidize surface hydroxyl groups into harmful hydroxyl radicals (·OH), while the inert silica framework acts as a physical isolation barrier to prevent reactive carrier migration.
- Superior UV Protection Efficiency: Despite its enhanced transparency, MSN@TiO₂ maintained excellent UVB protection, demonstrating slightly higher theoretical SPF values than commercial TiO₂ across all tested concentrations (e.g., an SPF of 10.05 ± 0.91 for MSN@TiO₂ versus 8.91 ± 0.20 for commercial TiO₂ at 15% loading).
- Favorable Cytocompatibility: CCK-8 and Calcein-AM/PI assays on L929 fibroblasts showed that MSN@TiO₂ maintains cell viability above 80% across all tested concentrations up to 150 µg/mL with a high viable cell ratio of 96.59% ± 0.19% after 24 hours. In contrast, commercial TiO₂ cell viability dropped sharply at higher concentrations, maintaining 80% viability only at 30 µg/mL, with a live cell ratio of just 72.82% ± 6.48%.
The novelty of this research lies in its fundamental shift away from conventional physical core-shell coatings (such as SiO₂@TiO₂, where the active TiO₂ remains exposed on the outer surface and can exhibit elevated photoactivity). Instead, this study successfully encapsulates and chemically bonds the anatase TiO₂ nanoparticles inside the internal mesoporous channels of a dendritic silica carrier. This unique configuration isolates the active metal oxides from direct external contact, suppresses free-radical generation, and dramatically reduces skin whitening without compromising the material’s inherent UV-filtering performance.
The future implications of this work are highly promising for the cosmetic industry, offering a template for formulating “invisible” and highly biocompatible physical sunscreens. To transition this composite into commercial formulations, future research must address current evaluation gaps by conducting in vivo toxicity testing, evaluating ROS generation under active irradiation, performing phototoxicity assays under simulated sunlight, and confirming safety on human skin models (such as human keratinocytes or 3D reconstructed epidermis). Additionally, direct kinetic experiments (e.g., photoluminescence or electron spin resonance) are required to fully validate the underlying carrier recombination dynamics.
Link to the study: https://www.mdpi.com/2079-9284/13/5/226
In the figure: (a) SEM images of MSN, (b,c) TEM images of MSN, (d) SEM images of MSN@TiO2, (e,f)
TEM images of MSN@TiO2, and (g) HAADF/EDS mapping images of MSN@TiO2.