Sun protection and nanoparticles: the hidden side of TiO₂

Titanium dioxide (TiO₂) is among the most widely used materials in cosmetics and numerous industrial products. Why? Because of its high refractive index and its ability to absorb UV radiation, features that make it ideal as a white pigment and as a UV filter in sunscreen creams.

TiO₂ is often subjected to surface silanization, a treatment that improves nanoparticle dispersion, enhances compatibility with matrices, and, above all, is expected to reduce the formation of reactive oxygen species (ROS) under light exposure. ROS are free radicals—highly aggressive and oxidizing molecules that can be harmful to the skin.
But the question remains: is this treatment enough to neutralize the problem?

A study that changes expectations

The Materials Chemistry group of the Department of Industrial Engineering (DII – UniTrento), in collaboration with Prof. Massimiliano D’Arienzo’s team at the University of Milano-Bicocca, published in the prestigious journal Angewandte Chemie a study that sheds new light on the issue: “Singlet Oxygen Photocatalytic Generation by Silanized TiO₂ Nanoparticles” (link to the article).

The key result is surprising: silanization does not eliminate the formation of reactive species; instead, it promotes the generation of singlet oxygen (¹O₂), a highly reactive and oxidizing form of molecular oxygen.

How does it work?

Silanization modifies the coordination of the titanium surface centers. This process stabilizes Ti³⁺ electronic defects in the subsurface regions of the material which, under UV and near-infrared irradiation, transfer energy to the adsorbed molecular oxygen, selectively promoting it to the excited singlet oxygen state.

Thus, it is not an electron transfer (typical in photocatalysis), but an energy transfer of the Förster type (FRET), a rare and particularly interesting phenomenon in this context.

From Cosmetics to Green Chemistry

To concretely test the material’s behavior, silanized TiO₂ was used as a photocatalyst for the synthesis of limonene epoxide from limonene. The result? A selective and efficient reaction, demonstrating how this material can also find applications in sustainable organic synthesis (Green Chemistry).

This dual nature of silanized TiO₂ is crucial:

• On one hand, it raises questions about its safety in cosmetics, where the formation of singlet oxygen could have undesirable effects on the skin;
  
• On the other, it offers new opportunities to steer the material’s reactivity toward high value-added industrial products. Limonene epoxide, for example, is a precursor to poly(limonene carbonate), a bio-based polymer with interesting physico-chemical properties that make it a green alternative to petroleum-derived polycarbonates.
  

This research therefore represents an important step forward in the field of safety, but above all in the understanding and control of the fundamental reaction mechanisms of photoactive materials, enabling their use in selective oxidation processes. This is the key to effectively replacing petroleum refineries with more sustainable and environmentally friendly solar refineries.