White iron rust material provides safe UV protection

   A research team consisting of NIMS, Hokkaido University and Hiroshima University has developed an iron oxide-based ultraviolet (UV)-absorbing powder material by stabilizing normally unstable, colorless, UV-absorbing di-nuclear iron species (iron atoms are illustrated as purple spheres in the figure below) in porous silica (silicon dioxide). If its performance can be further improved, this material may serve as a viable alternative to potentially carcinogenic titanium dioxide (TiO₂), which has been widely used in cosmetics and sunscreens.

   TiO₂ has been used in a wide variety of products in various applications (e.g., as a white pigment, in UV protection (mainly by scattering and neutralizing UV radiation) and as a photocatalyst), including cosmetics, everyday items, food products, medical products and building materials. However, the European Union classified this substance as a category 2 carcinogen in 2020, causing its use and production to decline and leading France to ban the use of food-grade TiO₂. Although Japan hasn’t restricted the use of TiO₂, developing alternatives to it is an important national issue considering the size of the Japanese TiO₂ market.

   A di-nuclear iron species is a type of iron oxide in which a pair of iron atoms are linked by ligands of water molecules or hydroxy groups. This species exhibits higher photocatalytic activity than TiO₂ when absorbing UV radiation. This ability separates it from other iron oxides that are used as a red food coloring. Although di-nuclear iron species are commonly found in enzymes and other proteins, they are unstable and difficult to synthesize. Their safe and stable use has long been a focus of research interest. This research team recently succeeded in stabilizing a di-nuclear iron species by embedding it in porous silica powder, thereby restricting it from transforming into a higher-order multinuclear iron species or crystallized iron oxide and reducing its harmful photocatalytic activities. The resulting product is a UV-absorbing white powder material. The team also prepared a sunscreen cream using this material as an active ingredient and found that its performance and stability were comparable to those of the TiO₂ materials currently used in sunscreens.

   This research team devised a procedure to synthesize the white powder material capable of absorbing and neutralizing UV radiation using ingredients safer than TiO₂. This material may potentially be used to develop new cosmetics and sunscreens. In addition, the microporous structure of silica, which is used to stabilize di-nuclear iron species, can be modified to maximize the iron species’ photocatalytic activities. This approach may be applicable to the development of photocatalysts that can be used in air purifiers and other technologies. The team will pursue this possibility.

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   This project was carried out by a research team led by Yusuke Ide (Principal Researcher, International Center for Materials Nanoarchitectonics, NIMS), Shinya Mine (Postdoctoral Researcher, Institute for Catalysis (ICAT), Hokkaido University (HU)), Takashi Toyao (Assistant Professor, ICAT, HU), Ken-ichi Shimizu (Professor, ICAT, HU) and Nao Tsunoji (Assistant Professor, Graduate School of Advanced Science and Engineering, Hiroshima University). This work was supported by the JSPS Grants-in-Aid for Scientific Research (grant number: 21H02034).