Lignin coating to quench photocatalytic activity of titanium dioxide nanoparticles for potential skin care applications

Ultraviolet light can cause photodamage to the skin, such as sunburns and melanomas. TiO2 is introduced in sunscreen formulations to reflect and scatter UV radiation. However, it can also photocatalyze the production of reactive species like O2−˙ and OH˙. Here, we aimed to remove the photocatalytic activity of TiO2 (anatase and rutile), while preserving the UV filter property. Anatase and rutile were modified through two preparative protocols. The first used HCl lignin precipitation of ethylene glycol lignin solution in the presence of the cross-linker glutaraldehyde and anatase or rutile nanoparticles. The second protocol used HNO3 lignin precipitation of lignin aqueous solution in the presence of anatase or rutile nanoparticles. Both methodologies were performed at room temperature and ambient pressure in green media, with vigorous mixing followed by 20 kHz sonication. The composite materials obtained were fully characterized by SEM, XRD analysis and FT-IR spectroscopy, and their photostability, and photo and shielding activities were evaluated through reference reactions: oxidation of 2-propanol, an ene-reaction conducted on an α,β-unsaturated carboxylic derivative and photochemical transformation of o-nitrobenzaldehyde to o-nitrosobenzoic acid. Therefore, in the near future, industrial use of these new clusters can help to minimize TiO2 phototoxicity in sunscreen formulations, while preserving the sunscreen photoprotection activity.