Lignin improves TiO2 sunscreens

LIGNIN, a waste byproduct from paper production, can be used as a sustainable alternative additive in sunscreen to stabilise titanium dioxide particles in suncream, according to new research.

TiO₂ is one of the most common sunscreen ingredients, as it reflects and scatters UV light, reducing damage to skin. However, it is also a highly reactive photocatalyst which can generate reactive oxygen species, or free radicals, and has been implicated in enzyme deactivation and potential health effects. As a result, suncream manufacturers coat TiO₂ nanoparticles using SiO₂ or Al₂O₃ to reduce the photocatalytic activity. A team of researchers led by the University of Ottawa’s Juan Scaiano, has instead proposed a different technique – coating the particles with a lignin shell, which rather than blocking TiO₂’s catalytic activity, absorbs the free radicals as soon as they are formed.

The researchers experimented with both water- and oil-soluble forms of lignin, which is the second most abundant biopolymer on earth. To coat the TiO₂ particles, the researchers dissolve the lignin in either water or an organic solvent, and mix in the particles. After being kept in the dark overnight, the mixture is subjected to UVA radiation for two hours while being stirred. After being separated and washed three times, the researchers dried the particles for at least an hour at 100–120?C.

In tests with 2-propanol, which photo-oxidises to acetone, Scaiano and the team found that after three hours of UVA radiation with the lignin-coated nanoparticles, the 2-propanol had not oxidised into acetone. In control tests with uncoated particles, the 2-propanol completely oxidised. The researchers also tested the lignin-coated nanoparticles for their enzyme inhibition, using alkaline phosphatase as the model enzyme, subjecting a mixture of the enzyme and a suspension of TiO₂ nanoparticles to 30 minutes of UV irradiation. In control experiments with uncoated TiO₂, the enzyme was inactivated, while the enzyme remained active with the coated nanoparticles. In all experiments, the particles coated with oil-soluble lignin were found to be slightly more resilient than those made with water-soluble lignin.

In an additional benefit, the researchers found that the lignin-coated nanoparticles act as a photoprotector to avobenzone, another suncream ingredient used to absorb UVA rays, but which can be prone to photodegradation

“We believe that the particles described here, showing a nanometric size and a very light colour, are promising candidates as ingredients in skincare formulations, especially for sunscreens, given that they are nontoxic and waterproof. Additionally, our approach regarding the use of a safe and extremely versatile material, mainly a byproduct of the paper industry, also contributes to the development of eco-friendly processes for the cosmetic industry,” the researchers conclude.

ACS Omega DOI: 10.1021/acsomega.6b00177