The season of sun, sand . . . and nanoparticles?

Lynn M. Savage, Features Editor, lynn.savage@photonics.com

There is no doubt that natural, unfiltered sunlight is a primal force. It charges ions in the atmosphere, providing iridescent fireworks known as the polar lights. It warms the seas and churns the air, providing weather. It feeds plants via photosynthesis, helps create vitamin D in your body, adds highlights to your hair, and can even improve one’s mood and increase one’s willingness to buy things. Of course, beach goers and other outdoor aficionados know that the sun bronzes the skin as well, the risk of melanoma be damned.

But sunlight also affects artificial materials. It strips the color from ink, leaving bluish tones where vibrant greens, yellows and reds once displayed on signs. It also is being found to have a direct effect on the nanoscopic chemicals that are used more and more in personal care products, such as shampoo, makeup and sunscreen lotion.

Because nanoparticles are being used increasingly in consumer products, there is a growing sense of urgency in discovering whether they will have a negative effect on the environment or on human health. After all, once flushed down the drain or tossed into the trash, soap, skin care products and the like enter the ecosystem. Once there, they commence to break down, releasing minute particles and leaving them likely to be taken up by plants, animals and, ultimately, people.

With this nanopollution in mind, researchers globally are examining the mechanisms of product breakdown, how nanoparticles are released and the role light plays in these processes.

One team, composed of scientists from CNRS/Aix-Marseille Université in Aix-en-Provence and from Université d’Aix Marseille, both in France, is looking at the behavior of nanoparticle-infused sunscreens in particular. In a recent paper published in Environmental Pollution, the investigators describe the roles played by sunlight, water and the passage of time in breaking down such lotions.

The group studied sunscreen infused with nanoscale composites with cores of TiO₂ coated with layers of Al(OH)₃ and poly-dimethylsiloxane (PDMS) – a common additive that reflects UV rays while remaining mostly clear, unlike the opaque white lotions of years past.

The researchers, led by Jean-Yves Bottero, found that sunlight breaks down the PDMS layer, leaving the TiO₂ core and Al(OH)₃ layer open to degradation from exposure to water, potentially freeing the TiO₂ particles into lakes, oceans and the broader world. They also noted that the stability of the PDMS-less composite was affected by organic matter naturally present in the water, leaving open the question of whether tests of nano-materials in the laboratory can take into account real-world reactions.

Ultimately, research such as that performed by Bottero and his colleagues should inspire others to carefully consider how something as simple yet powerful as sunlight might affect new technologies.