Sweat and food stains vanish under blue light

"Our method utilizes visible blue light in combination with ambient oxygen, which acts as the oxidizing agent to drive the photobleaching process," says Tomohiro Sugahara, the study's corresponding author. "This approach avoids the use of harsh chemical oxidants typically required in conventional bleaching methods, making it inherently more sustainable."

Yellow clothing stains are caused by squalene and oleic acid from skin oils and sweat, as well as natural pigments like beta carotene and lycopene, present in oranges, tomatoes and other foods. UV light is a potential stain-removing alternative to chemical oxidizers like bleach and hydrogen peroxide, but it can damage delicate fabrics. Sugahara and Hisanari Yoneda previously determined that a high-intensity blue LED light could remove yellow color from aged resin polymers, and they wanted to see whether blue light could also break down yellow stains on fabric without causing damage.

Initially, they exposed vials of beta-carotene, lycopene and squalene to high-intensity blue LED light for three hours. All the samples lost color, and spectroscopic analyses indicated that oxygen in the air helped the photobleaching process by breaking bonds to produce colorless compounds. Next, the team applied squalene onto cotton fabric swatches. After heating the swatches to simulate aging, they treated the samples for 10 minutes, by soaking them in a hydrogen peroxide solution or exposing them to the blue LED or UV light. The blue light reduced the yellow stain substantially more than the hydrogen peroxide or UV exposure. In fact, UV exposure generated some new yellow-colored compounds. Additional tests showed that the blue LED treatment lightened squalene stains on silk and polyester without damaging the fabrics. The method also reduced the color of other stain-causing substances, including aged oleic acid, orange juice and tomato juice, on cotton swatches.

High-intensity blue LED light is a promising way to remove clothing stains, but the researchers say they want to do additional colorfastness and safety testing before commercializing a light system for home and industrial use.

The authors do not have an external funding source for this work. They are employed by Asahi Kasei Corporation, a company that develops fiber products, chemicals and electronic materials.