Silk is a natural fiber produced by the silkworm Bombyx mori that is renowned for its luster and softness, making it a highly prized material for the production of luxury clothing and a wide variety of furnishings. Adding color to silk requires dyeing techniques which, on the industrial scale, require high consumption of water and energy and involve the addition of harsh chemicals that not only tarnish the quality of the silk but are also harmful to the environment.
Ming-Yong Han at the A*STAR Institute of Materials Research and Engineering and co-workers have now discovered a simpler, ‘greener’ way of introducing colors—and luminescence—into silk fibers. By adding fluorescent dyes to silkworm feed, they were able to prepare luminescent silk fibers in a palette of colors including pink, green and orange.
The color change in the silk fibers is induced by a family of compounds known as rhodamines. Traditionally rhodamine B and 6G have been used, for example, in applications including dye lasers and fluorometers. Han and his co-workers examined the effects of introducing rhodamine B, 110 and 101 into mulberry powder used for feeding silkworms. The amount of rhodamine they added was equivalent to 0.05% of the mixture by weight.
The researchers found that silk fibers produced by silkworms that have ingested rhodamine B, 110 and 101 appear dark pink, light yellow and light pink under sunlight and emit an orange, green and pink glow under ultraviolet radiation (see image). In comparison, silk fibers produced by normal silkworms appear white under sunlight and emit a blue glow under ultraviolet radiation.
The silkworms’ uptake of rhodamine appears to be extremely fast, as changes in body color were observed approximately one hour after ingesting the ‘rhodamine’ feed. Lowering the concentration of rhodamine B led to less intensely colored cocoons, while increasing the concentration did not result in a significant increase in color intensity.
Examination under scanning electron microscopy showed that the core filament of the silk fiber known as the fibroin absorbed most of the rhodamine dyes. The outer layer of the silk fiber known as sericin was left untamed. Aside from the color, the researchers found little differences between the colored silk and normal silk.
“We used advanced characterization techniques to show that it is possible to change the properties of silk fibroin by modifying the silkworm diet,” says Han. “The new approach will lead to the development of a new generation of ‘functional’ silk production which, for example, may have biomedical applications.”
The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering.