By Confining Light, Silk May Create Biosensors

AUTUM C. PYLANT, NEWS EDITOR, autum.pylant@photonics.com

Nobel laureate Philip Anderson predicted that electrons can be brought to a complete halt in materials because of their scattering and defects. Named after the prize winner, the Anderson localization of light proposes that light waves can be confined in the same manner. Because the phenomenon requires extremely condensed and densely packed disordered nanostructures, it has been elusive and difficult to capture.

By taking advantage of the quasi-2D nanoarchitecture of a silkworm’s fibers, a team of researchers from Purdue University has shown light can be localized in natural silk. Their findings relate to light transport.

Vibrant, sparkling and lustrous reflection of light from natural silk cocoons. Courtesy of Young Kim/Purdue University.

“Objects found in nature often have silvery or lustrous colors, including fish, insects and even human tissue. This type of optical appearance drives our research to figure out why such incredible reflections exist in nature,” said Young Kim, an associate professor in Purdue University’s Weldon School of Biomedical Engineering. “This is in contrast to the perception that light in natural and biological media is simply diffusing.”

For the Anderson localization to occur, there must be both scattering and interference between scattered light waves. Densely packed irregular nanostructures cause light waves to interfere with each other, both constructively and destructively. If constructively, the light is intensified. Scattering power is maximized when there are many scattering centers and when their sizes are comparable to the wavelength of the light. Both scattering and size criteria are met in the nanoarchitecture of a silkworm’s fibers.

Researchers demonstrated that the 10- to 20-μm-diameter silk fibers are capable of light confinement, as they have numerous scattering centers inside of them.

Silkworm rearing.Courtesy of Young Kim/Purdue University.

Kim told Photonics Media that the light confinement effect in biological and natural tissue was completely unexpected and could provide a range of direct applications.

“From a biosensing standpoint, based on strong light-matter interactions in silk, we can develop highly sensitive biosensors,” Kim said. “From a manufacturing standpoint, we can use the bioreactor or the insect factory of silkworms to mass-produce nanomaterials and nanostructures in a scalable, sustainable and eco-friendly manner.”

Kim and fellow researchers found most of the light transmission disappeared into the silk surface. However, there was a small area where the energy was confined and transmitted through biogenic light localization.

Although biological structures such as silk diffuse light, other natural materials with similar microstructures do not possess the localized modes that make Anderson localization of light possible.

“Such a difference makes silk particularly interesting for radiative heat transfer.” Kim said. The silk has a high emissivity for infrared light, meaning it readily radiates heat (or infrared radiation), while at the same time being a good reflector of solar light. Because the strong reflectivity from Anderson localization is combined with the high emissivity of the biomolecules in infrared radiation, silk radiates more heat than it absorbs, making it ideal for passive cooling or “self cooling.”

The work is led by researchers at Purdue’s Weldon School of Biomedical Engineering, the Department of Agricultural Biology of the National Institute of Agricultural Sciences in South Korea, and the Materials and Manufacturing Directorate of the U.S. Air Force Research Laboratory.