Tikalon Blog by Dev Gualtieri

Photonic Pigments

April 11, 2014

Colors make foods more appealing, as demonstrated by such comestibles as red velvet cake and jelly beans. The association is so strong that children will conflate flavors and their colors. As a child, I personally enjoyed red.

Jelly bean candy is associated with Easter, but a "jelly bean" component in the electronics industry is one that's generic, inexpensive, and available from many sources.

(Photo by Brandi Sims, via Wikimedia Commons.)

Although color additives were often used in foods, the US Food and Drug Administration (FDA) wasn't given jurisdiction over color additives until 1960, and that was only in response to reported toxic effects caused by some manufacturers using too much orange dye. In the 1970s, high doses of the popular food dye, FD&C Red No. 2, a.k.a., Red Dye No. 2, (amaranth, C₂₀H₁₁N₂Na₃O₁₀S₃) were shown to produce cancer in female rats, and the dye was withdrawn from use.

At the time FD&C Red No. 2 was discontinued, about a million pounds of the dye was used annually. Seeing Red Dye No. 2 on an ingredient label is one thing, but a consumer confronted with its chemical name, trisodium (4E)-3-oxo-4-[(4-sulfonato-1-naphthyl)hydrazono]naphthalene-2,7-disulfonate, might think twice about eating it. The dye was replaced by FD&C Red 40, C₁₈H₁₄N₂Na₂O₈S₂, lesser known as disodium 6-hydroxy-5-((2-methoxy-5-methyl-4-sulfophenyl)azo)-2-naphthalenesulfonate. Somehow, that doesn't sound that much better.

You don't always need a dye to produce a color. The wave nature of light allows creation of color by diffraction and interference effects. A film of oil on the surface of water functions as a Fabry–Pérot etalon to produce color. I noticed as a child, as have many of my readers, the colorful sheen on the surface of some cuts of luncheon meats. This arises from the periodic reflection, and subsequent interference, of light from striated rows of muscle cells.

This same process of color generation, called iridescence, occurs also in the nano-textured surface of butterfly wings, as shown in the photograph. The spiral groves on CDs and DVDs are spaced so closely that they will spread white light into its spectrum of colors like a diffraction grating. There are instructions on YouTube for creating a spectrograph using a DVD and a webcam.[1]

Dorsal view of a male Morpho didius butterfly.

The wing color is caused by nanoscale texture.

(Photo by Didier Descouens, via Wikimedia Commons.)

The problem with iridescent color is that it changes with viewing angle. This allows for some nice artistic effects, but iridescent materials cannot be used as a replacement for a single color dye. Now, an international team of scientists from Harvard University's School of Engineering and Applied Sciences, the Korea Advanced Institute of Science and Technology (KAIST, Daejeon, Korea), and the Korea Electronics Technology Institute (Gyeonggi-do, Korea) has reported on a way to produce a palette of individual colors using "photonic pigments."[2-3] The research team, led by Harvard's Vinothan N. Manoharan, reported on its experiments in a recent issue of Angewandte Chemie International Edition.[2]

The photonic pigments are simply microcapsules containing a dense amorphous packing of core–shell colloidal particles.[2] These microcapsules are assembled using microfluidics, and they have colors that span the visible spectrum (see figure).[2]

Examples of blue, green, and red photonic pigments. These images are by bright-field optical microscopy (top) and dark-field optical microscopy (bottom). (Harvard University image by Jin-Gyu Park.)[3]

This approach to photonic pigmentation began when one of the Harvard coauthors, Jin-Gyu Park, was at Yale University. In his research there, he found that he could create a blue color from aggregates of solid particles.[3] In the Harvard process, microcapsules are filled with a disordered solution of even smaller particles suspended in water. As the microcapsules dry out, the microcapsules shrink, bringing the particles closer together. The interior particles aren't ordered, but there's still an average separation between the particles sets the final color, as shown in the figure.[3]

Photonic pigment color shift with shrinkage

The color of a photonic pigment microcapsule shifts to shorter wavelengths as it shrinks to its final size. (Harvard University image by Jin-Gyu Park.)[3]

Aside from the advantage of being able to produce color from non-toxic materials, the colors produced have the advantage of permanence. Dyes will eventually fade through exposure to light, but the photonic pigments achieve their colors by structure, so their colors are essentially ageless.[3] The spherical particles could be used for sunlight-readable electronic ink displays.[3]

This research was supported by the National Science Foundation, both as a project grant and through support of Harvard's Center for Nanoscale Systems. A provisional patent application has been filed on this color capsule technology.[3]

Photonic Pigments

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