Using nature's iridescence

Cellulose twists shine

Daryl Wile

Derek Gray peers from behind cellulose.
Owen Egan

It's a good thing Derek Gray enjoys his work, since it's practically impossible for him to get away from it. Gray studies the chemistry of cellulose, earth's most abundant natural polymer. Cellulose is found just about everywhere, from wooden furniture to paper, and has been introduced into synthetic materials such as plastics, textiles, and even foods. "If you take a close look at the ingredients for your salad dressing, there's probably a small amount of cellulose derivative in it," Gray points out. That said, there is far more cellulose in the salad itself — lettuce, carrots, in fact all plants contain cellulose.

Plant cells make cellulose by linking sugar molecules together in long chains. These sugar molecules are "chiral" — asymmetric in such a way that they're not superimposable on their mirror image, like your hands. No matter which way you turn one hand, it is never the same as the other; this is also true of cellulose's basic building blocks. Groups of these sugar chains stick together to make a cellulose microfibril, a material that is as strong as steel.

Gray's work with cellulose involves some of the unusual properties of this structure. When his research group used powerful microscopes to look at cellulose, they found that some had twisted themselves into right-handed helical ribbons. Twisted structures like these ribbons are also chiral, because their mirror images twist in the opposite direction.

Gray has also discovered that cellulose's chiral properties are preserved when these chains are assembled into different materials, from newsprint to nanocrystals. For example, to make newsprint, wood has to be mechanically broken up into individual fibres whose chiral structure imparts a twist to newsprint as it dries: if you cut a newspaper printed top to bottom (like the Gazette) into strips and wet them, the strips will twist to the right. A newspaper printed sideways (like the McGill Reporter) twists to the left.

The twisting of these cellulose nanocrystals gives them the ability to reflect colours. "If the length of the helix is about the wavelength of light, you get a very peculiar diffraction off the surface," explains Gray. This diffraction gives cellulose nanocrystals a shiny, iridescent colour that changes slightly when viewed from different angles, much like fish scales.

Gray hopes that this discovery will have useful applications. "The great thing is that cellulose is completely natural. And green is in." Not only can one imagine outrageously colourful cellulose salad dressings; this natural iridescent material could also be used, for example, in cosmetic products.

Daryl Wile is a WARM-SPARK writer. WARM-SPARK (Writing About Research at McGill-Students Promoting Awareness of Research Knowledge) is a program supported by the VP Research Office, Associate Vice Principal (Communications), the faculties of Agricultural and Environmental Sciences, Arts, Engineering, Medicine and Science. See www.spark.mcgill.ca for more information and articles.