The Pacific cleaner shrimp (Lysmata amboinensis) that was the character Jacques in Pixar Animation Studio’s 2003 animated Disney movie Finding Nemo may hold the secret to whitening agents that don’t endanger health.
Inorganic nanoparticles such as titanium dioxide and zinc oxide are widely used as whitening agents in foods, cosmetics and paints, but due to health concerns, there is currently an intensive search to find organic, bio-compatible analogs to replace these materials.
Now, Dr. Ben Palmer and student Tali Lemcoff of Ben-Gurion University of the Negev (BGU) in Beersheba have discovered a new material in cleaner shrimp that produces one of the most efficient white reflectors in nature, which could inspire the development of novel, organic whitening materials.
By studying the white material found in cleaner shrimp, the researchers also discovered a completely new principle in optics. The findings have just been published in the prestigious Nature Photonics journal, under the title “Brilliant whiteness in shrimp from ultra-thin layers of birefringent nanospheres.”
How do shrimp keep things white?
Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. The study was supported by European Research Council starting grant.
The shrimp uses white stripes on its cuticle covering all its outer surfaces, including appendages, to attract fish, which it then proceeds to clean by eating parasites off the fish’s body. When the researchers took a closer look at these white stripes, they discovered something amazing: The white stripes are made of an ultra-thin layer of densely packed particles of a small molecule named isoxanthopterin.
Making white materials from thick materials is easy, but creating efficient white reflectors from thin, dense materials is challenging, due to an optical effect called “optical crowding,” in which reflectance decreases at higher packing densities. Despite being fewer than five microns thick, the whiteness produced by the shrimp is extremely bright, making it one of the thinnest and most efficient white materials that exist.
The key to the optics is in the arrangement of molecules in the particles; the molecules are arranged in a liquid crystal, stacked in columns which radiate radially from the center of the nanospheres like the spokes of a wheel.
To generate whiteness, photons of all wavelengths must be scattered multiple times and lose their directional information to produce broadband, angle-independent reflectance. Although this is easily achieved with thick samples, it is difficult to obtain with thin layers of material. Recent studies have explored how ultra-thin photonic structures can be tuned to produce efficient scattering that is relevant to sensors, solar cells, displays and enhanced absorbers, they wrote.
“At first, I thought it was not interesting because the nanospheres were not classic crystals. However, when we looked closer using cryo-SEM and TEM microscopes, we realized not only that the particles are liquid crystals, like those in LCD displays, but that they exhibit birefringence (dual refraction), which is exceedingly rare in the animal world,” enthused Lemcoff.
It turns out that this special arrangement of molecules is key to overcoming the optical crowding hurdle, allowing the particles to be packed densely, reducing the thickness of the layer required to produce bright whiteness.
“It is really one of the first times we have learned an entirely new principle from studying an organism, Palmer concluded. “The shrimp has overcome a seemingly fundamental hurdle in optics by creating particles with this special arrangement of molecules. Now the question is, how can we replicate this effect for creating new materials we could use as food additives in white bread, or in white paint and other applications?”