How squid and octopus might point the way to nanotechnology-based stealth coatings


QDOTS imagesCAKXSY1K 8(Nanowerk Spotlight) For a long time, scientists have  been fascinated by the dramatic changes in color used by marine creatures like  squids and octopuses, but they never quite understood the mechanism responsible  for this.


Only recently they found out that a neurotransmitter, acetylcholine,  sets in motion a  cascade of events that culminate in the addition of phosphate groups to a family  of unique proteins called reflectins. This process allows the proteins  to condense, driving the animal’s color-changing process. The latest findings  revealed that there is a nanoscale  mechanism behind cephalopods’ ability to change color.   Watch this amazing video of a camouflaging octopus:

Having begun to unravel the natural mechanisms behind these  amazing abilities, researchers are trying to use this knowledge to make  artificial camouflage coatings. New work from the lab of Alon A.  Gorodetsky, Assistant Professor at the  Henry Samueli School of Engineering  at the University of California, Irvine, addresses the challenge of making  something appear and disappear when visualized with standard infrared detection  equipment.

In a paper in the July 30, 2013, online edition of Advanced  Materials (“Reconfigurable Infrared Camouflage Coatings from a  Cephalopod Protein”), the team demonstrates graphene-templated, biomimetic  camouflage coatings that possess several important advantages.   “We used reflectin, a protein that is important for cephalopod  structural coloration, as a functional optical material,” Gorodetsky explains to  Nanowerk. “We fabricated thin films from this protein, whose reflectance – and coloration – could be  dynamically tuned over a range of over 600 nm  and even into the infrared (in the presence of an appropriate stimulus).

Our  approach is environmentally friendly and compatible with a wide range of  surfaces, potentially allowing many simple objects to acquire camouflage  capabilities.” The novelty of these findings lies in the functionality of the  team’s thin-films within the infrared region of the electromagnetic spectrum,  roughly 700nm to 1200nm, which matches the standard imaging range of infrared  visualization equipment. This region is not commonly accessible to biologically  derived materials. Gorodetsky notes that reflectin’s tunable optical properties  compare favorably to those of artificial polymeric materials.

“Given these advantages, our dynamically tunable,  infrared-reflective films represent a crucial first step towards the development  of reconfigurable and disposable biomimetic camouflage technologies for stealth  applications,”  says Gorodetsky. ” I can also imagine applications in energy efficient  reflective coatings and biologically inspired optics.”   The team began their studies by developing a protocol for the  production of the histidine-tagged reflectin A1 (RfA1). Experimenting with a  variety of substrates and surface treatments for the reliable formation of RfA1  thin films, they achieved best results by spincasting 5 to 10 nm films of  graphene oxide on glass substrates. They then spread RfA1 onto the graphene  oxide-coated substrates, yielding smooth films over centimeter areas.



           appearance of the RfA1 film in the absence and presence of an external stimulus, when visualized with an infrared camera

Illustration depicting the appearance of the RfA1 film in the  absence and presence of an external stimulus (acetic acid), when visualized with  an infrared  camera. (Reprinted with permission from Wiley-VCH Verlag)  

These films showed a distinct coloration, depending on their  thickness. For instance, a 125 nm-thick film was blue and a 207 nm-thick film  was orange. “Inspired by the dynamic optical properties of reflectin  nanostructures, we sought to shift the reflectance of our RfA1 films into the  infrared region of the electromagnetic spectrum,” says Gorodetsky.

“Given that  some squid can dynamically modulate their skin reflectance across the entire  visible spectrum and even out to near infrared wavelengths of ∼800 nm, we  postulated that it should also be possible to tune the reflectance of our RfA1  thin films across a similar, or even larger, wavelength range. Thus, we sought  conditions that would significantly increase the thickness of our RfA1 films  and, consequently, shift their reflectance spectra toward the infrared.”

To that end, the researchers explored the response of their RfA1  coatings to a variety of chemical stimuli. They discovered that exposing the  films to vapor from a concentrated acetic acid solution induced a large,  reversible shift in the reflectance spectra, caused by the acid-induced swelling  of the closely packed RfA1 nanoparticles in the film.   “With the goal of fabricating dynamically tunable camouflage  materials, which will self-reconfigure in response to an external signal, we are  currently developing alternative, milder strategies for triggering coloration  changes in our material,” Gorodetsky describes the team’s future work plans.

By Michael Berger. Copyright © Nanowerk

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