In the United States, turkeys usually only make the press around Thanksgiving, when they play the star role in this annual feast. In Korea and Japan, said Seung-Wuk Lee of the University of California–Berkeley, “they call the turkey the seven-faced bird because it can show seven different colors on its face.” For example, when a turkey is excited, the red skin on its face changes to white or blue. Now, the color-changing properties of their skin are inspiring biomimetic sensors that can detect the presence of environmental toxins or explosives.
In the January 21 issue of Nature Communications (DOI: 10.1038/ncomms4043), Lee and colleagues from UC–Berkeley, Pusan National University, Sungkyunkwan University, and Korea University have demonstrated that the turkey’s structural coloration mechanism can be mimicked using bioengineered phage viruses to create nanostructured surfaces that alter their color in the presence of specific molecules.
The turkey’s skin contains nanostructured bundles of collagen protein that interact with the incident light to provide structural coloration. Stimulation triggers swelling of the blood vessels in the turkey’s skin, stretching the collagen layer and altering its optical properties and perceived color.
Because of the difficulties associated with synthesizing ordered nanostructures out of unstable collagen bundles, Lee and his team instead used the M13 phage, a bacterial virus that behaves similarly to the turkey’s collagen bundles but which can be engineered to display functional peptides. “Tunability is quite challenging in the collagen, but it’s very easy to do in the phage,” said Lee.
The researchers genetically engineered the M13 phage to recognize target molecules and allowed it to self-replicate. By extracting a substrate from a phage solution and controlling the rate of deposition, researchers mediated the phages’ self-assembly into a thin-film surface composed of quasi-ordered phage bundles.
In the presence of a chemical vapor, the spacing between the bundles changes; this expansion or contraction alters the coherent scattering of the incident light and, consequently, the surface’s structural color. Using an iPhone app that analyzes the color change on an RGB scale, researchers can confirm the presence of a toxin and estimate its concentration.
When composed of wild-type M13 phage, the sensor displayed characteristic responses to humidity changes and volatile organic compounds. However, the sensor also demonstrated selectivity and specificity for molecules with lower vapor pressures, like many explosives and environmental toxins. They engineered a phage surface sensitive to TNT, a common explosive, by modifying the phage to display a TNT-binding peptide motif. As TNT vapor concentration increased, the binding of TNT to the substrate induced structural changes in the phage bundles and a color change of the material.
A key advantage of Lee’s design over other colorimetric sensors is the viewing-angle independent coloration of the phage bundles. This color fidelity, combined with the simplicity of fabrication and portability, makes these sensors powerful and practical tools for detecting toxins.
Marya Lieberman studies self-assembly of biomolecules at the University of Notre Dame. “Though the stability, reproducibility, and interferences still need to be characterized, this is a really clever way to translate a chemical interaction on the nanometer scale into a visual output on the centimeter scale. When a sensor can be read visually, it cuts the cost of making the sensor and makes it more practical for field use,” Lieberman said.
