(Nanowerk Spotlight) Chameleon-like adaptive camouflage is based on structural colors that have evolved in Nature to help animals like cephalopods and certain birds and fish to modify their colors to adapt to environmental changes.
"Adaptive colors are realized by the change in volume or thickness of cell or protein soft layers in response to environmental stimuli, such as a change in humidity, light, temperature, or concentrations of specific molecules," Young Jin Yoo from the Flexible OptoElectronics Laboratory at Gwangju Institute of Science and Technology (GIST), explains to Nanowerk. "Environmentally responsive, man-made color-changeable materials could facilitate the fabrication of ultrathin colorimetric sensors for the rapid detection of pathogens, toxins, or environmental changes which are not picked up by human senses."
Colorimetric sensing techniques require only the naked eye or ordinary visible color photography and are attractive because of their simple-to-understand results (the change in color). Generally, a colorimetric sensor consist of a recognition moiety and a signal moiety and they can be used for both qualitative analytic identification as well as quantitative analysis.
Colorimetric sensor fabrication generally is constrained by the requirement for highly ordered structures and periodically arranged refractive indices, concomitant co-assembling of multiple materials and micro - or nano building blocks to achieve uniform, highly saturated colors at a large (i.e. centimeter) scale.
To overcome these challenges, the GIST team has turned to M13 bacteriophages as a sensing layer. M13 is a nanoscale, benign virus with a shape that closely resembles collagen fibers. It can change its structure by shrinking or expanding in response to a changing surrounding environment.
<Left: Schematic illustration of colorimetric sensor display with visualization process insensitive/sensitive color difference. Middle and right: Color images of colorimetric sensor display in different states (before/after phage coating, and humidity indicating state, respectively). Scale bar is 1 cm. (Reprinted with permission from Wiley-VCH Verlag) (click on image to enlarge)>
"By using a highly lossy resonant promoter (HLRP) as the substrate, the spin-coated M-13 virus layer exhibits strong resonance even with ultrathin thickness variations, resulting in colorimetric behavior with enhanced chromaticity," notes Prof. Young Min Song, who led this work.
The researchers reported their findings in Advanced Science ("Large-Area Virus Coated Ultrathin Colorimetric Sensors with a Highly Lossy Resonant Promoter for Enhanced Chromaticity").
By optimizing the conditions of the virus layer spin-coating deposition, the researchers coated a ∼60 nm thick virus layer with high uniformity. The highly lossy ultra-thin resonance promoter with resonance enhancement was able to realize a distinct color change even with only a nanoscale thickness variation of the M13 virus layer.
<Left: Response/recovery time measurement result by repeated exposure to relative humidity of 20% and 70%.. Middle: Schematic illustration of genetically engineered M13 phage and engineered sequences. Right: Schematic of multicolorimetric sensor array with four engineered phage coating layer (WT, 3A, 4E, and 3W) on HLRP. (click on image to enlarge)>
(Image: Song Research Group)
This drastically downsized colorimetric sensor was able to successfully distinguish substances having a similar molecular structure at extremely low concentration levels of only tens of ppb.
"Considering the complex refractive index changes, we improved the responsive chromaticity by optimizing the resonance area for the HLRP design," says Joo Hwan Ko, another co-author of the paper. "We then verified the wide chromatic responsivity by performing the optimization process for various material combinations."
To demonstrate a practical application, the team fabricated a colorimeter sensor for indicating humidity detection by revealing spatially designed droplet patterns with selective chromatic responsivity.
Due to the optimized ultrathin phage layer, the colorimetric humidity indicator displays a rapid response to drastic changes in humidity, such as breath blowing. After the moisture disappears, the fabricated indicator is reversibly returned to its initial hidden pattern with the deswelling of phage layer. (Video: Flexible OptoElectronics Laboratory, GIST)
By genetically engineering the M13 phage, the reactivity between certain target particles and the sensor surface can be controlled. This enables the fabrication of colorimetric sensors that react to specific harmful chemicals, pathogens and viral substances.
"We believe our concept will be used as a colorimetric sensor with highly saturated colors and ultra-sensitive detecting ability for detecting harmful substances ," the researchers conclude. "Moreover, advanced genetic engineering will allow us to improve the applicability with enhanced sensitivity into the medical industry with colorimetric sensing applications such as diagnostic kits for virus and pathogen sensing."
By Michael Berger – Michael is author of three books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology, Nanotechnology: The Future is Tiny, and Nanoengineering: The Skills and Tools Making Technology Invisible Copyright © Nanowerk