Portable vapor sensors or ‘sniffers’ have obvious applications in military, industrial, and gas-utility field operations. But conventional sensors lack the resolution and species selectivity of high-end laboratory instrumentation, which can identify and quantify unknowns down to part-per-billion or even part-per-trillion levels in complex mixtures containing hundreds or thousands of volatiles. Although portable devices have become smaller, more energy-efficient, and less costly,1 poor selectivity to individual compounds remains the major barrier for their practical implementation in situations with high levels of interference.
To change the status quo in vapor sensing, our group focused on fundamentally new detection concepts. Our goal became to improve selectivity using individual sensing materials rather than arrays of sensors. We looked at optical transducers based on wavelength-multiplexed spectral measurements. We discovered that the nanoscale morphology of the wing scales of some butterflies exhibited acute chemical-sensing properties.2 That discovery led to development of a completely novel sensing platform.
Our work was inspired by the pioneering results of Clyde Mason,3 Helen Ghiradella,4 and Pete Vukusic.5 More than 80 years ago, Mason noted changes in reflected light when liquids of different refractive index were applied to wings of iridescent butterflies. Figure 1illustrates his discovery using a butterfly of the genus Morpho. Ghiradella and Vukusic demonstrated that the iridescence of many butterflies is caused by interactions of light with photonic nanoscale structures of their wing scales.
Figure 1. Color changes of a Morpho butterfly upon exposure to liquids of different refractive index, n. (A) Before exposure to liquids. (B) Results of exposure of the left fore- and hindwing to ethanol (n = 1.362) and toluene (n = 1.497), respectively.
Based on research into their optical responses to pure solvents, we studied Morpho scales for selective detection of numerous vapors, including water, methanol, and ethanol vapors (see Figure 2). We collected reflectivity spectra of Morpho butterfly scales in differential-reflectivity mode2 and further processed them using principal components analysis, a commonly used technique for identifying covariances among multivariate data. The results demonstrated that these closely related vapors can be discriminated well at all measured vapor partial pressures (P), including the smallest tested P = 0.02Po, where Po is the species-specific saturation vapor pressure. It is critical to note that the contributions of the identified principal components (PCs) were quite remarkable from this sensing material (PC1 = 71.7, PC2 = 18.8, and PC3 = 6.9%), which is evidence of highly selective vapor detection. The significance of this finding is that the response selectivity of iridescent scales of butterfly wings to different vapors dramatically outperforms existing nano-engineered and other optical sensors.
Unexpected high selectivity of the spectral response of the photonic nanostructure of Morpho butterfly wings to different vapors. Discrimination of water (H2
O), methanol (CH3
OH), and ethanol (C2
OH) vapors was performed using principal components (PCs) analysis of differential-reflectivity spectra after mean-centering. Numbers are partial pressures of the three vapors, ranging from 0 to 0.2 times the saturation vapor pressure.2
We recently started a program to develop innovative, bio-inspired nanostructured sensors that would enable faster, more selective chemical detection.6 We have taken this knowledge of the natural optical responses of nanostructures of butterfly scales to pure solvents of different refractive index and expanded this optical phenomenon into the selective detection of numerous gases and their mixtures with a single bio-inspired photonic structure (see Figure 3).
Figure 3. A new concept for selective chemical detection using bio-inspired photonic nanostructures draws inspiration from our discovery that nanostructures on the wing scales of Morpho butterflies have acute chemical-sensing capabilities. In addition to enabling more acute chemical sensors for homeland security, our sensing platform could lead to other industrial and healthcare applications that include emissions monitoring at power plants and breath analysis for disease detection.
The main focus of our challenging new program is to demonstrate selective detection of specific analytes in the presence of several closely related chemicals using a single sensing material.
This work has been supported by GE Long Term Research Funds and Defense Advanced Research Projects Agency contract 18W911NF-10-C-0069. Our current program is a collaborative effort of GE Global Research with the Air Force Research Laboratory, State University of New York (Albany), and the University of Exeter (UK).
Radislav A. Potyrailo
GE Global Research
Radislav Potyrailo is a principal scientist at the GE Global Research Center, an industrial adjunct professor of chemistry at Indiana University (Bloomington, IN), and a SPIE Fellow. His research focuses on new microanalytical instrumentation, sensing technologies, and functional materials.
1. D. Janasek, J. Franzke, A. Manz, Scaling and the design of miniaturized chemical-analysis systems, Nature 442, pp. 374-380, 2006.
2. R. A. Potyrailo, H. Ghiradella, A. Vertiatchikh, K. Dovidenko, J. R. Cournoyer, E. Olson, Morpho butterfly wing scales demonstrate highly selective vapour response, Nat. Photon. 1, pp. 123-128, 2007.
3. C. W. Mason, Structural colors in insects. II., J. Phys. Chem. 31, pp. 321-354, 1927.
4. H. Ghiradella, D. Aneshansley, T. Eisner, R. E. Silberglied, H. E. Hinton, Ultraviolet reflection of a male butterfly: interference color caused by thin-layer elaboration of wing scales, Science 178, pp. 1214-1217, 1972.
5. P. Vukusic, J. R. Sambles, Photonic structures in biology, Nature 424, pp. 852-855, 2003.