| Abstract: | For almost a century, the iridescence of tropical Morpho butterfly scales has been known to originate from 3D vertical ridge structures of stacked periodic layers of cuticle separated by air gaps. Here we describe a biological pattern of surface functionality that we have found in these photonic structures. This pattern is a gradient of surface polarity of the ridge structures that runs from their polar tops to their less-polar bottoms. This finding shows a biological pattern design that could stimulate numerous technological applications ranging from photonic security tags to self-cleaning surfaces, gas separators, protective clothing, sensors, and many others. As an important first step, this biomaterial property and our knowledge of its basis has allowed us to unveil a general mechanism of selective vapor response observed in the photonic Morpho nanostructures. This mechanism of selective vapor response brings a multivariable perspective for sensing, where selectivity is achieved within a single chemically graded nanostructured sensing unit, rather than from an array of separate sensors.Structural colors in tropical butterflies have been of scientific interest for almost a century (1, 2), with many optical studies examining Morpho species (3, 4). The iridescence exhibited by scales of Morpho butterflies originates from 3D vertical ridge structures of stacked periodic layers of cuticle separated by air gaps (5).In this study, we have found a biological pattern of surface functionality within these ridge structures of Morpho scales. This pattern is a naturally formed gradient of surface polarity extending from the polar tops of ridges to their less-polar bottoms. We validated the existence of this gradient by applying a suite of complementary techniques to assess the chemistry of the ridges on the nanoscale. At first, we mapped the spatial polarity distribution of individual ridges by staining scales with polarity-sensitive dyes and performing spatially resolved optical characterization experiments. We further monitored the change of spatial polarity distribution within individual ridges before and after a control experiment aimed to reduce this initial polarity gradient. Finally, we performed detailed experiments of exposing Morpho scales to vapors of different polarity, observed the optical spectral responses, and compared these experimental effects with responses from simulated Morpho nanostructures with either a uniform or a gradient surface polarity. Results of these complementary techniques provided strong evidence of the existence of the naturally formed gradient of surface polarity on the ridges of Morpho scales.Although this surface polarity gradient may not be essential for butterfly survival, but rather is a by-product of the process of scale development, this perspective on biological pattern design can offer opportunities for a variety of technological applications. Examples of such applications include photonic security tags, self-cleaning surfaces, gas separators, protective clothing, sensors, and many others. As a first application, the knowledge of the gradient of surface polarity of the ridges allowed us to unveil a general mechanism of selective vapor response in photonic Morpho nanostructures. According to this mechanism, vapors of different polarity are preferentially adsorbed onto the respective regions of the ridges. This preferential adsorption is expressed in the corresponding spectral regions of the reflectance spectra. Our previous work on the vapor response of Morpho butterfly scales (6) did not provide a mechanism for the observed unusually selective vapor response.In vapor sensing, the quest for selective sensors started in the 1950s with studies of metal oxides (7). Following initial observations of vapor responses of different materials, competing requirements between selectivity and reversibility of sensors were discovered, explaining poor selectivity of individual sensors (8). The idea of combining individual sensors into arrays (9) became an accepted compromise to improve selectivity. At present, development of selectivity-tunable yet simplified sensor systems attracts tremendous attention (10, 11). The mechanism of selective vapor response reported here introduces a different perspective for selective vapor sensing, where the selectivity is achieved within a single chemically graded nanostructured sensing unit, rather than from an array of separate sensors. |
