Yuanjin Zhao et al. look at the development of bio-inspired structural color materials
Structural colorations appeared in the natural world during the Cambrian explosion 500 million years ago, when living creatures began to develop and rapidly diversify. This promoted the co-development of their optical and visual systems as a way of adapting to the selection pressures of survival and reproduction [1]. Because of their striking brilliance, people have always been fascinated by these natural structural colorations, despite the defining principles being unclear until several hundred years ago, when Hooke and Newton explained that the colors in silverfish and peacock originated from their physical structure. A more intensive investigation was reported in 1942 by Anderson and Richards, benefiting from the observation of detailed micro/nanostructures through electron microscopy.
From then on, structural coloration has been successively discovered in various organisms and has been gradually distinguished from chemical or pigment-based coloration according to coloring mechanisms; structural coloration relates to the physical interaction of periodic structures with light, such as through reflection and refraction, whereas the chemical or pigment-based colorations involves photoelectrical energy consumption and conversion [2]. Structural coloration has some obvious advantages over other types of coloring, as it can be free from photochemical degradation and its colors are purer because of the comparable length scale between their structural spacing and wavelength of light. Such intriguing structural color materials, together with their potential applications, continue to arouse widespread interest.
Natural structural color materials can be readily used out of the box. The most intuitive application is for ornaments and jewelry. For example, natural opal, the national gemstone of Australia, has an iridescent color and is esthetically pleasing. In addition, natural structural color materials possess periodic nanoarchitectures in ordered arrangements from one to three dimensions, and thus provide structure-dependent functions such as self-cleaning and vapor response. Moreover, their structure-derived ability to manipulate light propagation fits the requirements for modern optical systems and other extended applications. However, the biopolymeric components of these materials, often with poor mechanical strength, have largely limited their practical application. A direct solution to this problem is to convert the original structures from the natural materials into replicas by using a deposition-evaporation strategy or the sol-gel chemistry method. For example, through low-temperature atomic layer deposition and high-temperature template removal, the structure of Morpho butterfly wings, which exhibit optical features and also serves as waveguide and splitter, could be incorporated into an alumina replica with high-fidelity [3].
Besides direct application, natural structural color materials have also provided inspiration for scientists and engineers to mimic their delicate micro/nanostructures [4]. The processing principles can be generally classified into top-down and bottom-up methods. Top-down approaches attempt to use microfabrication tools such as electron beam or X-ray lithography to “sculpt” or “write” nanostructure patterns predesigned by computer on bulk material substrates. A large number of high-quality materials with photonic nanostructures have been successfully prepared with these methods. However, when it comes to three-dimensional (3D) fabrication, the technological complexity increases dramatically. This problem, together with the high manufacturing cost, low production efficiency, and resolution limit, still restricts batch production of advanced photonic materials. In contrast, bottom-up approaches seek to build hierarchical ordered structures through self-assembly and physicochemical interactions between basic building units, including molecular components such as block copolymers or liquid crystal molecules, and colloidal nanoparticles such as submicron silica or polystyrene beads. As the most favorable method, such structural color materials assembled from colloidal bead units have been intensively investigated [5], [6] and [7]. Through sedimentation, capillary force-induced deposition or lifting, colloidal nanoparticles can assemble in bulk or on flat surfaces to form colloidal crystal films. In addition, by spatially confined assembling, the morphology of colloidal crystals can be diversified into fibers, rods, spheres, or other complex hierarchical geometries.
Artificial structural color materials have also been extensively applied in optoelectronics, as well as in the biochemical and medical areas. As with periodic dielectric units, the structural color materials, termed “photonic crystals” (PhCs), can control the propagation of photons and generate a so-called photonic band gap. By tailoring the PhCs with certain surface microstructures, these structural color materials were imparted with angle-independence, and could thus be used to create decoration, coating, painting, and display units. Through the introduction of defects or cavities, light flow can be guided or trapped, thus creating waveguides or photonic circuits for photonic communication. Such a photonic device can also be integrated with electronic devices, which shed light on optoelectronic applications. Besides guiding the light, structural color materials also sense light. By incorporating responsive polymers into the photonic crystal structural framework, such materials can distinguish and detect a wide range of stimuli through different degrees of color shift, leading to important applications in anti-counterfeiting technology and smart sensors [8]. It is worth noting that, responsive structural color materials have recently attracted considerable attention in biomedical fields, serving as spectroscopic barcodes, label-free sensors, and cell microcarriers for multiplex biomolecule and cell assays [9] and [10].
Although structural color materials have been intensively studied both theoretically and practically, in addition to their prospects, there still remain challenges. In design, scientists have expended enormous time and effort to create sophisticated photonic nanostructures of structural color materials, while overlooking materials that were already in existence within natural creatures. This dilemma indicates the importance of interdisciplinary collaboration between materials scientists, engineers, biologists, and zoologists, for timely access to structural and biological information that could be used for mimicking. In fabrication, some elaborate natural photonic structures are beyond the current technical limits. Even when the materials can be manufactured, the output is too low to meet the industrial demand. However, plants and animals can make these structures under ambient conditions; it thus provides an alternative fabrication method directly through cell culture and genetic engineering. In application, new advantages of the structural color materials still need to be found, and materials with clear value are anticipated to be commercially exploited. For this purpose, it will be more beneficial for the technology holders to establish extensive cooperation with larger companies with mature product sales and promotion systems.
In a word, although the science of imitating natural structural coloration is still very much in its infancy, we firmly believe that these shiny materials will have a bright future.
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