Fibers with the Tunable Structure Colors Based on the ...link.springer.com/content/pdf/10.1007/978-981-4451-68-0_6-1.pdfCountless colors can be found in nature, including green leaves,

Fibers with the Tunable Structure Colors Based on the Ordered andAmorphous Structures

Wei Yuan, Chaojie Wu, Ning Zhou and Ke-Qin Zhang*National Engineering Laboratory for Modern Silk, College of Textile and Clothing Engineering, Soochow University,Suzhou, China

Abstract

Dyeing is a process that is critical in the coloring of fibers or fabrics in the textile industry. The wastewater discarded from the dyeing process gives rise to the severe pollution to the environment. In thischapter, a novel coloration strategy was presented, which uses structural colors by incorporatingordered photonic and amorphous structure onto fibers. This coloration strategy originates from colorstructures found in nature, such as butterfly wings and parrot feathers. Furthermore, recent results onthe preparation, and mechanical and optical properties of these structurally colored fibers that mimicnatural color structures, are discussed in detail. It is believed that structural coloration of fibers haspotential as the environment-friendly, non-fading, and economic solution demanded by the currenttextile industry.

Keywords

Colorful fibers; Structural color; Photonic crystal; Amorphous structures

Introduction

The Pollutions of the Dying Process in the Textile IndustryIn the clothing industry, color is the most distinctive trait that is prominent in determining an articleof clothing’s first impression to customers. Thus, color is a major concern for workers in the textileand clothing industry, who endlessly pursue the creation of fascinating garments and textile productsfor consumers. Textile products achieve various colors through the coloration processes, whichoften utilize dyes or pigments. Dyes can diffuse into fibers and have inherent affinity on the fibermaterials. Pigments, which can only adhere to the surface of fiber materials through other chemicalagents, have no inherent affinity on the fibers. Most manufacturers use synthetic pigments and dyesinstead of natural extracts; which are difficult and even impossible to naturally degrade.

The dyeing process involves the transfer dyes from the dyebath onto the fiber. During the dyeingprocess, dye molecules transfer from the dyebath into the fiber, and other dye molecules desorb fromthe fiber to reenter the dyebath. When the rates of dye molecules entering and leaving the fiber areequal, the amount of dye in the fiber does not change with additional dyeing time; an equilibriumcondition has been established. Exhaustion is expressed as the percentage of amount dye originallyadded to the dyebath, to the percentage that transfers to the fiber. For example, if 4/5 of the dyeoriginally in the dyebath transfers to the fiber, the exhaustion is 80 %. The most ideal and

*Email: [email protected]

Handbook of Smart TextilesDOI 10.1007/978-981-4451-68-0_6-1# Springer Science+Business Media Singapore 2014

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environmentally friendly scenario is 100 % exhaustion, where no dye is wasted in the bath.However, this ideal condition is not a reality in industry manufacturing. Dye transfers to fibergradually as dyeing time passes, but exhaustion increase is usually diminished as time increases. Theadditional time and costs required to produce higher exhaustion may be more expensive than thesavings achieved in dye and waste treatment costs [1]. Nearly 10 % of the input dyes are lost duringthe textile coloration process. Furthermore, auxiliary chemicals are often added to the dyebath toimprove dyeing results, and some of them are also lost in the bath. Thus, the complex wastewaterfrom dyeing processes contains unreacted dyestuffs (color), suspended solid, high amount ofdissolved solids, and the auxiliary chemicals used in the various stages of dyeing and processing[2]. The unnatural colors of the wastewater are aesthetically unpleasant and easily bring to mind anassociation with contamination. The wastewater should be controlled and disposed beforedischarge.

Currently, most common methods of wastewater treatment consist of physical and/or chemicalprocesses. The conventional coagulation process creates a problem of sludge disposal. Biologicaltreatments, such as using of cells or enzymes, show low degradation efficiency. Some newtechniques, such as ozonation, electrochemical destruction, treatment using Fenton’s regent, andphotocatalytic oxidation, usually involve complicated procedures or are economically unfeasible[3]. Many researchers are searching for better methods of wastewater treatment. Experts in theindustry often wonder if there are any good solutions to handle the wastewater problem. Someexperts are even turning to the dye process itself, exploring the possibility for coloration strategies toreplace the dyeing process. The answer may be found in nature.

The Colors in NatureThe world is full of light. Visible light is made of seven wavelength groups. Color is a result ofhuman visual perception to the wavelengths that correspond to red, green, blue, and other distin-guishable visible wavelengths. Color derives from the spectrum of light interacting in the eye withthe spectral sensitivities of the light receptors. Color categories and physical specifications of colorare also associated with objects, materials, light sources, and so on; these categories are based onphysical properties such as light absorption, reflection, or emission spectra.

Countless colors can be found in nature, including green leaves, red flowers, and blue sky, asshown in Fig. 1. These colors, which be seen on plants and animals, are produced in two quitedistinct ways. One of the methods occurs when light is absorbed in material; this is usually the casefor ordinary coloration mechanisms [4] in colored materials such as pigments and dyes. The othermethod of color production arises when the light is reflected, scattered, and deflected, not reachingthe eyes under the presence of a specific nanostructure. The coloration in the second case operatesbased on purely physical properties. Thus, in nature, there are two types of color: pigmentary color(chemical color) and structural color (physical color).

Pigmentary ColorPigments change the color of reflected or transmitted light as a result of wavelength-selectiveabsorption. Many materials selectively absorb certain wavelengths of light. For example, thegreen chlorophyll contained in leaves enables the plants to use sunlight as an energy source forassimilation, allowing for synthesis of sugar, starch, and cellulose from water and CO2. Chlorophyllcan absorb short-wavelength (blue) and long-wavelength (red) light, while medium wavelengths(green or yellow-green) are remitted, making the leaves appear green [5].


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Structural ColorBesides pigmentary color, the colors due to the interference of light from microstructures composedof different refractive index materials that are comparable to the visible wavelength of light could beobserved; such coloration is called structural color [6]. Because structural color is fadeless and noenergy is lost from the color mechanism, structurally colored materials have attracted great attentionin a wide variety of research fields.

The study of structural colors has long history originating from observations of the complexinteractions between light and the sophisticated nanostructures generated in the natural world. Thephysical mechanisms of natural structural color have been explored intensively [7–10]. One of themain mechanisms is ascribed to the photonic bandgap effects of regular photonic structures, whichmostly appears in the feathers of birds and the skins of beetles [11–13]. The incident light iscoherently scattered by the periodic structure of photonic crystals (PCs), which results in a part oflight with certain frequencies strongly reflected. The final color of the structure is determined by thefrequency of the constructively reflected light. The observed color of PCs depends on the angle ofthe incident light, which is usually called iridescent phenomena. Light scattering is another coloringmechanism for objects found in nature [14–18], such as the blue appearance of the sky due to thescattering of clouds. The incident light is dispersed by a single scatter, which results with certainfrequencies of light being strongly scattered. The frequency of the scattered light always corre-sponds to certain electromagnetic multipolar modes supported by the polarization of the moleculardipoles in scatters. In such case, the light scatters are randomly packed in space, significantlydifferent from the structure in its periodic arrangements. The incoherent isotropic scattering fromsuch structure occurs, and eventually, this results in the noniridescent color phenomena.

Fig. 1 Two types of coloration mechanism for natural colors. The left panels show the pigmentary color throughinteraction between the light and natural pigments. The right panels illustrate the structural colors originating from theperiodic nano-sized structures


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Because the technology of dyeing and finishing in textile industry has some drawbacks, such ashigh pollution, high energy consumption, and low resource utilization rates, an idea that colors fibersbased on physical methods has been put forth, without the assistance of chemical dyes. In thischapter, the methods of structurally colored fiber fabrication will be reviewed, based on the twocoloring mechanisms described above: light reflecting of photonic crystals and scattering ofamorphous structures.

Colorful Fibers Based on the Photonic Crystals

Description on the Photonic Crystal StructuresPhotonic crystals (PCs) [19, 20] are one type of well-known photonic nanomaterials that possess aperiodic refractive variance. Due to the periodicity in dielectric, PC materials possess a photonicbandgap (PBG), forbidding certain wavelengths of light located in the PBG from transmissionthrough the material. According to variations in the refractive index and period in space, PCs can beclassified as one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D), as shown inFig. 2a. Because of these distinct functions, PC materials have been utilized in optical fibers, displaydevices, sensors, and other technology.

Recently, PCs have attracted increasing interest from researchers due to their unique structuralcolor properties [10, 21]. Photonic materials with vivid structural colors exist commonly in natureand are found in species of birds, butterflies, and insects [22, 23]. The colorful appearance of the PCmaterials can be ascribed to interference and reflection, which can be described by Bragg’s law [24](Fig. 2b). The law is given by

Fig. 2 A schematic diagram of photonic crystal. (a) one-, two-, and three-dimensional photonic crystals. (b) Incidentlight with a wavelength predicted by a modified Bragg equation (Eq. 1) undergoes diffraction when propagating througha photonic crystal. The wavelength of light that is coherently scattered is centered on l and can be estimated by applyingEq. 1 using the incident angle, y; the effective refractive index of the photonic crystal, neff; and the periodicity of thestructure, D


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l ¼ 2D neff2 � sin2y

� �1=2(1)

Here, l is the wavelength of the reflected light, neff is the average reflective index of the constituentphotonic materials, D is the distance of diffracting plane spacing, and y is the incident angle. Basedon the equation, there are several methods for tuning structural color, such as changing thediffracting plane spacing D, the average refractive index neff, or the incident angle y.

Methods to Prepare the Fibers with the Photonic StructuresInspired by the structural colors in nature, structurally colored fibers can be fabricated through twodistinct methods. One is multilayer interference (1D photonic crystal) from radial direction of fibers,which widely exists in some insects, fish, and plant leaves. The other is 3D photonic crystal withfibrous sharp. In this section, the methods of structurally colored fibers preparation with multilayerinterference and 3D photonic crystal are presented.

Multilayer InterferenceStructural colors related tomultilayer interference aremost commonly found in nature [8, 12].Metal-lic reflection from the elytra of beetles is one of the most well-known examples of multilayerinterference [8]. Figure 3a shows photographs of the Japanese jewel beetle, Chrysochroafulgidissima. The change in viewing angle from the normal to its tail reveals a remarkable colorchange from yellowish green to deep blue. The cross section of the elytra was investigated by using atransmission electron microscope. The researchers found that the beetle’s frame consisted ofepicuticle on the outside and exocuticle on the inside. The epicuticle consists of five alternate layers,

0.40 mm

b

a

Fig. 3 (a) Viewing-angle dependence of the color change in jewel beetles. (b) TEM image of the cross section of theelytron (Reprinted with the permission from Ref. [8])


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as shown in Fig. 3b, which depends on the apparent color of the elytron, and changes from yellowishgreen to blue.

In the textile world, MORPHOTEX fibers [9] are the world’s first non-dyed, nanotechnology-based, structurally colored fibers. The technology marks a significant advance in the history ofdyeing based on the use of natural and chemical dyes. MORPHOTEX is composed of unstained,structurally colored fibers. This technology is a biomimetic conception based on the microscopicstructure of Morpho butterfly’s wings. No dyes or pigments are used. Energy consumption andindustrial waste are reduced because no dye processes are required.

This fiber is made of polyester and has a flattened shape with a thickness of 15–17 mm, withinwhich 61 layers of nylon 6, and polyester with a thickness of 70–90 nm, are incorporated, as shownin Fig. 4a. Because of the multilayer structure, wavelength-selective reflection and change inappearance with viewing angle are obtained. Four types of basic colors, red, green, blue, and violet,can be developed by precisely controlling the layer thickness according to visible wavelength.

Figure 4b demonstrates the weaving of a wedding dress using this fiber. Since the polymermaterials constituting the layer have similar refractive indexes (n = 1.60 for nylon and n = 1.55 forpolyester), the reflection bandwidth is limited within a small wavelength region. So the weddingdress seems to be pale blue, and it differs considerably from that of theMorpho butterfly. Therefore,some new techniques were prepared for fabrication of structurally colored fibers based on multilayerinterference.

Skorobogatiy et al. [25] fabricated solid- and hollow-core PBG Bragg fibers using layer-by-layerdeposition of polymer film, as well as co-rolling of commercial and homemade polymer films. Thetypical solid-core PBG Bragg fiber is presented in Fig. 5a. For fabrication of Bragg fibers, twomaterial combinations were used: polystyrene (PS)/poly (methyl methacrylate) (PMMA) andpolycarbonate (PC)/poly(vinylene difluoride) (PVDF), featuring the refractive index contrasts of1.6/1.48 and 1.58/1.4, respectively.

It was revealed that the hollow-core PBG fiber technology for photonic textiles is advantageousbecause such fibers can emit guided radiation sideways without the need of any mechanicaldeformations. Moreover, emission rate and the color of irradiated light can be controlled by varyingthe number of layers in the reflector and the reflector layer thicknesses, respectively. They alsodeveloped all-polymer low refractive index solid-core PBG fibers, which are economical and wellsuitable for industrial scale-up. The light emitted by the solid-core Bragg fibers appears very

15-17

61 layers

Alternating multiple layers

PA

a b

PET

PA

PET

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MIC

Fig. 4 (a) Cross section of the MORPHOTEX fiber (Reprinted with the permission from Ref. [9]). (b) The weddingdress fabricated by the MORPHOTEX fibers


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uniformly distributed over the fiber length, and no bright spots are typically observed, as shown inFig. 5b and c. Under white light illumination, emitted color is very stable over time, as it is definedby the fiber geometry rather than by spectral content of the light source.

In order for the color in this type of PBG Bragg fiber to appear, there must be a light source tolaunch white light into such fiber. The color of the PBG fiber under ambient illumination isnonsignificant. Therefore, such PBG fiber may have potential applications in smart textiles forentertainment.

Kolle et al. [26] revealed a new approach for fabrication of multilayer fibers with structural color.The fibers consist of two elastomeric dielectrics, polydimethylsiloxane (PDMS) and polyisoprene–-polystyrene triblock copolymer (PSPI), two inexpensive materials that are commercially available inindustrial quantities, and provide a sufficiently high refractive index contrast(nPDMS = 1.41 � 0.02, nPSPI = 1.54 � 0.02, determined by ellipsometry). As shown in Fig. 6a,multilayer fibers were produced by initially forming a bilayer of the two constituent materials, whichis subsequently rolled up onto a thin glass fiber with 15 mm to form the multilayer cladding. SEMimages of the cross section of the multilayer fiber with 80 periods wrapped around the core glassfiber are shown in Fig. 6b and c. The thickness of the two films in the initial bilayer can be tunedduring film deposition. Consequently, the spectral position of the reflection band of the fibers can befreely adjusted. As shown in Fig. 6d, three fibers with high reflectivity in different color ranges andtheir corresponding complementary colors in transmission are present.

In the process of multilayer fiber fabrication, the glass fiber acts as the substrate for the multilayersin the rolling process; the glass can be removed from the fiber by dissolution in hydrofluoric acid.Once the glass core is removed, the fiber is composed of two elastomers and can now be elasticallydeformed by stretching it along its axis. An elongation along the fiber axis leads to a compressionperpendicular to it, causing a decrease of its overall diameter and a reduction of the thickness of each

Fig. 5 (a) The fundamental structure and cross-section of solid-core plastic Bragg fiber perform (upper) and a resultantfiber (under ) withmagnified image of PS/PMMAmulitlayers. (b) Colorful PBGBragg fibers by launching white light intothe Bragg fibers. (c) Launching light into a PBG fiber-based textile (Reprinted with the permission from Ref. [25])


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individual layer. This way, the reflected and transmitted color can be reversibly tuned by axialextension of the fibers, as shown in Fig. 6e.

Cylindrical 3D Photonic CrystalFibrous or cylindrical photonic crystals are usually found in some insects and animals in nature.Andrew Parker et al. [11] first discovered photonic crystal structures in the sea mouse (Fig. 7a). Thesea mouse is partially covered with long hairs that produce a brilliant iridescence, as shown inFig. 7b; the sea mouse displays a range of colors that changes with the direction of the incident lightand the direction of observation.

Fig. 6 (a) Schematic representation of the manufacturing of artificial photonic fibers. (b) The cross-sectional SEMimage of the fiber. (c) SEM image of the individual layers in the cladding. (d) Optical micrographs of three rolled-upmultilayer fibers with different layer thicknesses and colors in reflection (top) and transmission (bottom). (e) Colortuning of a fiber with different layer thickness where the glass core was removed by a hydrofluoric acid etch (Reprintedwith the permission from Ref. [26])

Fig. 7 (a) The photography of the sea mouse. (b) Its iridescent threads. (c) The cross-sectional micrograph of a spine(Reprinted with the permission from Ref. [11])


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The cross-sectional micrograph of a hair, as shown in Fig. 7c, reveals a periodic nanostructuredpattern with hexagonal symmetry. Each hair contains thousands of hollow, close-packed, andlongitudinally oriented cylinders with a diameter of approximately 230 nm. Bright structural colorsare produced when the cylinders collectively diffract light that is incident on the sides of the hairfibers.

Liu et al. [27–29] exploit a method for fabrication of 3D structurally colored fibers based on PCs.Silica colloidal crystals are self-assembled onto a glass fiber by a heating evaporation self-assemblymethod in micro-space. The colloidal crystal self-assembled onto the fiber endows the fiber withiridescent color. The process of the experiment is shown in Fig. 8a; silica glass capillaries with aninner diameter of 530 mm and a polyimide outer coating were used as the microchannels. Subse-quently, a silica optical fiber with 300 mmwas put into the microcapillary. Then, the silica suspensionwas injected into a capillary through a Teflon tube. The capillary was horizontally placed in an ovenat 70 �C and dried overnight.

Figure 8b and c show typical SEM images of the fiber self-assembled by colloidal silica. This self-assembly usually results in an ordered lattice structure with a periodic arrangement of silica spherescorresponding to a (111) crystal plane with an fcc structure. As shown in Fig. 8d and e, differentstructural colors can be obtained by using different size silica particles at 215 and 240 nm.

There has also been success in fabricating a sort of structurally colored fiber with the assistance ofa magnetic field. As shown in Fig. 8f, the Fe3O4 colloidal suspension was mixed with a PEGDAresin containing DMPA as a photoinitiator. A plastic fiber was placed in the microcapillary. Then,using a syringe, the Fe3O4-PEGDA suspension was injected into a 20 cm long capillary through aTeflon tube. An NdFeB permanent magnet with a center magnetic field strength of 250 mTwas usedto generate a magnetic field. The capillary was immediately exposed to UV light. Then, the flexiblecolored fiber was taken out of the microcapillary.

The digital photos of the structurally colored fibers are shown in Fig. 8g–i; it can be clearly seenthat, when a magnetic field was applied, a strong structural color was immediately observed. Afterthe color is formed in the micro-space via magnetization, the color is fixed via UV light irradiation,which induces photopolymerization inside the Fe3O4-PEGDA suspension. The suspensionphotopolymerization solidifies the resin into a curving film and maintains the interparticle spacingof the colloidal nanoparticle chains. In this way, the structurally colored fibers are potentially veryfeasible for mass production.

Zhou et al. [30] exploited a new method for forming structurally colored fibers. As shown inFig. 9a, the electrophoretic deposition (EPD) technique was adopted to fabricate core–shell colloidalfibers with structural colors. The carbon fiber was fixed onto the copper plate via conductive silverglue. Subsequently, a voltage was imposed on the electrodes to drive the colloidal spheres to attach,forming cylindrical colloidal assembly on the surface of the carbon fibers. PS nanospheres withdiameters of 185, 230, and 290 nmwere selected to fabricate colorful core–shell fibers. According toBragg’s law, the photonic crystal assembled by the above PS spheres would present red, green, andblue colors with light normally incident on the (111) plane of the fcc structure. Similar structuralcolors were expected to appear on the colloidal fibers.

The optical dark-field images of colloidal fibers in red, green, and blue colors, respectively, areshown in Fig. 9b. The reflective spectra were measured by optical spectrometer, as shown in Fig. 9c.The peaks at 635, 525, and 435 nm corresponded to the individual structural color, shown in Fig. 9b.Surface and cross-sectional structures of colloidal fibers consisting of PS nanospheres with diam-eters of 230 nm are shown in Fig. 9d and e. The arrangement of the colloidal nanospheres deviatesfrom the ideal fcc structure. Ordered domains with the sizes of 10–20 mm2 are randomly oriented


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without the long-range order, as clearly observed from the enlarged surface and cross-sectionalstructures.

Differing from the abovementioned method for fabrication of structurally colored fibers based onself-assemble or electrophoretic deposition, Finlayson et al. [31] reported a new way to produce

1. Fiber2. Fe3O4@C-in-PEGDA CNPs3. Micro-space

UV light

UV 365 nm

oven

Meniscus

Solvent evoporation

Structural colored fiber for

desired reflectance property

H

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Optical fiber

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Removal of magnetic fieldReflectance-type PC fiber

32

a b

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Fig. 8 (a) A schematic diagram of the fabrication of a colloid self-assembly opal onto a fiber in micro-space. (b) SEM ofthe obtained fiber surface. (c) Magnified zone of (b). (d and e) Optical microscopy images of the corresponding fibersself-assembled with 215 and 240 nm silica (Reprinted with the permission from Ref. [27]). (f) Schematic diagram ofmagnetic field-induced formation of a structural colored fiber in micro-space. Digital photos of the colorful fibers underthe same external magnetic field with different sizes of Fe3O4 colloidal spheres (g) 120 nm, (h) 145 nm, (i) 180 nm. (j)Cross section of the structural colored fiber. (k) Visible chain-like structures (Reprinted with the permission from Ref.[29])


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high-quality polymer opal fiber in an industrially scalable process. First, the core–shell particleprecursors were prepared, with the particle consisting of a hard polystyrene (PS) core coated with athin polymer layer containing allyl methacrylate (ALMA) as a grafting agent and a soft polyethy-lacrylate (PEA) outer shell, as shown in Fig. 10a. Then, the core–shell particle precursors wereloaded into a mini-extruder, which consists of two counterrotating metallic screws with adjustablespeed in the range 1–150 rotations per minute (rpm) and adjustable temperatures between 25 �C and250 �C. In the extruder, the precursors form a melt, homogenized under the extreme shear forcesprovided by the screws. The overpressure generated then drives the shear-ordered granular materialthrough a stainless steel die, producing thin opal fibers with colors as shown in Fig. 10b–d.

The fibers have sufficient mechanical robustness to demonstrate the anticipated stretch-tunablestructural color. They show significant changes in color as they are stretched. Figure 10e and f bothvisually and spectroscopically shows how the color changes in a 1 mm diameter sample from redthrough green, blue, and finally a grayish color, as the strain increasing from 0 % to 50 %. Thesecolor changes are due to the decreases of the inter-planar distance during stretching, as explained byBragg diffraction. In the process of stretching the fibers, spheres within each plane parallel to thesurface move apart, but planes normal to the surface move closer to each other in order to keep thetotal volume constant, causing the Bragg wavelength to shift to lower values. The dark fieldreflectance for different strains is shown in Fig. 10f.

Fig. 9 (a) Schematic diagram of the fabrication of the structurally colored fibers. (b) Dark-field images of red-, green-,and blue-colored fibers under the microscope. (c) Reflective spectra. (d) Surface and (e) cross-sectional SEM images ofthe colloidal fiber (Reprinted with the permission from Ref. [30])


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These elastomeric polymer opal fibers have many attractive functional features, such as intensestructural color, with inherent stretch and bend tunability, in addition to excellent durability andmechanical robustness. These fibers are potential candidates for a novel range of nanomaterials andclothing fabrics, utilizing strong structural color effects as a replacement for toxic andphotodegradable dyes.

Colorful Fibers Based on the Amorphous Structure

Amorphous Structures in NatureLiving organisms have exploited photonic structures to produce striking structural coloration sincethe Cambrian period. A variety of ordered photonic structures that produce iridescent colors havebeen revealed, including thin films, multilayers, diffraction gratings, and PCs, found in birds,insects, sea animals, and even in plants. These iridescent colors depend on the angle of viewingand the incident light. However, such angle dependence presents a barrier for developing displaysand sensors using structurally colored materials.

In addition to ordered structures in the biological world, there are amorphous structures that canproduce noniridescent structural colors [17, 18, 32–34], as shown in Fig. 11. The caruncles of somebirds show noniridescent blue or green colors, which arise from 2D amorphous structures. The SEMimage shown in Fig. 11a reveals that the dermis of the caruncles consists of a thick layer of collagen afew hundred microns in thickness, with the parallel collagen fibers forming a quasi-ordered array[32]. A similar 2D amorphous structure is also found in the African mandrill (Fig. 11d) [34], which

Hard Core – Soft Shell Die aperture100-2000 μm

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Fig. 10 (a) Schematic of core–interlayer–shell system, based on PS-ALMA-PEA. (b) Fiber extrusion process. (c)Cross-linked fibers may be knitted into fabrics (d). (e) Dark-field images of the opal fiber at strains of 0 %, 20 %, 35 %,and 50 %. (f) Corresponding reflectance spectra for different strains (Reprinted with the permission from Ref. [31])


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has noniridescent blue skin under natural sunlight. These colorations are also caused by theconstructive interference of parallel dermal collagen fibers with short-range order.

The back feather barbs of the cotinga contain a 3D amorphous structure consisting of nearlyrandom close-packed spherical air cavities that sometimes have small interconnections, as shown inFig. 11b [18]. This 3D amorphous structure gives rise to a vivid noniridescent blue color. A similar3D amorphous structure is also found in the scales of the longhorn beetle Anoplophora graafi, asshown in Fig. 11c [33]. Overlapping each other in regular order on the beetle elytra, each needlelikescale has a distinct color, and the scale colors vary from blue, green, yellow, to red. Structuralcharacterization revealed that the scales possess a 3D amorphous structure consisting of randomclose-packed chitin nanoparticle. Different scale colors are due to different nanoparticles and brightgreenish-white lateral stripes on the elytra by color mixing.

A spinodal decomposition-like amorphous structure was found in the back feather barbs of theeastern bluebird. The structure leads to a noniridescent blue structural color, as shown in Fig. 11e[18]. Rod-connected amorphous-diamond-structured amorphous photonic crystal offer excellentPBGs. Such amorphous structures already exist in the biological world, found in the blue featherbarbs of the scarlet macaw, as shown in Fig. 11f [17].

Noniridescent structural coloration by amorphous structure has fascinated scientists for a longtime. The attempts to understand the mechanisms associated with biological materials often lead tothe development of new materials exhibiting unique and notable functions.

Creation of Amorphous StructuresTo fabricate amorphous structures [16, 35–38], both top-down and bottom-up methods have beenused. One of the most commonly used bottom-up methods is the self-assembly of colloids insuspensions. Based on phase-transition behaviors in colloidal systems, amorphous soft glassy

Fig. 11 (a) TEM image of collagen arrays in the light-blue-colored caruncle of the asity N. coruscans (Reprinted withthe permission from Ref. [32]). (b) TEM of a feather barb of the male plum-throated cotinga (Reprinted with thepermission from Ref. [18]). (c) SEM of a green scale of the longhorn beetle A. graafi (Reprinted with the permissionfrom Ref. [33]). (d) TEM of the blue skin of the mandrill (Reprinted with the permission from Ref. [34]). (e) TEM of afeather barb of the male eastern bluebird (Reprinted with the permission from Ref. [18]). (f) SEM of a blue feather barbof the scarlet macaw (Reprinted with the permission from Ref. [17])


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colloidal gel with only short-range order was successfully fabricated. Obtained samples showhomogeneous and angle-independent structural colors, as illustrated in Fig. 12a [35].

Ueno et al. [36] prepared an amorphous array composed of core–shell particles in an ionic liquid.When the concentration of monodisperse colloids is low in a liquid suspension, the colloids canmove freely, forming a fluid-like state, as shown in Fig. 12b (A–C). When the colloidal contentincreases, the electrostatic interactions among colloids increase, and the suspension forms anequilibrium crystalline state. As the concentration increases further and exceeds a certain criticalpoint, the viscosity of the suspension is so large that the relaxation time for forming crystallinearrangements approaches infinity. At this stage, the system forms a stable glassy state, and thecolloidal arrangements possess only short-range order, as shown in Fig. 12b (D–F).

Amorphous structures composed of dried colloids [37] were also successfully fabricated bymixing two sizes of colloids with formation of long-range order during the self-assembly process,as shown in Fig. 12c. The obtained samples of the bidisperse dried colloids show noniridescentstructural colors.

Nature provides delicate amorphous structures which serve as templates to model inorganicstructures [38]. In the peach-faced lovebird, the blue feather barbs possess a 3D amorphous structureconsisting of disordered bicontinuous random network of keratin backbones. By using the featherbarbs as hard templates, SiO2 and TiO2 3D amorphous structures were replicated with a sol–gelmethod. As shown in Fig. 12d , inverted SiO2 amorphous structure displays bright noniridescentstructural colors. The SEM image confirmed the faithful replication. Inspired by structural colora-tion in natural amorphous, electrically tunable full-color display pixels were obtained based on a 3Damorphous structure composed of a Fe3O4@SiO2 core–shell colloidal suspension, [16], as shown inFig. 12e. Pixel colors can be switched very fast by applying an electrical voltage due to the sensitiveelectrophoretic responses of the core–shell colloids.

50 um

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Fig. 12 (a) Photographs of gel colloidal suspensions with different polymer contents at different viewing angles(Reprinted with the permission from Ref. [35]). (b) Photograph of soft glassy colloidal gel with different colloidalconcentrations (Reprinted with the permission from Ref. [36]). (c) Photograph and SEM of a dried colloidal amorphousstructure film with mixed colloids (Reprinted with the permission from Ref. [37]). (d) Microphotograph and SEM of thetransverse cross section of a SiO2 disordered bicontinuous amorphous structure (Reprinted with the permission fromRef. [38]). (e) Photographs of electrically tunable structural-color display pixels at different applied voltages (Reprintedwith the permission from Ref. [16])


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Amorphous structures possess many interesting and unique optical properties such as isotropicPBGs or photonic pseudogaps, and noniridescent structural color productions, which result fromtheir unique structure features such as short-range order. These interesting properties demonstratethat amorphous structures are a new type of optical material and have potential in a variety ofimportant applications such as in photonics, color-related technologies, displays, and solar cells.

Fabrication of Colorful Fibers Through the Polymeric Phase SeparationPhase separation may be used to fabricate amorphous structures [17, 18, 39]. There are two types ofphase separation: nucleation and growth, and spinodal decomposition. In an immiscible polymerblend, for example, phase separation via nucleation and growth may give rise to morphology ofisolated sphere droplets dispersed in a matrix. In contrast, spinodal decomposition may lead toco-continuous morphology consisting of continuous and interconnected phases.

Nature may have already adopted such phase separations to produce 3D amorphous structures.Based on the morphological similarities, it was conjectured that random close-packed and disor-dered bicontinuous 3D amorphous structures found in some bird feather barbs may be self-assembled by phase separation [17, 18, 39], driven by the polymerization of keratin from the cellularcytoplasm via nucleation and growth, and spinodal decomposition.

The phase separation may be a feasible way to prepare functional materials of amorphousstructure with noniridescent structural colors. However, to the best of our knowledge, there are noexisting reports about the fabrication of structurally colored fibers with amorphous structure byphase separation. Meanwhile, in nature, there have some fibrous scales in beetles, which havenoniridescent colors owing to amorphous structure.

Dong et al. [39] studied the structural and optical properties of scales in the longhorn beetleSphingnotus mirabilis. As shown in Fig. 13a, the beetle is characterized by long antennae, as long asits body. Its elytra display a noniridescent greenish-blue color marked with bright lateral whitestripes. Under optical microscope, the white stripes are composed of fibrous scales. The scales wereobserved by optical microscopy as shown in Fig. 12b and c. Under the reflection mode, scales showa light blue color, while they display a dim red color under the transmission mode. Colors underreflection and transmission mode are obviously complementary, implying that the scale color is astructural color.

The structural observation as shown in Fig. 13d shows the scales aligned nearly parallel to eachother along the longitudinal direction. The cross sections show that the scales are composed of aninner part surrounded by an outer chitin cortex, as shown in Fig. 13e. Obviously, the scale interior isa disordered network of chitin, responsible for the scale color.

The optical properties of scales were characterized experimentally bymeasuring reflection spectrawith the microspectroscopic equipment which consists of a tungsten lamp light source, a microscopewith objective 50� and a numerical aperture 0.55, and an optical spectrometer. The measuredreflection is given in Fig. 13f. It is characterized by a broad reflection peak positioned at about560 nm. This reflection peak covers almost all visible wavelengths.

These fibrous scales with amorphous structures in some beetles have noniridescent blue color,which may be a good idea for fabrication of structurally colored fibers with noniridescent colorbased on phase separation.


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Applications of the Structurally Colored Materials in the Fields of Sensingand Monitoring

In nature, many living creatures can reversibly change their structural color in response to externalenvironmental stimuli. Based on the lessons learned from natural photonic structures, some specificexamples of photonic crystals colorimetric sensors are presented in detail to demonstrate theirunprecedented potential in practical applications, such as the detections of temperature, pH, ionicspecies, solvents, vapor, humidity, pressure, and biomolecules. To use photonic crystals as sensors,diffractions that fall into the visible range are usually preferred, as the optical output can be directlyobserved by the unaided eye, without the need of complicated and expensive apparatuses to read thesignals. Generally, the photonic bandgap of the colorimetric sensors can be reversibly changed inresponse to external physical or chemical stimuli, which are discussed below.

Vapor and Solvent SensorsSensors based on the variation of structural colors analyze vapors and solvents by measuring thereflective peak shift that often occurs during the change of effective refractive index and lattice

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Wavelength (nm)600 700 800

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Fig. 13 (a) Optical image of the beetle S. mirabilis. (b, c) Reflection and transmission optical microscopic image of ascale, respectively. (d) SEM image of the white strip on an elytrum. (e) SEM cross-sectional image of a single scale(Reprinted with the permission from Ref. [39])


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spacing. Song et al. [40] fabricated a colorful oil-sensitive carbon inverse opal by using poly(St-MMA-AA) colloidal crystals as a template. The peak positions showed a linear relationshipwith the refractive indices of the oils, by which different oils could be distinguished based on thecolor of the carbon inverse opal with a pore size of 240 nm. The color shift upon adsorption of oilwas reversible, and the response time of the carbon inverse opal was less than 30s. Good oil-sensingstability of carbon inverse opal suggests that it is a promising and economical alternative totraditional oil-sensing materials (Fig. 14). Ozin et al. [41] assembled mesoporous Bragg stacks(MBS) by alternate multiple coatings of meso-TiO2 and meso-SiO2. Their optical response isreversibly altered by the analyte in their pores, and the optical response of MBS to the infiltrationof alcohols and alkanes into its pores reveals better sensitivity and selectivity than conventionalBragg reflectors. Investigation of the response of MBS to a series of alcohols and alkanes revealedthat their sensitivity and selectivity are highly dependent on the properties of the mesoporous metaloxide layers constituting the MBS. Yang et al. [42] prepared bioinspired organic/inorganic hybridone-dimensional photonic crystals (1DPCs) by alternating thin films of titania and poly(2-hydroxyethyl methacrylate-co-glycidyl methacrylate) (PHEMA-co-PGMA) by spin coating.The color of the 1DPCs varies from blue to green, yellow, orange, and red under differinghumidities, covering the whole visible range, which successfully combines structural color andwater vapor sensitivity. The repeatability of the reversible response of the 1DPCs to water vapor isperfect, and the process can be repeated more than 100 times.

Fig. 14 (a) Typical SEM images of the photonic crystal templates with diameters of 228 nm. (b) The correspondingcarbon inverse opals with diameters of 215 nm. The inset shows the spreading of an oil droplet on the carbon inverseopal. (c) Reflection spectra of the carbon inverse opal adsorbing four different oils, representing the four differentrefractive index ranges. The insets show the colors of the carbon inverse opal with different oils absorbed (Reprintedwith the permission from Ref. [40])


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Temperature SensorsGenerally, temperature sensors can be divided into both inorganic and organic sensors. Polymer-based temperature sensors detect temperature change based on fast optical-switching behaviorcaused by thermally induced reversible swelling and shrinking of the hydrogels. Asher et al. [43]developed a robust nanosecond photonic crystal switching material by using poly(N-isopropylacrylamide) (PNIPAM) nanogel colloidal particles that self-assemble into crystallinecolloidal arrays. At a low temperature of 10 �C, these PNIPAM particles are highly swollen with adiameter of 350 nm. As the temperature increases, the particles shrink and expel water, with diameterdecreasing to 125 nm at 40 �C. On the other hand, inorganic sensors detect temperature changesbased on the change of refractive index. They are good exceptional alternatives for polymer-basedsensors in broader application ranges owing to its relatively high stability and wide refractive indexrange. For example, Sato et al. [44] fabricated composite materials comprised of nematic liquidcrystals (LCs) and SiO2 inverse opal films. The structural color changes when the phases of LCmolecules change, as the refractive index of the LCs depends on the phases of LC molecules. Theposition of reflected peak shifts rapidly as temperature increases, because the temperature affects theLCs phases change from the nematic to isotropic (Fig. 15). Lotsch et al. [45] obtained photoniccrystals with textural mesoporosity by bottom-up assembly based on sequential spin-coatingsuspensions of TiO2 and SiO2 nanoparticles on glass substrates. They found that the optical responseof the crystals to temperature can be significantly enhanced by varying the relative humidity of theenvironment. The humidity-enhanced thermal tuning causes shifts of the reflected spectra by up to�1.66 nm K�1. Owing to their high inherent porosities and ease of fabrication, nanoparticle-basedphotonic crystals offer a great potential for the development of sensitive temperature and humiditysensors.

Ion and pH SensorsQuantitative analysis of ions such as Pb2+ and K+ using PC sensors is accomplished by measuringthe reflected spectra shift caused by changes of diffracting plane spacing. Takeoka et al. [46] reportedthe design and synthesis of a novel porous gel with crown ether using a templating technique forcapturing K+ selectively. Such crown ethers swell and shrink reversibly, changing the lattice spacingof the PCs causing the rapid response of color change. Red shifts of diffraction were observed by thenaked eye as K+ concentration increased. However, the structural color was unaffected for sensingNa+ (Fig. 16). Thomas et al. [47] demonstrated full-color tunability of variously quaternized

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Fig. 15 (a) Photographs of sample film below phase transition temperature and (b) above phase transition temperature.(c) Reflection spectra of nematic liquid crystal 4-pentyl-4-cyanobiphenyl (5CB)-infiltrated inverse opal. The spectra areplotted for temperature increments of 1 �C (Reprinted with the permission from Ref. [44])


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polystyrene–poly(2-vinyl pyridine) block copolymer films via selection of appropriate anions. Theselective swelling of the block copolymer lamellar structure with various ions allows tunability ofthe film from transparent to blue to red depending on the hydration strength of the ions. The positionof the PBG of the photonic gel films can be controlled by choosing hydration characteristics of thecounteranions and degree of quaternization for quaternized P2VP microdomains. Recently, thephotonic crystals for pH sensors play an important parameter for many water-based reactions andanalysis. Wang et al. [48] fabricated a new type of light-diffracting hydrogel composite film for H+

recognition consisting of nanoparticles and polymer through a combined physical–chemical poly-merization process, which significantly reduces the complexity of established methods. Thereflected wavelength shift is so obvious that the visible structural color change can be visuallyidentified for the hydrogel.

Biological SensorsRecently, photonic crystal materials have been employed to design an optical biosensor for bio-assays when appropriate physical structures have been attached by recognition groups. Takeokaet al. [49] created a colorimetric glucose sensor that can provide the desired monitoring of glucoselevels by the naked eye, which is easier to fabricate and to control than those similar colorimetricsystems. Furthermore, the time required for porous gels to reach swelling equilibrium is shorter(Fig. 17). Recently, sensors based on particle plasmon resonance (PPR) have played an importantrole in biomedical engineering. For example, Zhang et al. [50] reported an optical biosensor based

Fig. 16 Reflection spectra and photographs of periodically ordered interconnecting porous gels at 36 �C with differentionic concentrations. (a) Na+. (b) K+ (Reprinted with the permission from Ref. [46])


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on waveguided metallic photonic crystals (MPCs) for the sensitive testing of biomolecular interac-tions, which provides practical approaches for the label-free detection of specific bioreactions. Thesuccess of this sensor is demonstrated by sensing the specific reaction between the HIV-1 capsidprotein (p24) antigen and the monoclonal anti-p24 antibody. The strong and reliable sensor signalevaluated by way of the optical extinction spectra indicates that this kind of sensor device can easilyresolve p24 antigens at a concentration lower than 20 ng •ml�1, with a large space for improvement.

Summary

In this chapter, recent progress in the study of structurally colored fibers was reviewed, includingtheir fabrication, optical properties, and applications. Although significant progress has been madein recent years, there are many interesting problems that do not have solutions due to the lack ofmore in-depth studies. For example, the fabrication of structurally colored fibers with multilayerinterference is a feasible way to produce commercial fibers with structural color as MORPHOTEXfibers. However, research has not yet concluded a reliable method of providing the high refractiveindex contrast of the alternating layers, which may be a barrier for the commercial applications.Owing to the low mechanical properties, the 3D photonic crystal fibers assembled by colloidalnanoparticles may not have practical applications. Amorphous structures can bring moderatenoniridescent structural colors, which are more suitable for the visualization. However, the colorsmay almost appear blue based on amorphous structure.

Dye-free, nanomaterial-based, structurally colored fibers were prepared from various methods.The technology marks a significant advance in the history of dyeing based on the use of natural andchemical dyes, which is a conception based on the biomimetic study of the microscopic structure ofsome insects or birds. It is believed that this technology will reduce the energy consumption andindustrial waste due to its dye-free process.

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Fig. 17 Reflection spectra and photographs of periodically ordered interconnecting porous poly(NIPA-co-AAPBA) gelin a 2-(cyclohexylamino)-ethanesulfonic acid (CHES) buffer aqueous solution including different concentrations ofglucose at 28 �C (Reprinted with the permission from Ref. [49])


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Documents

Fibers with the Tunable Structure Colors Based on the ...link.springer.com/content/pdf/10.1007/978-981-4451-68-0_6-1.pdfCountless colors can be found in nature, including green leaves,