Preparation and Applications of Photonic Crystal Structural Color Fibers

Abstract

Photonic crystal fibers are fibers that are made up of photonic crystals and have bright structural colors. Their unique structural color is conducive to the green transformation of the textile dyeing and finishing industry, and some photonic crystal fibers can also show color-changing characteristics according to changes in process, material and external environment, which can play a significant role in smart wear. In this paper, the preparation methods of photonic crystal fibers with different structures are introduced, and their applications in the fields of textiles, sensing and anti-counterfeiting are introduced. The future development direction and prospects are predicted, which are of great significance for the development of preparation methods and potential applications of photonic crystal fibers.

Graphical Abstract:

Introduction

Since the evolution of human eyes and the development of cones, color has become an inseparable part of our lives. We can determine the seasons by discerning the colors of leaves, assess the danger of insects through their warning colors, and appreciate the beauty of nature by observing rainbows, snow-capped mountains, and rainforests. It was not until Isaac Newton ¹ used a prism to divide sunlight into its spectral components of red, orange, yellow, green, blue, and violet that humans began to systematically explore the nature of colors. This endeavor continues to the present day.

With further research into color, it has been discovered that natural colors can be categorized into two main types: chemical colors and structural colors, distinguished by their underlying mechanisms. ² Chemical colors are primarily produced through the action of pigments, which result from electronic transitions within molecular orbitals. By selectively absorbing or reflecting different wavelengths of light, pigment molecules enable us to perceive various colors. Chemical colors are widely present in nature—for instance, the color of leaves is the result of a combination of various pigments such as chlorophyll and carotenoids, while the colors of human blood, skin, and hair are also pigment-based. As technology advanced, people started using pigments for dyeing objects. However, since pigment molecules do not possess directional properties during light absorption and reflection, the perceived color remains consistent from any viewing angle. Objects dyed with chemical colorants are prone to fading due to reactions between pigment molecules and chemicals in the air. Additionally, environmental pollution can occur during the processing and application of chemically dyed textiles.^3,4 In contrast, the use of structural colors is more environmentally friendly. Structural colors, in simple terms, arise from the interaction between the fine structure of an object and light. They are independent of the object’s inherent color and can be altered by modifying the surface structure of the object. Examples of structural colors found in nature include rainbows, peacock feathers, and opals. Unlike colorants used in chemical dyeing, structural colors are more persistent and resistant to fading as long as the surface structure remains intact. Structurally colored objects tend to be brighter, more saturated, and possess a vivid sheen compared to chemically colored objects. Structural colors hold enormous potential for development in the field of dyeing.

The textile and dyeing industries have an inseparable relationship, but today chemical dyeing is under significant pressure for environmental protection, in order to seek green development. The reform of the dyeing industry is particularly important. Fibers are the most basic fabric structure, and their dyeing has great significance. In addition to structure, colored fiber can also bring unique function.^5–7 Detailed study of structural color in fibers will improve the production of the textile industry. Solving the problem of dyeing fibers, requires solution of the problem of structural color in yarn and structural color fabric production. In this paper, the author summarizes the production method of structural color fiber and the performance and its application to fibers, and I believe that it can provide some help for the subsequent study of structural color fiber.

Color Generation Principle of Photonic Crystal Structure

Photonic crystals^8–10 are a rapidly developing class of novel structured materials in which the dielectric constant varies periodically in space. They are artificial microstructures formed by periodically arranging different refractive index media. Due to their unique structure, when the variation in the dielectric constant is sufficiently large and the period of variation is comparable to the wavelength of light, the dispersion relation of light waves exhibits bandgap and passband structures, resulting in the generation of structural colors. This phenomenon can be explained using Bragg diffraction. When light within the photonic bandgap^11–12 is incident on the photonic crystal unit, diffraction occurs initially, and then the diffracted light undergoes mutual interference and returns to the original propagation medium. In the visible light range, as a portion of the light passes through the photonic crystal structure, the light within the photonic bandgap undergoes interference through Bragg diffraction and returns to the incident medium (typically air). As a result, one can observe the structural colors produced by the enhanced interference of a certain wavelength within the photonic bandgap, which is known as the structural color of the photonic crystal. According to the Bragg formula, ¹³ whether a certain wavelength of light undergoes Bragg diffraction is determined by the material refractive index value (n) of the photonic crystal, the angle of incidence (θ) of light, and the lattice spacing (d) of the photonic crystal structure, as shown in equation (1)

$$\mathrm{m}\mathrm{\lambda }=2\mathrm{d}\sqrt{{\mathrm{n}}^2-\mathrm{si}{\mathrm{n}}^2\mathrm{\theta }}$$

(1)

There are numerous examples of natural structures in the natural world that serve as typical examples of photonic crystals, such as opal produced in Australia, ¹⁴ peacock feathers, ¹⁵ the green iridescence of beef cuts, ¹⁶ green duck feathers, ¹⁷ and so on. These are all results of the action of photonic crystals.

The structural colors generated by photonic crystals exhibit both rainbow colors and non-rainbow colors. When the micro-particle structures constituting the photonic crystal are arranged periodically, the colors produced by the photonic crystal exhibit significant angular dependence and vary with the change in the viewing angle, known as the iridescence effect. On the other hand, amorphous photonic crystals represent the “defect state” of photonic crystals, where the micro-particle structures are not arranged periodically. The resulting photonic bandgap is incomplete, and the lattice arrangement is disordered, showcasing short-range order characteristics. This unique bandgap in amorphous photonic crystals is referred to as the photonic pseudo gap.^18–20 The corresponding photon density of states is non-zero and rapidly responsive to frequency, causing light within the region of low photon density to undergo intense reflection, thereby producing structural colors. Unlike traditional photonic bandgaps, the photonic pseudo gap is direction-independent, and light scatters uniformly in all directions, resulting in non-rainbow effects. ²¹

Preparation of the Structural Color Fibers

According to current research findings, the primary method for fabricating structural color fibers is through the utilization of photonic crystals, which differs from previously reported articles on structural color fibers.^22–24 In this review, the author will provide a comprehensive overview of the preparation methods and advancements in fabricating structural color fibers using one-dimensional, two-dimensional and three-dimensional photonic crystals.

One-Dimensional Photonic Crystal Fibers

Photonic crystals are classified according to the distribution of permittivity, as shown in Figure 1, which can be divided into one-dimensional photonic crystals, two-dimensional photonic crystals, and three-dimensional photonic crystals. A one-dimensional photonic crystal refers to a material whose dielectric constant is periodically arranged in one direction. Usually, the one-dimensional photonic crystal material is alternately stacked by two or more dielectric layers, and its structure is shown in Figure 1(a). In the alternating layer, the blue layer is one medium, and two layers of blue layer are filled by another material. This periodic arrangement structure makes the refractive index and dielectric constant of the fiber change periodically along the radial spatial position, while the refractive index and dielectric constant of the fiber axis do not change. One-dimensional photonic crystals mainly produce structural color through thin film interference and grating diffraction. In order to get the bright structure colored one-dimensional photonic crystal each medium thickness has certain rules. One-dimensional photonic crystal fiber preparation is relatively simple and the current application of the coating process can prepare a complete photon band gap structure. The main methods of atomic layer deposition (ALD) technology, ²⁵ magnetron sputtering technology, ²⁶ and multilayer film winding method are used. ²⁷

Figure 1.

Photonic crystal structure model: (a) 1-D photonic crystal fiber, (b) 2-D photonic crystal fiber, (c) 3-D photonic crystal fiber.

Atom-Layer Deposition Method

Atomic layer deposition (ALD) technology ²⁸ is an atomic-scale film preparation technology, through which ultra-thin films with controllable thickness and adjustable composition can be uniformly deposited. The principle is a layer-by-layer atomic growth film preparation technology, in which different precursors are alternately exposed to the surface of the substrate and form a sedimentary film on the surface of the substrate by chemisorption. 1D photonic crystal fibers with controllable thickness can be prepared by using ALD technology. As shown in Figure 2(a), in 2019, Zhang Shufen’s research group ²⁹ utilized ALD technology to construct a one-dimensional photonic crystal film on the black surface of carbon fibers, using zinc oxide (ZnO) and alumina (Al₂O₃) layers with a significant refractive index contrast as periodic component materials. By modifying the thickness of the ZnO and Al₂O₃ layers, the black surface of the carbon fibers exhibited vivid colors. The photonic crystal carbon fibers prepared through this method demonstrated excellent mechanical robustness, wash resistance, and reduced incoherent scattering due to the wideband absorption of carbon fibers in the visible spectrum. This, in turn, enhanced color saturation. ALD technology allows precise control of film thickness at the atomic level, minimizing the presence of impurities. However, this method also has certain limitations, such as slow deposition rates and relatively low efficiency due to the constraints of the atomic layer mechanism.

Figure 2.

Preparation of 1D photonic crystal fibers. (a) The flow chart of structural color carbon fiber prepared by the atomic layer deposition method. Reprinted (adapted) with permission from Niu et al. ²⁹ Copyright (2019) American Chemical Society. (b) Structural color carbon fiber prepared by the magnetron sputtering method. Reprinted (adapted) with permission from Zhao et al. ³⁰ Copyright (2021) John Wiley and Sons. (c) Flow chart of structural color fiber prepared by the winding method. Reprinted (adapted) with permission from Kolle et al. ²⁷ Copyright (2013) John Wiley and Sons.

Magnetron Sputtering Method

Magnetron sputtering is a commonly used surface coating technique that utilizes magnetically controlled arc discharge to convert solid sputtering materials into ions, which are then guided by electric and magnetic fields to deposit onto a substrate, forming thin films. Compared to ALD technology, magnetron sputtering has a higher deposition rate. ²⁶ It exhibits high deposition rates when depositing most metal films, especially those with high melting points, and oxide films such as sputtering tungsten, aluminum films, and reactively sputtered titanium oxide (TiO₂) and zirconia (ZrO₂) films. Moreover, it is easily controllable. During the coating process, as long as the sputtering conditions such as working pressure and electrical power remain relatively stable, a consistent deposition rate can be achieved. As shown in Figure 2(b), in 2021, Professor Ni Wenbin’s team ³⁰ utilized Al₂O₃ and TiO₂ films as building blocks for photonic crystals and alternately sputtered them onto carbon fiber surfaces using magnetron sputtering. By harnessing the directional nature of sputtering and the curved structure of carbon fibers, the thickness of the sputtered photonic nanostructures decreased along the curved surface of the fiber from the top. ³¹ The sputtered photonic shell layers enabled the carbon fibers to exhibit multiple consecutive structural color changes. By varying the thickness of the TiO₂ film, a range of structural colors such as blue, green, orange, and magenta could be easily obtained in one-dimensional photonic crystal fibers.

Thin Film Winding Method

The film winding method is a technique for fabricating photonic crystal fibers by wrapping double-layered or patterned films around fibers. ²⁷ As shown in Figure 2(c), in 2013, based on the findings of layered photonic structures in the seed coats of Margaritaria nobilis, Peter Vukusic’s team ²⁷ combined two elastic dielectric materials, polydimethylsiloxane (PDMS) and polyisoprene-polystyrene triblock copolymer (PSPI), to create a double-layered film. This film was then wound onto the surface of a glass fiber to form a one-dimensional photonic crystal fiber. During the winding process, the glass fiber, which acted as a multi-layer substrate, was either dissolved in hydrofluoric acid or simply pulled out from the fiber to remove it. Once the glass core was removed, the fiber, composed of the two elastic materials, could undergo elastic deformation by stretching along its axis. As the fiber underwent stretching and contraction, the color of the fiber exhibited a blue shift, demonstrating its characteristic of a mechanical color-changing response.

Two-Dimensional Photonic Crystal Fibers

Compared with the preparation of one-dimensional photonic crystal fibers, the preparation of two-dimensional photonic crystal fibers is more complex. Two-dimensional photonic crystal fibers are usually composed of periodic array of staggered dielectric rods, and the pores are filled by low refractive index material. As shown in Figure 1(b), the typical structure of 2D photonic crystals is composed of many parallel and periodic arranged media columns. In general, the dielectric number of two-dimensional photonic crystal fibers only presents a periodic arrangement in the radial space, while the permittivity along the axial direction is independent of the spatial position. ²³ This is similar to one-dimensional photonic crystal fibers, but one-dimensional photonic crystal fibers exhibit this characteristic mainly due to the two-dimensional special structure of the fiber, and the spatial periodic characteristics of the two-dimensional photonic crystal structure are due to the different composition and structure of their own photonic crystals. There are not many preparation methods for two-dimensional photonic crystal fibers, and the main preparation methods are the one-dimensional grating winding method ³² and the iterative size reduction method. ³³

One-Dimensional Grating Winding Method

When preparing one-dimensional photonic crystal fibers, multiple layers of coiled films are used, forming smooth planar films. In contrast, Jeon’s team ³² employed two-dimensional photonic crystal films for their approach. Inspired by the surface microstructure of the black-billed magpie feathers, they conducted detailed observations and research to fabricate two-dimensional photonic crystal fibers. As shown in Figure 3(a), they utilized laser interference lithography to form a one-dimensional grating structure on a flat surface, sequentially coated with polyvinyl alcohol (PVA) and polymethyl methacrylate (PMMA) materials. After dissolving and removing the PVA material, a flexible film with line corrugations was obtained. The grating lines were then wound parallel to the fiber axis in multiple layers, generating a two-dimensional (2D) array of densely stacked air columns in the cross-section, resembling the hollow melanosomes of the black-billed magpie’s hook-shaped feathers. This resulted in the creation of a two-dimensional photonic crystal fiber resembling the structure the black-billed magpie feathers. By controlling the thickness of the grating polymer film, the structural color on the surface of the fiber can be modified.

Figure 3.

Preparation of 2D photonic crystal fibers. (a) Structural color fiber flow and structure drawing using the winding method. Reprinted (adapted) with permission from Han et al. ³² Copyright (2017) Optica Publishing Group. (b) Schematic diagram of the flow of preparing structural color fibers by size iteration method. Reprinted (adapted) with permission from Khudiyev et al. ³³ Copyright (2014) Macmillan Publishers Limited.

Iterative Size Reduction Method

The iterative size reduction method refers to a technique used to prepare two-dimensional photonic crystal fibers by repeatedly stretching the fiber array to reduce its diameter. In 2014, Mehmet Bayindir and his team ³³ observed and simulated the structure of neck feathers from the green-headed mallard duck to create two-dimensional photonic crystal fiber structures as shown in Figure 3(b). The process involves several steps. In step one, polyvinylidene fluoride (PVDF) and polycarbonate (PC) films are sequentially wrapped around a PC rod. After thermal stretching and removal of the outermost PC layer, a composite fiber is obtained. In step two, the PC film is wrapped and thermally bonded to a bare glass substrate, followed by the dissolution of the glass rod, leaving behind a rectangular preform. Subsequently, 500 composite fibers measuring 4 cm in length are placed in the preform from step two and subjected to thermal stretching. Step three is similar to step two, except that the composite fiber is replaced with a preform. After step three, it is possible to produce two-dimensional photonic crystals with a wide range of lattice parameters (including lattice constants > 100 nm) and arbitrary lengths. The process can be repeated to achieve smaller structural diameters. By controlling the size reduction, one can control the lattice constant and structural color of the two-dimensional photonic crystal fiber.

Three-Dimensional Photonic Crystal Fiber

Compared with one-dimensional and two-dimensional photonic crystal fibers, it is much more difficult to control three-dimensional photonic crystal fibers. The 3D photonic crystal structure has a complete crystal band gap, which can adjust the photon propagation in the full direction of three-dimensional space. It is the most important thing in the development of photonic crystal, and also the difficulty in photonic crystal research. As shown in Figure 1(c), three-dimensional photonic crystal structure color fiber refers to the dielectric constant in three directions with a periodic arrangement of the fiber material. The photon band appears in the three-dimensional space. The most common three-dimensional photonic crystal fiber structure is the uniform particle size of nano particles on the fiber. Compared with one-dimensional photonic crystal fibers and two-dimensional photonic crystal fibers, three-dimensional photonic crystal fibers can be observed along either the radial or the axial direction. Most of the 3D photonic crystal structure color is derived from the surface center cubic structure of the nanoparticles, similar to the opal principle in nature. Since the concept of photonic crystals was put forward, the majority of scientific researchers have prepared three-dimensional photonic crystal fibers through self-assembly. After years of research, the preparation of 3D photonic crystal fibers has become more common. In general, the current preparation methods of 3D photonic crystals can be mainly divided into the colloidal microsphere self-assembly method, ³⁴ the microfluidic spinning method, ³⁵ the electrospinning method, ³⁶ the extrusion forming method, ³⁷ and the thin film tensile method. ³⁸

Self-Assembly Method

Self-assembly is the process of achieving structural sequences of different scales without any direct external influence. ³⁹ Various building blocks can be used to create periodic nanostructures, such as block copolymers, ⁴⁰ liquid crystals, ⁴¹ and colloids. ⁴² This phenomenon is often observed in nature, where it creates optimal colors and multifunctional materials. Therefore, understanding the structures and coloring mechanisms of biological systems in nature is crucial for effectively replicating them and creating perfectly structured, colorized materials. In recent years, several self-assembly methods for colloidal systems have been proposed and developed, including capillary assembly, ⁴³ electrophoretic deposition, ⁴⁴ convection-driven self-assembly, ⁴⁵ and more.

Capillary assembly is a method that utilizes the capillary effect of liquids, using capillaries as templates to allow nanospheres to self-assemble in a periodic arrangement on the inner wall of the capillary, thereby forming three-dimensional photonic crystal structures. ⁴³ This method is a common approach for fabricating three-dimensional photonic crystal fibers, and it offers relatively simple operation and a wide range of applicability. As shown in Figure 4(a), in 2011, Ming Wang’s team ⁴⁶ achieved the self-assembly of composite photonic crystals consisting of polystyrene (PS) gel templates and infiltrated silica gels by combining PS gel suspensions with hydrolyzed silica precursor solutions. They prepared continuous hollow cylindrical composite colloidal films on the inner and outer surfaces of capillaries and removed the PS gel crystals from the composite colloidal crystals, resulting in hollow cylindrical inverse opal and opal columns. As shown in Figure 4(b), in the same year, Yaogang Li’s team ⁴⁷ achieved the self-assembly of photonic crystal structures on glass fibers by placing them in capillaries containing silica colloidal microsphere solutions, where the silica microspheres self-assembled on the glass fiber surface due to the capillary effect during solvent evaporation. In 2017, Zhongze Gu’s team ⁴⁸ vertically immersed hollow glass fibers into an open glass bottle containing silica nanoparticles dispersed in ethanol at a normal concentration. As shown in Figure 4(c), as the solvent evaporated and the liquid level dropped, the silicon dioxide microspheres self-assembled on the inner wall of the fiber, and the silicon dioxide nanoparticles in the tape completely self-assembled into densely packed face-centered cubic arrays with fewer defects and more stacked layers, making it better than colloidal crystal stripes on structurally disordered and limited thin films on flat substrates. Consequently, a striped photonic crystal fiber with ideal three-dimensional ordered microstructures was formed. In 2019, their team ⁴⁹ used peacock feathers as inspiration and prepared bio-inspired inverse opal crystal fibers by using scratch-marked silica crystal templates. As shown in Figure 4(d), in summary, the hollow glass capillary is thoroughly cleaned and then inserted into a colloidal solution containing silica microspheres. Due to the capillary force, the colloidal solution automatically dynamically fills the glass capillary. The silica microspheres then self-assemble into photonic crystal structures inside the capillary through solvent evaporation. As shown in Figure 4(e), in 2021, the team led by Mathias Kolle ⁵⁰ used transparent elastic hollow fibers made of the thermoplastic elastomer Daikin T530 to serve as templates. They formed photonic crystal microspheres by emulsifying nanoscale colloid particles suspended in oil-in-water emulsion and then injected the concentrated suspension of photonic microspheres into the interior of the hollow fibers using a syringe needle. This resulted in three-dimensional photonic crystal fibers with reversible color changes in response to mechanical stimuli.

Figure 4.

Preparation of 3D photonic crystal fibers by capillary self-assembly method. (a) Schematic representation of the growth of composite colloid crystals on the capillary inner wall drawn by the sol-gel self-assembly method. Reprinted (adapted) with permission from Haibin et al. ⁴⁶ Copyright (2011) Optica Publishing Group. (b) Schematic representation of colloidal self-assembled opals assembled on fibers in the microspace. Reprinted (adapted) with permission from Liu et al. ⁴⁷ Copyright (2011) Royal Society of Chemistry. (c) Schematic representation of the self-assembly of colloidal crystals with stripe patterns in glass capillaries. Reprinted (adapted) with permission from Zhao et al. ⁴⁸ Copyright (2017) John Wiley and Sons. (d) Schematic diagram of silica self-assembly of microspheres on the surface of glass fiber into structure. Reprinted (adapted) with permission from Gao et al. ⁴⁹ Copyright (2019) Elsevier S.A. (e) The highly charged nanoscale gel colloids self-assemble in the hydrogel matrix to form a non-tightly packed photonic crystal structure. Reprinted (adapted) with permission from Isapour et al. ⁵⁰ Copyright (2021) John Wiley and Sons.

The electrophoretic deposition method is also a common method to prepare three-dimensional photonic crystal fibers. Its main process is to have a similar quality of monodisperse material evenly distributed in the solvent. Under the action of an electric field, the colloidal microspheres in the DC field to the anode (or cathode) form a three-dimensional periodic structure on the fiber surface. ⁴⁴ As shown in Figure 5(b), in 2013, Zhang Keqin’s research group ⁵¹ fixed carbon fibers on electrodes and immersed them in a polystyrene (PS) colloid solution. By applying voltage to the electrode, the colloidal spheres attached and assembled into cylindrical colloidal structures on the carbon fiber surface. By changing the particle size of the PS microspheres, the structural color of the three-dimensional photonic crystal fibers could be modified, and fibers of red, green, and blue colors were assembled using this method. In 2018, Xin Li’s research group ⁵² used wet spinning graphene fibers as templates and immersed them in a PS colloid microsphere solution for electrophoretic deposition, resulting in three-dimensional photonic crystal fibers with few surface defects and uniform thickness. PS microspheres densely arranged in a hexagonal close-packed manner on the graphene fiber surface, and by varying the particle size, fibers with various structural colors were obtained. As shown in Figure 5(a), in 2016, Yao-Gang Li’s research group ⁵³ prepared poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAM-co-AAc) hydrogel microspheres using emulsion polymerization. These microspheres, with solvent-responsive functionality, were electrophoretically deposited on carbon fibers, resulting in three-dimensional photonic crystal fibers with interconnected microporous structures. The ordered and interconnected micropores on the carbon nanotubes (CNTs) enabled efficient diffusion and sensing of vapors. In 2015, Hui-Sheng Peng’s research group ⁵⁴ fabricated three-dimensional photonic crystal fibers by electrophoretic deposition of polymer microspheres (such as polystyrene) onto aligned carbon nanotube (CNT) sheets. These CNTs were wrapped around an elastic polydimethylsiloxane (PDMS) fiber and further embedded in PDMS to form coaxial photonic crystal fibers. Due to the elastic nature of PDMS, the photonic crystal fibers exhibited significant deformation without fracture, enabling mechanical responsiveness. These large deformations caused changes in the inter-particle spacing and structural colors, resulting in mechanically responsive functionality. Although electrophoretic deposition allows for the rapid fabrication of three-dimensional photonic crystal fibers, its methodological characteristics require the fiber templates to be conductive, which imposes certain limitations.

Figure 5.

Preparation of 3D photonic crystal fibers by capillary self-assembly method. (a) Schematic diagram of the electrophoretic deposition device for assembling microspheres onto carbon fibers to form structural color fibers and the prepared structural color carbon fiber. Reprinted (adapted) with permission from Yuan et al. ⁵³ Copyright (2016) Royal Society of Chemistry. (b) Structurally colored fiber was prepared by electrophoresis deposition on the surface of the conductive carbon fiber with the use of polystyrene (PS) nanospheres of different sizes . Reprinted (adapted) with permission from Zhou et al. ⁵¹ Copyright (2013) American Chemical Society.

Convection self-assembly is a process that controls the uniform growth of particle arrays on the air-solution interface of a substrate. In this process, hydrophilic fibers are vertically immersed into a colloidal suspension. Upon drying, the convection effects during stretching and liquid evaporation lead to the self-assembly of microspheres on the fiber surface, forming a particle film. ⁴⁵ This film represents a three-dimensional photonic crystal structure and exhibits vivid structural colors. As shown in Figure 6(a), in 2019, Professor Keqin Zhang’s research group ⁵⁵ utilized a simple method of stretching bare fibers from a colloidal suspension to prepare structural color fibers. Hydrophilic polyethylene terephthalate (PET) fibers were vertically immersed into the colloidal suspension and slowly lifted, and the detachment and evaporation at the fiber tip caused upward convective flow, carrying the particles to the fiber tip. The lateral capillary forces between the particles connected them to each other, forming a closely packed array of particles on the substrate. Furthermore, the experimental parameters such as particle concentration, humidity, and temperature can be appropriately adjusted to achieve control over the colloidal coating on the fiber. As shown in Figure 6(b), in 2016, Professor Huisheng Peng’s research group ⁵⁶ used commercially available black spandex as the fiber template, fixed it to a transmission structure powered by an electric motor, and continuously passed it through a dispersion of polystyrene/poly(methyl methacrylate)/poly(ethyl acrylate) (PS/PMMA/PEA) microspheres with hard cores and soft shells. Through the convection effects during fiber immersion and stretching, the microspheres completed assembly on the spandex fiber surface. By varying the size of the microspheres used, spandex fibers with different structural colors were prepared, which can also be applied to thin films, textiles, and other materials. In 2017, the Kazumasa Hirogaki team ⁵⁷ soaked the fibers in an aqueous suspension of evenly sized silica colloidal particles and then dried them. During drying, the particles self-assembled through lateral capillary forces, ordered along the fiber surface. The colloidal crystal structure of the fibers was controlled in this way. The particles lining the surface become the first layer of the crystal lattice, from which the crystals grow. The crystal structure is controlled by affecting the mobility of the particle through its interaction with the surface. In their study, they not only prepared the 3D photonic crystal structure color fibers, but also studied the influence of hydrophilic and fiber surface charge on the formation of fiber colloidal crystals.

Figure 6.

Preparation of 3D photonic crystal fibers by convective self-assembly. (a) Schematic representation of structural color fibers by extracting naked fibers directly from colloidal suspension. Reprinted (adapted) with permission from Yuan et al. ⁵⁵ Copyright (2019) American Chemical Society. (b) Schematic of the continuous preparation of mechanical discoloration fibers by two motors. Reprinted (adapted) with permission from Zhang et al. ⁵⁶ Copyright (2016) Royal Society of Chemistry.

Electrospinning Method

Electrospinning is a method used to produce nanofibers and is widely employed in the preparation of three-dimensional photonic crystal fibers. This method ejects polymer or other material solution from the tip through a high voltage electric field to form fibers, and then deposits them on the collector to form a fiber film. ³⁶ In the production of three-dimensional photonic crystal fibers, a mixture of monodisperse latex particles and polymer solution is commonly used as the spinning solution, and under the influence of the electric field, it is ejected as a jet. As the solvent continuously evaporates, the internal colloids self-assemble to form an ordered three-dimensional photonic crystal structure. During this process, the polymer acts as a binder, encapsulating the microspheres to form fiber-like structures in the electrostatic field. As shown in Figure 7(a), in 2012, Zhang Keqin’s research group ⁵⁸ electrospun a spinning solution composed of a mixture of polystyrene (PS) microspheres and polyvinyl alcohol (PVA), obtaining various morphologies of electrospun fibers by adjusting the mass ratio of PS microspheres to PVA and the concentration of the PVA solution. As shown in Figure 7(b), in 2015, the same research group ⁵⁹ mixed a colloidal dispersion of 40% polystyrene–methyl methacrylate–acrylic acid [P(St-MMA-AA)] microspheres (1 mL) with a 13% polyvinyl alcohol (PVA) solution to prepare the electrospinning precursor solution. After electrospinning, the colloidal microspheres are arranged locally in a hexagonal pattern on the fiber surface, presenting a white appearance. After removing excess PVA through water treatment, the resulting fibers exhibited structural color due to the photonic band gap formed by the regular arrangement of colloidal microspheres and the Mie scattering caused by the disordered arrangement of colloidal microspheres. As shown in Figure 7(c), in 2018, Geunbae Lim’s team ⁶⁰ used electrospinning and the hydrothermal growth of zinc oxide (ZnO) to prepare structural color fibers. They prepared an oriented seed layer through electrospinning and controlled the hydrothermal growth time to generate various structural colors. The structural colors varied depending on the angle of incident light. When the light was parallel to the aligned nanofibers, no pattern was observed. This phenomenon is referred to as “light-exchange mode.” In 2019, Wang Jingxia’s team ⁶¹ prepared an electrospinning precursor solution by mixing poly(styrene–methyl methacrylate–acrylic acid) (P(St-MMA-AA)) latex particles (40%) and PVA (15%) in a weight ratio of 4:1. After electrospinning, electrospun fibers were obtained, with the soft polyvinyl alcohol acting as a binder to bond the colloidal spheres. The encapsulated colloidal spheres were wrapped and stretched into the fibers. Local hexagonal accumulation of latex particles appeared on the surface of the colloidal fibers. By printing water onto the colloidal fiber membrane, a designed colored pattern could be achieved. The formation of the pattern primarily relied on the morphological transition from fiber aggregation to colloidal aggregation.

Figure 7.

Preparation of 3D photonic crystal fibers by the electrospinning method. (a) Schematic diagram of colloidal electrospinning in the psn and polyvinyl alcohol blend solution. Reprinted (adapted) with permission from Yuan and Zhang. ⁵⁸ Copyright (2012) American Chemical Society. (b) Schematic diagram of the preparation process of colloid electrospinning and the preparation of colored FMs. Reprinted (adapted) with permission from Yuan et al. ⁵⁹ Copyright (2015) American Chemical Society. (c) Schematic diagram of the color manufacturing process of the arranged zinc oxide structure. Reprinted (adapted) with permission from Kim et al. ⁶⁰ Copyright (2018) Springer Nature.

Microfluidic Spinning Method

Microfluidic spinning is a rapidly developing three-dimensional photonic crystal fiber fabrication technology. This technique uses microfluidics in a microfluidic chip to control the flow of liquid in the medium, enabling the preparation of microscale fibers. ³⁵ As shown in Figure 8(a), in 2019, the research group led by Professor Su Chen ⁶² utilized a uniform mixture of viscous polyvinylpyrrolidone (PVP) and monodisperse silica particles to prepare micro-scale fibers. By controlling the rotational speed of the microfluidic spinning machine, colloidal fibers with different diameters can be efficiently produced. Monodisperse silica particles can be assembled into photonic crystal structures by calcination to exhibit vibrant structural colors. The structure color of the three-dimensional photonic crystal fibers can be easily adjusted by changing the size of the silica colloidal microspheres. As shown in Figure 8(b), in 2021, the team led by Chang-Soo Lee ⁶³ generated jets of photocurable dispersions by flowing a dispersion of surface-active agents in a carrier liquid through a microfluidic device. Monodisperse silica particles were dispersed in a resin to form elastic dispersions. Due to the repulsive particle–particle interactions in the resin, the particles self-organized into loosely packed regular arrays, resulting in structural colors. The jets of dispersions were photo-cured in situ by ultraviolet irradiation at the microfluidic channel outlet, continuously producing photon crystal fiber. As shown in Figure 8(c), in 2018, the team led by Michinari Kohri ⁶⁴ prepared structurally colored fibers using microfluidic emulsification and solvent diffusion. An inner aqueous solution composed of polystyrene−divinylbenzene@polydopamine (P(St-DVB)@PDA) core–shell particles (5 wt%) and polyvinyl pyrrolidone (PVP) (5 wt%), and a continuous external fluid (THF), were respectively injected into the inner and outer tubes. The water-to-THF ratio of the core–shell particle and PVP solution was set to 6/1. When the coaxial laminar flows of the two fluids merged at the outlet of the inner tube, a stable W/O jet was formed due to the miscibility between water and THF. Subsequently, water from the inner fluid started diffusing into the THF of the outer fluid. As the core–shell particles of P(St-DVB)@PDA were not dispersed in THF and PVP was insoluble in THF, a fiber-like structured material composed of the core–shell particles and PVP was obtained during the solvent diffusion process.

Figure 8.

Preparation of 3D photonic crystal fibers by the microfluidic spinning method. (a) Schematic representation of the microfluidic spinning process of. Reprinted (adapted) with permission from Li et a.l ⁶² Copyright (2019) Elsevier BV. (b) Schematic and OM image of continuous production of photonic fibers by in situ photopolymerization of the jet in the microfluidic device. Reprinted (adapted) with permission from Kim et al. ⁶⁰ Copyright (2021) John Wiley and Sons. (c) Structural color fiber was prepared by microfluidic emulsion and solvent diffusion. Reprinted (adapted) with permission from Kohri et al. ⁶⁴ Copyright (2018) American Chemical Society.

Extruding and Forming Method

Extrusion molding is a method of creating three-dimensional photonic crystal fibers by dispersing a colloidal microsphere suspension under pressure from a mold or micropores into air or a solvent and solidifying it. As shown in Figure 9(a), In 2011, Jeremy J. Baumberg’s team ³⁸ achieved the self-assembly of polymer particles into a three-dimensional face-centered cubic (fcc) lattice through high-temperature extrusion. As shown in Figure 9(a), the precursor of the core–shell particles had a diameter of approximately 300 nm and consisted of a hard polystyrene (PS) core and a soft polyacrylic acid ester (PEA) shell. The PS core was enveloped by a thin polymer layer containing methyl methacrylate (MMA) as a grafting agent. The miniature extruder (Thermo Scientific, Minilab) comprised two counter-rotating metal screws with adjustable speeds ranging from 1 to 150 revolutions per minute (rpm) and temperature control between 25°C and 250°C. Up to 6 g of Opal precursor material was manually fed into the extruder, where it melted and was homogenized under the extreme shear force provided by the screws. The resulting shear-driven and ordered particle material exceeded the pressure barrier through a narrow-hole stainless steel mold, thus generating three-dimensional photonic crystal fibers. As shown in Figure 9(b), In 2018, A. John Hart’s team ⁶⁵ extruded an aqueous solution of spherical polystyrene particles through a needle for fine position control relative to the substrate in a temperature-controlled environment. The formation of colloidal structures was initiated by creating liquid bridges between a small amount of suspension distributed between the substrate and the needle hole. These liquid bridges provided confinement for the assembly particles deposited at the bottom of the liquid bridge and allowed them to compact into a solid layer while retracting downward on the substrate during particle accumulation. The substrate was further moved downward at a speed that matched the vertical growth rate of the particle structure. This enabled the construction of fiber-like columnar structures with high aspect ratios.

Figure 9.

Preparation of 3D photonic crystal fiber by extrusion molding method. (a) Flow chart of extrusion molding of structural color fiber. Reprinted (adapted) with permission from Han et al. ³⁸ Copyright (2019) John Wiley and Sons. (b) Structural fiber was constructed by precisely dispensing the colloidal solution from a fine needle and then controlling the downward or multiaxial substrate motion. Reprinted (adapted) with permission from Tan et al. ⁶⁵ Copyright (2018) John Wiley and Sons.

Thin Film Tensile Method

The thin film tensile method is a method of preparing 3D photonic crystal fiber by stretching the 3D photonic crystal fiber film with high tensile properties. In 2019, Zhou’s team ³⁸ demonstrated a supramolecular organic gel with a highly tensile supramolecular organogel between silica nanoparticles (SiO₂ NPs), silicone oil and hydrogen bonding polymer gels through an unconventional cascade microphase separation process. The sparse and abundant SiO₂ NP aggregates in the gel matrix with appropriate size, shape and random distribution, as rigid structural color gamut guarantee the color of the entire soft sample under extreme mechanical deformation. In addition, the dynamic hydrogen bonding of the organic gel matrix provides the characteristics of shear thinning and easy to shape for the nanocomposite material. The photonic crystal structure color fiber with low angle dependence can be obtained through the large stretching of the organic gel membrane, and its own structural color will not change during the stretching process.

Functional Applications of Structural Color Fibers

Application in the Textile Field

Structural color fibers have significant prospects in the field of wearable technology. By using structural color fibers, it is possible to produce structurally colored textiles with vibrant colors, which have great potential in the fashion industry. In the study conducted by Wang Jingxia’s research group, ⁶¹ colloidal fiber films were prepared using electrospinning, and water was subsequently jet-printed onto the surface of the fiber films to create structural color patterns. The electro-spun colloidal film can be easily transferred onto any substrate. For example, by transferring the fiber film onto a flexible base material, a green flexible bracelet can be manufactured, and specific characters can be printed on the flexible bracelet. The preparation of various patterns on electro-spun colloidal fibers is of great significance for the development of new wearable optical materials/devices. In the study by Peng Huisheng’s research group, ⁵⁶ photonic crystal fibers were obtained by coating core-shell microspheres with a hard polystyrene/poly(methyl methacrylate) (PS/PMMA) core and a soft poly ethyl acrylate (PEA) shell onto spandex fibers. As shown in Figure 10(b), these fibers can be further designed to achieve complex patterns or woven into products such as bracelets and fabrics. The fabrics produced using this method also have mechanical color-responsive functionality, where the color shifts towards the blue end of the spectrum as the fabric is stretched. The colors are vibrant, making them suitable for application in smart textiles. In addition, as shown in Figure 10(a), their research group ⁵⁴ weaves fabrics and patterns using photonic crystal fibers prepared by electrophoretic deposition. The team led by Jeremy J. Baumberg ⁶⁵ fabricates photonic crystal fibers with high-saturation structural colors, strain-responsive properties, excellent durability, and mechanical performance through extrusion methods. As shown in Figure 10(c), these fibers can be woven into fabrics, and the structural colors change when the woven fabric is stretched. The research group of Zhang Shufen ³⁰ obtains one-dimensional photonic crystal fibers through magnetron sputtering, which can easily achieve a range of structural colors such as blue, green, orange, and magenta in carbon fibers. Impressively, the resulting structural color carbon fibers exhibit excellent mechanical strength and wash durability under repeated friction and washing conditions. Notably, as shown in Figure 10(d), by integrating them with wireless transmission systems, a novel thermal management device based on structural color carbon fibers has been demonstrated. Its temperature can be precisely controlled remotely through Bluetooth using mobile devices, offering broad prospects in colorful smart textiles and personal healthcare.

Figure 10.

Wearability of structural color fibers. (a) Colored patterns and fabric made from elastic light-based microcrystalline fibers. Reprinted (adapted) with permission from Sun et al. ⁵⁴ Copyright (2015) John Wiley and Sons. (b) Structural color fabric and bracelets made of structural color spandex fiber woven. Reprinted (adapted) with permission from Zhang et al. ⁵⁶ Copyright (2016) Royal Society of Chemistry. (c) The cross-linked fiber prepared by the extrusion method can be woven into a structural color fabric. Reprinted (adapted) with permission from Tan et al. ⁶⁵ Copyright (2018) John Wiley and Sons. (d) Application in wireless-operated wearable thermal management devices. Reprinted (adapted) with permission from Zhao et al. ³⁰ Copyright (2021) John Wiley and Sons.

Applications in the Sensing Field

Using the principle of Bragg diffraction in structural color fibers, photon crystal fibers with various functional responses can be prepared by combining photonic crystal materials with flexible fibers or other responsive materials. The main manifestation of these fibers can be observed through the structural color changes of the fibers. By employing different processes and materials, photonic crystal fibers find wide applications in sensing and responsive systems. The visual characteristics of these fibers also indicate promising development prospects.

Currently, the collected sensing applications mainly include light response, temperature response, magnetic field response, humidity response, solvent response, and electrothermal response. Peijun Xu’s team ⁶⁶ deposited graphene oxide on the surface of carbon fibers to form photonic crystal fibers. By studying the responsive properties of the prepared structured color carbon fibers to solvents, temperature, and stress, intelligent responses of colored carbon fibers were achieved. It was found that colored carbon fibers can change color under the stimulation of different solvent vapors, and the color-changing process is reversible. In addition, heating magnesium sulfate heptahydrate (MgSO₄·7 H₂O) under the colored carbon fibers can achieve a cyclic color-changing response. By embedding the structured color carbon fibers into epoxy resin and stretching it at both ends, a “stress-induced color change” response of the structured color carbon fibers can be achieved. Therefore, the color-changing response of this fiber has multiple potential applications in carbon fiber-based temperature, humidity, and strain detectors. Su Chen’s team ⁶² used microfluidics to prepare photonic crystal fibers with functional polyacrylamide (PAM) as the matrix to immobilize colloidal particles, endowing the fibers with humidity-sensing capability. As shown in Figure 11(c), The prepared photonic crystal fibers exhibit significant color changes across the entire visible light range when the environmental humidity is altered, showing potential applications in the field of microsensors. Yao Gang Li’s research group ⁶⁷ utilized magnetic nanoparticles to prepare photonic crystal fibers with magnetic field responsiveness. As shown in Figure 11(a), when a magnetic field is applied to the fibers, strong optical diffraction is immediately observed, leading to color changes. The visible light (from red to blue) can be observed through diffraction in three samples under the magnet, caused by varying sizes of the magnetic nanoparticles (CNPs). As shown in Figure 11(b), Zhongze Gu’s research group ⁴⁸ incorporated responsive graphene hydrogel into the prepared striped photonic crystal fibers. They used parallel near-infrared light with a constant radiation radius to verify its dynamic tuning ability. Reduced graphene oxide (rGO) produces photothermal heating due to its strong near-infrared absorption, and the poly(N-isopropyl acrylamide) (PNIPAM) hydrogel exhibits temperature-responsive swelling and shrinking behavior due to the transition between hydrophilic and hydrophobic states. Combining these two characteristics, the composite PNIPAM/rGO hydrogel is expected to have photothermal response properties. Haibin Ni’s team ⁶⁸ constructed PS colloid monolayer films and silica inverse opal films on the surface of fiber bundles by using vertical self-assembly and sol–gel self-assembly methods. The prepared fibers can be used for relative humidity and optical performance sensors. The fiber color is influenced by relative humidity (RH), and the position of the reflection peak shows a linear relationship with relative humidity.

Figure 11.

The color response properties of structural color fibers. (a) The discoloration response of the structural color fiber under a magnetic field. Reprinted (adapted) with permission from Shang et al. ⁶⁷ Copyright (2016) Academic Press Inc. (b) Schematic diagram of the different changes in the stripe structure color pattern of the structural color hydrogel fiber at different bending angles. Reprinted (adapted) with permission from Zhao et al. ⁴⁸ Copyright (2017) John Wiley and Sons. (c) Schematic representation of the structural color fibers with humidity-sensing capability. Reprinted (adapted) with permission from Li et al. ⁶² Copyright (2019) Elsevier BV.

Application in the Field of Anti-Counterfeiting

Photonic crystal fibers exhibit rainbow colors and unique responsive color-changing functions, which determine their significant development potential in the field of anti-counterfeiting. Wang Jingxia’s team ⁶¹ used inkjet printing of water on electro-spun films to create structural colors that can be carefully designed as QR code patterns. Geunbae Lim’s team ⁶⁰ achieved color variations in structural color patterns prepared by electrospinning, depending on the observing direction and the transmitted light direction. As shown in Figure 12(b), they successfully created an optical switch pattern by aligning the nanofibers in one direction, making the pattern visible only from one side. This method of pattern fabrication will find broad applications in gas sensors and anti-tampering labels. Gu Zhongze’s team ⁴⁸ utilized patterned structural colors made of PNIPAM/rGO hydrogels as dynamic barcode labels for various products. As shown in Figure 12(a), the PNIPAM/rGO hydrogels impart not only simple, uniformly colored stripes but also complex striped patterns with multiple widths and colors. These stripes provide much more intricate information than existing barcodes and increase the counterfeiter’s difficulty in forgery. Interestingly, by integrating near-infrared (NIR) light sources into barcode readers, these structural color stripe patterns exhibit dynamic color shifts under NIR scanning due to their photothermal response. Monochromatic stripe barcodes display a rapid and pronounced blue shift in structural color from red to green during NIR scanning. In the case of composite dual-color stripe barcodes, the red stripes turn blue, and the green stripes become invisible, revealing hidden decoding information with dual changes in color and composition. It is worth mentioning that by combining two separate hydrogel stripes with their respective compositions and colors into a unified barcode, two-dimensional stripe barcodes with unique hidden coding information can also be obtained. These findings demonstrate the tremendous anti-counterfeiting application potential of multifunctional barcode labels with dynamic encoding information and colors.

Figure 12.

Anti-counterfeiting application of structural color fiber. (a) Structural color hydrogel stripes as dynamic barcode labels for the anti-counterfeiting application. Reprinted (adapted) with permission from Zhao et al. ⁴⁸ Copyright (2017) John Wiley and Sons. (b) The influence of the color of the structural pattern with the reflected substrate and the direction of the incident light relative to the arrangement direction of the nanofibers on the pattern visibility. Reprinted (adapted) with permission from Kim et al. ⁶⁰ Copyright (2018) Springer Nature.

Conclusions

In summary, the characteristics of photonic crystal structured color fibers are different according to different structures. Different photonic crystal fibers can be selected under different application conditions, and their special structure and color-changing functions can be more suitable for the field of smart clothing with functions that cannot be produced by traditional chemical dyed fibers. The emergence of photonic crystal structural color fibers can reduce the pollution to the environment caused by fabric dyeing to a certain extent. Through the in-depth research of photonic crystals, it has a role in promoting the green development of the dyeing industry. At present, research on the structural color of photonic crystals gives it an excellent role in the field of textiles, sensing, and anti-counterfeiting. In the subsequent process of combining textile products, it is believed that it will develop more properties and functions. Whether it is about intelligent manufacturing or environmental protection, follow-up development and research on photonic crystal fibers are very necessary.

With the progress of science and technology, the preparation method of structural color fiber is also becoming rich, and it is developing in the direction of low cost, high quality and easy operation, but the development of photonic crystal fiber is still in its infancy, and it still has a long way to go until it is fully put into commercial use. In the follow-up research, we believe that the following aspects should be studied in depth:

1. In the self-assembly method, explore a better way to combine the nanospheres with the fiber, and it is best to make it widely used in various fibers, and improve the mechanical stability of the fiber structure color while maintaining the original characteristics of the fiber in the finishing.

2. Explore simpler and large-scale ways to produce structural color fibers to improve the feasibility of their commercial use.

3. Continue to further develop its color-changing response function, improve its sensitivity, and explore more color-changing response methods, so that it can be widely used in the field of sensing. With the continuous improvement of these functions, the practicability of photonic crystal structured color fibers will be greatly improved.