Artificial opal films with switchable colors

Optical sensors for mechanical stress, chemical load, or biological exposure are used for the early detection of damage, for status monitoring, and to measure changes in the environment. Conventional piezoelectric, fiber-optic, or electrochemical sensors are predominantly used for important applications such as these. However, sensor devices remain relatively complex and are therefore expensive to manufacture. More convenient methods of fabrication could enhance and accelerate detection while eliminating the requirement for sophisticated equipment and time-consuming evaluation.

Photonic bandgap materials have attracted a great deal of attention as potential candidates for a variety of applications, including actuation systems and optical sensing.^1,2 Artificial opals are particularly popular in optical materials research due to their brilliant iridescent colors and inexpensive, bottom-up fabrication. Several techniques have been developed to build artificial opals from simple polymer architectures. Like natural opal gems, these materials feature a highly ordered 3D structure of close-packed, submicroscopic spheres. By altering the angle of incident light, every color in the visible spectrum can be expressed in the same sample—see Figure 1(a)—due to interfering light reflections within the underlying 3D structure: see Figure 1(b).³ Remarkably, these 3D opaline structures also feature stimuli responsiveness. Optical properties may be altered by external triggers, such as a change of temperature, light, ionic strength, electric field, mechanical stress, or solvent.^1,3 We have already demonstrated the reproducible scaling of synthetic opals to industrial lengths. Our approach can be used as a toolkit for tailoring a range of low-cost polymer-based soft sensors that provide useful optical responses.

Figure 1. (a) Photographs of polymer opal film reflecting different colors when tilted, due to interfering light reflections corresponding to (b) Bragg's law for opaline materials. When white light is shone on an opal, the reflected wavelength depends on the angle of the refracted light (δ) and the lattice plane spacing (a_hkl). Since the light is refracted in the transition from one medium (air) to an optically denser medium (opal), the angle of the incident light (θ) changes after refraction to angle δ. (c) Transmission electron microscopy (TEM) images of an ultrathin section of an elastomeric polymer opal film prepared by a melt shear process, demonstrating the hexagonally arranged close-packed polymer spheres.

In elastomeric polymer opal films, submicroscopic polymer spheres are arranged in a 3D crystal lattice embedded in a soft polymer matrix: see Figure 1(c). If the distance between spheres is in the region of the wavelength of visible light, intense colors are reflected due to the scattering effects described by Bragg's law: see Figure 1(b). Opal films such as these can be prepared by a melt-shear process of uniform all-organic polymer core-shell (CS) spheres, which consist of a hard polystyrene core and a soft, elastomeric polyacrylate shell.⁴ In this process, the soft shells are melted at an elevated temperature. When pressure is applied, the rigid cores in the flowing melt arrange themselves into a close-packed structure embedded in a soft polyacrylate matrix. In contrast to other methods of self-organization, large-area and crack-free films can be produced. Opal films on which we performed a subsequent cross-linking process behave like rubber and exhibit a reversible color change when mechanical stress is applied: see Figure 2(a).⁵ This strain-dependent color variation could find application in efficient mechanical sensors.

Figure 2. (a) Photographs of a cross-linked elastomeric polymer opal film exhibiting a reversible color change under the application of mechanical stress. (b) Optical and TEM images of an elastomeric polymer opal film after the pressure-induced deformation of embedded soft spheres, which results in a change to the reflected color. The material can undergo temperature-induced recovery of its initial spherical shape and original optical properties. (c) Photographs of a patterned elastomeric polymer opal film featuring a light- and temperature-sensitive fluorescent dye. The effects of light-induced activation and temperature-induced reversible erasing of the fluorescent pattern are shown. (d) Optical image of an elastomeric polymer opal-paper composite film after solvent-induced swelling.

Our recent synthetic efforts have enabled the fabrication of stimuli-responsive opal films on a large scale. Pressure-induced deformation of embedded soft spheres causes the formation of anisotropic oblate spheroids: see Figure 2(b).⁶ Since these flattened spheres still show long-range 3D order, they feature remarkable optical properties with reflected colors undergoing a large wavelength shift. In addition to this, thermal treatment of these opal films leads to a full recovery of the initial spherical shape and optical properties, making them suitable for deformation sensor applications.

By incorporating light- and temperature-sensitive fluorescent dye in either the core or the shell of the polymer spheres, we were able to combine the reversible mechanochromic behavior of elastomeric opal films with light and temperature responsiveness: see Figure 2(c).⁷ The encased fluorescent dye can be activated and deactivated by irradiation and thermal treatment, respectively. Due to its photochromic and thermochromic behavior, this material may be suitable for 3D rewritable optical data storage.

We have recently developed a new strategy for fabricating solvent-responsive elastomeric opal films as potential candidates for a wide range of optical sensor applications: see Figure 2(d).⁸ Shearing the melt of CS spheres on top of a highly porous paper sheet and following this with a chemical cross-linking reaction leads to the creation of polymer-paper composite films with high tensile strength. Due to the high porosity of the paper-based sheets used, these composites swell in the presence of various solvents. This swelling causes a shift in the wavelength of the reflected colors. After solvent evaporation, the original opal structure can be recovered, causing the reflected color to shift back to its initial state. These approaches enhance the usability of all-organic soft sensors.

In summary, we were able to switch the optical properties of our elastomeric polymer opals using various external triggers. Our work provides a useful toolkit for a variety of polymer opal-based optical sensor devices. In future work, we will focus on the synthesis of biologically, electrochemically, and magnetically responsive polymer spheres and inorganic nanoparticles. This could extend the range of potential applications to nanophotonics, nanoelectronics, plasmonics, and metamaterials.

The authors would like to thank the Landesoffensive zur Entwicklung Wissenschaftlich-ökonomischer Exzellenz (LOEWE Soft Control) for the financial support of this work.