Multiresponsive Inverse‐Opal Hydrogels

Photonic crystals have been extensively studied over the last several years because of their potential applications in optical communication. Many methods, including templating methods, have been used to fabricate photonic crystals, of which colloidal crystals are especially important because they offer a wide range of easy ways to create various photonic crystals with opal or inverse-opal structure by selecting different infiltration methods, such as sol–gel processes, chemical vapor deposition (CVD), electrochemical processes, chemical bath deposition, and atomic layer deposition. At present, photonic crystals of various types of materials have been synthesized, including semiconductors, metals, polymers and hydrogels. Several assembling methods have been utilized for forming colloidal crystals, but the vertical deposition–evaporation method is most often reported because of its easy operation and inexpensive cost. Recently, much attention has been paid to responsive photonic crystals (or sensitive photonic crystals) whose diffraction wavelength (location of the photonic bandgap, PBG) shifts as the external ambient is changed. Photonic crystals can exhibit brilliant color called structural color due to Bragg diffraction if the PBG is located in the visible-light region. The structural color depends on the PBG that is determined by the material and crystal lattice. Thus, a structure’s color naturally offers a signal of the lattice structure of the photonic crystals for specific materials. Many polymer hydrogels can change volume reversibly in response to external conditions, such as solvent, temperature, pH and ionic strength, and biomolecules. This makes hydrogels excellent candidate materials for optically based sensors when they are coupled to an appropriate signal transduction mechanism, such as structure color or optical diffraction. Up to now, there are three types of responsive photonic crystals based on hydrogels. One is the pioneering work of Asher and co-workers who created functional periodic hydrogel structures through polymerization inside colloidal crystals, called polymerized colloidal crystal arrays (PCCAs). They and other groups have demonstrated reversible diffraction shifts of PCCAs due to stimuli such as mechanical force, metal ions, pH and ionic strength, and glucose. Another type is opals from microgels such as poly(N-isopropylacrylamide) (PNIPAM) microgel. These microgel opals displayed thermally induced shifts in optical diffraction due to lower critical solution temperature (LCST) behavior. The last one is inverse-opal hydrogel (IOH). Takeoka formed an IOH of PNIPAM that showed a reversible diffraction shift versus temperature. Lee synthesized mechanically robust IOH based on copolymers that exhibited optical diffraction sensitive to pH, ionic strength, and glucose. Barry recently created IOH of polyacrylamide (PAM), which responded to relative humidity by causing a rapid shift in the reflectance spectra due to interlayer swelling and shrinking of the structure in some specific conditions where other sensors may not be suited. As for the designs of sensors, a rapid response to the external environment and good mechanical stability are the main factors. PCCA is easily prepared and shows a high diffraction intensity. However, it is very fragile and unstable owing to the non-closest-packing crystal structure, and low polymer content hydrogels allow rapid diffusion of analyte. The IOH based on the closest-packing crystal has a rapid response and good mechanical stability. The rapid change in structural colors and diffraction wavelength responding to environment variations offers quickly synchronized signals, and thus the material has a potential application as a sensor. In this report, we fabricated two kinds of IOH through the colloidal crystal templating method. They respond to various external stimuli such as solvent, pH, L-lysine, and mechanical pressing. In addition to the advantages of rapid response and good mechanical stability, this IOH exhibits brilliant structural colors that can be observed easily by the naked eye. The polystyrene (PS) opal templates were obtained by the vertical deposition–evaporation method on a treated glass slide. An IOH of PAM or PAM/PAA (polyacrylic acid) hydrogel was prepared by using a capillary-attraction-induced method created in our lab as shown in Figure 1 (see the Experimental section). This method can lower infiltration defects and avoid the collapse of the PS opal template effectively, so that a large IOH thin film of at least 1 cm can be prepared. Such a size is large enough for applications such as sensing and detecting. The PS opal template has a bright blue color. It comes from the well-ordered face-centered cubic structure and uniform C O M M U N IC A IO N