Multiresponsive Inverse‐Opal Hydrogels

Abstract

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, [1][2][3][4][5][6][7] metals, [8][9][10] polymers [11][12][13][14][15][16][17][18][19] and hydrogels. [20][21][22][23] 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. [24][25][26][27][28][29] 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, [30,31] temperature, [32,33] pH and ionic strength, [34] and biomolecules. [35][36][37][38] 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).[20] They and other groups have demonstrated reversible diffraction shifts of PCCAs due to stimuli such as mechanical force, [20,39] metal ions, [21,22,40] pH and ionic strength, [23] and glucose. [22,41,42] Another type is opals from microgels such as poly(N-isopropylacrylamide) (PNIPAM) microgel. [43][44][45][46] 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. [47,48] Barry recen...

Tunable Colors in Opals and Inverse Opal Photonic Crystals

“…[ 93 ] Wang et al synthesized an inverse opal hydrogel that responded to ethanol/water mixtures, pH, L -lysine and, like most of these soft photonic crystals, to mechanical pressure. [ 94 ] A PS opal template was infi ltrated with polyacrylamide (PAM) and polyacrylamide/polyacrylic acid (PAM/PAA). The fraction of polyacrylic acid determined the extent of the blue shift of the refl ection peak relative to the inverse opal containing only PAM, making it possible to choose an initial color for the material.…”

Facile fabrication of free-standing colloidal-crystal films by interfacial self-assembly

“…So, in order to fabricate free-standing colloidal crystals, the interaction between the microspheres must be designed to be further strengthened. By infiltration with polymerizable monomers into the voids of the pre-prepared colloidal crystal, free-standing colloidal-crystal films with functional properties have been reported [22][23][24][25]. Using an organic molecular as a cross-linking agent, Hu et al [26] obtained hydrogel opal with the mechanical and thermal stability via covalently bonding selfassembled hydrogel nanoparticles.…”

Tunable multicolor pattern and stop-band shift based on inverse opal hydrogel heterostructure

“…Interference of the light waves scattered from the periodical photonic crystal structure results in the appearance of a photonic stop band that forbids propagation of certain wavelength of light. To date, photonic crystals with tunable structural colors are widely studied, the colors of which can be tuned by controlling various external stimuli, such as temperature [8][9][10][11][12][13], humidity [14,15], mechanical stress [16][17][18], solvent [19][20][21], pH [22,23], ions [24,25], light [26,27], electric/magnetic field [28][29][30][31] and other biological species [32,33] for various specific applications. Of great importance is that the structural colors are generated by the structural properties, not by selective absorption or reflection of light by pigments.…”