Responsive Photonic Crystals

Porous Hydrogel-Encapsulated Photonic Barcodes for Multiplex Detection of Cardiovascular Biomarkers

Early detection of cardiac troponin I (cTnI), B-type natriuretic peptide (BNP), and myoglobin (Myo) is essential for the diagnosis of acute myocardial infarction (AMI) and heart failure (HF). We designed a porous hydrogel-encapsulated photonic crystal (PhC) barcode-based suspension array for multiple cardiovascular marker detection. The hybrid hydrogel was composed of polyethylene glycol diacrylate (PEGDA) and gelatin, resulting in a porous and hydrophilic scaffold which ensured stability of the PhC in aqueous solutions. The encapsulated PhC barcodes had stable diffraction peaks for the corresponding markers. Using a sandwich format, the proposed suspension array was used for simultaneous multiplex detection of cardiovascular biomarkers in a single tube. The immunoassay results we tested on cTnI, BNP, and Myo could be assayed in the ranges of 0.01 to 1000 ng/mL, 0.1 to 10 000 pg/mL, and 1 to 10 000 ng/mL with limits of detection of 0.009 ng/mL, 0.084 pg/mL, and 0.68 ng/mL at 3σ, respectively. This method also showed acceptable accuracy and repeated detection, and the results were consistent with the results of conventional clinical methods for detecting actual clinical samples. Therefore, suspension arrays based on hydrogel-encapsulated PhC barcodes are highly promising for AMI diagnosis.

Flory-Huggins Photonic Sensors for the Optical Assessment of Molecular Diffusion Coefficients in Polymers

The lack of cost-effective systems for the assessment of air pollutants is a concern for health and safety in urban and industrial areas. The use of polymer thin films as label-free colorimetric sensors featuring specific interactions with pollutants would then represent a paradigm shift in environmental monitoring and packaging technologies, allowing to assess air quality, formation of byproducts in closed environment, and the barrier properties of the polymers. To this end, all-polymer distributed Bragg reflectors represent a promising approach toward a reliable and cost-effective transduction of chemical stimuli and effective colorimetric label-free selective detectors. We show selectivity attained by specific interactions between the polymer and analytes. Such interactions drive the analyte intercalation through the polymer structure and its kinetics, converting it in a dynamic optical response which is at the basis of the Flory-Huggins photonic sensors. The multivariate analyses of the response kinetics also allow distinguishing binary mixtures. Additionally, we demonstrate that such optical responses can be used to esteem the diffusion coefficients of small molecules within polymer media via simple UV-vis spectroscopy retrieving data comparable to those obtained with state-of-the-art gravimetric procedures. Last, we assess the figures of merit of the sensors in terms of lower detection limit, sensitivity, and reversibility, demonstrating that such devices can pave the way to an innovative, simple, and low-cost detection method integrable to in situ assessment of barrier polymers used for the encapsulation of optoelectronic devices, food packaging, and goods storage in general.

Pressure-Responsive Hierarchical Chiral Photonic Aerogels

Pressure-responsive chiral photonic aerogels are fabricated by combining liquid crystal self-assembly and ice-templating processes. The aerogels have a hierarchical structure in which the primary 2D chiral nematic structured walls of cellulose nanocrystals form ribbons that support a secondary 3D cellular network. Owing to the flexibility of the aerogels in solvent, the 3D structure of the aerogel can easily be transformed to a 2D structure by pressure-induced rearrangement. The aerogels vary from white in color, which arises from light scattering, to a reflective photonic crystal displaying bright iridescent colors that depend on the immersed solvent. A solvent-sensitive ink that shows quick color response to different solvents is designed using the pressure-responsive photonic aerogel. This material demonstrates a new response mechanism for the design of smart and mechanoresponsive photonic materials.

Gel-Based Artificial Photonic Skin to Sense a Gentle Touch by Reflection

This work demonstrates that engineering a three-dimensional photonic crystal (3DPC) structure in a highly flexible gel is a potential method to achieve flexible tactile artificial photonic skin (p-skin) for future visible-light communication (VLC). We investigated the photonic output modes of 3DPC-coated gel-based pressure sensors and explored their ability to sense low pressures (<10 kPa) through reflection. Such sensors with high sensitivity, fast response, and adjustable detection range can be fabricated in arrays of dots covering large, complex/uneven surfaces and are promising in the development of stimuli-responsive soft materials for future artificial intelligence, health monitoring, and photonic communication systems.

Ellipsoidal Artificial Melanin Particles as Building Blocks for Biomimetic Structural Coloration

Inspired by the structural coloration of anisotropic materials in nature, we demonstrate the preparation of structural color materials by the assembly of anisotropic particles. Spherical artificial melanin particles consisting of a polystyrene core and polydopamine shell were stretched asymmetrically to form uniform-sized ellipsoidal particles with different aspect ratios. The aspect ratio and assembly method of the ellipsoidal particles influence the structural coloration, indicating that the particle shape is one of the important parameters for controlling the structural coloration. The discovery of a method to control the structural color using ellipsoidal particles is useful in basic research on structural colors in nature and provides flexibility in material design and extends the application range of structural color materials.

Magnetically Actuated Dynamic Iridescence Inspired by the Neon Tetra

Inspired by the tropical fish neon tetra, we report a mechanism to achieve dynamic iridescence that can be magnetically tuned. This approach is based on the tilting of periodic photonic nanostructures, as opposed to the more common strain-induced color tuning. In this method, a periodic array of magnetic nanopillars serves as a template to guide the assembly of iron oxide nanoparticles when magnetized in a liquid environment. The periodic local fields induced by the magnetic template anchor the assembled particle columns, allowing the structure to tilt about the base when the angle of the applied field is changed. This effect emulates a microscopic "Venetian blind" and results in dynamic optical properties through structural coloration that is tunable in real time. The fabricated prototype demonstrates tunable reflectance spectra with peak wavelength shift from 528 to 720 nm. The magnetic actuation mechanism is reversible and has a fast response time around 0.3 s. This structure can be implemented on an arbitrary surface as dynamic camouflage, iridescent display, and tunable photonic elements, as well as in other applications such as active fluidic devices and particle manipulation.

Rapid Fabrication of Noniridescent Structural Color Coatings with High Color Visibility, Good Structural Stability, and Self-Healing Properties

Artificial construction of amorphous photonic structures (APSs) is an important approach for obtaining noniridescent structural colors and shows a great potential for practical applications in paints, textile coloring, display, or other color-related fields. However, the structural colors are usually dim because of the influence of incoherent scattering, and the point contact among the microspheres leads to poor structural stability. This paper presents an innovative strategy for constructing noniridescent structural color coatings with high color visibility, good structural stability, and self-healing properties by combining APSs with polymers. Color visibility is significantly improved without the addition of black light-absorbing substances because of the inherent properties of polysulfide microspheres. At the same time, the introduction of waterborne polyurea in the system enhanced the structural stability and imparted the self-healing properties. The prepared coatings can be applied to various substrates and even to the coloration of soft fabrics, which not only achieves excellent performance but can also be easily patterned on the bulk scale.

Stimuli-Responsive Optical Nanomaterials

Responsive optical nanomaterials that can sense and translate various external stimuli into optical signals, in the forms of observable changes in appearance and variations in spectral line shapes, are among the most active research topics in nanooptics. They are intensively exploited within the regimes of the four classic optical phenomena-diffraction in photonic crystals, absorption of plasmonic nanostructures, as well as color-switching systems, refraction of assembled birefringent nanostructures, and emission of photoluminescent nanomaterials and molecules. Herein, a comprehensive review of these research activities regarding the fundamental principles and practical strategies is provided. Starting with an overview of their substantial developments during the latest three decades, each subtopic discussion is led with fundamental theories that delineate the correlation between nanostructures and optical properties and the delicate research strategies are elaborated with specific attention focused on working principles and optical performances. The unique advantages and inherent limitations of each responsive optical nanoscale platform are summarized, accompanied by empirical criteria that should be met and perspectives on research opportunities where the developments of next-generation responsive optical nanomaterials might be directed.

Designing Responsive Photonic Crystal Patterns by Using Laser Engraving

Soft photonic crystals are periodic nanostructures that have attracted much attention for their applications in sensors, owing to their tunable structural colors in response to external stimuli. Patterned photonic crystals provide a novel strategy for constructing high-performance photonic materials with unique structures and functions. In this work, laser engraving is used for the first time to design patterns on a layered photonic hydrogel. This approach is based on the integration of laser power and chemical modifications to embed different polymer composites (polyelectrolyte and neutral polymers) along a prescribed laser-printed path. The polyelectrolyte and neutral composites show differential swelling or shrinking, causing a mechanical instability in the layered hydrogel. The resultant soft polymeric materials appear as synchronous tuning in the photonic band gaps in response to external stimuli. This approach is favorable for designing responsive photonic crystals with controllable optical properties and 3D shape transformation. Moreover, it is of great use in developing advanced photonic crystals for applications in sensors, soft actuators, and drug release.

Hollow Colloid Assembled Photonic Crystal Clusters as Suspension Barcodes for Multiplex Bioassays

Barcode particles have a demonstrated value for multiplexed high-throughput bioassays. Here, a novel photonic crystal (PhC) barcode is presented that consists of hollow colloidal nanospheres assembled through microfluidic droplet templates. Due to their gas-filled core, the resultant barcode particles not only show increased refractive index contrast, but also remain in suspension by adjusting the overall density of the PhC to match that of a detection solution. In addition, magnetic nanoparticles can be integrated to give the barcodes a magnetically controllable motion ability. The encoding ability of the barcodes is demonstrated in microRNA detection with high specificity and sensitivity, and the excellent features of the barcodes make them potentially very useful for biomedical applications.

A Versatile Strategy for Transparent Stimuli-Responsive Interference Coloration

The bioinspired stimuli-responsive structural coloration offers a wide variety of potential applications, ranging from sensing to camouflage to intelligent textiles. Because of its design simplicity, which does not require multilayers of materials with alternative refractive indices or micro- and nanostructures, thin film interference represents a promising solution toward scalable and affordable manufacturing of high-quality responsive structural coloration systems. However, thin films of polymers with appropriate thickness generally do not exhibit visible structural colors if they are directly deposited on transparent substrates such as glass. In this work, a versatile new strategy that enables transparent stimuli-responsive interference coloration (RIC) in the polymer-metal-substrate system is presented. The key concept is to use an ultrathin metal layer as an optical filter instead of high refractive index substrate or highly reflective substrate. Such an optical filter layer allows tuning of the degree of transparency, the constructive interference reflection light, and complementary destructive interference transmission light via changing the metal layer thickness. Real-time, continuous, colorimetric RIC sensors for humidity, organic vapor, and temperature are demonstrated by using different stimuli-responsive polymers. The transparent RIC film on glass shows strong coupling of constructive interference reflected colors and complementary destructive interference transmitted colors on opposite sides of the film. Such transparent RIC film allows for the proof-of-concept demonstration of a self-reporting, humidity-sensing window.

Cardiomyocytes-Actuated Morpho Butterfly Wings

Morpho butterflies are famous for their wings' brilliant structural colors arising from periodic nanostructures, which show great potential value for fundamental research and practical applications. Here, a novel cellular mechanical visualizable biosensor formed by assembling engineered cardiac tissues on the Morpho butterfly wings is presented. The assembled cardiomyocytes benefit from the periodic parallel nanoridges of the wings and can recover their autonomic beating ability with guided cellular orientation and good contraction performance. As the beating processes are accompanied by the cardiomyocytes' elongation and contraction, the elastic butterfly wing substrate undergoes the same cycle of deformations, which causes corresponding synchronous shifts in their structural colors and photonic bandgaps for self-reporting of the cell mechanics. It is demonstrated that this self-reporting performance can be further improved by adding oriented carbon nanotubes in the nanoridges of the wings for the culture. In addition, taking advantage of the similar size of the cardiomyocyte and a single Morpho wing scale, the investigation of single-cell-level mechanics can be realized by detecting the optical performance of a single scale. These remarkable properties make these butterfly wings ideal platforms for biomedical research.

Metallosupramolecular Photonic Elastomers with Self-Healing Capability and Angle-Independent Color

Photonic elastomers that can change colors like a chameleon have shown great promise in various applications. However, it still remains a challenge to produce artificial photonic elastomers with desired optical and mechanical properties. Here, the generation of metallosupramolecular polymer-based photonic elastomers with tunable mechanical strength, angle-independent structural color, and self-healing capability is reported. The photonic elastomers are prepared by incorporating isotropically arranged monodispersed SiO₂ nanoparticles within a supramolecular elastomeric matrix based on metal coordination interaction between amino-terminated poly(dimethylsiloxane) and cerium trichloride. The photonic elastomers exhibit angle-independent structural colors, while Young's modulus and elongation at break of the as-formed photonic elastomers reach 0.24 MPa and 150%, respectively. The superior elasticity of photonic elastomers enables their chameleon-skin-like mechanochromic capability. Moreover, the photonic elastomers are capable of healing scratches or cuts to ensure sustainable optical and mechanical properties, which is crucial to their applications in wearable devices, optical coating, and visualized force sensing.

A Two-Step Strategy for Fabrication of Biocompatible 3D Magnetically Responsive Photonic Crystals

Extremely stable and biocompatible 3D magnetically responsive photonic crystals (MRPCs) are successfully prepared in aqueous solution. Classic hydrothermal synthesis was applied for preparation of the Fe₃O₄@C core. Modified Stöber method was then employed for synthesis of the different size of Fe₃O₄@C@SiO₂. Unlike the traditional magnetic nanoparticles, the highly negative charged superparamagnetic nanospheres (SMNs), i.e., the double-shell structure Fe₃O₄@C@SiO₂ are capable of rapidly self-assembling into 3D MRPCs with full visible and various colors that can be periodically and reversibly tuned under different kinds of external magnetic fields (EMFs) within 1 s. The assembling behavior and mechanism of the 3D MRPCs under EMF were monitored and analyzed. The preparation is simple and the size of the SMN is easily controllable by adjusting the amount of catalyst. Compared with the previous works, the synthesized 3D MRPCs are hydrophilic, and exhibit extremely high stability after 6-month storage. To conclude, our study provides an effective two-step strategy for fabrication of biocompatible 3D MRPCs and it reveals great potentials in biological fields.

2D Au Nanosphere Arrays/PVA-PBA-Modified-Hydrogel Composite Film for Glucose Detection with Strong Diffraction Intensity and Linear Response

A novel glucose sensor was reported that consisted of two-dimensional (2D) Au nanosphere arrays and glucose-responsive hydrogel film. This sensor exhibited an intense diffraction signal and an obvious diffraction color on a quartz slide due to the strong diffraction intensity of the Au nanosphere arrays. Thus, glucose was detected via the variation of diffraction wavelength and diffraction color, without a high reflective mirror. In addition, by introducing poly(vinyl alcohol) (PVA) to crosslink the phenylboronic acid (PBA)-modified hydrogel film, the diffraction wavelength of the 2D Au nanosphere arrays/hydrogel composite film shifted in the same direction in high ionic strength condition. In particular, it showed a nearly linear red-shift when the glucose concentration increased from 0 mM to 20 mM. Moreover, this glucose sensor displayed good reproducibility. The nearly linear response and good reproducibility were highly helpful for improving practical application of this glucose sensor.

Ultrafast assembly of nanoparticles to form smart polymeric photonic crystal films: a new platform for quick detection of solution compositions

Photonic crystals (PCs) are an important subset of photonic materials with specific optical properties, which can be utilized for structural color printing, anti-counterfeiting technologies, chemical sensors and so on. However, the fabrication of scalable, high-quality and uniform photonic crystal films at room temperature still remains a big challenge. Herein, a fast, energy efficient and scalable process is reported for the first time. A high-quality polymeric photonic crystal film can be fabricated from the uniform core/shell particle slurry within several seconds by a calendering process. The obtained crystalline structure can be rapidly captured by photo-curing, and the resultant PC films show extremely strong iridescent tunable structural colors. Because the as-designed PC film matrix is sensitive to solutions with different solubility parameters, a prototype demo sensor is firstly set up for quick detection of the composition of the alcohol/H2O mixture as a model of white spirits, which has the feature of reversible and linear quantitative sensing performance. In addition, the as-prepared PC film is further developed as an inexpensive test strip showing quick detection of ethanol/octane mixtures (possessing different solubility parameters) as a model of ethanol gasoline. This facile, scalable and energy efficient fabrication procedure is exceedingly promising for high-throughput production, showing great potential for industrialization of PC sensors and detectors. The combination of uniform particles and a dispersion medium can be potentially designed for different stimuli responsive systems, which is beneficial for applications ranging from sensing, anti-counterfeiting, to some special optical devices.

Reversibly photoswitchable alkoxy azobenzenes connected benzenetricarboxamide discotic liquid crystals with perpetual long range columnar assembly

Reversibly photoswitchable discotic liquid crystals (DLCs) with no change in columnar assembly.

Synthesis of monodisperse ferromagnetic CoxFe3−xO4 colloidal particles with magnetically tunable optical properties

Monodisperse Co_xFe_3–xO₄ colloidal particles with uniform size and tunable composition have been prepared using a one-step hydrothermal method.

Electroactive Soft Photonic Devices for the Synesthetic Perception of Color and Sound

Color, as perceived through the eye, transcends mere information in the visible range of electromagnetism and serves as an agent for communication and entertainment. Mechanochromic systems have thus far only aimed at satisfying the sense of vision and have overlooked the possibility of generating acoustic vibrations in concert with their visual color responses that would enable the simultaneous satisfaction of the auditory system. Transcending the boundaries of the two senses (i.e., sound and color), herein a strategy for their concurrent and synesthetic fulfillment is elucidated by electrically actuating an organogel photonic device, controlled by a single input signal. This new class of devices that integrate a color module with a speaker is fabricated from a mechanochromic layer that comprises close-packed photonic lattice with an organogel matrix pervading the void fraction. Exploiting a dielectric elastomer actuator, the system's mechanical response permits the simultaneous, yet independent, exploration of visible-light reflection alongside audible sound-wave generation. Large areal strains at low frequencies of actuation tune the photonic stop-band, whereas the layer remains incompressible and exhibits negligible strain when actuated at higher frequencies (e.g., tens of Hz), thereby making it amenable to modulate sound and color simultaneously yet independently.

Handwritable one-dimensional photonic crystals prepared from dendronized brush block copolymers

Second generation dendrons with six long alkyl groups and six benzyl groups as peripheral groups were modified into cis-5-norbornene-exo-2,3-dicarboxylicanhydride to prepare dendronized monomers.

Environmentally responsive photonic polymers

This feature article focuses on photonic polymers that change colouration due to an environmental stimulus and highlights their industrial feasibility.

Flexible construction of cellulose photonic crystal optical sensing nano-materials detecting organic solvents

We developed a simple and efficient method to construct 3D and 2D opal and inverse opal cellulose photonic crystal films (CPCF) by embedding 3D or 2D polymethyl methacrylate (PMMA) colloidal arrays into carboxymethyl cellulose (CMC), respectively.

Magneto-thermochromic coupling Janus sphere for dual response display

This work demonstrates a simple microfluidic device to synthesize a magneto-thermochromic sphere with Janus inner structure. The Janus sphere is composed of Fe₃O₄ microspheres, thermochromic particles, and polyacrylamide matrix. Because the Fe₃O₄ microspheres are assembled together in one pole, the Janus sphere can turn around by varying the direction of the external magnetic field. Originating from the temperature-dependent property of the thermochromic particles, the final Janus sphere can change its color from red to pale blue when the temperature is increased from 5 to 45 °C. The detailed formation process and the magneto-thermochromic mechanism are carefully investigated. Due to the magnetic switch and thermochromism, these Janus spheres can be applied as colorful displays by controlling the magnetic field and temperature. The results demonstrate that the dual responsive Janus spheres possess broad application potential in temperature sensors and displays.

This work demonstrates a simple microfluidic device to synthesize a magneto-thermochromic sphere with Janus inner structure.

Bio-inspired sensing and actuating materials

Biological systems contain various amazing examples that can display adaptive and active behaviors in response to external stimuli.

Robust magnetic double-network hydrogels with self-healing, MR imaging, cytocompatibility and 3D printability

Herein, we have fabricated a novel robust self-healing magnetic double-network hydrogel by multiple interactions between bondable magnetic Fe3O4 and chitosan-polyolefin matrix, and the hydrogel also exhibits an excellent magnetogenic effect and MR imageability. The practical potential of the magnetic double-network hydrogel is further revealed by its 3D-printing performance.

Designs of Biomaterials and Microenvironments for Neuroengineering

Recent clinical research on neuroengineering is primarily focused on biocompatible materials, which can be used to provide electroactive and topological cues, regulate the microenvironment, and perform other functions. Novel biomaterials for neuroengineering have been received much attention in the field of research, including graphene, photonic crystals, and organ-on-a-chip. Graphene, which has the advantage of high mechanical strength and chemical stability with the unique electrochemical performance for electrical signal detection and transmission, has significant potential as a conductive scaffolding in the field of medicine. Photonic crystal materials, known as a novel concept in nerve substrates, have provided a new avenue for neuroengineering research because of their unique ordered structure and spectral attributes. The "organ-on-a-chip" systems have shown significant prospects for the developments of the solutions to nerve regeneration by mimicking the microenvironment of nerve tissue. This paper presents a review of current progress in the designs of biomaterials and microenvironments and provides case studies in developing nerve system stents upon these biomaterials. In addition, we compose a conductive patterned compounded biomaterial, which could mimic neuronal microenvironment for neuroengineering by concentrating the advantage of such biomaterials.

Effects of Microsphere Size on the Mechanical Properties of Photonic Crystals

Photonic crystal (PC) thin films that are self-assembled from different-sized silica microspheres were prepared for studying mechanical properties via nanoindentation at the submicron scale. We found that the silica photonic crystals (PCs) possessed a face-centered cubic (FCC) microstructure and their elastic modulus and hardness were in the range of ~1.81–4.92 GPa and 0.008–0.033 GPa, respectively. The calculated results proved that there were size-dependent properties in the silica PCs, in that the elastic modulus and hardness increased as the diameter decreased from 538 nm to 326 nm. After studying the total work and plastic work in the progressive deformation of silica PCs during the nanoindentation tests, we developed a two-stage deformation model to explain how the microsphere size affects the mechanical properties of PC thin films. The phenomenon of “smaller is stronger” is mainly due to the energy consumption, which combines the effects of microstructure collapse, microsphere slide, and reduced porosity during the whole loading and unloading process. In addition, the results of numerical simulation matched the experimental data and reflected the energy change rules of PCs during the indentation process. Furthermore, the study affords useful guidance for constructing high-performance films with proper design and potential application in next-generation PC materials.

Tracking Molecular Diffusion in One-Dimensional Photonic Crystals

The intuitive use, inexpensive fabrication, and easy readout of colorimetric sensors, including photonic crystal architectures and Fabry-Pérot interference sensors, have made these devices a successful commercial case, and yet, understanding how the diffusion of analytes occurs throughout the structure is a key ingredient for designing the response of these platforms on demand. Herein, the diffusion of amines in a periodic multilayer system composed of two-dimensional nanosheets and dielectric nanoparticles is tracked by a combination of spectroscopic measurements and theoretical modelling. It is demonstrated that diffusion is controlled by the molecular size, with larger molecules showing larger layer swelling and slower diffusion times, which translates into important sensor characteristics such as signal change and saturation time. Since the approach visualizes the analyte impregnation process in a time- and spatially resolved fashion, it directly relates the macroscopic color readout into microscopic processes occurring at the molecular level, thus opening the door to rational sensor design.

Biomimetic Structural Color Films with a Bilayer Inverse Heterostructure for Anticounterfeiting Applications

The unique brilliant and angle-independent structural colors of morpho butterfly wings were derived from the multilayer interference, diffraction, and scattering of light with a composite structure including ordered and quasiamorphous arrays. Inspired by the biological heterostructure of ordered and quasiamorphous arrays in the wings, a bilayer inverse heterostructure (BLIHS) containing ordered array layers inverse structure (OALIS) and quasiamorphous array layers inverse structure (Q-AALIS) of polyvinylidene fluoride were successfully prepared through the template method. The BLIHS films selectively displayed iridescent structural color derived from Bragg diffraction of OALIS, whereas the color states transform to noniridescent color because of Q-AALIS just by rotating the sample. Furthermore, the patterning process could be realized by using the spray-coating method on the BILIS films as quasiamorphous array layers. By virtue of this novel photonic structure, the switch between hiding and displaying patterns could be easily realized by changing the viewing angles, and the as-prepared films exhibited inherent excellent durability, which is crucial to their potential for practical applications as anticounterfeiting materials.

Manipulating Luminescence of Light Emitters by Photonic Crystals

The modulation of luminescence is essential because unwanted spontaneous-emission modes have a negative effect on the performance of luminescence-based photonic devices. Photonic crystals are promising materials for the control of light emission because of the variation in the local density of optical modes within them. They have been widely investigated for the manipulation of the emission intensity and lifetime of light emitters. Several groups have achieved greatly enhanced emission by depositing emitters on the surface of photonic crystals. Herein, the different modulating effects of photonic crystal dimensions, light-emitter positions, photonic crystal structure type, and the refractive index of photonic crystal building blocks are highlighted, with the aim of evaluating the fundamental principles that determine light propagation. The applications of using photonic crystals to manipulate spontaneous emission in light-emitting diodes and sensors are also reviewed. In addition, potential future challenges and improvements in this field are presented.

Vivid structural colors from long-range ordered and carbon-integrated colloidal photonic crystals

A facile strategy to prepare high-quality colloidal photonic crystals (PCs) with good visibility is proposed. Based on a high refractive-index material (zinc sulfide), highly monodispersed colloidal particles are successfully produced and assembled into long-range ordered crystalline colloidal arrays. The carbon-based materials are in situ incorporated with the long-range ordered colloidal PCs, which endows PCs with the combined characteristics to simultaneously achieve an intense photonic stop band and excellent control of incoherent light scattering. Owing to these merits, the obtained ZnS colloidal PCs have demonstrated strong brightness with the maximum reflectivity of 98%. Moreover, the coloration, saturation, and viewing angle are all improved. This study provides a straightforward and cost-effective strategy to create structural colors with high-quality visibility, which is expected to facilitate future applications of colloidal PCs.

Hairy Polydopamine Particles as Platforms for Photonic and Magnetic Materials

By selecting the core materials and grafted-hair polymers, hairy particles with polymer brushes can create various types of functional materials. In recent years, polydopamine (PDA) particles that are obtained by polymerizing dopamine, which is an amino acid derivative, have attracted attention for various applications. Herein, we present a novel approach for creating photonic and magnetic materials from hairy PDA particles. By grafting a hydrophilic hair polymer, we have succeeded in producing photonic materials capable of structural color changes. Furthermore, we have demonstrated the preparation of magnetic materials by immobilizing holmium, which is one of the lanthanide elements, by electrostatic interactions onto a cationic hair polymer. These results demonstrate the possibility of hairy PDA particles for a wide range of applications, such as for photonic and magnetic materials.

Melanin Precursor Influence on Structural Colors from Artificial Melanin Particles: PolyDOPA, Polydopamine, and Polynorepinephrine

Polydopamine (PDA) is of interest as a mimetic material of melanin to produce structural color materials. Herein, to investigate the influence of the material composition of the artificial melanin particles on structural color, we demonstrated the preparation of core-shell particles by polymerization of norepinephrine or 3,4-dihydroxyphenylalanine, which are melanin precursors similar to dopamine, in the presence of polystyrene particles. It was revealed that the arrays of the prepared particles produce high-visibility structural color because of absorption of scattering light. Although poly(3,4-dihydroxyphenylalanine) showed the same tendency as PDA which was previous studied, polynorepinephrine can easily produce a smooth and thick shell layer compared with that of PDA, and pellets consisting of the particles showed angle-dependent structural color. Therefore, the artificial melanin particles that produce angle-dependent structural color became stable than ever before. These results indicated that material compositions of artificial melanin particles have influence on structural color, and an important finding for application as a coloring material was obtained.

Photonic Capsule Sensors with Built-In Colloidal Crystallites

Technologies to monitor microenvironmental conditions and its spatial distribution are in high demand, yet remain unmet need. Herein, photonic microsensors are designed in a capsule format that can be injected, suspended, and implanted in any target volume. Colorimetric sensors are loaded in the core of microcapsules by assembling core-shell colloids into crystallites through the depletion attraction. The shells of the colloids are made of a temperature-responsive hydrogel, which enables the crystallites to rapidly and widely tune the structural color in response to a change in temperature while maintaining close-packed arrays. The spherical symmetry of the microcapsules renders them optically isotropic, i.e., displaying orientation-independent color. In addition, as a solid membrane is used to protect the delicate crystallites from external stresses, their high stability is assured. More importantly, each microcapsule reports the temperature of its microenvironment so that a suspension of capsules can provide information on the spatial distribution of the temperature.

Strong Photonic-Band-Gap Effect on the Spontaneous Emission in 3D Lead Halide Perovskite Photonic Crystals

Stimulated emission in perovskite-embedded polymer opal structures is investigated. A polymer opal structure is filled with a perovskite, and perovskite photonic crystals are prepared. The spontaneous emission of the perovskite embedded in the polymer opal structures exhibits clear signatures of amplified spontaneous emission (ASE) via gain modulation. The difference in refractive-index contrast between the perovskite and the polymer opal is large enough for retaining photonic-crystals properties. The photonic band gap has a strong effect on the fluorescence emission intensity and lifetime. The stimulated emission spectrum exhibits a narrow ASE rather than a wide fluorescence peak in the thin film.

Colloidal Assembly Approaches to Micro/Nanostructures of Complex Morphologies

The ability to programmatically assemble colloidal micro/nanostructures into highly ordered superstructures is of great importance in both fundamental studies and practical applications. In addition to the sophisticated manipulation of the short-range and long-range interactions imposed on the colloidal building blocks, the intrinsic shape elements including face, edge, corner, concave, convex, and curvature also play very important roles in solving the "jigsaw puzzle" of the superstructures. Here, the recent progress in the development of colloidal assembly strategies for the formation of complex superstructures is reviewed, with a primary focus on the unique effects of the morphology of the building blocks to the assembly processes and the final structures. Overall, this Review aims to shed light on the fundamental understanding of the colloidal behaviors of complex micro/nanostructures and promote the continued development of effective strategies for the creation of functional materials with complex compositions and morphologies.

Recent Trends in Metallopolymer Design: Redox-Controlled Surfaces, Porous Membranes, and Switchable Optical Materials Using Ferrocene-Containing Polymers

Metallopolymers with metal functionalities are a unique class of functional materials. Their redox-mediated optoelectronic and catalytic switching capabilities, their outstanding structure formation and separation capabilities have been reported recently. Within this Minireview, the scope and limitations of intriguing ferrocene-containing systems will be discussed. In the first section recent advances in metallopolymer design will be given leading to a plethora of novel metallopolymer architectures. Discussed synthetic pathways comprise controlled and living polymerization protocols as well as surface immobilization strategies. In the following sections, we focus on recent advances and new applications for side-chain and main-chain ferrocene-containing polymers as (i) remote-switchable materials, (ii) smart surfaces, (iii) redox-responsive membranes, and some recent trends in (iv) photonic structures and (v) other optical applications.

Bioinspired Color Materials Combining Structural, Dye, and Background Colors

Human beings have developed many dyes and pigments and use them for printed and display materials to share information. Today's information society is not possible without these color materials. Some living organisms utilize body color for information exchange and protection by skilfully combining dye, structural, and background colors to realize a body color change based on circumstances. In this study, inspired by the extraordinary body color changes of living things, a composite color material combining photochromic dyes, a black substance, a spherical colloidal crystal exhibiting a structural color, and a background color is prepared. In addition to combining a dye color and a structural color that changes upon light irradiation, the contribution of the different effects of the background color on each coloring property allows the construction of a color material that can reversibly change into various colors under different conditions.

Field-Directed Self-Assembly of Mutually Polarizable Nanoparticles

Directed assembly of dielectric and paramagnetic nanoparticles can be used to synthesize diverse functional materials that polarize in response to an externally applied electric or magnetic field. However, theories capable of predicting the self-assembled states are lacking. In the proposed work, we develop a complete thermodynamic description of such assemblies for spherical nanoparticles. We show how an important physical feature of these types of particles, mutual polarization, sculpts the free energy landscape and has a remarkably strong influence on the nature of the self-assembled states. Modeling the mutual polarization among nanoparticles requires solving a many-bodied problem for the particle dipole moments. Typically, this computationally expensive task is avoided by neglecting mutual polarization and assuming that each particle in a concentrated dispersion acquires the same dipole moment as a single, isolated particle. Although valid in the limit of small dielectric or permeability contrasts between particles and solvent, this constant dipole assumption leads to qualitatively incorrect predictions for coexisting phases in equilibrium at large dielectric or permeability contrasts. Correctly accounting for mutual polarization enables a thermodynamic theory that describes the equilibrium phase diagram of polarizable dispersions in terms of experimentally controllable variables. Our theoretical predictions agree with the phase behavior we observe in dynamic simulations of these dispersions as well as that in experiments of field-directed structural transitions. In contrast to predictions of a constant dipole model, we find that dispersions of particles with different dielectric constants or magnetic permeabilities exhibit qualitatively different phase behavior. This new model also predicts the existence of a eutectic point at which two crystalline phases and a disordered phase of nanoparticles all simultaneously coexist.

Biomimetic enzyme cascade reaction system in microfluidic electrospray microcapsules

Mimicking subcellular compartments containing enzymes in organisms is considered a promising approach to substitute for missing or lost cellular functions. Inspired by the multicompartment structures of cellular architectures, we present a novel multienzyme system based on hollow hydrogel microcapsules with flexible enzymatic inverse opal particles. Benefiting from the precise operation capability of the microfluidic electrospray and the remarkable structural color marks in the inverse opal particles, we developed a multienzyme system with controllable number, type, and spatial arrangement of the encapsulated enzymes. The hydrogel shells also could improve enzyme stability against proteolysis in the system. The multienzyme system containing alcohol oxidase and catalase could act as a cascade biocatalyst and reduce alcohol levels in media, providing an alternative antidote and prophylactic for alcohol intoxication. These features indicated that our strategy provides an ideal enzyme cascade reaction system for complex biocatalysis and biomimetic functions of some organelles or organs.

Instantaneous, Simple, and Reversible Revealing of Invisible Patterns Encrypted in Robust Hollow Sphere Colloidal Photonic Crystals

The colors of photonic crystals are based on their periodic crystalline structure. They show clear advantages over conventional chromophores for many applications, mainly due to their anti-photobleaching and responsiveness to stimuli. More specifically, combining colloidal photonic crystals and invisible patterns is important in steganography and watermarking for anticounterfeiting applications. Here a convenient way to imprint robust invisible patterns in colloidal crystals of hollow silica spheres is presented. While these patterns remain invisible under static environmental humidity, even up to near 100% relative humidity, they are unveiled immediately (≈100 ms) and fully reversibly by dynamic humid flow, e.g., human breath. They reveal themselves due to the extreme wettability of the patterned (etched) regions, as confirmed by contact angle measurements. The liquid surface tension threshold to induce wetting (revealing the imprinted invisible images) is evaluated by thermodynamic predictions and subsequently verified by exposure to various vapors with different surface tension. The color of the patterned regions is furthermore independently tuned by vapors with different refractive indices. Such a system can play a key role in applications such as anticounterfeiting, identification, and vapor sensing.

One-Step Anionic Copolymerization Enables Formation of Linear Ultrahigh-Molecular-Weight Block Copolymer Films Featuring Vivid Structural Colors in the Bulk State

Ultrahigh-molecular-weight (UHMW) tapered block copolymers (BCPs) consisting of polyisoprene- block-poly(4-methylstyrene) featuring overall molar masses in the range of 1101-2033 kg mol^-1 ( M_w) are synthesized via a convenient one-step anionic copolymerization protocol. The obtained UHMW BCPs are investigated by differential scanning calorimetry, size exclusion chromatography, and ¹H NMR spectroscopy. Microphase separation for the UHMW BCPs in the bulk state is investigated by transmission electron microscopy (TEM) measurements and scanning electron microscopy (SEM), revealing well-ordered lamellar and spherical domains with large domain sizes in the range of 100-200 nm. Excellent order and periodicity are observed for lamellar morphologies over large film areas of 90 × 120 μm. Because of this high order of the underlying domains and the different refractive indices of the block segments, vivid structural colors can be observed in the bulk state. Structural colors of BCP films are investigated by angle-dependent UV/vis measurements, revealing intensive reflection colors according to Bragg's law of diffraction. The optical characteristics are directly correlated to TEM and SEM results. Moreover, colored BCP films featuring spherical domains for one block segment with domain sizes of 97-122 nm revealed blue structural colors stemming from disordered particle scattering.