Responsive Photonic Crystals

Photo-Responsive Behavior of Azobenzene Based Polar Hockey-Stick-Shaped Liquid Crystals

A study on the photoswitching behavior of azobenzene-based polar hockey-stick-shaped liquid crystals (HSLCs) has been presented. Two new series of five phenyl rings based polar HSLCs have been designed and synthesized. Solution state photoisomerization of the synthesized materials was investigated thoroughly via UV-visible and ¹ H NMR spectroscopic techniques, whereas solid-state photochromic behavior was elucidated via physical color change of the materials, solid-state UV-visible study, powder XRD, and FE-SEM techniques. The materials exhibited decent photochromic behavior for different potential applications. The thermal phase behavior of the superstructural assembly has been characterized via polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and temperature-dependent small and wide-angle X-ray scattering (SAXS/WAXS) studies. Depending upon the length of the terminal alkyl chain, nematic (N) and partially bilayer smectic A (SmA_d ) phases were observed. DFT calculations revealed the favorable anti-parallel arrangement of the polar molecules that substantiate the formation of SmA_d phase.

Unconventional-Phase Crystalline Materials Constructed from Multiscale Building Blocks

Crystal phase, an intrinsic characteristic of crystalline materials, is one of the key parameters to determine their physicochemical properties. Recently, great progress has been made in the synthesis of nanomaterials with unconventional phases that are different from their thermodynamically stable bulk counterparts via various synthetic methods. A nanocrystalline material can also be viewed as an assembly of atoms with long-range order. When larger entities, such as nanoclusters, nanoparticles, and microparticles, are used as building blocks, supercrystalline materials with rich phases are obtained, some of which even have no analogues in the atomic and molecular crystals. The unconventional phases of nanocrystalline and supercrystalline materials endow them with distinctive properties as compared to their conventional counterparts. This Review highlights the state-of-the-art progress of nanocrystalline and supercrystalline materials with unconventional phases constructed from multiscale building blocks, including atoms, nanoclusters, spherical and anisotropic nanoparticles, and microparticles. Emerging strategies for engineering their crystal phases are introduced, with highlights on the governing parameters that are essential for the formation of unconventional phases. Phase-dependent properties and applications of nanocrystalline and supercrystalline materials are summarized. Finally, major challenges and opportunities in future research directions are proposed.

Bioinspired Dynamic Camouflage from Colloidal Nanocrystals Embedded Electrochromics

Camouflage is often seen in animals, and it presents in both passive and active forms. For instance, the wings of Closterocerus coffeellae exhibit distinct appearances against different backgrounds, while the chameleon actively changes its skin colors to morph into the environment. Herein, we report an artificial skin-like optoelectronic device that enables actively changing appearances and passively morphing into the environment by manipulating light-matter interactions with electrochromic polymers and photonic colloid nanocrystals. To construct the new electrochromic device, highly reflective, yet transmissive photonic nanocrystals are introduced into the gel electrolyte and sandwiched between the layers of electrochromic polymers and ion storage materials. Through voltage-controlled color switching of electrochromic polymers from colored state to bleached state, the degree of light absorbance, transmittance, and reflectance can be finely balanced and precisely modulated with the device. A broad synthesized color gamut and angle-dependent visual effects can be realized on this electronic skin-like device.

Magnetically Assembled Photonic Crystal Gels with Wide Thermochromic Range and High Sensitivity

Thermochromic poly(N-isopropyl acrylamide) (PNIPAM) photonic crystal gels based on 1D magnetically assembling colloidal nanocrystal clusters have attracted much attention due to its convenient preparation process, striking color response, and good mechanical strength. However, there remain challenges to broaden the thermochromic range and improve the sensitivity for their iridescent color display. Here, a PNIPAM photonic gel with wide thermochromic range and high sensitivity is prepared by using four-arm star poly(ethylene glycol) acrylamide (PEGAAm) as cross-linker at appropriately reduced magnetic field strength as well as cross-linker content. PEGAAm improves the homogeneity of the microstructure in PNIPAM photonic gel and thus maintains the structure colors at a wide temperature range from room temperature to 44 °C. The reduced magnetic field strength of 70 Gs and low cross-linker content (the molar ratio of monomer to cross-linker of 300:1) lead to a large initial lattice spacing of the photonic gel and thus wide diffraction wavelength migration of 194 nm. This optimized PNIPAM gel exhibits vivid iridescent colors from orange-red to indigo blue as temperature increases from 20 to 44 °C with satisfactory repeatability. Therefore, it may be an ideal candidate for temperature sensors and displays with utility and accuracy such as low-temperature burns.

Rapid Responsive Mechanochromic Photonic Pigments with Alternating Glassy-Rubbery Concentric Lamellar Nanostructures

Photonic pigment particles prepared via self-assembly have been suffering from their poor mechanical performances; i.e., they can easily be damaged and lose structural color under a compression force. This greatly limits their uses as mechanochromic pigments. Here, a nanoscale concentric lamellar structure of alternating glassy-rubbery microdomains is successfully created within photonic microparticles through a confined self-assembly and photo-cross-linking strategy. The glassy domain is composed of polystyrene, and cross-linked bottlebrush polydimethylsiloxane served as the supersoft elastic domain. The obtained photonic structure not only shows large deformation and visible color changes under a loaded compression force but also rapidly recovers to its original state in less than 1 s (∼0.16 s) upon unloading. Continuously loading-unloading micro compression test indicates that no obvious damage can be identified after 250 cycles, indicating the high durability of the pigments against deformation. These pigments with different reflected colors are simply obtained using bottlebrush block copolymer formulations with tunable weight percentages of polymer additives. The mechanical robust photonic pigments may be useful in many important applications.

From colloidal particles to photonic crystals: advances in self-assembly and their emerging applications

Over the last three decades, photonic crystals (PhCs) have attracted intense interests thanks to their broad potential applications in optics and photonics. Generally, these structures can be fabricated via either "top-down" lithographic or "bottom-up" self-assembly approaches. The self-assembly approaches have attracted particular attention due to their low cost, simple fabrication processes, relative convenience of scaling up, and the ease of creating complex structures with nanometer precision. The self-assembled colloidal crystals (CCs), which are good candidates for PhCs, have offered unprecedented opportunities for photonics, optics, optoelectronics, sensing, energy harvesting, environmental remediation, pigments, and many other applications. The creation of high-quality CCs and their mass fabrication over large areas are the critical limiting factors for real-world applications. This paper reviews the state-of-the-art techniques in the self-assembly of colloidal particles for the fabrication of large-area high-quality CCs and CCs with unique symmetries. The first part of this review summarizes the types of defects commonly encountered in the fabrication process and their effects on the optical properties of the resultant CCs. Next, the mechanisms of the formation of cracks/defects are discussed, and a range of versatile fabrication methods to create large-area crack/defect-free two-dimensional and three-dimensional CCs are described. Meanwhile, we also shed light on both the advantages and limitations of these advanced approaches developed to fabricate high-quality CCs. The self-assembly routes and achievements in the fabrication of CCs with the ability to open a complete photonic bandgap, such as cubic diamond and pyrochlore structure CCs, are discussed as well. Then emerging applications of large-area high-quality CCs and unique photonic structures enabled by the advanced self-assembly methods are illustrated. At the end of this review, we outlook the future approaches in the fabrication of perfect CCs and highlight their novel real-world applications.

Tuning the photonic properties of graphene oxide suspensions with nanostructured additives

Photonic materials that can selectively reflect light across the visible spectrum are valuable for applications in optical devices, sensors, and decoration. Although two-dimensional (2D) colloids that stack into layers with spacing of hundreds of nanometers are able to selectively diffract light, controlling their separation in solution has proven challenging. In this work, we investigate the role of additives to control the photonic properties of hybrid colloidal suspensions of graphene oxide (GO). We discovered that low concentrations of colloidal additives like cellulose nanocrystals (CNCs) and clay nanoparticles (hectorite) added to GO suspensions lead to dramatic color changes. These hybrid colloidal suspensions demonstrate tunable structural colors and temperature-sensitive properties that likely originate from the entropically driven ejection of guests between the sheets, and from the interactions between colloidal electrical double layers and additional counterions. On the other hand, blending polymeric or molecular additives with GO suspensions either deteriorates or does not impact the photonic properties. These results are helpful to understand the interaction between GO suspensions and additives over different length scales, and open a path to advancing photonic materials based on hybrid colloidal suspensions.

Form Birefringence in Resonant Transducers for the Selective Monitoring of VOCs under Ambient Conditions

In this work, we have developed a new kind of nanocolumnar birefringent Bragg microcavity (BBM) that, tailored by oblique angle deposition, behaves as a selective transducer of volatile organic compounds (VOCs). Unlike the atomic lattice origin of birefringence in anisotropic single crystals, in the BBM, it stems from an anisotropic self-organization at the nanoscale of the voids and structural elements of the layers. The optical adsorption isotherms recorded upon exposure of these nanostructured systems to water vapor and VOCs have revealed a rich yet unexplored phenomenology linked to their optical activity that provides both capacity for vapor identification and partial pressure determination. This photonic response has been reproduced with a theoretical model accounting for the evolution of the form birefringence of the individual layers upon vapor condensation in nanopores and internanocolumnar voids. BBMs that repel water vapor but are accessible to VOCs have been also developed through grafting of their internal surfaces with perfluorooctyltriethoxysilane molecules. These nanostructured photonic systems are proposed for the development of transducers that, operating under environmental conditions, may respond specifically to VOCs without any influence by the degree of humidity of the medium.

Nonintrusively Adjusting Structural Colors of Sealed Two-Dimensional Photonic Crystals: Immediate Transformation between Transparency and Intense Iridescence and Their Applications

Responsive photonic crystals (PCs), which can adjust structural colors in response to external stimuli, show great potential applications in displays, sensors, wearable electronics, encryption, and anticounterfeiting. In contrast, conventional structure-intrusive adjustment manners that external stimuli directly interact with the ordered arrays may lead to structural damage or longer response time. Here, a noninvasive adjustment of the structural colors of two-dimensional (2D) PCs (2D-PCs) is explored based upon diffraction theory. Sealed 2D-PCs and 2D inverse opal photonic crystal (IOPC) flexible devices are prepared. They are highly transparent in air but immediately exhibit intense viewing angle-dependent structural colors after being dipped in water. The mechanism of transparent-iridescent immediate transformation is explained by Bragg's law. The design mechanism is examined by numerical simulation and spectral shifts in different external media. We demonstrate its applications in the fields of information encryption and anticounterfeiting by using the transparent-iridescent immediate transformation of sealed 2D-PC patterns and 2D IOPC free-standing films sealed on the product surface. Because of the strong contrast between transparency and intense iridescence, reversible and immediate transformation, and durability, sealed 2D-PCs and 2D IOPC flexible devices designed by the noninvasive adjustment strategy will lead to a variety of new applications in displays, sensors, wearable electronics, encryption, and anticounterfeiting.

Self-Assembled Photonic Microsensors with Strong Aggregation-Induced Emission for Ultra-Trace Quantitative Detection

Ultratrace quantitative detection based on fluorescence is highly desirable for many important applications such as environmental monitoring or disease diagnosis, which however has remained a great challenge because of limited and irregular fluorescence responses to analytes at ultralow concentrations. Herein the problem is circumvented via local enrichment and detection of analytes within a microsensor, that is, photonic porous microspheres grafted with aggregation-induced emission gens (AIEgens). The obtained microspheres exhibit dual structural and molecular functions, namely, bright structural colors and strong fluorescence. Large fluorescence quenching induced by nitrophenol compounds in an aqueous environment is observed at ultralow concentrations (10^-12-10^-8 mol/L), enabling quantitative detection at a ppb level (ng/L). This is achieved within a porous structure with good connectivity between the nanopores to improve analyte diffusion, an internal layer of poly(ethylene oxide) (PEO) for analyte enrichment via hydrogen bonding, and homogeneous distribution of AIEgens within the PEO layer for enhanced fluorescence quenching. The fluorescent porous microspheres can be readily obtained in a single step templated by well-ordered water-in-oil-in-water double emulsion droplets with AIE amphiphilic bottlebrush block copolymers as the effective stabilizer.

Wearable and Implantable Intraocular Pressure Biosensors: Recent Progress and Future Prospects

Biosensors worn on or implanted in eyes have been garnering substantial attention since being proven to be an effective means to acquire critical biomarkers for monitoring the states of ophthalmic disease, diabetes. Among these disorders, glaucoma, the second leading cause of blindness globally, usually results in irreversible blindness. Continuous intraocular pressure (IOP) monitoring is considered as an effective measure, which provides a comprehensive view of IOP changes that is beyond reach for the "snapshots" measurements by clinical tonometry. However, to satisfy the applications in ophthalmology, the development of IOP sensors are required to be prepared with biocompatible, miniature, transparent, wireless and battery-free features, which are still challenging with many current fabrication processes. In this work, the recent advances in this field are reviewed by categorizing these devices into wearable and implantable IOP sensors. The materials and structures exploited for engineering these IOP devices are presented. Additionally, their working principle, performance, and the potential risk that materials and device architectures may pose to ocular tissue are discussed. This review should be valuable for preferable structure design, device fabrication, performance optimization, and reducing potential risk of these devices. It is significant for the development of future practical IOP sensors.

Stimuli-Responsive Photonic Crystals

Recently, tunable photonic crystals (PhCs) have received great research interest, thanks to the wide range of applications in which they can be employed, such as light emission and sensing, among others. In addition, the versatility and ease of fabrication of PhCs allow for the integration of a large range of responsive elements that, in turn, can permit active tuning of PhC optical properties upon application of external stimuli, e.g., physical, chemical or even biological triggers. In this work, we summarize the most employed theoretical tools used for the design of optical properties of responsive PhCs and the most used fabrication techniques. Furthermore, we collect the most relevant results related to this field, with particular emphasis on electrochromic devices.

Designable structural coloration by colloidal particle assembly: from nature to artificial manufacturing

Structural color attracts considerable scientific interests and industrial explorations in various fields for the eco-friendly, fade-resistant, and dynamic advantages. After the long-period evolution, nature has achieved the optimized color structures at various length scales, which has inspired people to learn and replicate them to improve the artificial structure color. In this review, we focus on the design of artificial structural colors based on colloidal particle assembly and summarize the functional bioinspired structure colors. We demonstrate the design principles of biomimetic structural colors via the precise structure engineering and typical bottom-up methods. Some main applications are outlined in the following chapter. Finally, we propose the existing challenges and promising prospects. This review is expected to introduce the recent design strategies about the artificial structure colors and provide the insights for its future development.

Self-assembly of colloidal inorganic nanocrystals: nanoscale forces, emergent properties and applications

The self-assembly of colloidal nanoparticles has made it possible to bridge the nanoscopic and macroscopic worlds and to make complex nanostructures. The nanoparticle-mediated assembly enables many potential applications, from biodetection and nanomedicine to optoelectronic devices. Properties of assembled materials are determined not only by the nature of nanoparticle building blocks, but also by spatial positions of nanoparticles within the assemblies. A deep understanding of nanoscale interactions between nanoparticles is a prerequisite to controlling nanoparticle arrangement during assembly. In this review, we present an overview of interparticle interactions governing their assembly in a liquid phase. Considerable attention is devoted to examples that illustrate nanoparticle assembly into ordered superstructures using different types of building blocks, including plasmonic nanoparticles, magnetic nanoparticles, lanthanide-doped nanophosphors, and quantum dots. We also cover the physicochemical properties of nanoparticle ensembles, especially those arising from particle coupling effects. We further discuss future research directions and challenges in controlling self-assembly at a level of precision that is most crucial to technology development.

Controllable Radical Polymerization of Selenide Functionalized Vinyl Monomers and Its Application in Redox Responsive Photonic Crystals

Selenium-containing monomer (p-phenylseleno) styrene (p-PhSeSt) is polymerized by reversible addition-fragmentation chain transfer polymerization. Polymer, (P(p-PhSeSt)), with controlled molecular weight and narrow molecular weight is obtained. The selenide moiety in obtained P(p-PhSeSt) can be selectively oxidized to selenoxide or selenone groups by H₂ O₂ or NaClO, respectively. These oxidized groups can be further reduced to selenide by Na₂ S₂ O₄ . The structure changing of polymers during such redox cycle is characterized by nuclear magnetic resonance, X-ray photoelectron spectroscopy, and size exclusion chromatography. Properties, such as thermal performance, glass transition temperature, water contact angles, and refractive indices, of the resulting polymers are systematically investigated before and after oxidation. In addition, SiO₂ inverse opal photonic crystal (IOPC) is fabricated by sacrificial polymer colloidal template method. Owing to changes of the RIs of P(p-PhSeSt) after selective oxidation, the predictable change of PC bandgap as a redox-responsive PC sensor is successfully realized, which provides new perspectives for modulating photonic crystals.

Geminate labels programmed by two-tone microdroplets combining structural and fluorescent color

Creating a security label that carries entirely distinct information in reflective and fluorescent states would enhance anti-counterfeiting levels to deter counterfeits ranging from currencies to pharmaceuticals, but has proven extremely challenging. Efforts to tune the reflection color of luminescent materials by modifying inherent chemical structures remain outweighed by substantial trade-offs in fluorescence properties, and vice versa, which destroys the information integrity of labels in either reflection or fluorescent color. Here, a strategy is reported to design geminate labels by programming fluorescent cholesteric liquid crystal microdroplets (two-tone inks), where the luminescent material is 'coated' with the structural color from helical superstructures. These structurally defined microdroplets fabricated by a capillary microfluidic technique contribute to different but intact messages of both reflective and fluorescent patterns in the geminate labels. Such two-tone inks have enormous potential to provide a platform for encryption and protection of valuable authentic information in anti-counterfeiting technology.

Progress in polydopamine-based melanin mimetic materials for structural color generation

Structural color is a color derived from optical interaction between light and a microstructure and is often seen in nature. Natural melanin plays an important role in bright structural coloration. For example, the vivid colors of peacock feathers are due to structural colors. The periodic arrangement of melanin granules inside the feathers leads to light interference, and the black granules absorb scattered light well, resulting in bright structural color. In recent years, polydopamine (PDA) has attracted attention as a melanin mimetic material. This review article summarizes recent research on structural coloration using PDA-based artificial melanin materials. It also outlines possible applications using bright structural colors realized by artificial melanin materials and future perspectives.

Wettability-Controlled Directional Actuating Strategy Based on Bilayer Photonic Crystals

Although the water-triggered bending behavior of bilayer films has been a wide concerned, there are few reports on wettability-controlled directional actuators with visible color changes. Using photonic crystals as carriers, bilayer directional bending structural color actuators were prepared based on the hydrophilic difference. Top inverse opal with strong hydrophilicity can promote water penetration and strengthen the effect of swelling. While, bottom inverse opal with weak hydrophilicity can inhibit water penetration and weaken the effect of swelling. When the bilayer structure is immersed in water, its wettability differences will produce different optical responses for visualization and will bring different swelling performances, resulting in directional bending. Infiltration differences are visualized as structural color red shifts or transparency. The mechanism of the design involves optical diffractions in the fabricated periodic nanostructures, differences in the surface wettability and swelling rate, uses the infiltration and capillary evaporation of water to realize the spectral diversity of reflectance, and the enhancement of bending by gradient infiltration. This work deeply analyzes the improvement of the photonic crystal structure on the optical and bending performance of the wettability-controlled actuator, provides a basic model for the design of bionic components, and opens an idea for the combination of bilayer photonic crystals and actuators.

Robust synthesis of highly charged superparamagnetic Fe3O4 colloidal nanocrystal clusters for magnetically responsive photonic crystals

Herein, highly charged superparamagnetic Fe3O4 colloidal nanocrystal clusters with uniform and widely tunable sizes have been synthesized by a robust and easily scalable solvothermal method for magnetically responsive photonic crystals.

High-Performance Colorimetric Humidity Sensors Based on Konjac Glucomannan

High-humidity conditions (85-100% relative humidity (RH)) have very diverse effects on many aspects of people's daily lives. Despite remarkable progress in the development of structural coloration-based humidity sensors, how to significantly improve the sensitivity and visual humidity resolution of these humidity sensors under a high-humidity environment remains a great challenge. In this study, high-performance colorimetric humidity sensors based on environment-friendly konjac glucomannan (KGM) via thin-film interference are developed using a simple, affordable, and scalable preparation method. An effective strategy is demonstrated for substantially improving the sensor sensitivity and visual humidity resolution under a high-humidity environment via synergistic integration of multiorder interference peaks, sensor array technology, and superior water-absorbing polymer. The KGM full-range humidity sensors exhibit fast and dynamic response toward the humidity change without power consumption, and they also show high sensitivity and selectivity, little hysteresis, and excellent stability against high-humidity conditions. The KGM humidity sensors display extraordinary red shift of the reflection peak (e.g., 385 nm) and the visual humidity resolution as high as 1.5% RH in the visible range from 85 to 100% RH, which represent the largest spectra shift and highest visual humidity resolution, respectively, for structural coloration-based humidity sensors in high-humidity conditions.

Responsive Photonic Crystal Microcapsules of Block Copolymers with Enhanced Monochromaticity

Self-assembly of block copolymers (BCPs) has been developed as a promising approach for constructing photonic crystal (PC) microspheres for dynamic optical modulation. However, high curvature in the center of microspheres usually distorts the periodic core structure, leading to an inconsistency of photonic bandgap and poor monochromaticity of structural color. Herein, we report a simple yet robust strategy for fabricating responsive PC microcapsules of polystyrene-b-poly(2-vinylpyridine) through self-emulsification strategy. Interestingly, the microcapsules exhibit bright structural color with significantly enhanced monochromaticity, compared to their solid counterpart, since the microcapsules have no irregular cores. The structural colors of the PC microcapsules not only exhibit a variability through binary mixing of BCPs but also show a responsiveness to pH value. As a colored microcarrier, the PC microcapsules show a potential for visualizing the pH-dependent release behavior of encapsulated hydrophilic cargos on account of pH-responsive structural color.

Photonic Janus Balls with Controlled Magnetic Moment and Density Asymmetry

Colloidal crystals show structural colors through wavelength-selective diffraction at photonic stopbands. Here, we design photonic Janus balls with a controlled magnetic moment for programmable structural color switching. The Janus balls are produced from microfluidically produced paired drops of two distinct photocurable resins. The lighter resin contains magnetic nanoparticles and carbon black, whereas heavier one contains silica particles at a high volume fraction. The paired drops spontaneously align vertically due to the density asymmetry. The magnetic moment is assigned in the vertically aligned drops by aligning magnetic nanoparticles with an external field and capturing them through photopolymerization. Silica particles in the heavier compartment spontaneously form crystalline arrays due to interparticle repulsion, developing structural colors. The resulting photonic Janus balls vertically align without an external field, like a roly-poly toy, so that carbon-black-laden compartments face upward. With an external magnetic field, the Janus balls align their magnetic moment to the field and display structural colors. Importantly, the direction of the magnetic moment is set by the direction of the external field during photopolymerization, which enables the simultaneous manipulation of orientations of distinct photonic Janus balls in a programmed manner. These photonic Janus balls are potentially useful as active color inks for anti-counterfeiting tags.

Mechanochromism in Structurally Colored Polymeric Materials

Mechanochromic effects in structurally colored materials are the result of deformation-induced changes to their ordered nanostructures. Polymeric materials which respond in this way to deformation offer an attractive combination of characteristics, including continuous strain sensing, high strain resolution, and a wide strain-sensing range. Such materials are potentially useful for a wide range of applications, which extend from pressure-sensing bandages to anti-counterfeiting devices. Focusing on the materials design aspects, recent developments in this field are summarized. The article starts with an overview of different approaches to achieve mechanochromic effects in structurally colored materials, before the physical principles governing the interaction of light with each of these materials types are summarized. Diverse methodologies to prepare these polymers are then discussed in detail, and where applicable, naturally occurring materials that inspired the design of artificial systems are discussed. The capabilities and limitations of structurally colored materials in reporting and visualizing mechanical deformation are examined from a general standpoint and also in more specific technological contexts. To conclude, current trends in the field are highlighted and possible future opportunities are identified.

Magnetically Tunable Plasmon Coupling of Au Nanoshells Enabled by Space-Free Confined Growth

We report the unconventional space-free confined growth of Au nanoshells with well-defined plasmonic properties and active tuning of their plasmon coupling by the nanoscale magnetic assembly. The seeded growth of Au exclusively occurred at the hard-soft interfaces between the Fe₃O₄ core and phenolic resin without the need of creating a limiting space, which represents a general and elegant approach to various core-shell nanostructures. The deformability of permeable phenolic layers plays an essential role in regulating the interfacial growth of Au nanoshells. While the polymer elasticity suppresses the radial deposition of Au atoms, their high deformability can afford enough spaces for the formation of conformal metallic shells. The coupled magnetic-plasmonic properties allow active tuning of the plasmon coupling and the resonant scattering of Au nanoshells by the magnetic assembly of the hybrid nanoparticles into plasmonic chains, whose potentials in applications have been demonstrated in designing transparent displays and anticounterfeiting devices.

Thermally Responsive Photonic Fibers Consisting of Chained Nanoparticles

Fibers that can reversibly and passively change colors along with body temperatures are highly desired for potential applications including temperature sensors, smart wearables, and photonic devices. Here, we develop a facile strategy to fabricate thermochromic photonic fibers, which could exhibit tunable structural colors as a function of temperatures. The thermochromic fibers are prepared by aligning superparamagnetic, carbon-encapsulated Fe₃O₄ colloidal nanocrystal clusters (Fe₃O₄@C CNCs) in a thermoresponsive hydrogel, poly(N-isopropylacrylamide) (PNIPAM), forming chain-like structures under an external magnetic field before gelation. When the fiber is transferred from air to water at room temperature, it changes color from dark green to red as it swells. The red color can be reversibly changed back to green as the temperature is raised to 36 °C, while the fiber shrinks and the reflection peak shifts from 642 to 494 nm. The swelling of the fiber is anisotropic: by 60% in the diameter direction but 45% in the length direction. Therefore, the fiber can act as a thermochromic actuator.

Tunable and asymmetric optical bistability of one-dimensional photonic crystals based on InSb and nonlinear materials

In this paper, one-dimensional (1D) photonic crystals (PCs) based on InSb and nonlinear materials are studied using the transmission matrix method. It is found that in the terahertz band, due to the magneto-optical characteristics of InSb, when the incident light propagates through such 1D PCs in two different directions (forward and backward propagation), an asymmetric optical bistable state can be obtained. Propagating from the forward direction, the bistable state is achieved. Spreading from the backward direction, the multistable state can be observed. Compared with the bistable threshold value for the case of forward incidence, the similar value that appeared in the multistable curve is significantly reduced. The polarization-sensitive features of the asymmetric optical bistability of the presented 1D PCs are also investigated under TM waves, which focuses on the tuning effects of incident angle, external magnetic field, temperature, and the thicknesses of the InSb and nonlinear dielectric layers on the asymmetric optical bistability. The results show that when the incident light enters from two different directions, the tailoring effects of the incident angle and thickness of the InSb layer on the bistable state are obviously different, but the tuning influences of the magnetic field, temperature, and the thickness of the nonlinear material layer on the bistable state are similar. The asymmetric optical bistability we made in this paper can be applied to multifunctional devices and nonreciprocal optical isolators.

Endowing Zeolite LTA Superballs with the Ability to Manipulate Light in Multiple Ways

Advances in zeolites research emerging from interdisciplinary efforts have opened new opportunities beyond conventional applications. Colloids drive much current research owing to their distinct collective behaviors, but so far, using zeolites as a colloidal building block to construct ordered superstructures remains unexplored. Herein we show that self-assembly of colloidal zeolite LTA superball (ZAS) by tilted-angle sedimentation forms macroscopic films with micro-mesoporosity and 3D long-range periodicity featuring a photonic band gap (PBG) that is tunable through the superball geometry and responds reversibly to chemical vapors. Remarkably, self-assembly of ZAS at elevated temperature forms 3D chiral photonic crystals that enable negative circular dichroism, selective reflection of right-handed circularly polarized (CP) light and left-handed CP luminescence based on PBG. We present a novel class of functional colloids and zeolite-based photonic crystals with the ability to manipulate light in several ways.

Integration of bio-responsive silver in 1D photonic crystals: towards the colorimetric detection of bacteria

The colour purity and versatility of fabrication of one-dimensional photonic crystals (1D PhCs) make them ideal candidates for colorimetric sensing of a variety of analytes. For instance, the detection of bacterial contaminants in food via colorimetric sensors can be highly appealing, as most of the existing detection techniques are in general time-consuming and the read-out requires specialised personnel. Here, we present a colorimetric sensor based on hybrid plasmonic/photonic 1D crystals. We demonstrate that the modification of the silver plasmon resonance brought about by the effective silver/bacterium interaction can be translated into the visible spectral region, producing a change in the structural colour. In addition, we observe a superior colorimetric sensitivity against the Gram negative Escherichia coli compared to the Gram positive Micrococcus luteus, a result that we attribute to the more efficient electrostatic interaction and cellular adhesion between the silver surface and the Gram-negative bacteria outer membrane. This approach demonstrates that in principle an easy colorimetric detection of bacterial contaminants can be achieved through the use of bio-responsive plasmonic materials, such as silver, whose selective electrostatic interaction with bacterial cell wall is well-known and occurs without the need of chemical functionalisation.

Colloidal assembly in droplets: structures and optical properties

Colloidal assembly in emulsion drops provides fundamental tools for studying optimum particle arrangement under spherical confinement and practical means for producing photonic microparticles. Recent progress has revealed that energetically favored cluster configurations are different from conventional supraballs, which could enhance optical performance. This paper reviews state-of-the-art emulsion-templated colloidal clusters, and particularly focuses on recently reported novel structures such as icosahedral, decahedral, and single-crystalline face-centered cubic (fcc) clusters. We classify the clusters according to the number of component particles as small (N < O(10²)), medium (O(10²) ≤N≤O(10⁴)), and large (N≥O(10⁵)). For each size of clusters, we discuss the detailed structures, mechanisms of cluster formation, and optical properties and potential applications. Finally, we outline current challenges and questions that require further investigation.

Libraries of Uniform Magnetic Multicore Nanoparticles with Tunable Dimensions for Biomedical and Photonic Applications

Multicore iron oxide nanoparticles, also known as colloidal nanocrystal clusters, are magnetic materials with diverse applications in biomedicine and photonics. Here, we examine how both of their characteristic dimensional features, the primary particle and sub-micron colloid diameters, influence their magnetic properties and performance in two different applications. The characterization of these basic size-dependent properties is enabled by a synthetic strategy that provides independent control over both the primary nanocrystal and cluster dimensions. Over a wide range of conditions, electron microscopy and X-ray diffraction reveal that the oriented attachment of smaller nanocrystals results in their crystallographic alignment throughout the entire superstructure. We apply a sulfonated polymer with high charge density to prevent cluster aggregation and conjugate molecular dyes to particle surfaces so as to visualize their collection using handheld magnets. These libraries of colloidal clusters, indexed both by primary nanocrystal dimension (d_p) and overall cluster diameter (D_c), form magnetic photonic crystals with relatively weak size-dependent properties. In contrast, their performance as MRI T₂ contrast agents is highly sensitive to cluster diameter, not primary particle size, and is optimized for materials of 50 nm diameter (r₂ = 364 mM^-1 s^-1). These results exemplify the relevance of dimensional control in developing applications for these versatile materials.

A responsive photonic crystal film sensor for the ultrasensitive detection of uranyl ions

As an effective nuclear energy resource, uranium plays an important role in industry and energy but the wastes of uranium also cause radioactive contamination, which is harmful to the environment and the human body. Herein, a responsive photonic crystal (PC) film sensor for the ultrasensitive and label-free detection of uranyl ions (UO₂²⁺) has been proposed, which is easy to construct and does not need to be combined with a hydrogel. The PC film is not pH-sensitive because it is obtained by the self-assembly of methyl methacrylate-acrylonitrile co-polymeric nanospheres (PMMA-AN). These nanospheres were modified with amidoxime groups, which have a good coordination ability with UO₂²⁺. The bindings between nanospheres and UO₂²⁺ change the refractive index and disturb the face-centered cubic structure of the film, which leads to a decrease in the diffraction peak intensity of the PC film. The sensor works in the concentration range of 10 pM to 100 μM for UO₂²⁺ determination and the decreased intensities of the diffraction peaks are linearly correlated with the logarithm of UO₂²⁺ concentration in the range from 1 nM to 100 μM. Moreover, the sensor shows good selectivity for UO₂²⁺ and can also perform the determination of UO₂²⁺ in a real sample. The responsive PC film sensor shows great potential in the label-free and ultrasensitive detection of UO₂²⁺.

Full-color natural rubber latex with a photonic nanostructure composite

As environmental problems increase, there is an urgent demand for new eco-friendly materials. Natural rubber latex (NRL) is a natural material extracted from rubber trees. But its dyeing process with chemical dyes might result in contamination and environmental degradation. Here, NRL is composited with a photonic crystal (PhC) structure by spin coating for the first time. The polymethyl methacrylate (PMMA) photonic nanostructure has been embedded into NRL to give it colors and provide it with optical functionalities. Colors of the composite could be designed and controlled by the sizes of the nanocolloids from 180 nm to 295 nm. The colors have strong stability under external stretching. The 3D natural rubber latex photonic crystal (NRLPC) is used as a responsive material to detect volatile organic compounds (VOCs) including formaldehyde, acetone, toluene, xylene and styrene. With its visual color appearance, biocompatibility and flexibility, NRLPC has promising potential in various sensing applications.

Bioinspired conductive cellulose liquid-crystal hydrogels as multifunctional electrical skins

Bionic electronic skin (E-skin) that could convert external physical or mechanical stimuli into output signals has a wide range of applications including wearable devices, artificial prostheses, software robots, etc. Here, we present a chameleon-inspired multifunctional E-skin based on hydroxypropyl cellulose (HPC), Poly(Acrylamide-co-Acrylic acid) (PACA), and carbon nanotubes (CNTs) composited liquid-crystal hydrogel. We found that the HPC could still form cholesteric liquid-crystal photonic structures with the CNTs additive for enhancing their color saturation and PACA polymerization for locating their assembled periodic structures. As the composite hydrogel containing HPC elements and the PACA scaffold responds to different stimuli, such as temperature variations, mechanical pressure, and tension, it could correspondingly change its volume or internal nanostructure and report these as visible color switches. In addition, due to the additive of CNTs, the composite hydrogel could also output these stimuli as electrical resistance signals. Thus, the hydrogel E-skins had the ability of quantitatively feeding back external stimuli through electrical resistance as well as visually mapping the stimulating sites by color variation. This dual-signal sensing provides the ability of visible-user interaction as well as antiinterference, endowing the multifunctional E-skin with great application prospects.

Thermal Responsive Photonic Crystal Achieved through the Control of Light Path Guided by Phase Transition

Responsive photonic crystal is widely considered in the field of anti-counterfeiting and information encryption because of their structural color changes caused by external stimulation. However, the response signal is usually achieved by adjusting the periodic lattice constant based on Bragg's law with volume changes. Thus, it is a great challenge to achieve the response of photonic crystals by non-array parameter control. Herein, novel thermal responsive photonic crystal (TRPC) with low angle dependent structural color is fabricated by introducing poly(ethylene glycol) into the structure of low angle dependent SnO₂ inverse opal. The response is achieved through the control of light path guided by phase transition and the significant volume change caused by the change of traditional array parameters can be effectively avoided. Meanwhile, the low angle dependent structural color of TRPC can effectively reduce the interference of observation angle change to response signal caused by external thermal stimulation. Patterned responsive photonic crystals with temperature gradient response are easily obtained by combining confinement self-assembly and direct template method, and the patterns can be presented and hidden by the control of light path, showing great potential in anti-counterfeiting and information encryption fields.

Tunable Photonic Microspheres of Comb-Like Supramolecules

Photonic crystals (PCs) are ideal candidates for reflective color pigments with high color purity and brightness due to tunable optical stop band. Herein, the generation of PC microspheres through 3D confined supramolecular assembly of block copolymers (polystyrene-block-poly(2-vinylpyridine), PS-b-P2VP) and small molecules (3-n-pentadecylphenol, PDP) in emulsion droplets is demonstrated. The intrinsic structural colors of the PC microspheres are effectively regulated by tuning hydrogen-bonding interaction between P2VP blocks and PDP, where reflected color can be readily tuned across the whole visible spectrum range. Also, the effects of both PDP and homopolymer (hPS) on periodic structure and optical properties of the microspheres are investigated. Moreover, the spectral results of finite element method (FEM) simulation agree well with the variation of structural colors by tuning the periodicity in PC microspheres. The supramolecular microspheres with tunable intrinsic structural color can be potentially useful in the various practical applications including display, anti-counterfeit printing and painting.

Dynamic Interfacial Regulation by Photodeformable Azobenzene-Containing Liquid Crystal Polymer Micro/Nanostructures

Photoresponsive materials offer local, temporal, and remote control over their chemical or physical properties under external stimuli, giving new tools for interfacial regulation. Among all, photodeformable azobenzene-containing liquid crystal polymers (azo-LCPs) have received increasing attention because they can be processed into various micro/nanostructures and have the potential to reversibly tune the interfacial properties through chemical and/or morphological variation by light, providing effective dynamic interface regulation. In this feature article, we highlight the milestones in the dynamic regulation of different interfacial properties through micro/nanostructures made of photodeformable azobenzene-containing liquid crystal polymers (azo-LCPs). We describe the preparation of different azo-LCP micro/nanostructures from the aspects of materials and processing techniques and reveal the importance of mesogen orientation toward dynamic interfacial regulation. By introducing our recently developed linear azo-LCP (azo-LLCP) with good mechanical and photoresponsive performances, we discuss the challenge and opportunity with respect to the dynamic light regulation of two- and three-dimensional (2D/3D) micro/nanostructures to tune their related interfacial properties. We have also given our expectation toward exploring photodeformable micro/nanostructures for advanced applications such as in microfluidics, biosensors, and nanotherapeutics.

Coupling magnetic and plasmonic anisotropy in hybrid nanorods for mechanochromic responses

Mechanochromic response is of great importance in designing bionic robot systems and colorimetric devices. Unfortunately, compared to mimicking motions of natural creatures, fabricating mechanochromic systems with programmable colorimetric responses remains challenging. Herein, we report the development of unconventional mechanochromic films based on hybrid nanorods integrated with magnetic and plasmonic anisotropy. Magnetic-plasmonic hybrid nanorods have been synthesized through a unique space-confined seed-mediated process, which represents an open platform for preparing next-generation complex nanostructures. By coupling magnetic and plasmonic anisotropy, the plasmonic excitation of the hybrid nanorods could be collectively regulated using magnetic fields. It facilitates convenient incorporation of the hybrid nanorods into polymer films with a well-controlled orientation and enables sensitive colorimetric changes in response to linear and angular motions. The combination of unique synthesis and convenient magnetic alignment provides an advanced approach for designing programmable mechanochromic devices with the desired precision, flexibility, and scalability.

Oriented boronate affinity-imprinted inverse opal hydrogel for glycoprotein assay via colorimetry

A biomimetic antibody is described for colorimetric determination of glycoprotein, and horseradish peroxidase (HRP) is used as model analyte. Use is made of oriented surface imprinted inverse opal hydrogel particles functionalized with phenylboronic acid. The inverse opal hydrogel particles were negatively replicated from silica colloidal crystal beads (SCCBs), so that they were endowed with larger specific surface area than the bulk structure. Benefit from that, there were abundant surface molecularly imprinting sites in the hydrogel particles. Because the imprinting sites match the structure of the template molecules, they can recognize HRP with high selectivity and sensitivity. The recognized glycoprotein was bonded with the phenylboronic acid within the sites. The bonded HRP was determined by colorimetry of 3, 3', 5, 5'-tetramethylbenzidine (TMB) single-component solution at 450 nm, and it shows a 16.03 imprinting factor under optimized conditions. Alpha-fetoprotein (AFP) was also investigated and demostrated the value of this strategy in practical applications. The results show that the absorbance increases linearly in the 1-50 ng mL^-1 AFP concentration range and has a 1.32 ng mL^-1 detection limit. The assay of human serum was realized by the standard addition method. This strategy is promising to open new horizons for glycoprotein assay. Graphical abstract Schematic representation of the preparation of oriented boronate affinity-imprinted inverse opal hydrogel particles for glycoprotein assay.

Photonic crystal-based smart contact lens for continuous intraocular pressure monitoring

Glaucoma is a very common disease after cataracts and is dangerous enough to cause irreversible blindness. However, often the main symptom of glaucoma is difficult to recognize because it may be absent or appear late, so the risk of blindness is greater. Intraocular pressure (IOP) is a well-known primary factor indicating glaucoma. In this study, we demonstrate a smart IOP sensor embedded in a contact lens that works through visual color changes without an external power source such as a battery or RF-based wireless power transfer. A microhydraulic amplification mechanism is adopted to enhance the range of color change from a photonic crystal (PC)-based flexible membrane whose lattice distance between nanostructures varies according to the morphology changes of an eyeball caused by IOP. The performance of the sensor is quantitatively demonstrated using an artificial silicone eye model for in vitro evaluation and a porcine eyeball for ex vivo verification. It has a limit of detection (LOD) of 3.2 and 5.12 mmHg, which was measured and evaluated using a spectrometer and a smartphone camera, respectively. The results prove that our sensor embedded in the contact lens can continuously monitor the IOP change using color change, and a smartphone camera can be used as a quantitative IOP measurement system in a noninvasive manner without an expensive optical spectrometer.

Structurally coloured contact lens sensor for point-of-care ophthalmic health monitoring

Point-of-care (POC) diagnosis is of great significance in offering precise and personalized treatment for patients with eye diseases. Contact lenses, as a kind of popular wearable device on the eye, provide a suitable platform for the integration of biosensors for the POC diagnosis of eye diseases. However, existing contact lens sensors usually involve complex electronics and circuits, the manufacturing of which is complicated and signal readout requires additional instruments. To realize the instrument-free detection of pathologically relevant signals of eye diseases, we successfully established a structurally coloured contact lens sensor with a tunable colour in this investigation, which can directly report changes in moisture and pressure that are critical signs for xerophthalmia and glaucoma diagnosis, respectively, by altering colours. Importantly, this structurally coloured contact lens sensor is made solely from a biocompatible hydrogel, without the addition of any chemical pigments, therefore exhibiting superior biosafety and wearing comfort for wearable applications. With both excellent biocompatibility and sensing capabilities, this structurally coloured contact lens sensors thus holds great promise for instrument-free ophthalmic health monitoring, which will benefit a large proportion of the population that have a high risk of eye disease.