Thermal-Responsive Photonic Crystal with Function of Color Switch Based on Thermochromic System

In Situ Growth of Multiresponsive Structural Color Patterns within Hydrogels for Multiple Information Encryption

Mimicking natural organisms to directly fabricate multiresponsive structural color patterns from small molecules is of great significance for information encryption but remains challenging. Herein, we present a bionic entanglement-interlocking microphase separation strategy for in situ growth of multiresponsive structural color patterns within hydrogel matrixes. The precursor solutions of common polymerization-induced phase-separated materials are used as small-molecule nutrients. The newly generated polymer networks can thus spontaneously collapse into phase-separated microspheres within hydrogels during polymerization. In particular, the dense internetwork entanglements form between the microspheres and hydrogel networks during phase separation, anchoring the microspheres firmly onto hydrogel networks to hinder their gathering. Consequently, these newly grown microspheres can be maintained at the desired nanoscale for yielding the structural blue color by light scattering. Multiresponsive schemochrome patterns can be readily created by growing different microspheres within hydrogel matrixes for multiple information encryptions. We demonstrate that this facile self-growth strategy is applicable to different polymerization-induced phase-separated materials and hydrogel matrixes, regardless of cross-linking modes and geometries.

Shape-stabilized flexible thermochromic films with one-sided adhesion via gradient crosslinking strategy for temperature indicating

Thermochromic dyes (TCDs) based on a three-component color change system suffer from solid rigidity and liquid leakage issues because of the intrinsic solid-liquid phase change performance, resulting in difficulty in temperature visualization applications for smart wearable fields. Despite considerable efforts in microencapsulation of thermochromic dyes, designing and fabricating essentially flexible thermochromic phase change films still need to be explored. Herein, a one-sided adhesive gradient-crosslinked thermochromic film is reported to address these issues to make a trade-off between stability and flexibility, excellent thermochromic performance, and temperature visualization. The thermochromic wearable films have been fabricated exploiting tea polyphenol thermochromic dyes, vinyl dimethylsiloxane, and hydrosilicone oil via the salt-template-assisted method and gradient crosslinking strategy, which have porous structures with an average pore size of 12.8 μm and a porosity of 28 %. Due to the spatial limiting threshold effect of the porosity structure, interconnected 3D polysiloxane porous networks can provide ample support for tea polyphenol thermochromic dyes and effectively prevent liquid leakage. Upon heating, the thermochromic film changes from blue to white with the K/S value decreasing from 7.69 to 0.78 and the ΔE* increasing from 2.7 to 16.1 at 610 nm, and the color-changing temperature is 42 °C. Gradient crosslinked thermochromic films exhibit excellent temperature-responsive color change properties, desirable one-side adhesion, and thermal energy storage, enabling multicolor temperature displays and temperature-controlled multilevel information transfer.

Structural Color Colloidal Photonic Crystals for Biomedical Applications

Photonic crystals are a new class of optical microstructure materials characterized by a dielectric constant that varies periodically with space and features a photonic bandgap. Inspired by natural photonic crystals such as butterfly scales, a series of artificial photonic crystals are developed for use in integrated photonic platforms, biosensing, communication, and other fields. Among them, colloidal photonic crystals (CPCs) have gained widespread attention due to their excellent optical properties and advantages, such as ease of preparation and functionalization. This work reviews the classification and self-assembly principles of CPCs, details some of the latest biomedical applications of large-area, high-quality CPCs prepared using advanced self-assembly methods, summarizes the existing challenges in CPC construction and application, and anticipates future development directions and optimization strategy. With further advancements, CPCs are expected to play a more critical role in biosensors, drug delivery, cell research, and other fields, bringing significant benefits to biomedical research and clinical practice.

Bioinspired Low-Angle-Dependent Photonic Crystal Elastomer for Highly Sensitive Visual Strain Sensor

Photonic crystals (PCs) possess unique photonic band gap properties that can be used in the field of sensors and smart displays if modulated on the micronano structure. Both nonclose-packed (NCP) structure and high refractive index (RI) contrast of PC play important roles in response sensitivity during stretching. Herein, we constructed an NCP-structured PC strain sensor with high RI by a novel coating-etching strategy. Stretch-induced changes in structural color correspond to the strength of the force, enabling the detection of the strength of the acting force by the naked eye. The flexible 3D cross-linked network constructed by poly(ethylene glycol) phenyl ether acrylate and pentaerythritol tetrakis(3-mercaptopropionate) endows the sensor with excellent elasticity and robustness. The designed PC strain sensor achieves high mechanochromic sensitivity (∼8.3 nm/%, 0.02 to 4.21 MPa) and a substantial reflection peak shift (Δλ = 249 nm). More importantly, the high RI contrast (Δn = 0.43) between CdS and polymers imparts isotropic optical properties, ensuring a broad viewing angle while avoiding misleading signals. The research provides a novel fabrication strategy to construct sensitive PC strain sensors, expanding the prospective applicability to human movement monitoring and secure message encryption.

Multicolor recordable and erasable photonic crystals based on on-off thermoswitchable mechanochromism toward inkless rewritable paper

Mechanochromic photonic crystals are attractive due to their force-dependent structural colors; however, showing unrecordable color and unsatisfied performances, which significantly limits their development and expansion toward advanced applications. Here, a thermal-responsive mechanochromic photonic crystal with a multicolor recordability-erasability was fabricated by combining non-close-packing mechanochromic photonic crystals and phase-change materials. Multicolor recordability is realized by pressing thermal-responsive mechanochromic photonic crystals to obtain target colors over the phase-change temperature followed by fixing the target colors and deformed configuration at room temperature. The stable recorded color can be erased and reconfigured by simply heating and similar color-recording procedures respectively due to the thermoswitchable on-off mechanochromism of thermal-responsive mechanochromic photonic crystals along with solid-gel phase transition. These thermal-responsive mechanochromic photonic crystals are ideal rewritable papers for ink-freely achieving multicolor patterns with high resolution, difficult for conventional photonic papers. This work offers a perspective for designing color-recordable/erasable and other stimulus-switchable materials with advanced applications.

Color construction of multi-colored carbon fibers using glucose

Carbon fibers (CFs) have attracted attention in the automotive, aviation, and aerospace industries. However, the coloration of CFs is challenging due to their brittleness, inertness, complexity, and time/energy-intensive processes. Herein, inspired by the naturally grown protrusive nanostructures on the green central surface of peacock back feathers, we report an in-situ self-growing strategy for developing carbon spheres (CSs) on the CFs surface to achieve color tuning. This is achieved via the dynamic growth of CSs using glucose as the feeding material. Combined with the coloration process, the interaction between CSs and CFs promotes stable interfacial forces in integrated molding. This strategy allows the coloring system to continuously vary its color in a designated manner, thereby, endowing it with satisfactory mechanical robustness, acid durability, and light fastness. We anticipate this developed approach can be potentially competitive in the color construction of CFs with multi-colors due to its low-cost manufacturing.

2D Information Security System Based on Polyurethane Inverse Photonic Glass Structure

Information security has become a major global problem in recent years. Thus, people continue to exert much effort in developing new information security technologies based on encryption and storage. In this study, a 2D information security technology based on polyurethane optical devices with inverse photonic glass structure (PU-IPG) is introduced. Based on 1) the swelling and plasticizing effects of various solvents on PU-IPG and 2) the capillary force that can produce geometric deformation on micro/nanostructures when solvents evaporate, a 2D information security system with two modules of decryption (structural color information display) and anticounterfeiting (structural color transformation) is successfully constructed. The spraying method adopted can be simple and fast and can provide a large area to build photonic glass templates, which greatly improves the capacity and category of information in the encryption system. The prepared PU-IPG optical devices can produce large-area multicolor output capability of information. These devices also have excellent mechanical properties, strong cycle stability, environmental friendliness, and low price. Therefore, the preparation strategy has great reference value and application prospects in the field of information security.

Thermochromic Photonic Crystal Paper with Integrated Multilayer Structure and Fast Thermal Response: A Waterproof and Mechanically Stable Material for Structural-Colored Thermal Printing

Thermochromic photonic crystals are promising materials for thermal printing due to their unfaded colors under chemical/illuminated environments and the absence of toxic chemicals. However, the slow thermochromic response, the multistep printing procedures, the use of inks or developing liquids, and the requirement of expensive parts in printers limit their applications. Here, a thermochromic polyurethane/hydrophobic-SiO2 photonic crystal/paraffin (PU/HPO-SiO2 -PC/Para) film with an integrated multilayer structure is fabricated for all-solid-state and single-step thermal printing that is fully compatible with commercial printers. The fast thermochromic response in milliseconds enables high-resolution and grayscale printing as the paraffin infiltration and the color change can be finely controlled in a microscale range. The integrated and hydrophobic multilayer structure renders the thermochromic film good stability in daily liquids, which addresses the long-existing concern of print fading. Meanwhile, the integrated multilayer structure also enhances the mechanical stability when it is deposited on fibrous paper so that people can fold, cut, or staple the thermal papers, and make notes confidently in practical usage.

Permanent irreversible structural color based on core-shell chemically bonded SiO2@P(St-BA) particles

Novel core-shell chemically bonded SiO₂@P(St-BA) particles were designed and self-assembled to prepare photonic crystals. Due to the irreversible collapse of polymer shells during hot-pressing, SiO₂@P(St-BA) particles could provide new ideas for high-stability and bright red-shifted structural color patterns.

An Angle-Independent Multi-Color Display Electro-Responsive Hydrogel Film

In nature, some organisms have the ability to camouflage to adapt to environmental changes; they blend with the environment by changing their skin colors. Such a phenomenon is of great significance for the research of adaptive camouflage materials. In this study, we propose a novel design scheme for the study of angle-independent photonic materials and successfully prepare an electrically tunable multi-color display angle-independent inverse opal photonic gel (IOPG). After photopolymerization of hydroxyethyl methacrylate with ionizable monomer acrylic acid (AA) in a long-range disordered opal template and etching, the angle-independent inverse opal photonic gel is obtained, presenting a single structural color. The electrically responsive color changes can be achieved at different angles. The color of the disordered AA-IOPG changes from green to blue-green when applying +4 V bias voltage and from green to orange when applying -4 V bias voltage. The electrochromism of the disordered AA-IOPG is mainly due to the local pH change caused by water electrolysis under bias voltage, which leads to a change of the swelling ratio. The disordered AA-IOPG shows high color tunability and durability through repeated opposite bias voltage tests, indicating that it is a promising conductive photonic material.

Self-Assembly of Shark Scale-Patterned Tunable Superhydrophobic/Antifouling Structures with Visual Color Response

The stacked riblet-like shark scales, also known as dermal denticles, allow them to control the boundary layer flow over the skin and to reduce interactions with any biomaterial attached, which guide the design of antifouling coatings. Interestingly, shark scales are with a wide variation in geometry both across species and body locations, thereby displaying diversified antifouling capabilities. Inspired by the multifarious denticles, a stretchable shark scale-patterned silica hollow sphere colloidal crystal/polyperfluoroether acrylate-polyurethane acrylate composite film is engineered through a scalable self-assembly approach. Upon stretching, the patterned photonic crystals feature different short-term antibacterial and long-term anti-biofilm performances with a distinguished color response under varied elongation ratios. To gain a better understanding, the dependence of elongation ratio on antiwetting behaviors, antifouling performances, and structural color changes has also been investigated in this research.

Portable Comestible-Liquid Quality Test Enabled by Stretchable and Reusable Ion-Detection Photonic Papers

Currently, there have been widespread investigation conducted into responsive photonic crystal hydrogels (RPCHs) characterized by high selectivity and sensitivity for colorimetric indicators and physical/chemical sensors. In spite of this, it remains challenging to use RPCHs for sensing due to their limited mechanical property and molding capability. In the present study, a double-network structure is proposed to design highly stretchable, sensitive, and reusable ion-detection photonic papers (IDPPs) for assessing the quality of visual and portable comestible liquids (e.g., soy sauce). It is constructed by integrating polyacrylamide and poly-methacryloxyethyl trimethyl ammonium chloride with highly ordered polystyrene microspheres. The double-network structure improves the mechanical properties of IDPPs with their elongation at break increasing from 110 to 1600%. Meanwhile, the optical properties of photonic crystals are retained. The IDPPs achieve a fast ion response by applying control on the swelling behavior of the hydration radius of the counter ions through ion exchange. Given a certain concentration range (0.01-0.10 M), chloride ions can be detected fast (3-30 s) by exchanging ions with a small hydration radius through an IDPP, which is clearly observable. Due to the improvement of mechanical properties and the reversible exchange of ions derived from IDPPs, their reusability is significantly enhanced (>30 times). Characterized by a simple operation, high durability, and excellent sustainability, these IDPPs are promising for practical application in food security and human health assessment.

Dual Responsive Dependent Background Color Based on Thermochromic 1D Photonic Crystal Multilayer Films

In this paper, we present dual responsive one-dimensional (1D) photonic crystal (PC) multilayer films that utilize a high-humidity environment and temperature. Dual responsive 1D PC multilayer films are fabricated on precoated thermochromic film by sequential alternate layer deposition of photo-crosslinkable poly(2-vinylnaphthalene-co-benzophenone acrylate) (P(2VN-co-BPA)) as a high refractive index polymer, and poly(4-vinylpyrollidone-co-benzophenone acrylate) P(4VP-co-BPA) as a low refractive index polymer. The thermochromic film shows a vivid color transition from black to white at 28 °C. Three different colors of thermochromic 1D PC multilayer films are prepared by thickness modulation of P(4VP-co-BPA) layers, and the films on a black background exhibit visible spectrum color only in a high-humidity environment (over 90% relative humidity (RH)). For the three films placed on a hands display, three different composite colors are synthesized by the reflection of light, including yellow, magenta, and cyan, due to the changing of backgrounds from black to white with temperature. Additionally, the films show remarkable color transitions with reliable reversibility. The films can be applied as anti-counterfeiting labels and can be used for smart decoration films. To the best of our knowledge, this is the first report of dual response colorimetric films that change color in various ways depending on temperature and humidity changes, and we believe that it can be applied to various applications.

Bioinspired Colloidal Photonic Composites: Fabrications and Emerging Applications

Organisms in nature have evolved unique structural colors and stimuli-responsive functions for camouflage, warning, and communication over millions of years, which are essential to their survival in harsh conditions. Inspired by these characteristics, colloidal photonic composites (CPCs) composed of colloidal photonic crystals embedded in the polymeric matrix are artificially prepared and show great promise in applications. This review focuses on the summary of building blocks, i.e., colloidal particles and polymeric matrices, and constructive strategies from the perspective of designing CPCs with robust performance and specific functionality. Furthermore, their state-of-the-art applications are also discussed, including colorful coatings, anti-counterfeiting, and regulation of photoluminescence, especially in the field of visualized sensing. Finally, current challenges and potential for future developments in this field are discussed. The purpose of this review is not only to clarify the design principle for artificial CPCs but also to serve as a roadmap for the exploration of next-generation photonic materials.

Color-Tuning Mechanism of Electrically Stretchable Photonic Organogels

In contrast to nano-processed rigid photonic crystals with fixed structures, soft photonic organic hydrogel beads with dielectric nanostructures possess advanced capabilities, such as stimuli-responsive deformation and photonic wavelength color changes. Recenlty, advanced from well-investigated mechanochromic method, an electromechanical stress approach is used to demonstrate electrically induced mechanical color shifts in soft organic photonic hydrogel beads. To better understand the electrically stretchable color change functionality in such soft organic photonic hydrogel systems, the electromechanical wavelength-tuning mechanism is comprehensively investigated in this study. By employing controllable electroactive dielectric elastomeric actuators, the discoloration wavelength-tuning process of an electrically stretchable photonic organogel is carefully examined. Based on the experimental in-situ response of electrically stretchable nano-spherical polystyrene hydrogel beads, the color change mechanism is meticulously analyzed. Further, changes in the nanostructure of the symmetrically and electrically stretchable organogel are analytically investigated through simulations of its hexagonal close-packed (HCP) lattice model. Detailed photonic wavelength control factors, such as the refractive index of dielectric materials, lattice diffraction, and bead distance in an organogel lattice, are theoretically studied. Herein, the switcing mechanism of electrically stretchable mechanochromic photonic organogels with photonic stopband-tuning features are suggested for the first time.

Materials with Tunable Optical Properties for Wearable Epidermal Sensing in Health Monitoring

Advances in wearable epidermal sensors have revolutionized the way that physiological signals are captured and measured for health monitoring. One major challenge is to convert physiological signals to easily readable signals in a convenient way. One possibility for wearable epidermal sensors is based on visible readouts. There are a range of materials whose optical properties can be tuned by parameters such as temperature, pH, light, and electric fields. Herein, this review covers and highlights a set of materials with tunable optical properties and their integration into wearable epidermal sensors for health monitoring. Specifically, the recent progress, fabrication, and applications of these materials for wearable epidermal sensors are summarized and discussed. Finally, the challenges and perspectives for the next generation wearable devices are proposed.

Dual-Responsive Photonic Crystal Sensors Based on Physical Crossing-Linking SF-PNIPAM Dual-Crosslinked Hydrogel

Flexible wearable materials have frequently been used in drug delivery, healthcare monitoring, and wearable sensors for decades. As a novel type of artificially designed functional material, photonic crystals (PCs) are sensitive to the changes in the external environment and stimuli signals. However, the rigidity of the PCs limits their application in the field of biometric and optical sensors. This study selects silk fibroin (SF) and poly-N-isopropylacrylamide (PNIPAM) as principal components to prepare the hydrogel with the physical crosslinking agent lithium silicate (LMSH) and is then integrated with PCs to obtain the SF-PNIPAM dual-crosslinked nanocomposite for temperature and strain sensing. The structural colors of the PCs change from blue to orange-red by the variation in temperature or strain. The visual temperature-sensing and adhesion properties enable the SF-PNIPAM dual-crosslinked nanocomposite to be directly attached to the skin in order to monitor the real-time dynamic of human temperature. Based on its excellent optical properties and biocompatibility, the SF-PNIPAM dual-crosslinked nanocomposite can be applied to the field of visual biosensing, wearable display devices, and wound dressing materials.

Ultrafast and Irreversibly Thermochromic SiO2 -PC/PEG Double Layer for Green Thermal Printing

Traditional thermochromic photonic crystal (PC) usually has a slow and reversible thermal response, which limits its application in thermal printing. Here, the authors develop a thermochromic "SiO₂ -PC/PEG" double layer structure with a responding time of milliseconds for fast thermal printing. Controlled by the print-head, the polyethylene glycol (PEG) melts, infiltrates, and solidifies within the interparticle voids, which instantly and irreversibly changes the refractive index and produces the PC pattern. Multicolor printing can be realized by tuning the size and type of colloidal particles. Resolution as high as 300 DPI is achieved to print the high-resolution patterns and then the grayscale patterns based on the control of pixel densities. Different from fiber thermal paper, the "SiO₂ -PC/PEG" film has no toxic bisphenol A and possesses superior light stability for keeping the images longer. It is fully compatible with the commercial printer, which provides a mature solution for fast and convenient preparation of PC patterns.

Robust Large-Sized Photochromic Photonic Crystal Film for Smart Decoration and Anti-Counterfeiting

Photochromic materials are widely investigated due to their vivid color transformation for many real applications. In this work, a new kind of multiangle photochromic photonic crystal (PC) material with high robustness and long durability for smart phone decoration and anticounterfeiting features is fabricated. After thermal mixing of spiropyran powder and monodisperse core-interlayer-shell (CIS) particles, a large-area and high-quality photochromic PC film has been prepared by the self-designed bending-induced ordering technique (BIOT). The spiropyran powder can be well dispersed in the order-structured PC system, so the perfect synergistic combination of photochromism and angle-dependent structure colors can be achieved. The color-switching test for the as-prepared photochromic PC film proved its excellent reversibility and stability. Because of the excellent flexibility of the photo-cross-linked PC films, they can be designed and cut into various shapes with high robustness and long durability. Interestingly, a temperature-controlled photochromic effect was found in this photochromic PC system. Therefore, the as-prepared photochromic PC films can play a significant role in the fields of smart decoration and anticounterfeiting by their unique color switching effects under different stimuli. More importantly, our work verified the feasibility of this route to prepare a series of large-sized advanced smart PC devices by adding versatile functional materials.

Upconversion Nanoparticle-Integrated Bilayer Inverse Opal Photonic Crystal Film for the Triple Anticounterfeiting

Optical anticounterfeiting plays a vital role in information security because it can be recognized by the naked eye and is difficult to imitate. Herein, a hydrophilic modified upconversion nanoparticle (M-UCNP)-integrated bilayer inverse opal photonic crystal (IOPC) film was designed in which the luminescent M-UCNPs were deposited on the surface of the optimized bilayer structure with double photonic stop bands. The structure which can modulate light to produce structural colors can also enhance the upconversion luminescence (UCL) to improve the anticounterfeiting effect synergistically. On the one hand, the reflection colors from green to blue were observed in the specular angles on the front (540-layer) of the film. Meanwhile, the scattering colors under nonspecular angles from red to blue on the back (808-layer) appeared in the natural light. On the other hand, the bilayer structure in which the 808-layer functions as a "secondary excitation source" to improve the intensity of the excitation light on M-UCNPs and the 540-layer reflects the emission light of the M-UCNPs to enhance the UCL intensity endows the film with good night vision ability. Finally, the dual-mode structural colors and enhanced UCL of the patterned film work together to realize triple anticounterfeiting in banknotes.

Fabrication of multi-responsive photonic crystals based on selenium-containing copolymers

Responsive photonic crystals (PCs) have attracted great interest due to their adjustable structure color.

Supramolecular DNA Photonic Hydrogels for On-Demand Control of Coloration with High Spatial and Temporal Resolution

Hydrogels embedded with periodic arrays of nanoparticles display a striking photonic crystal coloration that may be useful for applications such as camouflage, anticounterfeiting, and chemical sensing. Dynamically generating color patterns requires control of nanoparticle organization within a polymer network on-demand, which is challenging. We solve this problem by creating a DNA hydrogel system that shows a 50 000-fold decrease in modulus upon heating by ∼10 °C. Magnetic nanoparticles entrapped within these DNA gels generate a structural color only when the gel is heated and a magnetic field is applied. A spatially controlled photonic crystal coloration was achieved by photopatterning with a near-infrared illumination. Color was "erased" by illuminating or heating the gel in the absence of an external magnetic field. The on-demand assembly technology demonstrated here may be beneficial for the development of a new generation of smart materials with potential applications in erasable lithography, encryption, and sensing.

Colloidal Self-Assembly Approaches to Smart Nanostructured Materials

Colloidal self-assembly refers to a solution-processed assembly of nanometer-/micrometer-sized, well-dispersed particles into secondary structures, whose collective properties are controlled by not only nanoparticle property but also the superstructure symmetry, orientation, phase, and dimension. This combination of characteristics makes colloidal superstructures highly susceptible to remote stimuli or local environmental changes, representing a prominent platform for developing stimuli-responsive materials and smart devices. Chemists are achieving even more delicate control over their active responses to various practical stimuli, setting the stage ready for fully exploiting the potential of this unique set of materials. This review addresses the assembly of colloids into stimuli-responsive or smart nanostructured materials. We first delineate the colloidal self-assembly driven by forces of different length scales. A set of concepts and equations are outlined for controlling the colloidal crystal growth, appreciating the importance of particle connectivity in creating responsive superstructures. We then present working mechanisms and practical strategies for engineering smart colloidal assemblies. The concepts underpinning separation and connectivity control are systematically introduced, allowing active tuning and precise prediction of the colloidal crystal properties in response to external stimuli. Various exciting applications of these unique materials are summarized with a specific focus on the structure-property correlation in smart materials and functional devices. We conclude this review with a summary of existing challenges in colloidal self-assembly of smart materials and provide a perspective on their further advances to the next generation.

Armored colloidal photonic crystals for solar evaporation

Colloidal photonic crystals (CPCs) with a highly ordered crystal structure have attracted great attention in displays, colorimetric sensors and solar energy utilization fields. However, the easily cracking microstructure, inferior assembly efficiency and low refractive index contrast result in poor structural colors. Herein, we develop core-shell poly(styrene-acrylic)@polypyrrole (P(St-AA)@PPy) colloidal nanoparticles by the in situ chemical coupling reaction via droplet microfluidic technology. By membrane separation-assisted assembly (MSAA) and electrostatic spraying strategies, the P(St-AA)@PPy colloidal nanoparticles are assembled into the CPC film, which presents high assembly efficiency and saturated angle-independent structural colors, due to the light-absorbing PPy shell and hydrogen-bond interaction between nanoparticles. Benefitting from these outstanding performances, the P(St-AA)@PPy film shows excellent photothermal properties, which can realize a solar vaporization rate of 1.5825 kg m^-2 h^-1, corresponding to a light-to-vapor efficiency of 94.20%, under 1.0 sun solar irradiance conditions. Our findings open a path for the design of functional CPCs and new-generation photothermal applications.

Fabrication of robust and reusable mold with nanostructures and its application to anti-counterfeiting surfaces based on structural colors

In this study, we report a method to fabricate molds and flexible stamps with 2D photonic crystal structures. This includes self-assembly of polystyrene particles into monolayer, oxygen reactive ion etching, thin film (chromium (Cr)) deposition, and polydimethylsiloxane replication. By tuning the thickness of Cr layer, reusable master molds with nano bumps or nano concaves could be prepared selectively. We showed that the replicated flexible stamps out of these molds exhibited structural colors. Characteristics of the colors depended on viewing angle, brightness of background and light source. And the colors even faded out when the background is white or when the stamp was bent. By using this feature, possible strategies for anti-counterfeiting applications have been suggested in this study. Since the molds are reusable and the fabrication method is simple and cost-effective, this study is expected to contribute to nano devices for industries in future.

Magneto-sensitive photonic crystal ink for quick printing of smart devices with structural colors

In this paper, we report a facile strategy to combine magneto-responsive photonic crystal (MRPC) ink with 3D printing technology. The building blocks of MRPC are based on Fe₃O₄ magnetic nanoparticle clusters (MNCs) with uniform and tunable size. The MNC dispersion is able to change its photonic band gap from red to blue as the external magnetic field strength is increased. The magneto-responsive photonic crystal ink can be readily obtained by taking advantage of an ethylene glycol (EG)-in-oil emulsion with a reinforced silicone rubber prepolymer as the outer phase. Using the well-designed formula, the MNC dispersion can be well-preserved inside the emulsion droplets of the ink, maintaining its original contactless magnetic field response. As a proof of concept, custom quick response code and butterfly patterns were successfully printed and showed vivid and tunable color as a function of the external magnetic field strength with good repeatability.

A self-healing smart photonic crystal hydrogel sensor for glucose and related saccharides

A self-healing smart PhC hydrogel sensor that combines the optical property of photonic crystal and the dynamic regeneration property of boronate ester bond has been prepared for determination of glucose and related saccharides using Debye diffraction ring detection. The boronate ester bond formed through phenylboronic acid and dopamine endows the hydrogel network self-healing ability, and the tensile stress of the healing hydrogel can recover to 94.4%; this excellent self-healing property can effectively improve the reliability and lifetime of the hydrogel. Due to the high bonding capacity between 1,2- and 1,3-diol and phenylboronic acid, the hydrogel sensor has a good recognition ability for glucose and related saccharides. The reaction between the monosaccharides and the phenylboronic acid group makes the sensor swell and the diameter of the Debye diffraction ring decrease. The sensor shows good reuse and responsive ability for saccharides; the RSD of the recoverability assays is 4.3%. The determination range of the sensor to glucose is 0.5 to 12 mM. The sensor also has good response to glucose in urine, exhibiting potential application value in the preliminary screening of diabetes. Although the sensor has poor selectivity for specific monosaccharides, the process of measuring the Debye ring makes the determination no longer rely on expensive and complicated equipment and greatly simplifies the determining process and reduces the cost of determination, which shows a broad application prospect. The boronate ester bond formed through phenylboronic acid and dopamine results in the self-healing property of hydrogel network, which can effectively improve the reliability and lifetime of hydrogel. And due to the high bonding capacity between 1,2- and 1,3-diol and phenylboronic acid, the smart hydrogel sensor has a good recognition ability for glucose and related saccharides. The reaction between the monosaccharides and the phenylboronic acid group breaks the original boronate ester bond; this will lead to a decrease in cross-linking density of the PhC hydrogel sensor and further makes the sensor swell and the diameter of the Debye diffraction ring decrease.

Exploring Reversible Thermochromic Behavior in a Rare Ni(II)-MOF System

Thermochromic metal-organic frameworks (MOFs) are promising functional materials for a wide range of applications due to their ability to exhibit color variation under external temperature stimuli, yet the development of them with high cyclability and efficient regeneration processes remains challenging. Here, presented is a rare example of an ultrastable Ni(II)-MOF exhibiting an unprecedented reversible four-step color change between two complementary colors in a wide temperature range, which could be repeated for at least 500 cycles without losing crystallinity and thermochromic performance. Notably, the regeneration can be achieved within 1 min by simply letting the crystals cool naturally in the air, facilitated by the unique nature of the channels' inner surface. The reversible thermochromic behavior is owing to a series of reversible crystal structure changes with temperature, including the stepwise dehydration/rehydration process, and structural changes. This work facilitates the future development of more MOF-based reversible thermochromic materials with excellent performance and improved practical applicability.

A two-dimensional photonic crystal hydrogel biosensor for colorimetric detection of penicillin G and penicillinase inhibitors

A simple penicillinase functionalized two-dimensional photonic crystal hydrogel (2DPPCH) biosensor was developed for colorimetric detection of penicillin G and penicillinase inhibitors. The penicillinase can specifically recognize penicillin G and catalyze it to produce penicilloic acid, which decreases the pH of the hydrogel microenvironment and shrinks the pH-sensitive hydrogel. The particle spacing decrease of the 2D photonic crystal array induced by the hydrogel shrinkage further causes a blue-shift in the diffraction wavelength. While the hydrolysis reaction is repressed upon treatment with clavulanate potassium (a kind of penicillinase inhibitor), no significant change in the diffraction wavelength is found. The detection of targets can be achieved by measuring the Debye diffraction ring diameter or observing the structural color change in the visible region. The lowest detectable concentrations for penicillin G and clavulanate potassium are 1 μM and 0.1 μM, respectively. Moreover, the 2DPPCH is proved to exhibit high selectivity and an excellent regeneration property, and it shows satisfactory performance for penicillin G analysis in real water samples.

Simple and efficient fabrication of multi-stage color-changeable photonic prints as anti-counterfeit labels

Color changeable photonic prints (CCPPs) show their potential applications in high-level information storage and anti-counterfeiting, but usually suffer from the complex fabrication process and limited color variation. Here, a simple and efficient method is developed to generate CCPPs with multilevel tunable color contrasts by packing the solvent responsive photonic crystals with diverse cross-linking degrees and desired way. The key to the successful fabrication is to create and control over the optical response of each part of the CCPPs through altering the cross-linking degree of PCs and thus the affinity between the CCPPs and solvents. A CCPPs based anti-fake label with the encrypted information functionality which originates from reversible color change between dried state and swelling with the mixture of acetic acid and ethanol is investigated. Compared with conventional CCPPs, the as-prepared CCPPs can reveal multistage information depending on the volume fraction of ethanol. This work provides a new insight for the simple fabrication of CCPPs and will facilitate their applications in the information protection and high-level anti-counterfeiting.

Transmittance Tunable Smart Window Based on Magnetically Responsive 1D Nanochains

Smart optical materials are drawing more and more attention because of their wide application in energy conservation, wearable sensors, optical tuning, and medical devices. However, current smart optical materials, including electroresponsive, thermoresponsive, and mechanoresponsive materials, are greatly restricted in practical applications because of their long response time, complicated preparation, and high cost. This study develops a novel, magnetically tunable, smart optical material with swift and high-contrast optical switching based on one-dimensional (1D) Fe₃O₄@SiO₂ nanochains (NCs), which have the large shape anisotropy of the 1D structure and the superparamagnetic properties of Fe₃O₄ particles. The material exhibited a clear transparent state when NCs were arranged parallel to the viewing direction under an applied magnetic field, whereas it showed good shielding effect when the NCs were randomly oriented upon removal of the field. The light transmittance could be dynamically adjusted over the wide range of 20-80% through a small applied magnetic field of 50-100 Oe, which is superior to most of the currently reported systems. This swift, sensitive, and reversible response is attributed to the good responsivity of magnetic NCs. Also, an effective model was proposed to explain the transmittance modulation scheme and forecast its optical potential. The large tunable range and the low triggered field make Fe₃O₄@SiO₂ NCs an advantageous candidate for application in smart windows, optical switchers, and other fields.

Study on the Twisted and Coiled Polymer Actuator with Strain Self-Sensing Ability

Twisted and coiled polymer actuators (TCAs) are a kind of efficient artificial muscles, which have good prospects for application in soft robots, bionic devices, and biological, medical, and other high-tech fields. However, the inability to dynamically sense and adjust the strain of the actuator will lead to uncertainty in the accuracy of deformation and strain, resulting in imprecise target action. Herein, TCAs with strain self-sensing ability (TCASA) are prepared by integrating the stretchable optomechanical film (SOMF) sensors into TCAs, which provides a simple strategy for dynamical strain sensing. These SOMFs have a wide range of color changes during deformation of TCAs, and the strain is perceived by observing the color change according to the corresponding relationship between color change and strain. Furthermore, the proposed TCASA maintain excellent cycling stability of strain self-sensing during cyclic tests (200 cycles) and excellent strain self-sensing performance to perform strain control compared to TCAs without SOMFs. The results indicate that the proposed structure is a promising soft actuator with excellent strain self-sensing ability, which is well suited for soft robots, bionic devices, biological and medical fields, smart wearable technologies, and so forth, especially when controlled, repetitive deformations are required.

Liquid photonic crystal detection reagent for reliable sensing of Cu2+ in water

A traditional hydrogel photonic crystal sensor is prepared by multiple steps, including colloidal assembly, polymerization, and recognition group modification, and its measurement repeatability is a challenge due to the inevitable deviations in sensor fabrication and application. In this work, a salicylic acid-containing “SiO₂/propylene carbonate” liquid photonic crystal (Sal-LPC), as a new photonic sensing material, was developed to demonstrate reliable measurement of Cu²⁺ in water. When the Sal-LPC reagent was mixed with the test sample, the Cu²⁺ promoted the release of H⁺ from Sal and shrank the photonic crystal lattice, so that the Cu²⁺ concentration could be determined by the reflection blueshift of liquid PC. The Sal-LPC reagent showed a stronger response to Cu²⁺ than to other cations, and its sensitivity and measurement range could be optimized by the particle fraction and Sal dosage. Compared to traditional PC hydrogel sensors, the liquid PC reagent was composed of colloidal particles and responsive species, which required no strict control in synthesis. More importantly, the optical response of the liquid PC reagent was scarcely affected by changes in synthesis, storage, or application, and it could interact with the analyte quickly and quantitatively, which ensured accurate and repeatable measurement in either chemical analysis or environmental monitoring.

A salicylic acid-containing liquid photonic crystal can detect Cu²⁺ through its reflection blueshift due to the release of H⁺.

Spherical Colloidal Photonic Crystals with Selected Lattice Plane Exposure and Enhanced Color Saturation for Dynamic Optical Displays

While structural color materials have nonfading properties and contribute significantly to the sustainable development of pigments or dyes, they are plagued by low color saturation and limited color tunability. Here, we describe a new type of spherical colloidal photonic crystals (CPCs) prepared by a droplet-based microfluidic strategy, featuring enhanced color saturation and tunable structural colors. Methyl viologen (MV) functionalized SiO₂ colloids were synthesized and used for the preparation of CPCs in microdroplets. Because of the absorption of incoherently scattered light by MV, the ratio of peak-to-background amplitude in the reflectance spectra of CPCs is increased, leading to brilliant structural color with enhanced saturation. The lattice plane exposure of spherical CPCs depends on the refractive index contrast between the filling medium and SiO₂ building blocks, and this offers an alternative way to tune the structural color in a spherical CPC. Accordingly, a dynamic optical display was constructed, providing valuable insights to the future development of structural color-based sensors, surface coatings, or displays.

Influence of Thermochromic Pigment Powder on Properties of Waterborne Primer Film for Chinese Fir

This study chose organic thermochromic pigment powder and waterborne wood primer as the paint base, and Chinese fir board as the substrate to prepare thermochromic waterborne coatings with different concentrations of thermochromic pigment powder. The best concentration of thermochromic pigment powder for waterborne primer film on Chinese fir surface was explored. The experimental results showed that the color-changing property of the primer film was the best when the concentration of pigment powder in primer film was 5.0%–10%. There was a negative correlation between the gloss of the primer and the concentration of pigment powder. The gloss of the primer film was the highest when the concentration of pigment powder was 5%. When the concentration of pigment powder is 0%–20% and 25.0%–30%, the adhesion of the coating is grade 0 and grade 1, respectively. The resistance to the impact of primer film increased with the increase of concentration of pigment powder, but the resistance to the impact of primer film with 0%–30% of thermochromic pigment powder concentration was similar. Scanning electron microscopy showed that the higher the concentration of thermochromic pigment powder, the more particles and agglomeration. When the concentration of pigment powder was 5%, the distribution of particles was uniform and no agglomeration, and the microstructure of primer film was the best. Infrared spectroscopy showed that there was no difference in the composition of the paint film from 0% to 30%. The results showed that the comprehensive property of waterborne primer film on Chinese fir was better when the pigment concentration was 5%. Waterborne thermochromic primer film provides a potential application for the development of intelligent furniture in different temperature ranges.