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

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.

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.

A Disposable Thermally Triggered Photonic Crystal Anti-Counterfeiting Tag with Irreversible Response and Multi-Step Color Changes

Thermochromic photonic crystal (PC) is a promising material for anti-counterfeiting applications, but there are still challenges to further improve the anti-counterfeiting performance and the practicability in usage. Here, a disposable thermally triggered PC anti-counterfeiting tag with irreversible response and multi-step color changes is developed based on the thermochromic Silica/(Polyethylene glycol-Ethoxylated trimethylolpropane triacrylate) (SiO2/(PEG-ETPTA)) double-layer film. The fast and irreversible thermal response come from the quick melting and infiltration of PEG-ETPTA into the PCs upon heating. The multi-step color change at different temperatures originated from the regioselective control of the UV curing degree of the PEG-ETPTA layer and the resulting thermochromic temperature of the double-layer film. Therefore, the invisible PC pattern on the tag can be revealed part by part upon heating and became invisible again after overheating, which offered diversified visual effects and enhanced anti-counterfeiting performances.

Electrically Responsive Photonic Crystals with Enhanced Suspension Stability and Color Saturation for Electrophoretic Displays and Smart Windows

Electrophoretic displays (EPDs) based on photonic crystals show great potential due to their reduced eye fatigue and low power consumption. However, the current image quality and service life of this system still face great challenges. In this work, we fabricated a new kind of electrically responsive photonic crystal (ERPC) device based on PSMA@SiO2 liquid colloidal crystals (LCCs) for EPDs. By introduction of the PSMA core with lower density and higher refractive index, the suspension stability and color saturation of PSMA@SiO2 LCCs were greatly enhanced compared with those of bare SiO2 LCCs. The PSMA@SiO2 LCCs showed brilliant colors, wide color tuning range (∼200 nm), and good reversibility under low voltages (<4 V). Interestingly, the transparency of PSMA@SiO2 LCCs could also be obviously regulated by an electric field, which was different from the traditional ways that change the thickness of PCs or contrast of refractive index (Δn) between the nanospheres and matrix. This transparency modulation offered a novel idea for the transmittance control of smart windows. As a proof of concept, we fabricated a new type of patterned ERPC device to demonstrate their potential in electrophoretic displays and smart windows with controllable transmittance under an electric field.

Tunable and Non-Invasive Printing of Transmissive Interference Colors with 2D Material Inks

Interference colors hold significant importance in optics and arts. Current methods for printing interference colors entail complex procedures and large-scale printing systems for the scarcity of inks that exhibit both sensitivity and tunability to external fields. The production of highly transparent inks capable of rendering transmissive colors has presented ongoing challenges. Here, a type of paramagnetic ink based on 2D materials that exhibit polychrome in one magnetic field is invented. By precisely manipulating the doping ratio of magnetic elements within titanate nanosheets, the magneto-optical sensitivity named Cotton-Mouton coefficient is engineerable from 728 to a record high value of 3272 m-1 T-2, with negligible influence on its intrinsic wide optical bandgap. Combined with the sensitive and controllable magneto-responsiveness of the ink, modulate and non-invasively print transmissive interference colors using small permanent magnets are precised. This work paves the way for preparing transmissive interference colors in an energy-saving and damage-free manner, which can expand its use in widespread areas.

Smart colloidal photonic crystal sensors

Smart colloidal photonic crystals (PCs) with stimuli-responsive periodic micro/nano-structures, photonic bandgaps, and structural colors have shown unique advantages (high sensitivity, visual readout, wireless characteristics, etc.) in sensing by outputting diverse structural colors and reflection signals. In this review, smart PC sensors are summarized according to their fabrications, structures, sensing mechanisms, and applications. The fabrications of colloidal PCs are mainly by self-assembling the well-defined nanoparticles into the periodical structure (supersaturation-, polymerization-, evaporation-, shear-, interaction-, and field-induced self-assembly process). Their structures can be divided into two groups: closely packed and non-closely packed nano-structures. The sensing mechanisms can be explained by Bragg's law, including the change in the effective refractive index, lattice constant, and the order degree. The sensing applications are detailly introduced according to the analytes of the target, including solvents, vapors, humidity, mechanical force, temperature, electrical field, magnetic field, pH, ions/molecules, and so on. Finally, the corresponding challenges and the future potential prospects of artificial smart colloidal PCs in the sensing field are discussed.

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.

Rewritable Structurally Colored Paper Based on Hollow SiO2-Polyurethane Composite Photonic Crystal Film

Rewritable paper, which can be used multiple times as an effective solution for sustainable development and lessen the heavy environment pollution, has received widespread attention. A photonic crystal with dye-free character and tunable structure color has attracted significant interest in this area. Generally, handwriting on the photonic crystal structure containing a responsive polymer or hydrogel ingredient was based on the change of lattice spacing. It is necessary to enrich the diversities of color adjustment mechanism for further application. Herein, a flexible rewritable photonic crystal structurally colored paper with excellent mechanical strength based on the hollow SiO2 (h-SiO2) particle and polyurethane was developed. Owning to the varied optical response of h-SiO2 photonic crystal film in different solvents, handwriting on this paper was realized by applying polarity solvents such as EG as colorless ink directly, which could also be erased by resoaking the film in water. Writing and erasing on this paper were totally reversible. The color adjustment mechanism and the realization of handwriting on this paper are totally different from those of the previous reported photonic crystal-based rewritable paper. The combination of quick handwriting and flexibility is significant for potential application as rewritable paper.

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.

Butterfly-Inspired Tri-State Photonic Crystal Composite Film for Multilevel Information Encryption and Anti-Counterfeiting

Counterfeiting is a worldwide issue and has long troubled legitimate businesses, while nowadays anti-counterfeiting materials and technology are still insufficient to combat the escalating counterfeit behaviors. Inspired by hindwing structure of Troides magellanus, a new kind of anti-counterfeiting material taking advantage of both physical and chemical structures to display multiple optical states is prepared. The chemical units (luminescent lanthanide) are blended with physical units (monodispersed colloidal particles) and mediating molecules, which are then assembled into a photonic crystal structure at room temperature in less than 10 s through a new assembly technique called molecule-mediated shear-induced assembly technique (MSAT). The as-prepared photonic crystal films feature three unique optical states, each displaying structural, fluorescent, and phosphorescent color under different lighting conditions, which integrates colors from both physical and chemical origins. Furthermore, by incorporating different luminescent materials into different parts of the photonic crystal pattern, a high-level information encryption system is designed to be capable of carrying three distinct types of information. Thanks to this powerful tool of MSAT, it is now possible to assemble different-sized, even irregular non-spherical units with monodispersed spherical units into high-quality photonic crystal films, which provides easy access to incorporating new features into photonic crystal systems.

Bioinspired Polypeptide Photonic Films with Tunable Structural Color

We report a new synthetic strategy of combining N-carboxyanhydride (NCA) chemistry and photonic crystals for the fabrication of polypeptide structural color films. Driven by surface-initiated ring-opening polymerization, the di-NCA derivative of l-cystine (Cys) is introduced to replicate the functionalized colloidal crystal templates and construct freestanding P(Cys) films with tunable structural color. Furthermore, the feasibility of preparing patterned polypeptide photonic films is demonstrated via template microfabrication. Because of the incorporation of l-glutamate (Glu) components, the P(Cys-co-Glu) co-polypeptide films are endowed with a visual color responsiveness toward pH changes. Additionally, the polypeptide photonic films show on-demand degradability. Given the large family of amino acid building blocks, this powerful and versatile approach paves the way for chemical derivatization of multifunctional peptide-based optical platforms.