Structural Color Circulation in a Bilayer Photonic Crystal by Increasing the Incident Angle

Ethanol-responsive structural colors with multi-level information encryption based on the patterned three-layer inverse opal photonic crystal

Stimulus-responsive inverse opal photonic crystals (IOPCs) with tunable structural colors show significant promise in information security. To improve upon the traditional bilayer structure with limited color information and single decoding mode, this work developed an ethanol-responsive structure with multi-level information encryption ability by inserting a functional layer into two shielding layers (red Layer A with a photonic stop band (PSB) at 640 nm and green Layer C with a PSB at 530 nm). The functional layer was composed of colorless Layer B, a quick response (QR) code pattern made of TiO2 nanoparticles, and a dense polymer. Due to the isolation of distinct layers, different reflectance values, and different PSB positions of the three-layer IOPC, the structural color of Layer B could only be "turned on" by wetting the entire structure when its PSB redshifted from 360 nm to 460 nm. Specifically, when either side was individually wetted, the PSB of Layer A or C redshifted to 825 nm or 685 nm, and the color of the QR code was dominated by the unwetted red or green layer. After the entire structure had been soaked, the blue QR code was decoded. Meanwhile, when the detecting angle increased from 5° to 60°, the PSBs of Layers B and C in the wetted three-layer IOPC blueshifted from 460 nm to 365 nm and from 685 nm to 540 nm, respectively, which resulted in a cascade decoding process with a single- or mixed-color output. This structure provides a good foundation for multi-level information encryption.

Polyethylene imine-modified photonic crystal microfluidic chip for highly sensitive detection of microbial spores

To address the lengthy cycles, complex operations, high costs, and insufficient sensitivity of biomarker detection in traditional biological control agents, photonic crystal treated with PEI was developed for highly sensitive detection of Sclerotinia sclerotiorum microbial spores. By incorporating gelatin molecules, photonic crystal is endowed with excellent photothermal stability and high stability in aqueous solutions. The photonic crystal surface is conferred a positive charge by PEI, which can be used to enhance the adsorption of spores. Efficient enrichment of Sclerotinia sclerotiorum and Purpureocillium lilacinum spores is achieved, with coefficients of determination (RYe et al. (2014)2) 0.963 and 0.971, respectively. The detection range is from 102 to 106 spores/ml, and the photonic crystal exhibited good reusability. The prepared photonic crystal enables rapid, non-destructive, and accurate quantitative detection of microbial spores.

Chitosan-Based Structural Color Films for Humidity Sensing with Antiviral Effect

This scientific investigation emphasizes the essential integration of nature's influence in crafting multifunctional surfaces with bio-inspired designs for enhanced functionality and environmental advantages. The study introduces an innovative approach, merging color decoration, humidity sensing, and antiviral properties into a unified surface using chitosan, an organo-biological polymer, to create cost-effective multilayered films through sol-gel deposition and UV photoinduced deposition of metal nanoparticles. The resulting chitosan films showcase diverse structural colors and demonstrate significant antiviral efficiency, with a 50% and 85% virus inhibition rate within a rapid 20 min reaction, validated through fluorescence cell expression and real-time qPCR (polymerase chain reaction) assays. Silver-deposited chitosan films further enhance antiviral activity, achieving remarkable 91% and 95% inhibition in independent assays. These films exhibit humidity-responsive color modifications across a 25-90% relative humidity range, enabling real-time monitoring validated through simulation studies. The proposed three-in-one functional surface can have versatile applications in surface decoration, medicine, air conditioning, and the food industry. It can serve as a real-time humidity sensor for indoor and outdoor surfaces, find use in biomedical devices for continuous humidity monitoring, and offer antiviral protection for frequently handled devices and tools. The customizable colors enhance visual appeal, making it a comprehensive solution for diverse applications.

In Situ Metal Deposition on Perhydropolysilazane-Derived Silica for Structural Color Surfaces with Antiviral Activity

Structural coloration has recently sparked considerable attention on the laboratory and industrial scale. Structural colors can create vivid, saturated, and long-lasting colors on metallic surfaces for optical filters, digital displays, and surface decoration. This study used an all-solution, low-cost method, free of a specific setup procedure, to fabricate structural colors of a multilayered metal-dielectric structure based on interference effects within a Fabry-Perot cavity. The insulating (dielectric) layer was produced from perhydropolysilazane, an inorganic silicon-containing polymer, from which hydrogen was liberated during conversion into silica and applied in situ to reduce metallic nanoparticles on the silica surface. This simple manufacturing technique contributes to the fabrication of large, high-quality surfaces, which could potentially be employed for surface decoration. The fabricated surfaces also exhibited excellent hydrophobic properties with contact angles up to 137°, endowing them with self-cleaning properties. In addition, the antiviral and antibacterial impact of the silver (Ag)/silica (SiO2)/stainless steel (SUS) film was also examined, as Ag has been reported to have antimicrobial and, recently, antiviral properties. According to three independently conducted antiviral assays, the fluorescence expression of virus-infected cells, PCR analysis, and modified tissue culture infectious dose assay, the film inhibited lentivirus by 75, 97, and 99% when exposed to the virus for 20 min, 1 h, and 20 min, respectively. Furthermore, the film had exceptional antibacterial activity with no colony growth observed for 24 and 12 h of inoculation. It is thus conceivable that these structural color-based films can be used to not only decorate metal surfaces with aesthetic colors but also limit virus and bacterium propagation successfully.

Trace Explosive Detection Based on Photonic Crystal Amplified Fluorescence

With increasing demand for public security and environmental protection, it is highly desirable to develop strategies to identify trace explosives (e. g., 2,4,6-trinitrotoluene (TNT)). Herein, we report novel photonic crystal (PC)-based sensor chips for trace TNT detection by using amplification effect of PCs on fluorescence (FL) signals. The sensor chips are constructed by integrating silica nanoparticles (NPs) modified with (3-aminopropyl)triethoxysilane (APTES) and fluorescein isothiocyanate isomer (FITC) and PC substrates. The amino groups on FITC-APTES-silica NPs can specifically bind with TNT molecules to form Meisenheimer complexes and strongly quench the FL signal of neighboring fluorophores FITC through Förster resonance energy transfer. PCs with matched PBG can amplify the FL signal of FITC-APTES-silica NPs about 24.4-fold and significantly improve sensitivity and resolution of trace TNT detection with the limit of detection of 0.23 nM. The PC-based sensor chips are stable, sensitive, and reliable TNT sensing platforms, showing great potential in homeland safety and environmental protection.

Rapid Color-Switching of MnO2 Hollow-Nanosphere Films in Dynamic Water Vapor for Reversible Optical Encryption

Drop-casting manganese oxide (MnO₂ ) hollow nanospheres synthesized via a simple surface-initiated redox route produces thin films exhibiting angle-independent structural colors. The colors can rapidly change in response to high-humidity dynamic water vapor (relative humidity > 90%) with excellent reversibility. When the film is triggered by dynamic water vapor with a relative humidity of ≈100%, the color changes with an optimal wavelength redshift of ≈60 nm at ≈600 ms while there is no shift under static water vapor. The unique selective response originates from the nanoscale porosity formed in the shells by randomly stacked MnO₂ nanosheets, which enhances the capillary condensation of dynamic water vapor and promotes the change of their effective refractive index for rapid color switching. The repeated color-switching tests over 100 times confirm the durability and reversibility of the MnO₂ film. The potential of these films for applications in anti-counterfeiting and information encryption is further demonstrated by reversible encoding and decoding initiated exclusively by exposure to human breath.

Low-Angle-Dependent Anticounterfeiting Label Decoded by Alcohol Tissue Wiping Based on a Multilayer Photonic Crystal Structure

The application of photonic crystals (PCs) as anticounterfeiting materials has been widely investigated because of their tunable photonic stop band and corresponding changeable structural colors. In this work, we designed a composite PC structure including an information CdS PC layer at the bottom and a polymer-based layer composed of an inverse opal PC (IOPC) layer and a disordered porous layer on the top, which can be decoded by an alcohol tissue. The high refractive index of the bottom patterned CdS PC layer provides the structure with a vivid low-angle-dependent structural color in the decoded mode, which ensures the stability of the information conveyed by this label. When the incident angle changed from 5 to 45°, the structural color of the patterned CdS layer changed slightly. In the hidden mode, the low transmittance shields the structural color of the CdS layer. When the structure was wiped with the alcohol tissue, the transmittance of the upper IOPC layer could be increased quickly due to the similar refractive indexes of the used polymer and alcohol, and the pattern of the CdS layer was decoded. Thus, the designed composite PC can act as an anticounterfeiting label, in which the encrypted pattern can be decoded by alcohol tissue wiping and shows a vivid low-angle-dependent structural color. To enhance the anticounterfeiting ability of the designed structure, a double-sided label with different encryption patterns on both sides was designed. Based on the simple reversible encryption and decryption process as well as the color stability, the label shows great application potential in the daily anticounterfeiting field.

Iridescent Colloidal Crystals Composed of SiO2 Porous Hollow Sphere for SERS Application

In recent years, the application of low-refractive-index materials in the optical field has attracted considerable attention due to it high transmittance and high optical sensitivity. In this study, we synthesized SiO₂ porous hollow spheres (SPHS) with an ultralow refractive index (n = 1.05) by using a templating method. Their refractive indices could be easily controlled from 1.05 to 1.08 by tuning the thickness of shell. In addition, a droplet coatings method is proposed for SPHS colloidal crystal (CC) by controlling the temperature and humidity. The SPHS CCs displayed distinct structural colors when the incident angle was adjusted and demonstrated high angular resolution. Moreover, the iridescent color changes could be observed with the naked eye. For surface-enhanced Raman scattering application, more analyte could be absorbed by the porous shells, and metal nanoparticles were coated on the SPHSs surface to increase the hot spot density for improving the SERS intensity.

A Biomimetic Human Lung-on-a-Chip with Colorful Display of Microphysiological Breath

Lung-on-a-chip models hold great promise for disease modeling and drug screening. Herein, inspired by the iridescence phenomenon of soap bubbles, a novel biomimetic 3D microphysiological lung-on-a-chip system with breathing visualization is presented. The system, with an array of pulmonary alveoli at the physiological scale, is constructed and coated with structural color materials. Cyclic deformation is induced by regular airflow, resembling the expansion and contraction of the alveoli during rhythmic breathing. As the deformation is accompanied with corresponding synchronous shifts in the structural color, the constructed system offers self-reporting of the cell mechanics and enables real-time monitoring of the cultivation process. Using this system, the dynamic relationships between the color atlas and disease symptoms, showing the essential role of mechanical stretching in the phenotypes of idiopathic pulmonary fibrosis, are investigated. These features make this human lung system ideal in biological study, disease monitoring, and drug discovery.

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.

Photo-Luminescent Photonic Crystals for Anti-Counterfeiting

The conventional photonic crystals (PCs) are usually prepared by the self-assembly of silica or polystyrene particles. However, their applications are limited significantly due to the lack of the functions of the building blocks. Here, a new kind of photo-luminescent photonic crystals (PLPCs) with brilliant PL and structural colors were prepared by the self-assembly of dye-doped silica particles. The PL and structural colors of PCs can be well-controlled by altering the species of dyes and the size of the particles, respectively. Based on these advantages, PLPC patterns with encrypted information were fabricated through the combination of PLPCs and PCs with similar structural colors but diverse PL colors. These patterns can reversibly hide and display the encrypted information under sunlight and UV illumination, respectively. This work paves a new way for constructing functional PCs and will promote their applications in anti-counterfeiting, smart labels, and optical devices.

Bioinspired Quasi-3D Multiplexed Anti-Counterfeit Imaging via Self-Assembled and Nanoimprinted Photonic Architectures

Innovative multiplexing technologies based on nano-optics for anti-counterfeiting have been proposed as overt and covert technologies to secure products and make them difficult to counterfeit. However, most of these nano-optical anti-counterfeiting materials are metasurfaces and metamaterials with complex and expensive fabrication process, often resulting in materials that are not damage tolerant. Highly efficient anti-counterfeiting technologies with easy fabrication process are targeted for intuitive and effective authentication of banknotes, secure documents, and goods packing. Here, a simple strategy exploiting self-assembling and nanoimprinting technique to fabricate a composite lattice photonic crystal architecture featuring full spatial control of light, multiplexed full-pixel imaging, and multichannel cryptography combined with customized algorithms is reported. In particular, the real-time encryption/recognition of mobile quick response codes and anti-counterfeiting labels on a postage stamp, encoded by the proposed photonic architecture, are both demonstrated. The wave optics of scattering, diffraction, and polarization process involved are also described, validated with numerical simulations and experiments. By introducing a new degree of freedom in the 3D space, the multichannel image switching exhibits unprecedented variability of encryption, providing a promising roadmap to achieve larger information capacity, better security, and higher definition for the benefit of modern anti-counterfeiting security.

Reconfigurable Inverse Opal Structure Film for a Rewritable and Double-Sided Photonic Crystal Paper

A rewritable photonic crystal (PC) paper as an environmentally friendly and low-resource-consuming material for information storage and spreading has gradually become a research hotspot. In this work, a novel rewritable PC paper with inkless writing and double-sided rewritability properties was developed. A double-sided epoxy resin PC paper exhibiting an inverse opal structure and a bright structural color was fabricated using the sacrificial template method. Carbon black was doped into the material to increase color saturation and purity while preventing light transmission and protecting the double-sided structural color from interference. The force of sliding friction and deformation triggered by capillary pressure as well as swelling-triggered recovery of the inverse opal structure led to an easy rewriting of the PC paper. The PC paper exhibited excellent rewritability even after 50 runs of the rewriting process. Given the inkless and double-sided rewriting, this study provides a new method for the preparation of rewritable PC papers.

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.

Periodic Stratified Porous Structures in Dynamic Polyelectrolyte Films Through Standing-Wave Optical Crosslinking for Structural Color

Periodic porous structures have been introduced into functional films to meet the requirements of various applications. Though many approaches have been developed to generate desired structures in polymeric films, few of them can effectively and dynamically achieve periodic porous structures. Here, a facile way is proposed to introduce periodic stratified porous structures into polyelectrolyte films. A photo-crosslinkable polyelectrolyte film of poly(ethylenimine) (PEI) and photoreactive poly(acrylic acid) derivative (PAA-N3 ) is prepared by layer-by-layer (LbL) self-assembly. Stratified crosslinking of the PEI/PAA-N3 film is generated basing on standing-wave optics. The periodic stratified porous structure is constructed by forming pores in noncrosslinked regions in the film. Thanks to the dynamic mobility of polyelectrolytes, this structural controlment can be repeated several times. The size of pores corresponding to the layer spacing of the film contributes to the structural colors. Furthermore, structural color patterns are fabricated in the film by selective photo-crosslinking using photomasks. Although the large-scale structural controlment in thick (micron-scale and above) films needs to be explored further, this work highlights the periodic structural controlment in polymeric films and thus presents an approach for application potentials in sensor, detection, and ink-free printing.

Noniridescent structural color from enhanced electromagnetic resonances of particle aggregations and its applications for reconfigurable patterns

HYPOTHESIS

The conventional noniridescent structural colors refer to the coherent scattering of visible light by the short-range ordered structures assembled from the small colloids (100-250 nm). Our hypothesis is that noniridescent structural color can be generated by the random aggregations of large silica particles through the enhanced electromagnetic resonances.

EXPERIMENTS

The random aggregations of large silica particles (350-475 nm) were prepared through the infiltration of silica particles solution with the porous substrate. The mechanism of the structural color is investigated. Reconfigurable patterns are prepared.

FINDINGS

Dissimilar to the conventional noniridescent colors, the angle-independent colors of silica aggregations originate from the enhanced electromagnetic resonances due to the random aggregation of the particles. The colors (blue, green, and red) and corresponding reflection peak positions of the particle aggregations can be well controlled by simply altering the size of the silica particles. Compared to the traditional prints with permanent patterns, reconfigurable patterns with large-area and multicolor can be fabricated by the repeatedly selective spray of water on the substrate pre-coated with noniridescent colors. This work provides new insight and greenway for the fabrication of noniridescent structural colors and reconfigurable patterns, and will promote their applications in soft display, green printing, and anti-counterfeiting.

Lotus Seedpod Inspiration: Particle-Nested Double-Inverse Opal Films with Fast and Reversible Structural Color Switching for Information Security

The integration of novel structures into colloidal crystals provides the possibility of constructing stimuli-responsive photonic materials. However, in most opal and inverse opal structures, replacing the interior air with an infiltrated liquid will cause partial refractive index matching, resulting in the reduction or even disappearance of the photonic band gap. Herein, inspired by the lotus seedpod, an innovative particle-nested double-inverse opal film with fast and reversible structural color switching (≈1 s) is first fabricated by introducing polystyrene (PS) spheres into an inverted opal backbone. Importantly, refractive index matching can be effectively avoided due to the existence of internal PS spheres, and optical switching from diffusive to photonic behavior is achieved by a liquid with low surface tension for the response. Furthermore, a reversible ethanol stimuli-response bilayer double-inverse opal film with multistate switching for information encryption is proposed by combining optical scattering and diffraction. The scattered light from the top layer caused by the randomly distributed and weakly scattering PS spheres within the pores makes the pattern at the bottom invisible. Simultaneously, the display and discoloration of the pattern can be realized instantaneously by ethanol response. Thus, this new preparation strategy exhibits great potential in the security fields.

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.

Fabrication of Transparent Polymer Optical Device Combined with Selective Visible-Light Transmission and Zero-Birefringence

The formation of optical products using the traditional molten processing methods is a direct and extensive application of optical fields, and it suffers from intrinsic birefringence and optical distortion due to polymer orientation and residual stress. Here, a unique concept is proposed by assembling photonic crystal nanospheres without orientation in a rubbery state to realize transparent optical devices with zero-birefringence and high transparency. By developing fabrication techniques for transparent zero-birefringence optical devices, certain outstanding performances are realized, including no optical distortion and excellent mechanical properties. Simultaneously, by controlling the particle size of the photonic crystal, one has successfully obtained transparent optical devices with the visible light selective transmission are successfully obtained. The transparent zero-birefringence optical devices are promising candidates for potential applications for fine optical devices. The work opens up an exciting new fabrication route for zero-birefringence and highly transparent polymer devices that have been difficult to create using traditional methods.

Robust, Portable, and Specific Water-Response Silk Film with Noniridescent Pattern Encryption for Information Security

In modern days, information is a key resource for accelerating the development of society, economy, and culture. Thus, information security has always been a high priority for any country, business, and department. Herein, a simple and effective strategy for preparing an independent optical device for information security is proposed by using silk fibroin materials with a quasiamorphous inverse structure. Given the reversible hydrogen bonds between silk fibroin materials and water molecules, a multicolor high-resolution pattern with a variable color can be obtained by using a simple spray coating method. Furthermore, a reversible water stimulus-response silk film with a laminated structure that consists of hidden and patterned layers and carries quick response (QR) code information is prepared. This device effectively hides (encryption) the QR code pattern in a normal environment and quickly displays the information (decryption) in water. Simultaneously, the silk film shows good mechanical strength, excellent biocompatibility, long-term structural stability, and a unique response mechanism, which make it a suitable carrier of optical information. Thus, this new preparation strategy of an optical device has a potential application value and is an important reference in the fields of information security and functional materials.

Green and Efficient Inkjet Printing of Cotton Fabrics Using Reactive Dye@Copolymer Nanospheres

Digital inkjet printing of textiles possesses great advantages like high efficiency and flexible production, but the challenges like the risk of causing serious environmental problems due to the large usage of dyes and chemicals still remain a matter of concern. In response to this problem, herein, a novel kind of reactive dye@copolymer nanosphere was prepared through the adsorption of C. I. Reactive Red 218 dyes (RR218) onto cationic poly(styrene-butyl acrylate-vinylbenzyl trimethylammonium chloride) (PSBV) nanospheres and applied in inkjet printing on woven cotton fabric. Results show that the prepared RR218@PSBV nanospheres possessed homogeneous size and good stability for ink preparation. In comparison with the original RR218 solution, the color depth of RR218@PSBV-printed fabric increased by 1.4 times and the dye residues in the printing effluent were reduced by about 45%. Meanwhile, the consumptions of sodium carbonate and urea in conventional inkjet printing were reduced by about 3.3 and 22.8 mg/cm², respectively, and the printing process was simplified with 30% energy saving. Furthermore, the mechanism of the color enhancement by nanospheres was revealed by the calculation of absorption and scattering coefficients based on the Kubelka-Munk function. This work provides a potential application of dye@polymer nanospheres to promote the optimization of the textile inkjet printing technique and alleviates the environmental impact of conventional textile coloration.

Highly Efficient Detection of Homologues and Isomers by the Dynamic Swelling Reflection Spectrum

Precise and efficient detection of solvents with similar refractive index is highly desired but remains a big challenge for the conventional opal because the shift of its reflection wavelength only depends on the refractive index of the solvent to be detected. Here, homologues (alcohols, acids, alkalis, esters, and aromatic hydrocarbons), isomers, and other solvents with similar refractive index and structures were precisely distinguished through the dynamic swelling reflection spectrum (DSRS) pattern based on the different swelling behavior of swellable photonic paper in solvents. The one reflection signal of photonic paper will split into two reflection peaks, which then tend to merge together during the swelling process. The variation of the reflection signals and merging time are highly sensitive to the polarity and refractive index of the solvent, and the differences can be significantly amplified in DSRS, resulting in the distinction of the solvent from its unique geometric pattern. Moreover, the variation tendency of the reflectance provides an additional parameter in recognition of the solvent, which can be explained by calculation and comparison of the practical volume ratio of the solvent swelled into the photonic paper and the corresponding critical volume ratio of the solvent determined by its refractive index.

Real-Time and Label-Free Monitoring of Biomolecular Interactions within Complex Biological Media Using a Silica Colloidal Crystal Film

A method capable of real-time and label-free monitoring of biomolecular interactions within whole blood, without any sample separation and label process, is described. This was accomplished using silica colloidal crystal (SCC) films, three-dimensionally ordered silica particle arrays whose interference effect is a function of their optical thickness, as interference-sensitive substrates. Interactions between immunoglobulin G (IgG) and protein A from Staphylococcus aureus (SPA) conjugates with changes in the optical thickness of SCC films were monitored spectroscopically. Successful detection of IgG was achieved in the buffer and whole blood. This system constitutes a simple label-free analysis showing great potential in monitoring interactions between biomolecules in complex biological media.

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.

Reflection and transmission two-way structural colors

Management of reflection and transmission two-way structural colors is significant in color displays, projections, and anticounterfeiting. Here, inspired by the Lycurgus Cup, we fabricated photonic crystals with opal and inverse opal structures with controlled thickness, which show reflection and transmission two-way structural colors. In order to balance the reflection and transmission intensities, we first studied the effect of the order layer thickness on the reflection and transmission spectra and found that a thickness of about 5 μm can help the structural colors achieve high saturation in both directions. The photonic crystal film built with 295 nm SiO2 spheres shows bright red and green structural colors in the reflection and transmission directions, respectively. These two-way colors can be projected onto substrates, similar to a transflective color filter. The color displays can be tuned by adjusting the angle between the incident light and the sample. Furthermore, we also patterned the photonic crystal film with two-way structural colors, which shows clear patterns and rich colors in both directions. The photonic crystals assembled on a small wine glass display two-way structural colors similar to those of the Lycurgus Cup. More importantly, a flexible inverse opal photonic crystal film with two-way structural colors was also fabricated, which can be applied in multimode anticounterfeiting. This work will greatly expand the application field of photonic crystals in double sided displays, transflective color filters and anticounterfeiting.

Nanosphere-Aggregation-Induced Reflection and Its Application in Large-Area and High-Precision Panchromatic Inkjet Printing

Artificial structural colors have attracted more and more attention due to their high photostability, low toxicity, and brilliant colors. Inkjet printing of photonic crystals or amorphous photonic structures can realize large-scale structural color patterns, while plasma printing of metals can achieve high-precision color images. However, still no method is available to fabricate structural color patterns on both a large scale and with high precision. Here, nanosphere-aggregation-induced reflection (NAIR) is first theoretically and experimentally demonstrated and vivid full-spectrum structural color can be generated based on NAIR. Dramatically different from photonic crystals, the accumulation of only a few monodisperse dielectric spheres with an appropriate refractive index and diameter can produce bright structural colors, which makes high resolution possible. By introducing commercial inkjet printers, this aggregate structure can be constructed at high speed in a large scale. Importantly, the color mixing is easily performed by simultaneously applying spheres with different sizes, which allow us to sophisticatedly control the generated color. The demonstrated NAIR printing paves the way toward a full-spectrum, large-scale, and high-precision structural color, offering great potential for daily commercial utilization.

Rotational Periodicity Display of the Tunable Wettability Pattern in a Photoswitch Based on a Response Bilayer Photonic Crystal

Although the forward diffraction of the three-dimensional (3D) photonic crystal is easily applied to a photoswitch, backward diffraction rainbows are rarely reported. The first rotational photoswitch based on a bilayer 3D photonic crystal with backward diffractions similar to those of two-dimensional photonic crystals was fabricated by vertically combining different thicknesses of nanoparticle templates. When rotating the bilayer photonic crystal, the opening or closing of the rotational photoswitch shows periodic reproducibility values of 30 and 60°. Different periods are regulated by the thickness and scattering effect of the top layer. Moreover, invisible patterns can be encoded and erased by changing the wettability via pH. Because of the decreasing of the refractive index differentials, it will be revealed rapidly when immersed in water. The revealed pattern can be periodically turned on and off by rotating the bilayer photonic crystal. It has great application prospects in optical prism, warning board, anti-counterfeiting, steganography, watermarking, and complex information coding.

Asymmetric structural colors based on monodisperse single-crystal Cu2O spheres

Structural colors have attracted broad attention owing to their anti-photobleaching capability and brilliant metallic color. In particular, the asymmetric structural colors generated by a simple material will have great practical significance in the fields of biomimetic materials, double-side display and anti-counterfeiting. The asymmetric optical effects were usually achieved by the plasmonic effects of Ag or Au nanocrystals. Here, for the first time, we realized the asymmetric structural colors based on the asymmetric scattering of Cu₂O single-crystal spheres. By spray-coating Cu₂O spheres on a glass slide, different structural colors were viewed from the Cu₂O film side and the glass slide side. The FDTD simulations confirmed that the asymmetric colors were ascribed to the inhomogeneous distribution of the electric field intensity. The film built by 200 nm Cu₂O spheres on a glass slide shows green and cyan structural colors from the front and back sides, respectively. The colors on both sides of the Cu₂O films were proved to be tuned by changing the diameters of the Cu₂O single-crystal spheres. Different substrates were used to examine the influence of substrates on the asymmetric colors. Finally, inspired by different brilliant colors from the front and back of natural creatures, the patterns of butterfly and petals were fabricated by Cu₂O spheres. Impressively, similar to nature, the patterns show completely different colors viewed from the front and back sides. The asymmetric structural colors of Cu₂O single-crystal spheres will open up new avenues to realize multi-mode color output and pave their applications in display, biomimetic materials and anti-counterfeiting materials.

Biomimetic Color-Changing Hierarchical and Gradient Hydrogel Actuators Based on Salt-Induced Microphase Separation

There have been more challenges for hydrogel actuators to meet the combined requirement of discoloration, complex deformation, and simple preparation. Structural coloration is widely used to fabricate discolored hydrogel via microrearrangement of photonic crystals in the hydrogel framework. However, precise regulation is usually required. Besides, the macro-optical properties are unstable. Herein, we develop a hierarchical and gradient hydrogel actuator with complex deformation and color-changing functions using an electrophoresis method. A simple but effective strategy is presented for fabrication of hierarchical hydrogel composed of homopolymers and copolymers via salt-induced microphase separation during the polymerization of the N-isopropylacrylamide (NIPAm) and [2-(methacryloyloxy)ethyl]trimethylammonium chloride (DMC). Meanwhile a gradient distribution of DMC is also formed during the polymerization due to migration of DMC under electric field. The hierarchical and gradient structures are characterized by confocal laser scanning microscope (CLSM), small-angle X-ray scattering measurement (SAXS), temperature-variable Fourier transform infrared (FTIR), etc. The discoloration mechanism is proposed. The as-prepared hydrogel can undergo fast and complex thermo-triggered deformation and discoloration. Bionic movements of discoloration flowering and information encryption are successfully demonstrated, promising great potential in the application of biomimetic materials.

A facile fabrication strategy for anisotropic photonic crystals using deformable spherical nanoparticles

A 2D anisotropic photonic crystal (APC) of bowl-shaped nanoparticles has been fabricated using deformable spherical nanoparticles. The prepared 2D isotropic photonic crystal (IPC) of spherical nanoparticles is transformed into a 2D APC by a chemical etching process, in which the interiors of the spherical nanoparticles are preferentially dissolved to eventually form a bowl-like morphology. Due to the accurate and controllable deformability of the spherical nanoparticles, the arrangement and orientations of the bowl-shaped nanoparticles are highly ordered and uniform. The morphology, optical properties and surface wettability of the 2D APC are all distinct from those of the prepared 2D IPC. This facile strategy provides an easy and low-cost way to fabricate highly ordered and uniform APCs.

Different Structural Colors or Patterns on the Front and Back Sides of a Multilayer Photonic Structure

The application of photonic crystals in the field of color display and anticounterfeiting has been widely studied because of their brilliant and angle-dependent structural colors. Most of the research is focused on structural colors on the front side of photonic crystals, and both sides of the crystals usually display the same or similar optical properties. Here, multilayer photonic crystals with different structural colors or different patterns on the front and back sides were designed. In a trilayer photonic structure, an amorphous SiO₂ layer with a thickness of about 10 μm was inserted into two layers of highly ordered photonic crystals with band gaps of 625 and 470 nm. The amorphous SiO₂ layer acts as a gate to prohibit light transmission, and thereby, the structural colors of the two photonic crystals were separated. Hence, the trilayer structure shows red and blue colors on each side. Then, a light window was opened in the disordered layer using a patterned mask; thus, a pattern with a mixed color of both ordered layers was observed on each side in the window field, which was obviously different from the background color. Finally, completely different patterns on each side were also realized by building a multilayer structure. The different structural colors or patterns on each side of the photonic structures provide them with enriched color range and enhanced display or anticounterfeiting ability.

Bio-inspired transparent structural color film and its application in biomimetic camouflage

The transparent wings of insects with intelligent structural colors or good invisibility in different surroundings provide them with unique camouflage capability for protection and information exchange. Inspired by the existence of ordered biological nanostructures on the surface of the wings, freestanding composite photonic crystal (PC) films were prepared by infiltrating polydimethylsiloxane (PDMS, n = 1.41) into the interstices of a highly ordered opal PC using poly(methyl methacrylate) (PMMA, n = 1.49) spheres as building blocks. The appropriate refractive index contrast (Δn = 0.08) endowed the composite film with high transparency and vivid structural colors. Consequently, the PC film was invisible in shaded surroundings and showed brilliant structural color under sunlight. Also, 186, 229 and 257 nm PMMA spheres were used to obtain composite PC films with different structural colors. Moreover, as a proof of concept, a biomimetic dragonfly-shaped film was fabricated using a patterned substrate. When it was placed on a green tree under sunlight, abundant structural colors appeared at different specular viewing angles. However, it camouflaged in the environment when the shadows of the green tree shielded the sunlight or when viewed in non-specular angles with sunshine. This unique property indicated their potential applications in biomimetic camouflage and smart stealth materials for bionic machines.