A photonic crystal material for the online detection of nonpolar hydrocarbon vapors

A modern level of nanotechnology allows us to create conceptually new test systems for chemical analyses and to develop sensitive and compact sensors for various types of substances. However, at present, there are very few commercially available compact sensors for the determination of toxic and carcinogenic substances, such as organic solvents that are used in some construction materials. This article contains an overview of how 3D photonic crystals are used for the creation of a new test system for nonpolar organic solvents. The morphology and structural parameters of the photonic crystals, based upon a crystalline colloidal array with a sensing matrix of polydimethylsiloxane, have been determined by using scanning electron microscopy and by the results of specular reflectance spectroscopy based on the Bragg-Snell law. A new approach has been proposed for the application of this sensor in chemical analysis for the qualitative detection of saturated vapors of volatile organic compounds due to configuration changes of the photonic bandgap, recorded by diffuse reflectance spectroscopy. The exposure of the sensor to aromatic (benzene, toluene and p-xylene) and aliphatic (n-pentane, n-heptane, n-octane and n-decane) hydrocarbons has been analyzed. The reconstitution of spectral parameters of the sensor during the periodic detection of saturated vapors of toluene has been evaluated.

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.

Wettability-Controlled Directional Actuating Strategy Based on Bilayer Photonic Crystals

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

Structural Color Materials for Optical Anticounterfeiting

The counterfeiting of goods is growing worldwide, affecting practically any marketable item ranging from consumer goods to human health. Anticounterfeiting is essential for authentication, currency, and security. Anticounterfeiting tags based on structural color materials have enjoyed worldwide and long-term commercial success due to their inexpensive production and exceptional ease of percept. However, conventional anticounterfeiting tags of holographic gratings can be readily copied or imitated. Much progress has been made recently to overcome this limitation by employing sufficient complexity and stimuli-responsive ability into the structural color materials. Moreover, traditional processing methods of structural color tags are mainly based on photolithography and nanoimprinting, while new processing methods such as the inkless printing and additive manufacturing have been developed, enabling massive scale up fabrication of novel structural color security engineering. This review presents recent breakthroughs in structural color materials, and their applications in optical encryption and anticounterfeiting are discussed in detail. Special attention is given to the unique structures for optical anticounterfeiting techniques and their optical aspects for encryption. Finally, emerging research directions and current challenges in optical encryption technologies using structural color materials is presented.

Colorimetric Ethanol Indicator Based on Instantaneous, Localized Wetting of a Photonic Crystal

Easy-to-use sensors for ethanol solutions have broad applications ranging from monitoring alcohol quality to combating underage drinking. Although there are a number of electronic and colorimetric sensors available for determining alcohol concentration, there is currently no device that can concurrently provide a prompt, well-defined, quickly recoverable readout and remain readily affordable. Here, we developed a field-ready, colorimetric indicator that provides a fast, clear identification of ethanol-water mixtures between 0 and 40% based on the discoloration of a wetted photonic crystal. We cooperatively exploit the iridescence and the geometrical gating in silica inverse opal films (IOFs), together with a fine-tuned surface chemistry gradient, to distinguish ethanol concentrations by their wettability patterns in the different segments of the IOFs. The resultant all-in-one colorimetric sensor delivers a striking and instantaneous optical response at an ethanol concentration as low as 5%. We further improve the ease of use by seamlessly integrating this colorimetric platform with drinking glassware (a glass stirrer and a vial). This research provides an optimal means for colorimetric ethanol detection and is a step toward the immersible sensing of diverse molecules (e.g., biomarkers) in aqueous solutions without expensive laboratory tests.

Functional Micro–Nano Structure with Variable Colour: Applications for Anti-Counterfeiting

Colour patterns based on micro-nano structure have attracted enormous research interests due to unique optical switches and smart surface applications in photonic crystal, superhydrophobic surface modification, controlled adhesion, inkjet printing, biological detection, supramolecular self-assembly, anti-counterfeiting, optical device and other fields. In traditional methods, many patterns of micro-nano structure are derived from changes of refractive index or lattice parameters. Generally, the refractive index and lattice parameters of photonic crystals are processed by common solvents, salts or reactive monomers under specific electric, magnetic and stress conditions. This review focuses on the recent developments in the fabrication of micro-nano structures for patterns including styles, materials, methods and characteristics. It summarized the advantages and disadvantages of inkjet printing, angle-independent photonic crystal, self-assembled photonic crystals by magnetic field force, gravity, electric field, inverse opal photonic crystal, electron beam etching, ion beam etching, laser holographic lithography, imprinting technology and surface wrinkle technology, etc. This review will provide a summary on designing micro-nano patterns and details on patterns composed of photonic crystals by surface wrinkles technology and plasmonic micro-nano technology. In addition, colour patterns as switches are fabricated with good stability and reproducibility in anti-counterfeiting application. Finally, there will be a conclusion and an outlook on future perspectives.

Aptamer-based hydrogel barcodes for the capture and detection of multiple types of pathogenic bacteria

Rapid and sensitive diagnosing hematological infections based on the separation and detection of pathogenic bacteria in the patient's blood is a significant challenge. To address this, we herein present a new barcodes technology that can simultaneously capture and detect multiple types of pathogenic bacteria from a complex sample. The barcodes are poly (ethylene glycol) (PEG) hydrogel inverse opal particles with characteristic reflection peak codes that remain stable during bacteria capture on their surfaces. As the spherical surface of the particles has ordered porous nanostructure, the barcodes can provide not only more surface area for probe immobilization and reaction, but also a nanopatterned platform for highly efficient bioreactions. In addition, the PEG hydrogel scaffold could decrease the non-specificity adsorption by its anti-adhesive effect, and the decorated aptamer probes in the scaffolds could increase the sensitivity, reliability, and specificity of the bacteria capture and detection. Moreover, the tagged magnetic nanoparticles in the PEG scaffold could impart the barcodes with controllable movement under magnetic fields, which can be used to significantly increase the reaction speed and simplify the processing of the bioassays. Based on the describe barcodes, it was demonstrated that the bacteria could be captured and identified even at low bacterial concentrations (100 CFU mL^-1) within 2.5h, which is effectively shortened in comparison with the "gold standard" in clinic. These features make the barcodes ideal for capturing and detecting multiple bacteria from clinical samples for hematological infection diagnostics.

Colorimetric gas detection by the varying thickness of a thin film of ultrasmall PTSA-coated TiO2 nanoparticles on a Si substrate

Colorimetric gas sensing is demonstrated by thin films based on ultrasmall TiO₂ nanoparticles (NPs) on Si substrates. The NPs are bound into the film by p-toluenesulfonic acid (PTSA) and the film is made to absorb volatile organic compounds (VOCs). Since the color of the sensing element depends on the interference of reflected light from the surface of the film and from the film/silicon substrate interface, colorimetric detection is possible by the varying thickness of the NP-based film. Indeed, VOC absorption causes significant swelling of the film. Thus, the optical path length is increased, interference wavelengths are shifted and the refractive index of the film is decreased. This causes a change of color of the sensor element visible by the naked eye. The color response is rapid and changes reversibly within seconds of exposure. The sensing element is extremely simple and cheap, and can be fabricated by common coating processes.

Label-free colorimetric detection of tetracycline using analyte-responsive inverse-opal hydrogels based on molecular imprinting technology

Analyte-responsive inverse-opal hydrogels based on molecular imprinting technology were fabricated for selective, sensitive, and label-free colorimetric detection of tetracycline.

Lithographically Encrypted Inverse Opals for Anti-Counterfeiting Applications

Colloidal photonic crystals possess inimitable optical properties of iridescent structural colors and unique spectral shape, which render them useful for security materials. This work reports a novel method to encrypt graphical and spectral codes in polymeric inverse opals to provide advanced security. To accomplish this, this study prepares lithographically featured micropatterns on the top surface of hydrophobic inverse opals, which serve as shadow masks against the surface modification of air cavities to achieve hydrophilicity. The resultant inverse opals allow rapid infiltration of aqueous solution into the hydrophilic cavities while retaining air in the hydrophobic cavities. Therefore, the structural color of inverse opals is regioselectively red-shifted, disclosing the encrypted graphical codes. The decoded inverse opals also deliver unique reflectance spectral codes originated from two distinct regions. The combinatorial code composed of graphical and optical codes is revealed only when the aqueous solution agreed in advance is used for decoding. In addition, the encrypted inverse opals are chemically stable, providing invariant codes with high reproducibility. In addition, high mechanical stability enables the transfer of the films onto any surfaces. This novel encryption technology will provide a new opportunity in a wide range of security applications.

A photonic crystal hydrogel suspension array for the capture of blood cells from whole blood

Diagnosing hematological disorders based on the separation and detection of cells in the patient's blood is a significant challenge. We have developed a novel barcode particle-based suspension array that can simultaneously capture and detect multiple types of blood cells. The barcode particles are polyacrylamide (PAAm) hydrogel inverse opal microcarriers with characteristic reflection peak codes that remain stable during cell capture on their surfaces. The hydrophilic PAAm hydrogel scaffolds of the barcode particles can entrap various plasma proteins to capture different cells in the blood, with little damage to captured cells.

Bio-inspired photonic crystals with superwettability

This review focus on the recent developments in the mechanism, fabrication and application of bio-inspired PCs with superwettability.

Bioinspired materials: from low to high dimensional structure

The surprising properties of biomaterials are the results of billions of years of evolution. Generally, biomaterials are assembled under mild conditions with very limited supply of constituents available for living organism, and their amazing properties largely result from the sophisticated hierarchical structures. Following the biomimetic principles to prepare manmade materials has drawn great research interests in materials science and engineering. In this review, we summarize the recent progress in fabricating bioinspired materials with the emphasis on mimicking the structure from one to three dimensions. Selected examples are described with a focus on the relationship between the structural characters and the corresponding functions. For one-dimensional materials, spider fibers, polar bear hair, multichannel plant roots and so on have been involved. Natural structure color and color shifting surfaces, and the antifouling, antireflective coatings of biomaterials are chosen as the typical examples of the two-dimensional biomimicking. The outstanding protection performance, and the stimuli responsive and self-healing functions of biomaterials based on the sophisticated hierarchical bulk structures are the emphases of the three-dimensional mimicking. Finally, a summary and outlook are given.