Photonic crystal structures with tunable structure color as colorimetric sensors

1D topological photonic crystal based nanosensor for tuberculosis detection

In this study, we present a nanosized biosensor based on the photobiological properties of one-dimensional (1D) topological photonic crystals (PCs). A topological structure had been designed by combining two PC structures (PC 1 and PC 2) comprised of functional material layers, Si and SiO2. These two, PC 1 and PC 2, differ in terms of the thickness and arrangement of these dielectric materials. We carried out a comparison between two distinct topological PCs: one using random PCs, and the other featuring a mirror heterostructure. Tuberculosis may be diagnosed by inserting a sensor layer into 1D topological PCs. The sensing process is based on the refractive indexes of the analytes in the sensor layer. When the 1D-topological heterostructure-based PC and its mirror-image structures are stacked together, the sensor becomes more efficient for analyte detection than the conventional PCs. The random-based topological PC outperformed the heterostructure-based topological PC in analyte sensing. Photonic media witness notable blue shifts due to the analytes' variations in refractive index. The numerical results of the sensor are computed using the transfer matrix approach. Effective results are achieved by optimizing the thicknesses of the sensor layer and dielectric layers; number of periods and incident angle. In normal incident light, the developed sensor shows a high sensitivity of 1500 nm RIU-1with a very low limit of detection in the order of 2.2 × 10-06RIU and a high-quality factor of 30 659.54.

Surfactant-Free, Size-Controllable, and Scalable Green Synthesis of ZIF-8 Particles with Narrow Size Distribution by Tuning Key Reaction Parameters in Water Solvent

The chemical/physical properties and reliable performance of nanoporous materials are strongly influenced by the particle size and corresponding distribution. Among many types of MOFs, ZIF-8, is still widely used and many studies have been conducted to control the particle size and uniformity of ZIF-8 using surfactants and organic solvents. However, the use of surfactants and organic solvents process is expensive and may cause environmental pollution. For the first time, in this paper, a surfactant-free, size-controllable, and scalable green synthesis method of ZIF-8 particles is reported using four reaction parameters (temperature, concentration, pouring time, and reactant ratio) that affect the formation of nuclei and growth of ZIF-8 crystals. The as-synthesized ZIF-8 nanoparticles show great uniformity and controllable particle sizes in the wide range of 147-915 nm. In addition, a 2 L large-scale synthesis of ZIF-8 with narrow size distribution is developed by finely tuned particle size in water without any additives. To demonstrate the efficient utilization of nanopores according to the particle size and size distribution, an adsorption test is conducted on the ZIF-8 nanoparticles. This study will support the synthesis of size-controlled ZIF-8 with narrow size distribution and their composites for achieving high performance in the emerging applications.

Applications of inverse opal photonic crystal hydrogels in the preparation of acid-base color-changing materials

Hydrogels are three-dimensional (3D) crosslinked network hydrophilic polymers that have structures similar to that of biological protein tissue and can quickly absorb a large amount of water. Opal photonic crystals (OPCs) are a kind of photonic band gap material formed by the periodic arrangement of 3D media, and inverse opal photonic crystals (IOPCs) are their inverse structure. Inverse opal photonic crystal hydrogels (IOPCHs) can produce corresponding visual color responses to a change in acid or alkali in an external humid environment, which has wide applications in chemical sensing, anti-counterfeiting, medical detection, intelligent display, and other fields, and the field has developed rapidly in recent years. In this paper, the research progress on fast acid-base response IOPCHs (pH-IOPCHs) is comprehensively described from the perspective of material synthesis. The technical bottleneck of enhancing the performance of acid-base-responsive IOPCHs and the current practical application limitations are summarized, and the development prospects of acid-base-responsive IOPCHs are described. These comprehensive analyses are expected to provide new ideas for solving problems in the preparation and application of pH-IOPCHs.

A review on hybridization of plasmonic and photonic crystal biosensors for effective cancer cell diagnosis

Cancer causes one in six deaths worldwide, and 1.6 million cancer patients face annual out-of-pocket medical expenditures. In response to these, portable, label-free, highly sensitive, specific, and responsive optical biosensors are under development. Therefore, in this review, the recent advances, advantages, performance analysis, and current challenges associated with the fabrication of plasmonic biosensors, photonic crystals, and the hybridization of both for cancer diagnosis are assessed. The primary focus is on the development of biosensors that combine different shapes, sizes, and optical properties of metallic and dielectric nanoparticles with various coupling techniques. The latter part discusses the challenges and prospects of developing effective biosensors for early cancer diagnosis using dielectric and metallic nanoparticles. These data will help the audience advance research and development of next-generation plasmonic biosensors for effective cancer diagnosis.

Ultralight plasmonic structural color paint

All present commercial colors are based on pigments. While such traditional pigment-based colorants offer a commercial platform for large-volume and angle insensitiveness, they are limited by their instability in atmosphere, color fading, and severe environmental toxicity. Commercial exploitation of artificial structural coloration has fallen short due to the lack of design ideas and impractical nanofabrication techniques. Here, we present a self-assembled subwavelength plasmonic cavity that overcomes these challenges while offering a tailorable platform for rendering angle and polarization-independent vivid structural colors. Fabricated through large-scale techniques, we produce stand-alone paints ready to be used on any substrate. The platform offers full coloration with a single layer of pigment, surface density of 0.4 g/m², making it the lightest paint in the world.

Multifaceted Structurally Coloured Materials: Diffraction and Total Internal Reflection (TIR) from Nanoscale Surface Wrinkling

We investigate the combined effects of surface diffraction and total internal reflection (TIR) in the design of 3-dimensional materials exhibiting distinct structural colour on various facets. We employ mechanical wrinkling to introduce surface diffraction gratings (from the nano to the micron scales) on one face of an elastomeric rectangular parallelepiped-shaped slab and explore the roles, in the perceived colours, of wrinkling pattern, wavelength, the directionality of incident light and observation angles. We propose a simple model that satisfactorily accounts for all experimental observations. Employing polydimethylsiloxane (PDMS), which readily swells in the presence of various liquids and gases, we demonstrate that such multifaceted colours can respond to their environment. By coupling a right angle triangular prism with a surface grating, we demonstrate the straightforward fabrication of a so-called GRISM (GRating + prISM). Finally, using a range of examples, we outline possibilities for a predictive material design using multi-axial wrinkling patterns and more complex polyhedra.

Double-Network Hydrogel-Based Photonic Crystal Sensor for Mechanical Force Naked Eye Sensing and Its Application in Medical Compressive or Stretchy Instruments

Herein, we coalesced a poly(acrylamide-co-N-Acryloyl phenylalanine)/polyacrylamide double-network (P(AM-co-APA)/PAM DN) hydrogel with a photonic crystal array, fabricating a mechanochromic sensor for application in flexible medical instruments by naked eye monitoring. The intensified mechanical properties of the DN hydrogel were proved by the mechanical property tests, which are attributed to the interactions of chemical bonds and hydrogen bonds between the two polymer networks. In the range of stress from 0 to 328 kPa, the reflected light wavelength of this sensor changed from 659 to 480 nm and the color changed from red to blue in response; in the range of pressure from 0 to 85 kPa, the sensor exhibited a spectrum changing from 658 nm to 467 nm, covering almost the whole visible color range. The prepared sensor was incorporated into medical instruments including the femoral artery hemostat and bandage to indicate pressure and tensile stress in practical applications. Within the appropriate pressure for wound recovery, the sensitivity and correlation between the external stimulus of pressure and wavelength of this integrated sensor were 5.58 nm·kPa-1 and over 0.99, respectively. Ultimately, the sensor proved to be tough, sensitive, and durable, showing a broad prospect of a series of future applications.

Bioinspired Optical Flexible Cellulose Nanocrystal Films with Strain-Adaptive Structural Coloration

Recent advances of photonic crystals are driven to mechanical sensors and smart wearable devices; however, for chiral photonic cellulose nanocrystal (CNC) materials, vivid structural coloration and reversible mechanochromism like chameleon skin remain a big challenge. Here, we report a ternary co-assembly and post-UV-irradiation polymerization strategy to develop flexible and elastic CNC composite films, which, notably, have naked-eye-visible brilliant structural colors and stretching-induced color change covering a broad wavelength region at a moderate deformation (like skin). By adjusting the stretching, the film is designed as a smart skin to adapt to surrounding environments for camouflage. This work offers a universal strategy for constructing biomimic optically functional cellulose skins.

Nanocellulose as a promising substrate for advanced sensors and their applications

Nanocellulose has broad and promising applications owing to its low density, large specific surface area, high mechanical strength, modifiability, renewability. Recently, nanocellulose has been widely used to fabricate flexible, durable and environmental-friendly sensor substrates. In this contribution, the construction and characteristics of nanocellulose-based sensors are comprehensively reviewed. Various nanocellulose-based sensors are summarized and divided into colorimetric, fluorescent, electronic, electrochemical and SERS types according to the sensing mechanism. This review also introduces the applications of nanocellulose-based sensors in the fields of biomedicine, environmental monitoring, food safety, and wearable devices.

Nanochitin: Chemistry, Structure, Assembly, and Applications

Chitin, a fascinating biopolymer found in living organisms, fulfills current demands of availability, sustainability, biocompatibility, biodegradability, functionality, and renewability. A feature of chitin is its ability to structure into hierarchical assemblies, spanning the nano- and macroscales, imparting toughness and resistance (chemical, biological, among others) to multicomponent materials as well as adding adaptability, tunability, and versatility. Retaining the inherent structural characteristics of chitin and its colloidal features in dispersed media has been central to its use, considering it as a building block for the construction of emerging materials. Top-down chitin designs have been reported and differentiate from the traditional molecular-level, bottom-up synthesis and assembly for material development. Such topics are the focus of this Review, which also covers the origins and biological characteristics of chitin and their influence on the morphological and physical-chemical properties. We discuss recent achievements in the isolation, deconstruction, and fractionation of chitin nanostructures of varying axial aspects (nanofibrils and nanorods) along with methods for their modification and assembly into functional materials. We highlight the role of nanochitin in its native architecture and as a component of materials subjected to multiscale interactions, leading to highly dynamic and functional structures. We introduce the most recent advances in the applications of nanochitin-derived materials and industrialization efforts, following green manufacturing principles. Finally, we offer a critical perspective about the adoption of nanochitin in the context of advanced, sustainable materials.

Factors Influencing Recognition Capability of Inverse Opal Structured Photonic Crystal Sensors

Nowadays, many kinds of colloidal photonic crystal (PC) sensors with inverse opal (IO) structures have been developed. However, there are few systematic studies on the factors influencing their recognition capability and responsiveness capability. In this paper, the relationships between recognition capability of IO structured PC sensors and all the parameters in Bragg–Snell’s law have been explored. In addition, research on the recognition ability of PC sensors typically focuses only on the refractive index difference between the identified substances. Herein, we define two concepts, namely the absolute refractive index difference and the relative refractive index difference, and prove that the recognition ability not only relies on the absolute refractive index between the identified substances, but also on the relative refractive index. Bragg–Snell’s law analysis confirms that the responsiveness capability is directly proportional to the void size of the IO structure, which is also confirmed by the finite difference time domain (FDTD) method. It is believed that these systematic studies have important guiding significance for creating advanced IO structured PC sensors.

Bio-Alcohol Sensor Based on One-Dimensional Photonic Crystals for Detection of Organic Materials in Wastewater

In this work, we have explored a novel application of one–dimensional (1D) photonic crystals (PCs) as a biomarker for the detection of organic materials in wastewater. The high concentration of organic materials may lead to adverse impact on human life. In order to save human life from these adverse effects, we have investigated the bio-alcohol sensing properties of a 1D multilayer periodic structure (AB)N/C/(AB)N capable of detecting organic materials in wastewater. The proposed structure works on the principle to detect a very small change in the refractive index of the wastewater sample under investigation by means of producing a shift in the position of the defect mode inside the photonic band gap (PBG) of the proposed structure. The transfer matrix method (TMM) has been used to investigate the transmission properties of the proposed design with the help of MATLAB software. We have also studied the effect of changes in the defect layer’s thickness, the volume fraction of the nanocomposite material and the incident angle on the sensitivity of our proposed bio-alcohol sensing design. Our bio-alcohol sensor shows a high sensitivity value of 500 nm/RIU and a low detection limit value of 1 × 10−5 RIU. The figure of merit and quality factor values of our bio-alcohol sensor are 5 × 103 and 5.236 × 103, respectively. The damping rate of the design is ξ=95.4927×10−5.

The stochastic organization of genomes and the doctrine of energy-information evolution based on bio-antenna arrays

The article is devoted to the possibilities of considering the evolution of biological systems in connection with the unique emergent properties of antenna arrays, that is, systems of mutually matched antennas widely used in technology. Materials are presented in favor of the proposition that the evolution of biosystems can be formally considered as the evolution of systems of bio-antenna arrays and their energy-information wave activity, which participates in biological computation and contributes to the unification of body parts into a coherent whole. The use of digital antenna arrays in technology is based on their tensor-matrix theory. The author discovers a structural analogy of this theory with the tensor-matrix features of genetic coding systems, as well as algebraic modeling of the universal rules for the stochastic DNA organization of the genomes of higher and lower organisms. This analogy is just one of the facts presented in the article in favor of the usefulness of borrowing knowledge from modern antenna technology to consider the evolution of biosystems. The described new approach may exist along with other known approaches in evolutionary biology.

Recent Progress in Responsive Structural Color

Structural color has been regarded as an ideal alternative to pigments because of the advantages of environmental friendliness, resistance to fading, and dynamic regulation. Responsive structural color can give real-time visible feedback to external stimuli and thus has great prospects in many applications, such as displays, sensing, anticounterfeiting, information storage, and healthcare monitoring. In this Perspective, we elucidate basic concepts, controllable fabrications, and promising applications of responsive structural colors. In particular, we systematically summarize the general regulation mode of all kinds of responsive structural color systems. First, we introduce the basic chromogenic structures as well as the regulation modes of responsive structural color. Second, we present the fabrication methods of patterned structural color. Then, the promising applications of responsive structural color systems are highlighted in detail. Finally, we present the existing challenges and future perspectives on responsive structural colors.

Transparent Colloidal Crystals With Structural Colours

Spatially ordered arrangements of spherical colloids are known to exhibit structural colours. The intensity and brilliance of these structural colours typically improve with colloidal monodispersity, low concentrations of point and line defects and with increasing refractive index contrast between the colloids and the embedding medium. Here we show that suspensions of charge stabilised, fluorinated latex particles with low refractive-index contrast to their aqueous background form Wigner crystals with FCC symmetry for volume fractions between 13 and 40%. In reflection they exhibit both strong, almost angle-independent structural colours and sharp, more brilliant Bragg peaks despite the particle polydispersity and bimodal distribution. Simultaneously, these suspensions appear transparent in transmission. Furthermore, binary AB, A2B and A13B type mixtures of these fluorinated and similarly sized polystyrene particles appeared predominantly white but with clear Bragg peaks indicating a CsCl-like BCC structure and more complex crystals. We characterised the suspensions using a combination of reflectivity measurements and small-angle x-ray scattering, complemented by reflectivity modelling.

Two-Dimensional Material-Based Colorimetric Biosensors: A Review

Two-dimensional (2D) materials such as graphene, graphene oxide, transition metal oxide, MXene and others have shown high potential for the design and fabrication of various sensors and biosensors due to their 2D layered structure and unique properties. Compared to traditional fluorescent, electrochemical, and electrical biosensors, colorimetric biosensors exhibit several advantages including naked-eye determination, low cost, quick response, and easy fabrication. In this review, we present recent advances in the design, fabrication, and applications of 2D material-based high-performance colorimetric biosensors. Potential colorimetric sensing mechanisms and optimal material selection as well as sensor fabrication are introduced in brief. In addition, colorimetric biosensors based on different 2D materials such as graphene, transition metal dichalcogenide/oxide, MXenes, metal-organic frameworks, and metal nanoplates for the sensitive detection of DNA, proteins, viruses, small molecules, metallic ions, and others are presented and discussed in detail. This work will be helpful for readers to understand the knowledge of 2D material modification, nanozymes, and the synthesis of hybrid materials; meanwhile, it could be valuable to promote the design, fabrication, and applications of 2D material-based sensors and biosensors in quick bioanalysis and disease diagnostics.

Chiral Photonic Liquid Crystal Films Derived from Cellulose Nanocrystals

As a nanoscale renewable resource derived from lignocellulosic materials, cellulose nanocrystals (CNCs) have the features of high purity, high crystallinity, high aspect ratio, high Young's modulus, and large specific surface area. The most interesting trait is that they can form the entire films with bright structural colors through the evaporation-induced self-assembly (EISA) process under certain conditions. Structural color originates from micro-nano structure of CNCs matrixes via the interaction of nanoparticles with light, rather than the absorption and reflection of light from the pigment. CNCs are the new generation of photonic liquid crystal materials of choice due to their simple and convenient preparation processes, environmentally friendly fabrication approaches, and intrinsic chiral nematic structure. Therefore, understanding the forming mechanism of CNCs in nanoarchitectonics is crucial to multiple fields of physics, chemistry, materials science, and engineering application. Herein, a timely summary of the chiral photonic liquid crystal films derived from CNCs is systematically presented. The relationship of CNC, structural color, chiral nematic structure, film performance, and applications of chiral photonic liquid crystal films is discussed. The review article also summarizes the most recent achievements in the field of CNCs-based photonic functional materials along with the faced challenges.

Graphene Metapixels for Dynamically Switchable Structural Color

Structural coloration providing vibrant and tailored colors enables broad applications. Existing strategies of structural coloration either use resonances or diffraction induced by arrayed nanostructures with element sizes at a wavelength scale or are based on interference from vacuum-deposited large-area thin films. It is extremely challenging to achieve full color pixels with diffraction-limited resolution without sophisticated multiple-step nanostructure fabrication or externally applied field control. Realization of dynamically switchable full color displays with diffraction-limited resolution is even harder. This work demonstrates a structural color strategy with developed anisotropic graphene metapixels. The anisotropic optical property is given by the intrinsic birefringence of the layered structure of graphene metamaterials, and each metapixel is spatially encoded by direct laser printing with diffraction-limited resolution (250 nm). The colors can be dynamically and instantly switched by controlling the scattering of the light source to excite different modes based on the strong anisotropic optical properties of the graphene metapixels. The low-cost large-scale fabrication method allows experimental demonstration of a large-area (4 in.) flexible full color optical switchable display. Such a simple, effective and flexible method promises broad practical applications in color display and color image sensing related fields.

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

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

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.

Hyperchromatic structural color for perceptually enhanced sensing by the naked eye

Colorimetric sensors offer the prospect for on-demand sensing diagnostics in simple and low-cost form factors, enabling rapid spatiotemporal inspection by digital cameras or the naked eye. However, realizing strong dynamic color variations in response to small changes in sample properties has remained a considerable challenge, which is often pursued through the use of highly responsive materials under broadband illumination. In this work, we demonstrate a general colorimetric sensing technique that overcomes the performance limitations of existing chromatic and luminance-based sensing techniques. Our approach combines structural color optical filters as sensing elements alongside a multichromatic laser illuminant. We experimentally demonstrate our approach in the context of label-free biosensing and achieve ultrasensitive and perceptually enhanced chromatic color changes in response to refractive index changes and small molecule surface attachment. Using structurally enabled chromaticity variations, the human eye is able to resolve ∼0.1-nm spectral shifts with low-quality factor (e.g., Q ∼ 15) structural filters. This enables spatially resolved biosensing in large area (approximately centimeters squared) lithography-free sensing films with a naked eye limit of detection of ∼3 pg/mm², lower than industry standard sensors based on surface plasmon resonance that require spectral or angular interrogation. This work highlights the key roles played by both the choice of illuminant and design of structural color filter, and it offers a promising pathway for colorimetric devices to meet the strong demand for high-performance, rapid, and portable (or point-of-care) diagnostic sensors in applications spanning from biomedicine to environmental/structural monitoring.

Applications of Cellulose Nanomaterials in Stimuli-Responsive Optics

As one of the most abundant biopolymers, cellulose has been a basic but essential building block of human society, with its use dating back thousands of years. With recent developments in nanotechnology and increasing environmental concerns, cellulose-based nanomaterials are now gaining attention as promising green material candidates for many high-value applications as a result of their biocompatibility and advantageous physical and chemical properties. In particular, cellulose nanocrystals are notable for their optical properties that can respond to various environmental stimuli as a result of the unique chiral nematic structure of the material. Compositing cellulosic materials with functional polymers, small molecules, and other nanomaterials can further stabilize and amplify these responsive optical signals and introduce multiple new functionalities. On the basis of these capabilities, many advanced applications of cellulose nanomaterials have been proposed, including chemical sensors, photonic papers, decorative coatings, data security, and smart textiles. In this review, we discuss and summarize recent advances in this emerging field of stimuli-responsive optics based on cellulose nanomaterials.

Evaluation of shade matching of a novel supra-nano filled esthetic resin composite employing structural color using simplified simulated clinical cavities

OBJECTIVE

To evaluate the shade matching ability of a novel supra-nano filled esthetic resin composite employing structural color technology using simplified simulated clinical cavities. Filler morphology and light transmittance characteristics were also evaluated.

MATERIALS AND METHODS

One-hundred and twenty frames of resin composite were built in A1, A2, A3, and A4 shades to simulate Class I cavities (diameter = 4 mm, height = 2 mm). For each shaded frame, cavities were filled with three different types of filler containing resin composites (n = 10): supra-nano filled (SN filled) resin composite, microhybrid filled (MH filled) resin composite, and clustered-nano filled (CN filled) resin composite. Color parameters were calculated using CIELAB (△E_ab ) and CIEDE2000 (△E₀₀ ). Data were analyzed using one-way analysis of variance (ANOVA), followed by Duncan's test (α = .05). Filler morphology and light transmittance characteristics were measured to explore the role of structural color on shade matching.

RESULTS

△E_ab and △E₀₀ of SN filled resin composite were significantly lower in A2, A3, and A4 shades (P < .05).

CONCLUSIONS

The SN filled resin composite showed better shade matching with A2, A3, and A4 shades of resin composite frames compared to MH filled resin composite, and CN filled resin composite.

CLINICAL SIGNIFICANCE

Universal-shade resin composites, which were expected to match nearly all shades, simplify the restorative procedure. Resin composite, which contained spherical supra-nano filler particles, could contribute most to its shade matching by stimulating structural color. Structural color technology may provide additional benefits for shade matching of resin composites.

Photonics in nature and bioinspired designs: sustainable approaches for a colourful world

Biological systems possess nanoarchitectures that have evolved for specific purposes and whose ability to modulate the flow of light creates an extraordinary diversity of natural photonic structures. In particular, the striking beauty of the structural colouration observed in nature has inspired technological innovation in many fields. Intense research has been devoted to mimicking the unique vivid colours with newly designed photonic structures presenting stimuli-responsive properties, with remarkable applications in health care, safety and security. This review highlights bioinspired photonic approaches in this context, starting by presenting many appealing examples of structural colours in nature, followed by describing the versatility of fabrication methods and designed coloured structures. A particular focus is given to optical sensing for medical diagnosis, food control and environmental monitoring, which has experienced a significant growth, especially considering the advances in obtaining inexpensive miniaturized systems, more reliability, fast responses, and the use of label-free layouts. Additionally, naturally derived biomaterials and synthetic polymers are versatile and fit many different structural designs that are underlined. Progress in bioinspired photonic polymers and their integration in novel devices is discussed since recent developments have emerged to lift the expectations of smart, flexible, wearable and portable sensors. The discussion is expanded to give emphasis on additional functionalities offered to related biomedical applications and the use of structural colours in new sustainable strategies that could meet the needs of technological development.

Three-dimensional ordered macroporous magnetic photonic crystal microspheres for enrichment and detection of mycotoxins (I): Droplet-based microfluidic self-assembly synthesis

Ordered porous materials are attracting enormous attention due to their uniform pore structures, particularly the magnetic photonic crystal microspheres (PCMs) which not only possess unique photonic crystal structure but also can achieve separation easily based on magnet. Here, a two-phase microfluidic self-assembly synthetic system was established simply and employed for the preparation of three dimensional PCMs (3DPCMs) by using the emulsion droplet approach. One phase (dispersed phase) was an aqueous emulsion containing Fe₃O₄, silica (SiO₂) and polystyrene (PS) nanoparticles; another phase (continuous phase) was pure silicone oil. The droplets were formed by introducing the dispersed phase into the continuous phase through a tee valve. By heating the droplets, the water would evaporate and the nanoparticles would finally assemble into solid microspheres, which could be changed into macroporous 3DPCMs after removal of the PS nanoparticles by calcination. The contents and particle sizes of Fe₃O₄, SiO₂ and PS nanoparticles in the dispersed phase were investigated in detail and optimized to prepare macroporous magnetic 3DPCMs with high quality. The morphologies, surface crystal structure, magnetic property, particle size distribution, specific surface area and pore size of the macroporous magnetic 3DPCMs were characterized. The expected 3DPCM displayed regular and uniform photonic crystal structure, narrow particle size distribution and strong magnetic property. The macroporous magnetic 3DPCMs grafted with vomitoxin (DON)-antibodies could be applied for selective enrichment of DON in real samples.

Synthesis of High-Performance Photonic Crystal Film for SERS Applications via Drop-Coating Method

Silica nanospheres with a well-controlled particle size were prepared via a nucleation-to-growth synthesis process. A facile method is proposed for improving the self-assembly behavior of silica colloidal particles in droplet coatings by the simple controlling of the drying temperature. It is shown that a periodically arranged, opal-structured, photonic crystal film with a large area of approximately 4.0 cm2 can be prepared, even when the particle size is up to 840 nm. When the band gap of the silica photonic crystals falls in the visible-light region, the crystals exhibit distinct structural colors. Moreover, the wavelength of the reflected light increases with an increasing particle size of silica. When the photonic band gap overlaps the wavelength of the laser source, the overall Raman spectrum intensity is significantly enhanced. Accordingly, the proposed nucleation-to-growth process and drop-coating method provides a cheap and simple approach for the manufacture of uniform sized silica and surface-enhanced Raman scattering substrates, respectively.

Mechanism and control of "coffee-ring erosion" phenomena in structurally colored ionomer films

Ionomer polyesters have polymer backbones functionalized with charged groups that make them water-dispersible. Despite the widespread use of ionomer polymers in environmentally friendly coatings without volatile organic solvents, the fundamental understanding of their film formation properties is still limited. In the study, we deposited polyester nanofilms of brilliant structural colors and correlated the macroscale optical properties to the microscale thickness of the thin films. We found that sessile water droplets deposited on these films drive the formation of a rich variety of structures by an evaporation-induced effect of "coffee-ring erosion". The ionomers spontaneously get partially re-dispersed in the form of nanoparticles in the sessile droplets and driven by convective evaporation flows, become redistributed in multiple colorful ring patterns. By using the structural colors as means to follow the polymer redistribution, we characterized further the coffee-ring patterns and found that the generated patterns are dictated by polymer composition but are mostly independent on molecular weight. As expected by colloidal theory, this phenomenon was suppressed in presence of electrolytes. Furthermore, we show that the integrity of these thin polyester films can be significantly improved by thermal densification without any further chemical curing.

Photonic Crystal Stimuli-Responsive Chromatic Sensors: A Short Review

Photonic crystals (PhC) are spatially ordered structures with lattice parameters comparable to the wavelength of propagating light. Their geometrical and refractive index features lead to an energy band structure for photons, which may allow or forbid the propagation of electromagnetic waves in a limited frequency range. These unique properties have attracted much attention for both theoretical and applied research. Devices such as high-reflection omnidirectional mirrors, low-loss waveguides, and high- and low-reflection coatings have been demonstrated, and several application areas have been explored, from optical communications and color displays to energy harvest and sensors. In this latter area, photonic crystal fibers (PCF) have proven to be very suitable for the development of highly performing sensors, but one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) PhCs have been successfully employed, too. The working principle of most PhC sensors is based on the fact that any physical phenomenon which affects the periodicity and the refractive index of the PhC structure induces changes in the intensity and spectral characteristics of the reflected, transmitted or diffracted light; thus, optical measurements allow one to sense, for instance, temperature, pressure, strain, chemical parameters, like pH and ionic strength, and the presence of chemical or biological elements. In the present article, after a brief general introduction, we present a review of the state of the art of PhC sensors, with particular reference to our own results in the field of mechanochromic sensors. We believe that PhC sensors based on changes of structural color and mechanochromic effect are able to provide a promising, technologically simple, low-cost platform for further developing devices and functionalities.

Translucent in air and iridescent in water: structural analysis of a salamander egg sac

Females of some Asian salamanders of the genus Hynobius deposit in streams their eggs embedded in a translucent envelope called an 'egg sac'. The edges of the envelope exhibit a spectacular blue-to-yellow iridescent glow, which instantaneously disappears when the sac is removed from water. First, our scanning electron microscopy analyses reveal that the inner surface of the 100 μm-thick envelope displays striations (length scale of about 3 μm), which are themselves covered by much smaller (190 ± 30 nm) and quasi-periodic corrugations. The latter could constitute a surface diffraction grating generating iridescence by light interference. Second, our transmission electron microscopy and focused-ion-beam scanning electron microscopy analyses show that the bulk of the egg sac wall is composed of meandering fibres with a quasi-periodic modulation of 190 ± 60 nm along the thickness of the envelope, generating a photonic crystal. Third, Fourier power analyses of 450 electron microscopy images with varying incident angles indicate that changing the surrounding medium from water to air shifts most of the backscattered power spectrum to the ultraviolet range, hence, explaining that the egg sac loses visible iridescence when removed out of the water. Fourth, the results of our photography and optical spectroscopy experiments of submerged and emerged egg sacs rule out the possibility that the iridescence is due to a thin film or a multilayer, whereas the observed non-specular response is compatible with the backscattering expected from surface diffraction gratings and volumetric photonic crystals with spatial 1D modulation. Finally, although we mention several potential biological functions of the egg sac structural colours and iridescence, we emphasise that these optical properties might be the by-products of the envelope material internal structure selected during evolution for its mechanical properties.

Construction of an Array of Photonic Crystal Films for Visual Differentiation of Water/Ethanol Mixtures

A photonic crystal film (PCF) which consists of a porous layered structure with a highly ordered periodic arrangement of nanopores has been used to differentiate between various mixtures of water and ethanol (EtOH). The refractive index difference between the wall (silica) of the empty nanopore and air which occupies it results in the structural color of the PCF. This color disappears when the nanopores are infiltrated by a liquid with a similar refractive index to silica (or silicon dioxide). The disappearance of the structural color provides a means to construct a colorimetric sensor to differentiate between various water/EtOH mixtures based on their wettability of the nanopores in the PCF. In this study, an array of silica-based PCFs was synthesized on a silicon substrate with a precise control of nanopore properties using the co-assembly/sedimentation method. Using this method, we benefitted from having different PCFs on a single substrate. Chemical coatings, neck angles, and film thicknesses on each PCF were the three factors used to adjust the wettability of the pores. Nanopore wetting by water/EtOH mixtures was studied in a systematic manner based on the three factors, and the findings were used to develop a sensor for visual differentiation of various water/EtOH mixtures. The final developed sensor consisting of an array of six PCFs was able to differentiate between seven different water/EtOH mixtures: W10, W20, W30, W40, W50, W60, and W70, in which W10 means 10% of water in EtOH.

Structural color printing with a dielectric layer coated on a nanotextured metal substrate: simulation and experiment

The printing of plasmonic structural colors relies on noble metal nanostructures fabricated on Si, glass, or plastic substrates. This paper presents a simple surface structure for producing vivid structural colors directly from common metal substrates. The structure is formed by texturing the surface of stainless steel (STS) via imprinting and coating it with a dielectric layer. Diverse colors are generated simply by varying the thickness of the dielectric layer. The colors arise from surface plasmon resonance and guided-mode resonance of the incident light, which are excited on the textured STS surface and inside the dielectric layer, respectively. A finite-difference time-domain simulation shows that 500 nm is the optimum texture periodicity with regard to the tunability and vividness of the colors. This is experimentally verified by printing many differently colored images on the surface of STS substrates with a texture period of 500 nm. The proposed structure/method does not require a nanofabrication technique such as electron-beam lithography or focused ion beam etching. The results of the study provide a facile route for producing vivid structural colors on metals, which may find various applications, including surface decoration, product identification, anti-counterfeiting, and perfect absorbers.

Hybrid One-Dimensional Plasmonic-Photonic Crystals for Optical Detection of Bacterial Contaminants

Photonic crystal-based biosensors hold great promise as low-cost devices for real-time monitoring of a variety of biotargets, for example, bacterial contaminants in food. Here, we report the proof-of-concept for a new colorimetric sensor of bacterial contamination, which is based on a novel hybrid plasmonic-photonic device. Our system consists of a layer of silver, a plasmonic metal exhibiting a well-known bioactivity, on top of a one-dimensional photonic crystal. We attribute the bioresponsivity to the formation of polarization charges at the Ag/bacterium interface within a sort of "bio-doping" mechanism. Interestingly, this triggers a blue shift in the photonic response. As an example, we assessed the validity of our approach by detecting one of the most hazardous contaminants, Escherichia coli. This work demonstrates that our device can be a low-cost and portable platform for the detection of common bacterial contaminants.

Temperature-Responsive, Multicolor-Changing Photonic Polymers

A new principle is developed to fabricate temperature-responsive, multicolor photonic coatings that are capable of switching color. The coating is composed of a non-cross-linked liquid crystal siloxane-based elastomer that is interpenetrated through an acrylate-based liquid crystal network. Discrete temperature changes induce phase separation and mixing between the siloxane and the acrylate polymers and change the reflective colors correspondingly. The temperature-responsive color change of the coatings can be programmed by the processing conditions and coating formulation, which allows for the fabrication of photopatterned multicolor images. The photonic ink can be coated on flexible poly(ethylene terephthalate) films using roll-to-roll flexographic printing, making these temperature-responsive, multicolor-changing polymers appealing for applications such as responsive color decors, optical sensors, and anticounterfeit labels.

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.

Molecularly Imprinted Polymer-Based Hybrid Materials for the Development of Optical Sensors

We report on the development of new optical sensors using molecularly imprinted polymers (MIPs) combined with different materials and explore the novel strategies followed in order to overcome some of the limitations found during the last decade in terms of performance. This review pretends to offer a general overview, mainly focused on the last 3 years, on how the new fabrication procedures enable the synthesis of hybrid materials enhancing not only the recognition ability of the polymer but the optical signal. Introduction describes MIPs as biomimetic recognition elements, their properties and applications, emphasizing on each step of the fabrication/recognition procedure. The state of the art is presented and the change in the publication trend between electrochemical and optical sensor devices is thoroughly discussed according to the new fabrication and micro/nano-structuring techniques paving the way for a new generation of MIP-based optical sensors. We want to offer the reader a different perspective based on the materials science in contrast to other overviews. Different substrates for anchoring MIPs are considered and distributed in different sections according to the dimensionality and the nature of the composite, highlighting the synergetic effect obtained as a result of merging both materials to achieve the final goal.

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

Photonic crystals (PCs) have been widely applied in the anticounterfeiting field according to their easily tunable and angle-dependent structural colors. However, most studies are now focused on single-layer PCs assembled from monodisperse colloidal spheres, which have only one bandgap. Here, we prepared bilayer photonic crystal films by choosing 250 and 330 nm silica spheres as the bottom and top layer, respectively. The effect of the incident angle on the bandgap of PCs was investigated and the results showed that the bandgap of the bilayer PCs was incident angle dependent-the structure exhibited two strong bandgaps within small incident angles, while as the incident angle increases, both the bandgaps blue-shifted and more importantly, the bandgap of the bottom layer disappeared with a further increase in the incident angle. Furthermore, with the delicate design of the thickness of the top layer, this bilayer structure selectively displayed the structural colors of the bottom layer, overlap colors of both the top and the bottom layer, and the color of only the top layer, respectively. By changing the incident angle, the color circulation from green to magenta, orange, yellow, and green again was realized. The realization of the controllable color tunability further motived us towards the patterning of the bilayer PCs, which showed promising potential in the anticounterfeiting field.

2D Au Nanosphere Arrays/PVA-PBA-Modified-Hydrogel Composite Film for Glucose Detection with Strong Diffraction Intensity and Linear Response

A novel glucose sensor was reported that consisted of two-dimensional (2D) Au nanosphere arrays and glucose-responsive hydrogel film. This sensor exhibited an intense diffraction signal and an obvious diffraction color on a quartz slide due to the strong diffraction intensity of the Au nanosphere arrays. Thus, glucose was detected via the variation of diffraction wavelength and diffraction color, without a high reflective mirror. In addition, by introducing poly(vinyl alcohol) (PVA) to crosslink the phenylboronic acid (PBA)-modified hydrogel film, the diffraction wavelength of the 2D Au nanosphere arrays/hydrogel composite film shifted in the same direction in high ionic strength condition. In particular, it showed a nearly linear red-shift when the glucose concentration increased from 0 mM to 20 mM. Moreover, this glucose sensor displayed good reproducibility. The nearly linear response and good reproducibility were highly helpful for improving practical application of this glucose sensor.

Synthesis of monodisperse ferromagnetic CoxFe3−xO4 colloidal particles with magnetically tunable optical properties

Monodisperse Co_xFe_3–xO₄ colloidal particles with uniform size and tunable composition have been prepared using a one-step hydrothermal method.

Environmentally responsive photonic polymers

This feature article focuses on photonic polymers that change colouration due to an environmental stimulus and highlights their industrial feasibility.

Impact of Cubic Symmetry on Optical Activity of Dielectric 8-srs Networks

Photonic crystals are engineered structures able to control the propagation and properties of light. Due to this ability, they can be fashioned into optical components for advanced light manipulation and sensing. For these applications, a particularly interesting case study is the gyroid srs-network, a three-dimensional periodic network with both cubic symmetry and chirality. In this work we present the fabrication and characterization of three-dimensional cubically symmetric 8-srs photonic crystals derived from combination of eight individual gyroid srs-networks. We numerically and experimentally investigate optical properties of these photonic crystals and study in particular, the impact of cubic symmetry on transmission and optical activity (OA). Gyroid photonic crystals fabricated in this work can lead to the development of smaller, cheaper, and more efficient optical components with functionalities that go beyond the concept of lenses.

Structural Colored Balloon Composed of Temperature-Responsive Polymers Showing LCST Behavior

Structural colored balloons (SCBs) consisting of polymer thin film developed structural color by thin-layer interference on the shell. Thermoresponsive SCBs were prepared with poly(diethylene glycol monomethyl ether methacrylate)- co-poly( N-phenylacrylamide), which shows lower critical solution temperature (LCST) behavior. When cooling gelatin aqueous solution in which osmotic pressure was not operated, only hydration of the copolymer progressed due to LCST transition. The optical path length of the SCB increased due to swelling by water and subsequently decreased due to dissolution. The structural color changed according to the change in optical path length. In cold pure water, in addition to the hydration, osmotic pressure was operated to induce an influx of the outer solvent and the resulting diameter change also affected the shell thickness. The structural color change was analyzed to reveal that the dissolution of the polymer had significant effect on the developed structural color.