Reusable macroporous photonic crystal-based ethanol vapor detectors by doctor blade coating

Bioinspired Colorimetric Double Inverse Opal Photonic Crystal Indicators for Ethanol Concentration Sensing in Fermentation Engineering

Photonic crystal-based ethanol concentration indicators with rapid response and brilliant structural color output definitely take a place in colorimetric sensors. Here, based on the H-bond-regulated swelling of acrylate shape memory polymers (SMPs) and the solvent-induced structural color change of the double inverse opal photonic crystals (DIOPCs), new-type photonic crystals (PCs) colorimetric indicators were constructed, exhibiting a span of maximum reflection wavelength (λmax) up to ∼166 nm in response to alcohols with concentrations from 0 to 100 vol %. DIOPC indicators (DIOPCIs) show a rapid response to alcohols (<1.5 s) and output different structural colors (covering from blue to red). The colorimetric sensing mechanism includes the solvent-triggered recovery of the inverse opal skeleton, the cosolvency effect and H-bonds induced swelling/shrinkage of the polymer, the phase separation between polystyrene (PS) microsphere and polymer skeleton, and the light diffraction of DIOPCs. While ensuring a larger λmax span by regulating the H-bond interactions in polymer chains through acrylamide (AAm), AAm-modified DIOPCIs are sensitive to some specific ethanol concentrations. The real-time sensing of ethanol concentration during fermentation verified the practicability of DIOPCIs, thus establishing a visual model between structural color and corresponding fermentation kinetics. We envisage that the DIOPCIs will contribute to the intelligentization of the alcoholic fermentation and distillation industry.

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

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

Magnetic field responsive microspheres with tunable structural colors by controlled assembly of nanoparticles

Dynamic color tuning has many useful applications in nature for communication, camouflage, mood indication, etc. Structural colors have more advanced applications due to their ability to respond to external stimuli by dynamically changing color. In this work, we proposed an efficient method to prepare magneto-chromatic microspheres with tunable structural color. Through a microfluidic technique, the magneto-chromatic microspheres containing Fe₃O₄@C magnetic particles were continuously prepared. The size of the microspheres decreases with the increase of PVA solution phase to ETPTA phase flow rate ratio. Furthermore, the microspheres with larger sizes more easily form close packed structures. Microspheres can be constrained in PVA to form a free-standing film after the evaporation of water in PVA solution. The PVA film could display tunable brilliant structural colors when an external magnetic field is applied. Moreover, microspheres with fixed structural colors can also be acquired by polymerizing microspheres under UV light under an external magnetic field.

An efficient strategy was used for the preparation of magneto-chromatic microspheres with tunable structural color.

Colloidal Self-Assembly Approaches to Smart Nanostructured Materials

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

Multi-solvent large stopband monitoring based on the insolubility/superoleophilicity of PEDOT inverse opals

Monitoring and post-processing of organic solvents are important for environmental protection. Challenges remain in the development of a universal material which can detect any solvent with a large stopband shift and show excellent stability. Herein, we demonstrate a poly 3,4-ethylenedioxythiophene inverse opal (PEDOT-IO) with a large stopband shift toward various solvents based on the insolubility/superoleophilicity properties. The PEDOT-IO film was fabricated by the potentiostatic polymerization of 3,4-ethylene dioxythiophene using a three-electrode system, infiltrating the interstices of the photonic crystal template with PEDOT and subsequently removing the template. The surface of the PEDOT-IO film presented a composite structure: interconnected pores and hollow shells. When the solvent was introduced into the voids of PEDOT-IO film, the effective refractive index (n) of the whole sample increased due to the replacement of air with the solvent, and the pores and hollow shells showed different degrees of swelling. The synergistic effect of increased n and volume expansion contributed to a large redshift of the stopband of the PEDOT-IO film. PEDOT-IO film exhibited excellent resistance to various solvents and high/low temperature. This work further enriches the application of conductive polymers in solvent-responsive PC sensors and provides a novel means of creating PC-based optical materials and devices.

Bio-inspired gas sensing: boosting performance with sensor optimization guided by "machine learning"

The performance of existing gas sensors often degrades in field conditions because of the loss of measurement accuracy in the presence of interferences. Thus, new sensing approaches are required with improved sensor selectivity. We are developing a new generation of gas sensors, known as multivariable sensors, that have several independent responses for multi-gas detection with a single sensor. In this study, we analyze the capabilities of natural and fabricated photonic three-dimensional (3-D) nanostructures as sensors for the detection of different gaseous species, such as vapors and non-condensable gases. We employed bare Morpho butterfly wing scales to control their gas selectivity with different illumination angles. Next, we chemically functionalized Morpho butterfly wing scales with a fluorinated silane to boost the response of these nanostructures to the vapors of interest and to suppress the response to ambient humidity. Further, we followed our previously developed design rules for sensing nanostructures and fabricated bioinspired inorganic 3-D nanostructures to achieve functionality beyond natural Morpho scales. These fabricated nanostructures have embedded catalytically active gold nanoparticles to operate at high temperatures of ≈300 °C for the detection of gases for solid oxide fuel cell (SOFC) applications. Our performance advances in the detection of multiple gaseous species with specific nanostructure designs were achieved by coupling the spectral responses of these nanostructures with machine learning (a.k.a. multivariate analysis, chemometrics) tools. Our newly acquired knowledge from studies of these natural and fabricated inorganic nanostructures coupled with machine learning data analytics allowed us to advance our design rules for sensing nanostructures toward the required gas selectivity for numerous gas monitoring scenarios at room and high temperatures for industrial, environmental, and other applications.

Water-Soluble Chemical Vapor Detection Enabled by Doctor-Blade-Coated Macroporous Photonic Crystals

Water-soluble chemicals, involving a wide range of toxic chemicals in aqueous solutions, remain essential in both daily living or industrial uses. However, most toxicants are evaporated with water through their use and thus cause deleterious effects on the domestic environment and health in humans. Unfortunately, most current low-dose chemical vapor detection technologies are restricted by the use of sophisticated instruments and unable to promptly detect the quantity of diverse toxicants in a single analysis. To address these issues, this study reports the development of simple and fast chemical vapor detection using doctor-blade-coated macroporous poly(2-hydroxyethyl methacrylate)/poly(ethoxylated trimethylolpropane triacrylate) photonic crystals, in which the poly(2-hydroxyethyl methacrylate) has strong affinity to insecticide vapor owing to a favorable Gibbs free energy change for their mixing. The condensation of water-soluble chemical vapor therefore results in a significant reflection peak shift and an obvious color change. The visual colorimetric readout can be further improved by increasing the lattice spacing of the macroporous photonic crystals. Furthermore, the dependence of the reflection peak position on vapor pressure under actual conditions and the reproducibility of vapor detecting are also evaluated in this study.

Core-shell silica-rhodamine B nanosphere for synthetic opals: from fluorescence spectral redistribution to sensing

Photonic crystals are a unique tool to modify the photoluminescence of light-emitting materials. A variety of optical effects have been demonstrated by infiltrating opaline structures with photoactive media. On the other hand, the fabrication of such structures includes complex infiltration steps, that often affect the opal lattice and decrease the efficiency of light emission control. In this work, silica nanospheres were directly functionalized with rhodamine B to create an emitting shell around the dielectric core. Simple tuning of the microsphere preparation conditions allows selecting the appropriate sphere diameter and polydispersity index approaching 5%. These characteristics allow facile self-assembling of the nanospheres into three-dimensional photonic crystals whose peculiar density of photonic states at the band-gap edges induces spectral redistribution of the rhodamine B photoluminescence. The possibility to employ the new stable structure as sensor is also investigated. As a proof of principle, we report the variation of light emission obtained by exposure of the opal to vapor of chlorobenzene.

High Refractive Index Inverse Vulcanized Polymers for Organic Photonic Crystals

Photonic technologies are nowadays dominated by highly performing inorganic structures that are commonly fabricated via lithography or epitaxial growths. Unfortunately, the fabrication of these systems is costly, time consuming, and does not allow for the growth of large photonic structures. All-polymer photonic crystals could overcome this limitation thanks to easy solubility and melt processing. On the other hand, macromolecules often do not offer a dielectric contrast large enough to approach the performances of their inorganic counterparts. In this work, we demonstrate a new approach to achieve high dielectric contrast distributed Bragg reflectors with a photonic band gap that is tunable in a very broad spectral region. A highly transparent medium was developed through a blend of a commercial polymer with a high refractive index inverse vulcanized polymer that is rich in sulfur, where the large polarizability of the S–S bond provides refractive index values that are unconceivable with common non-conjugated polymers. This approach paves the way to the recycling of sulfur byproducts for new high added-value nano-structures.

Structural Colors Based on Amorphous Arrays Comprised Solely of Silica Particles

In this study, structural colors were fabricated by producing an amorphous array with atypical silica particles. The colors were controlled by an array of silica particles with different sizes. In previous research, the process required inducement of the amorphous array, which was complex. Meanwhile, in this paper, we proposed a simple one-step process. First, spherical silica nanoparticles were synthesized using the sol-gel process of the Stöber method. Atypical silica particles that induced an amorphous array were produced by adding a small amount of phenol-formaldehyde resin. Subsequently, the colloidal silica was converted to a powder using a convection oven. The characteristics of the synthesized silica particles were confirmed using a scanning electron microscope (SEM). All the synthesized silica powders obtained structural colors. Finally, the silica powders were dispersed in deionized (DI) water and coated on a glass slide. We confirmed that the silica particles showed different structural colors depending on the size of the particles. We also found that the color was highly independent of the viewing angle.

Patterned Full-Color Reflective Coatings Based on Photonic Cholesteric Liquid-Crystalline Particles

An easy approach to pattern angular-independent, multicolor reflective coatings based on cholesteric liquid-crystalline (CLC) particles is presented. CLC particles are fabricated by emulsification, which is a scalable, cost-effective, and environmentally friendly synthesis process. The photonic particles can be easily dispersed in a binder to produce reflective coatings. Furthermore, a simple strategy to remove the photonic cross-communication between the particles has been developed. By incorporating a reactive blue/green absorbing dye into the network structure of the CLC particles the cross-communication is absorbed by the dye, leading to well-defined structural colors. Moreover, we demonstrate the possibility of producing patterned multicolor images by controlled swelling of the particles by the binder.

Ultrafast assembly of nanoparticles to form smart polymeric photonic crystal films: a new platform for quick detection of solution compositions

Photonic crystals (PCs) are an important subset of photonic materials with specific optical properties, which can be utilized for structural color printing, anti-counterfeiting technologies, chemical sensors and so on. However, the fabrication of scalable, high-quality and uniform photonic crystal films at room temperature still remains a big challenge. Herein, a fast, energy efficient and scalable process is reported for the first time. A high-quality polymeric photonic crystal film can be fabricated from the uniform core/shell particle slurry within several seconds by a calendering process. The obtained crystalline structure can be rapidly captured by photo-curing, and the resultant PC films show extremely strong iridescent tunable structural colors. Because the as-designed PC film matrix is sensitive to solutions with different solubility parameters, a prototype demo sensor is firstly set up for quick detection of the composition of the alcohol/H2O mixture as a model of white spirits, which has the feature of reversible and linear quantitative sensing performance. In addition, the as-prepared PC film is further developed as an inexpensive test strip showing quick detection of ethanol/octane mixtures (possessing different solubility parameters) as a model of ethanol gasoline. This facile, scalable and energy efficient fabrication procedure is exceedingly promising for high-throughput production, showing great potential for industrialization of PC sensors and detectors. The combination of uniform particles and a dispersion medium can be potentially designed for different stimuli responsive systems, which is beneficial for applications ranging from sensing, anti-counterfeiting, to some special optical devices.

Achieving Color and Function with Structure: Optical and Catalytic Support Properties of ZrO2 Inverse Opal Thin Films

Taking inspiration from natural photonic crystal architectures, we report herein the successful fabrication of zirconia inverse opal (ZrO₂ IO) thin-film photonic crystals possessing striking iridescence at visible wavelengths. Poly(methyl methacrylate) (PMMA) colloidal crystal thin films (synthetic opals) were first deposited on glass microscope slides, after which the interstitial voids in the films were filled with a Zr(IV) sol. Controlled calcination of the resulting composite films yielded iridescent ZrO₂ IO thin films with pseudo photonic band gaps (PBGs) along the surface normal at visible wavelengths. The PBG position was dependent on the macropore diameter (D) in the inverse opals (and thus proportional to the diameter of the PMMA colloids in the sacrificial templates), the incident angle of light with respect to the surface normal (θ), and also the refractive index of the medium filling the macropores, all of which were accurately described by a modified Bragg's law expression. Au/ZrO₂ IO catalysts prepared using the ZrO₂ IO films demonstrated outstanding performance for the reduction of 4-nitrophenol to 4-aminophenol in the presence of NaBH₄, which can be attributed to the interconnected macroporosity in the films, which afforded a high Au nanoparticle dispersion and also facile diffusion of 4-nitrophenol to the catalytically active Au sites.

Bioinspired Stimuli-Responsive Color-Changing Systems

Stimuli-responsive colors are a unique characteristic of certain animals, evolved as either a method to hide from enemies and prey or to communicate their presence to rivals or mates. From a material science perspective, the solutions developed by Mother Nature to achieve these effects are a source of inspiration to scientists for decades. Here, an updated overview of the literature on bioinspired stimuli-responsive color-changing systems is provided. Starting from natural systems, which are the source of inspiration, a classification of the different solutions proposed is given, based on the stimuli used to trigger the color-changing effect.

Bioinspired Hydrogel Interferometer for Adaptive Coloration and Chemical Sensing

Living organisms ubiquitously display colors that adapt to environmental changes, relying on the soft layer of cells or proteins. Adoption of soft materials into an artificial adaptive color system has promoted the development of material systems for environmental and health monitoring, anti-counterfeiting, and stealth technologies. Here, a hydrogel interferometer based on a single hydrogel thin film covalently bonded to a reflective substrate is reported as a simple and universal adaptive color platform. Similar to the cell or protein soft layer of color-changing animals, the soft hydrogel layer rapidly changes its thickness in response to external stimuli, resulting in instant color change. Such interference colors provide a visual and quantifiable means of revealing rich environmental metrics. Computational model is established and captures the key features of hydrogel stimuli-responsive swelling, which elucidates the mechanism and design principle for the broad-based platform. The single material-based platform has advantages of remarkable color uniformity, fast response, high robustness, and facile fabrication. Its versatility is demonstrated by diverse applications: a volatile-vapor sensor with highly accurate quantitative detection, a colorimetric sensor array for multianalyte recognition, breath-controlled information encryption, and a colorimetric humidity indicator. Portable and easy-to-use sensing systems are demonstrated with smartphone-based colorimetric analysis.

UV and blue-light anti-reflective structurally colored contact lenses based on a copolymer hydrogel with amorphous array nanostructures

Novel anti-reflective structurally colored contact lenses not only block UV- and blue-light but also avoid the risk of chemical dyes.

Self-Assembled Curved Macroporous Photonic Crystal-Based Surfactant Detectors

Surfactants are extensively used as detergents, dispersants, and emulsifiers. Thus, wastewater containing high-concentration surfactants discharged to the environment pose a serious threat to the ecosystem. Unfortunately, conventional detection methods for surfactants suffer from the use of sophisticated instruments and cannot perform detections for various surfactants by a single analysis. The article reports the development of simple and sensitive surfactant detection using doctor-blade-coated three-dimensional curved macroporous photonic crystals on a cylindrical rod. The photonic crystals exhibit different hydrophobicities at various angular positions after surface modification. The penetration of aqueous surfactant solutions in the interconnected macropores causes red-shift as well as reduction in amplitude in the optical stop bands, resulting in surfactant detection with visible readout. The correlation between the surface tension, as well as the solution-infiltrated angular position, and the concentration of aqueous surfactant solutions has also been investigated in this study.

Toward high value sensing: monolayer-protected metal nanoparticles in multivariable gas and vapor sensors

This review provides analysis of advances in multivariable sensors based on monolayer-protected nanoparticles and several principles of signal transduction that result in building non-resonant and resonant electrical sensors as well as material- and structure-based photonic sensors.