Towards the Nanosheet-Based Photonic Nose: Vapor Recognition and Trace Water Sensing with Antimony Phosphate Thin Film Devices

Heat-Resistant and Color-Changing Luminescent Polysulfone for Information Encryption and Fire Alarming

An intrinsic difficulty with thermally responsive photoluminescent materials is that high temperatures usually destroy luminance due to the notorious thermal quenching effect. Limited by the vulnerable chemical structure and soft skeleton, most of the existing photoluminescent responsive materials fail to indicate or work at a surging temperature over 100 °C, thus limiting application in display and alarming in harsh conditions. Herein, enlightened by chameleon's adaptive nature to external stimulus, we introduce a topologically optimized electron donor-acceptor (DA) structure and supramolecular interactions of lanthanide ions into the polymer backbone. The emission color determined by the DA structure is stable at high temperatures, and metal-ligand interaction phosphorescence is temperature-adjustable. Owing to the excellent reproducibility and heat resistance of composite films, the sensors can be bent into different three-dimensional structures and adhered to metal surfaces as flexible thermometers with superior display resolution. The polymer composite film could be directly applied as a photoluminescent quick response (QR) code, with patterns simultaneously variable to a temperature from 30 to 150 °C free of manual operation. More importantly, the polymeric composite could be in-situ-oxidized to a "sulfone" structure with an enhanced glass transition temperature of 297-304 °C. The heat- and flame-resistant characteristics of the oxidized films give rise to the application of fire alarming devices since it can locate the fire source and respond exactly depending on the distance from the fire. The unique display, encryption, and alarming functions of the polymeric composite studied in this work bring forward a new concept of developing a great information security and disaster monitoring system with the application of temperature-responsive materials.

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.

High-Precision Colorimetric Sensing by Dynamic Tracking of Solvent Diffusion in Hollow-Sphere Photonic Crystals

Expensive instruments and complicated data processing are often required to discriminate solvents with similar structures and properties. Colorimetric sensors with high selectivity, low cost, and good portability are highly desirable to simplify such detection tasks. Herein, we report the fabrication of a photonic crystal sensor based on the self-assembled resorcinol formaldehyde (RF) hollow spheres to realize colorimetric sensing of polar solvents, including homologs and isomers based on the saturated diffusion time. The diffusion of solvent molecules through the photonic crystal film exhibits a unique three-step diffusion profile accompanied by a dynamic color change, as determined by the physicochemical properties of the solvent molecules and their interactions with the polymer shells, making it possible to accurately identify the solvent type based on the dynamic reflection spectra or visual perception. With its superior selectivity and sensitivity, this single-component colorimetric sensor represents a straightforward tool for convenient solvent detection and identification.

Intercalation and flexibility chemistries of soft layered materials

Layered materials, alternate stackings of two or more components, are found in a wide range of scales. Chemists can design and synthesize layered structures containing functional units. The soft-type layered materials exhibit characteristic dynamic functions originating from two-dimensional (2D) anisotropy and structure flexibility. This feature article focuses on "intercalation" and "flexibility" as two new perspectives for designing soft layered materials. Intercalation of guests is a characteristic approach for design of layered structures. Flexibility is an important factor to control the dynamic functions of the layered structures. As a model case, the intercalation-induced tunable stimuli-responsive color-change properties of layered polydiacetylene (PDA) are introduced to study the impact of the intercalation and flexibility on the dynamic functions. Recently, layered materials have drastically expanded the research area from conventional rigid inorganic compounds to new self-assembled nanostructures consisting of organic components, such as polymers, metal-organic frameworks, and covalent-organic frameworks. These new layered architectures have potentials for exhibiting dynamic functions originating from the structure flexibility beyond the static properties originating from classical intercalation and host-guest chemistries. Therefore, intercalation and flexibility chemistries of soft layered materials are regarded as new perspectives for design of advanced dynamic functional materials.

Diphylleia grayi-Inspired Intelligent Hydrochromic Adhesive Film

Diphylleia grayi-inspired hydrochromic nano/microstructured films have received much attention for its promising smart hydrochromic applications owing to their simple and low-cost but energy-effective strategy. A new type of water-switchable glazing film patterned with various nano/micro air-hole inverse opal arrays is introduced by selectively removing nano/microsphere polystyrene arrays embedded in the surface of polydimethylsiloxane (PDMS) films. Using the significant contrast ratio of the bleaching and the scattering states, we have optimized the switching properties of Mie scattered patterns. As a result, we obtained a single inverse opal layer-embedded PDMS adhesive film with hexagonally close-packed 1 μm air-hole arrays as an optimum scattered film. The differences of diffusive transmittance and optical haze values between the dry and the wet states of the best scattered film reached 44.93% (ΔT_D.T = 59.11-14.18%) and 54.88% (ΔH = 69.42-14.54%), respectively. In addition, using the best-optimized inverse opal layer-embedded PDMS film, we fabricated a perfectly imitated Diphylleia grayi structure for camouflage application and an intelligent hydrochromic window device. The dynamic water modulation of the scattered opaque and nonscattered transparent state of the inverse opal-patterned PDMS adhesive film can provide an advanced platform structure in the area of hydrochromic technology for smart windows, camouflage, and clear umbrellas for rainy days.

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.

Hydrochromic carbon dots as smart sensors for water sensing in organic solvents

Smart, stimuli-responsive, photoluminescent materials that undergo a visually perceptible emission color change in the presence of an external stimulus have long been attractive for use in sensor platforms. When the stimulus is the presence of water, the materials that undergo changes in their light emission properties are called hydrochromic and they can be used for the development of sensors to detect and quantify the water content in organic solvents, which is fundamental for laboratory safety and numerous industrial applications. Herein, we demonstrate the preparation of structurally different carbon dots with tunable emission wavelengths via a simple carbonization approach under controlled temperature and time, involving commercial brown sugar as a starting material. The detailed experimental analysis reveals the "structure-hydrochromic property" relationship of the carbon dots and assesses their capability as effective water sensors. The carbon dots that were proved most efficient for the specific application were then used to identify the presence of water in various aprotic and protic organic solvents via a sensing mechanism based either on the fluorescence wavelength shift or on the fluorescence intensity enhancement, respectively, attributed to the formation of intermolecular hydrogen bonds between carbon dots and water molecules. This is the first demonstration of structurally defined carbon dots in a specific application. The developed carbon dots, apart from being environmentally friendly, were proved to also be biocompatible, enabling this presented process to be a path to "green" sensors.

Visualization and Quantitative Detection of Friction Force by Self-Organized Organic Layered Composites

Visualization and quantitative detection of external stimuli are significant challenges in materials science. Quantitative detection of friction force, a mechanical stress, is not easily achieved using conventional stimuli-responsive materials. Here, the quantitative detection of friction force is reported, such as the strength and accumulated ammount, from the visible color of organic layered composites consisting of polydiacetylene (PDA) and organic amines without an excitation light source. The composites of the layered diacetylene monomer crystal and interlayer organic amine are synthesized through self-organization from the precursor solution. After topochemical polymerization, the layered composites based on PDA show tunable temperature-responsive and mechanoresponsive color-change properties depending on the types of interlayer amines. The layered composites are homogeneously coated on a filter paper. The change in color of the paper is quantitatively used to visualize the strength and accumulated amount of the applied friction force. Furthermore, writing pressure is measured by friction force using the paper device.

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.

Soft chemistry of ion-exchangeable layered metal oxides

Disassembly and re-assembly of layered metal oxides by soft chemical approaches can be used to tailor functionalities in artificial photosynthesis, energy storage, optics, and piezoelectrics.

Real-Time Imaging of 2D and 3D Temperature Distribution: Coating of Metal-Ion-Intercalated Organic Layered Composites with Tunable Stimuli-Responsive Properties

Organic layered materials have intercalation and dynamic properties. The dynamic properties are tuned by the intercalation of the guests. In general, however, it is not easy to achieve the homogeneous and thin coating of the layered materials on substrates with complex shapes because of the two-dimensional anisotropic structures. In the present work, the layered organic composites were homogeneously coated on a variety of substrates for application to practical devices. The metal-ion-intercalated layered polydiacetylene (PDA-Mⁿ⁺) with tunable stimuli-responsive color-change properties was coated on substrates, such as paper and cotton consisting of cellulose fibers. The homogeneous and thin coating of the precursor monomer crystal was achieved on the substrates through the controlled crystal growth. The intercalation and topochemical polymerization generated PDA-Mⁿ⁺ on the substrates. The PDA-Mⁿ⁺-coated devices visualized temperature distribution of two-dimensional surface and three-dimensional space in real time.

Toward Tunable Photonic Nanosheet Sensors: Strong Influence of the Interlayer Cation on the Sensing Characteristics

An approach toward intercalant tunable nanosheet-based Fabry-Pérot sensors is presented. The intercalant tetrabutylammonium significantly increases the sensitivity of the photonic nose sensor to volatile organic compounds with increasing polarity, enabling polarity-driven color-coded vapor differentiation. Paired with the improved millisecond response times for polar vapors, vapor imaging with spatio-temporal resolution is within reach.

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.