Optical Indicators based on Structural Colored Polymers

Aggregation-induced emission in novel multilayered 3D polymers: development of a fluorescent probe for selective metal ion detection

This study presents the characterization of a novel multilayered three-dimensional (3D) polymer exhibiting aggregation-induced emission (AIE) properties when excited at a low wavelength of 280 nm. Utilizing fluorescence spectroscopy, we demonstrate that the polymer displays a marked enhancement in luminescence upon aggregation, a characteristic behavior that distinguishes AIE-active materials from conventional fluorophores. Furthermore, we explore the potential application of this multilayered 3D polymer as a fluorescent probe for the selective detection of specified metal ions. By incorporating chelating functional groups into the polymer matrix, we facilitate specific interactions with target metal ions, leading to significant fluorescence intensity changes that correlate with ion concentration. According to their cyclic voltammetry characteristics, the polymers have potential applications in cutting-edge electrical and optoelectronic systems. Our findings indicate that this multilayered 3D polymer serves as an effective fluorescent sensor and offers tunable optical properties, paving the way for innovative applications in environmental monitoring and biomedical diagnostics. The results underscore the utility of AIE-active polymers in developing advanced materials for sensitive and selective detection of metal ions, contributing to the growing field of smart sensing technologies.

Fluorimetric Detection of Vapor Pollutants with Diketopyrrolopyrrole Polymer Microcavities

The increasing prevalence and detrimental effects of volatile organic compounds are driving the need for selective on-site sensors that do not require complex sampling or instrumentation. Broadband selective sensors exhibiting selectivity based on their distinct response mechanism is becoming of increasing technological relevance in both industrial and urban settings. In this context, we propose a label-free sensor based on a polymeric planar microcavity embedded with a fluorescent organic dye, designed to detect various pollutants in the vapor phase. The sensor consists of alternating layers of cellulose acetate and poly(N-vinylcarbazole) and contains a polystyrene defect layer doped with a quadrupolar diketopyrrolopyrrole. Both the structural properties of the polymer microcavity and the dye in the defect layer contribute to the sensor's response to analytes, creating a dual-probe system where a single photonic element translates chemical signals into optical signals, namely, transmission and fluorescence spectral variations. The discrimination capability of the photonic structure arises from the physicochemical interactions between the analytes and the polymer components. To validate our approach, we evaluate the sensor's response to four distinct volatile molecules and investigate the mechanisms influencing the optical response.

Patterned Photonic Actuators with Dynamic Shape-Morphing and Color-Changing Capabilities Fabricated by Athermal Embossing Technology

Stimuli-responsive patterned photonic actuators, characterized by their patterned nano/microscale structures and capacity to demonstrate synergistic color changes and shape morphing in response to external stimuli, have attracted intense scientific attention. However, traditional patterned photonic actuator systems still face limitations such as cumbersome and time-consuming preparation processes and small-scale deformations. Herein, we introduce a facile approach involving an athermal embossing technique to rapidly fabricate patterned photonic actuators based on near-infrared (NIR) light-responsive liquid crystal elastomers. The resulting patterned photonic actuators demonstrate remarkable features, including brilliant angle-dependent structural color, complex three-dimensional actuation, and good color durability under NIR light stimulation. As illustrative demonstrations of the proof-of-concept, we fabricate two light-fuelled patterned photonic soft actuators: a butterfly-inspired actuator that can produce wing-flapping dynamic changes in structural color, and an origami crane-shaped actuator with shape memory, structural color information storage, and dynamic display properties. This strategy provides distinct insights into the design and fabrication of various patterned photonic soft robotic devices and intelligent actuators.

Dual-Mode Patterns Enabled by Photofluidization of an Azobenzene-Containing Linear Liquid Crystal Copolymer

Creating dual-mode patterns in the same area of the material is an advanced method to increase the dimension of information storage, improve the level of encryption security, and promote the development of encoding technology. However, in situ, different patterns may lead to serious mutual interference in the process of manufacturing and usage. New materials and patterning techniques are essential for the advancement of noninterfering dual-mode patterns. Herein, noninterfering dual-mode patterns are demonstrated by combining the structural color and chromatic polarization, which is designed with an azobenzene-containing linear liquid crystal copolymer featuring a photofluidization effect. On the one hand, structural color patterns are imprinted via silicon templates with periodic microstructures after a UV-light-induced local transition of the polymer surface from a glassy to rubbery state. On the other hand, different polarization patterns based on the local photoinduced orientation of mesogens are created within the photofluidized region by the Weigert effect. Especially, the secondary imprinting is used to eliminate the partial damage to the structural color patterns during writing of the polarization patterns, thus obtaining dual-mode patterns without interference. This study provides a blueprint for the creation of advanced materials and sophisticated photopatterning techniques with potential cross-industry applications.

On the isomeric purity of endcap molecules in cholesteric liquid crystal oligomers for near-infrared thermochromic coatings

Structurally coloured responsive materials provide an interesting avenue for the development of autonomous temperature regulating window films. One interesting class of such thermochromic materials is cholesteric liquid crystals. However, cholesteric liquid crystals have rarely been applied in coatings for smart window applications. In this work, we report the synthesis of endcapped cholesteric liquid crystal oligomers and its application as near-infrared thermochromic coatings for windows. Two isomerically pure monoacrylate endcapping molecules and its isomeric mixture are synthesised. The molecules are used to synthesise a variety of endcapped cholesteric liquid crystal oligomers to study the effect of the isomeric purity on the thermochromic properties of the coatings. It is found that while the oligomers are almost identical in composition and phase behaviour, only one isomer produces a clear transparent coating, highlighting the significance of minute isomeric differences. Remarkably, the thermochromic behaviour of the coatings for all oligomers is the same. The best performing oligomer is able to reversibly blueshift by 250 nanometres when heated from room temperature to 100°C, opening the way of cholesteric liquid crystals for use in temperature regulating window films.

Self-indicating polymers: a pathway to intelligent materials

Self-indicating polymers have emerged as a promising class of smart materials that possess the unique ability to undergo detectable variations in their physical or chemical properties in response to various stimuli. This article presents an overview of the most important mechanisms through which these materials exhibit self-indication, including aggregation, phase transition, covalent and non-covalent bond cleavage, isomerization, charge transfer, and energy transfer. Aggregation is a prevalent mechanism observed in self-indicating polymers, where changes in the degree of molecular organization result in variations in optical or electrical properties. Phase transition-induced self-indication relies on the transformation between different phases, such as liquid-to-solid or crystalline-to-amorphous transitions, leading to observable changes in color or conductivity. Covalent bond cleavage-based self-indicating polymers undergo controlled degradation or fragmentation upon exposure to specific triggers, resulting in noticeable variations in their structural or mechanical properties. Isomerization is another crucial mechanism exploited in self-indicating polymers, where the reversible transformation between the different isomeric forms induces detectable changes in fluorescence or absorption spectra. Charge transfer-based self-indicating polymers rely on the modulation of electron or hole transfer within the polymer backbone, manifesting as changes in electrical conductivity or redox properties. Energy transfer is an essential mechanism utilized by certain self-indicating polymers, where energy transfer between chromophores or fluorophores leads to variations in the emission characteristics. Furthermore, this review article highlights the diverse range of applications for self-indicating polymers. These materials find particular use in sensing and monitoring applications, where their responsive nature enables them to act as sensors for specific analytes, environmental parameters, or mechanical stress. Self-indicating polymers have also been used in the development of smart materials, including stimuli-responsive coatings, drug delivery systems, food sensors, wearable devices, and molecular switches. The unique combination of tunable properties and responsiveness makes self-indicating polymers highly promising for future advancements in the fields of biotechnology, materials science, and electronics.

A Bifunctional Luminescent Whitening and Sensing Material Based on Photoluminescence and Mechanoluminescence

A bifunctional luminescent whitening and luminescent sensing composite material, BaMgAl12O17:Eu2+/polydimethylsiloxane (BAM/PDMS), that utilizes natural sunlight and mechanical energy is presented. By increasing the Eu2+ content, the photoluminescence (PL) excitation spectrum of the material shows a maximum redshift of 23 nm due to 5d level splitting of Eu2+, resulting in more spectral overlap with sunlight and an excellent PL whitening effect. Meanwhile, the self-recoverable mechanoluminescence (ML) of the material can be easily excited under mechanical stimuli due to contact electrification, exhibiting a unique stress sensing effect. Based on the unique features of PL whitening and ML sensing, the material is applied to model cars through a spray process, and the results demonstrate that the bifunctional BAM/PDMS material shows promising applications in automobile decoration.

Geometric phase-encoded stimuli-responsive cholesteric liquid crystals for visualizing real-time remote monitoring: humidity sensing as a proof of concept

Liquid crystals are a vital component of modern photonics, and recent studies have demonstrated the exceptional sensing properties of stimuli-responsive cholesteric liquid crystals. However, existing cholesteric liquid crystal-based sensors often rely on the naked eye perceptibility of structural color or the measurement of wavelength changes by spectrometric tools, which limits their practical applications. Therefore, developing a platform that produces recognizable sensing signals is critical. In this study, we present a visual sensing platform based on geometric phase encoding of stimuli-responsive cholesteric liquid crystal polymers that generates real-time visual patterns, rather than frequency changes. To demonstrate this platform's effectiveness, we used a humidity-responsive cholesteric liquid crystal polymer film encoded with a q-plate pattern, which revealed that humidity causes a shape change in the vortex beam reflected from the encoded cholesteric liquid crystal polymers. Moreover, we developed a prototype platform towards remote humidity monitoring benefiting from the high directionality and long-range transmission properties of laser beams carrying orbital angular momentum. Our approach provides a novel sensing platform for cholesteric liquid crystals-based sensors that offers promising practical applications. The ability to generate recognizable sensing signals through visual patterns offers a new level of practicality in the sensing field with stimuli-responsive cholesteric liquid crystals. This platform might have significant implications for a broad readership and will be of interest to researchers working in the field of photonics and sensing technology.

Monolithic integrated light-emitting-diode/photodetector sensor for photoactive analyte monitoring: design and simulation

We present the simulation and design optimization of an integrated light-emitting-diode/photodetector (LED-PD) sensor system for monitoring of light absorbance changes developing in analyte-sensitive compounds. The sensor integrates monolithically both components in a single chip, offering advantages such as downsizing, reduced assembly complexity, and lower power consumption. The changes in the optical parameters of the analyte-sensitive ink are detected by monitoring the power transmission from the LED to the PD. Ray tracing and coupled modeling approach (CMA) simulations are employed to investigate the interaction of the emitted light with the ink. In highly absorbing media, CMA predicts more accurate results by considering evanescent waves. Simulations also suggest that an approximately 39% change in optical transmission can be achieved by adjusting the ink-deposited layer thickness and varying the extinction coefficient from 10-4 to 3×10-4.

Exploring the structural color of micro-nano composite gratings with FDTD simulation and experimental validation

The significance of micro-nano composite gratings (MNCGs) resides in their applications, including optical devices, sensors, and diffractive elements, which drive research interest in their diffraction characteristics. This study investigates both the diffraction characteristics of MNCGs and the factors that influence them by employing both Finite-Difference Time-Domain (FDTD) methods and experimental validation. The initial focus lies in deciphering the differences in diffraction characteristics between micro-gratings (MGs) and MNCGs by analyzing the coupling effects, diffraction order, color distribution, and intensity variation. Additionally, this research emphatically investigates five aspects to discover the influencing factors of MNCG's diffraction characteristics, such as the height, groove angle of MGs and the period, blaze angle, and height of nano-gratings (NGs). Results show that the structural coloration and saturation of MNCG surpass that of MG. NG plays the actual spectral role, and a reduction in the period of NG leads to enhanced splitting light capability of the white light. The optical detection tests validated the simulation results. The present study reveals the diffractive properties of MNCGs, providing technical insights for the design and processing of optically variable devices.

Bioinspired humidity-responsive liquid crystalline materials: from adaptive soft actuators to visualized sensors and detectors

Inspired by nature, humidity-responsive materials and devices have attracted significant interest from scientists in multiple disciplines, ranging from chemistry, physics and materials science to biomimetics. Owing to their superiorities, including harmless stimulus and untethered control, humidity-driven materials have been widely investigated for application in soft robots, smart sensors and detectors, biomimetic devices and anticounterfeiting labels. Especially, humidity-responsive liquid crystalline materials are particularly appealing due to the combination of programmable and adaptive liquid crystal matrix and humidity-controllability, enabling the fabrication of advanced self-adaptive robots and visualized sensors. In this review, we summarize the recent progress in humidity-driven liquid crystalline materials. First, a brief introduction of liquid crystal materials, including liquid crystalline polymers, cholesteric liquid crystals, blue-phase liquid crystals and cholesteric cellulose nanocrystals is provided. Subsequently, the mechanisms of humidity-responsiveness are presented, followed by the diverse strategies for the fabrication of humidity-responsive liquid crystalline materials. The applications of humidity-driven devices will be presented ranging from soft actuators to visualized sensors and detectors. Finally, we provide an outlook on the development of humidity-driven liquid crystalline materials.

Machine Learning Enabled Image Analysis of Time-Temperature Sensing Colloidal Arrays

Smart, responsive materials are required in various advanced applications ranging from anti-counterfeiting to autonomous sensing. Colloidal crystals are a versatile material class for optically based sensing applications owing to their photonic stopband. A careful combination of materials synthesis and colloidal mesostructure rendered such systems helpful in responding to stimuli such as gases, humidity, or temperature. Here, an approach is demonstrated to simultaneously and independently measure the time and temperature solely based on the inherent material properties of complex colloidal crystal mixtures. An array of colloidal crystals, each featuring unique film formation kinetics, is fabricated. Combined with machine learning-enabled image analysis, the colloidal crystal arrays can autonomously record isothermal heating events - readout proceeds by acquiring photographs of the applied sensor using a standard smartphone camera. The concept shows how the progressing use of machine learning in materials science has the potential to allow non-classical forms of data acquisition and evaluation. This can provide novel insights into multiparameter systems and simplify applications of novel materials.

Photonic Liquid Crystal Polymer Absorbent for Immobilization and Detection of Gaseous Nerve Agent Simulants

Detection and sequestration of chemical warfare agents (CWAs), such as poisonous organophosphates, are highly desirable for both personal security and environmental protection. However, both sensing and absorption in a single device have been rarely reported. In this study, we describe a photonic absorbent based on a cholesteric liquid crystal polymer as a dual sensing and decontamination device for gas-type CWAs. Dimethyl methylphosphonate (DMMP) was used as a simulant compound. A blue reflective photonic polymer was fabricated that was able to detect DMMP vapor through absorption. Hydrogen bond interactions between DMMP and mesogenic carboxylic groups of the polymer allow selectivity and capture. A distinct optical change of the film from blue to bright green indicates the absorption of DMMP vapor molecules and confirms when full absorption of the polymer is achieved. The diffusion of DMMP vapor into the material was observed by the formation of a sharp boundary between swollen and unswollen material, as evidenced by scanning electron microscopy images and the structural color changes. In ambient conditions, DMMP molecules are retained in the photonic absorbent without release to the environment. Heating above approximately 60 °C releases the absorbed DMMP, leading to a reusable optical device. These results confirm the ability of photonic polymers to sense and immobilize dangerous vapor, paving the way for the realization of simple, battery-free optical devices able to simultaneously warn and protect.

Multicolor photonic patterns through an intensity-controlled single photopolymerization step

The UV intensity during photopolymerization allows control over the structural color of a cholesteric liquid crystal (CLC) polymer photonic coating in a single step. Simultaneously, the glass transition temperature (T_g) of the polymer can be tuned by the applied UV intensity. Most likely the low intensity photopolymerization increases the inhibition time, leading to in situ formation of polymer fragments through oxygen inhibition. The formation of polymer fragments changes the matrix during the inhibition time, which results in a color change before the polymer network is formed. Additionally, these fragments inside the network act as a plasticizer, effectively lowering the T_g. This method can be combined with temperature responsive properties based on shape memory to fabricate photonic coatings with multiple, responsive colored patterns. The presented work allows for new functionalities in responsive photonic polymers as multiple colors and response temperatures can be incorporated in a single polymerization step.

The UV intensity during photopolymerization allows control over the structural color of a cholesteric liquid crystal (CLC) polymer photonic coating in a single step.