An Optical Steam Sterilization Sensor Based On a Dual-Responsive Supramolecular Cross-Linked Photonic Polymer

Multiresponsive Self-Healing Lanthanide Fluorescent Hydrogel for Smart Textiles

Lanthanide organometallic complexes exhibit strong luminescence characteristics, owing to their antenna effects. The f-d energy level transition causes this phenomenon, which occurs when ligands and the external electrons of lanthanide metals coordinate. Based on this phenomenon, we used two lanthanide metals, europium (Eu) and terbium (Tb), in the present study as the metal center for iminodiacetic acid ligands. Further, we developed the resulting fluorescent organometallic complex as a smart material. The ligand-metal bond in the material functioned as a metal chelating agent and a cross-linking agent in a dynamically coordinated form, thereby prompting the material to self-heal. Temperature-sensitive poly-N-isopropylacrylamide was incorporated into the material as the polymer backbone. Afterward, we combined it with water-soluble poly(vinyl alcohol) and an additional ligand from poly(acrylic acid) to fabricate a high-performance hydrogel composite material. The shrinkage and expansion of the polymer form a grid between the materials. Because of the different coordination stabilities of Eu3+ and Tb3+, the corresponding material exhibits environmental responses toward excitation wavelength, temperature, and pH, thus generating different colors. When used in fabrics, the cross-linking mechanism of the material effectively looped the material between fabric fibers; furthermore, the temperature sensitivity of the polymer adjusted the size of pores between fabric fibers. At relatively higher temperatures (>32 °C), the polymer structure shrank, fiber pores expanded, and air permeability improved. Thus, this material appears to be promising for use in smart textiles.

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.

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.

Ink-Deposited Transparent Electrochromic Structural Colored Foils

Despite progress in the field of electrochromic devices, developing structural color-tunable photonic systems having both high transparency and flexibility remains challenging. Here, an ink-deposited transparent electrochromic structural colored foil displaying reflective colors, tuned by an integrated heater, is prepared in a single-substrate method. Efficient and homogeneous heating is induced by a gravure printed silver nanowire-based substrate, delivering an electrothermal response upon applying an electrical potential. On top of this flexible, transparent heater, a cholesteric liquid crystal ink is bar-coated and subsequently photopolymerized, yielding a structural colored film that exhibits temperature-responsive color changes. The transparent electrochromic foils appear colorless at room temperature but demonstrate structural color tuning with high optical quality when modifying the electrical potential. Both optical and electrothermal performances were preserved when deforming the foils. Applying the conductive and structural colored inks via the easy processable, continuous methods of gravure printing and bar-coating highlights the potential for scaling up to large-scale stimuli-responsive, transparent optical foils. These transparent structural colored foils can be potentially used for a wide range of photonic devices including smart windows, displays, and sensors and can be directly installed on top of curved, flexible surfaces.

Advanced Functional Liquid Crystals

Liquid crystals have been intensively studied as functional materials. Recently, integration of various disciplines has led to new directions in the design of functional liquid-crystalline materials in the fields of energy, water, photonics, actuation, sensing, and biotechnology. Here, recent advances in functional liquid crystals based on polymers, supramolecular complexes, gels, colloids, and inorganic-based hybrids are reviewed, from design strategies to functionalization of these materials and interfaces. New insights into liquid crystals provided by significant progress in advanced measurements and computational simulations, which enhance new design and functionalization of liquid-crystalline materials, are also discussed.

Optical Indicators based on Structural Colored Polymers

Polymer indicators are autonomous responsive materials that provide an optical signal of a specific exposure in time. This review describes the different polymer systems utilized to obtain indicators based on structural color. Structural color originates from the interaction of light with a periodic nanostructured polymer which causes a specific wavelength to be reflected. This reflected light can be used for fabricating battery-free indicators that show visible structural color changes upon exposure to a stimulus or analyte. In this review, the typical structural color response types categorized by stimulus are discussed and compared. Furthermore, the steps toward possible applications of optical indicators based on structural colored polymers are outlined.

Humidity-Responsive Photonic Crystals with pH and SO2 Gas Detection Ability Based on Cholesteric Liquid Crystalline Networks

Dynamic photonic crystals with tunable structural colors have been a hot topic in the research of anticounterfeiting devices, decoration, and detection. In this work, we prepared cholesteric liquid crystalline network (CLCN)-based photonic crystals that present humidity- and SO₂ gas-responsive behaviors. The covalently cross-linked CLCN film presents humidity-responsive color changes due to the swelling/deswelling of the matrix under different humidity conditions. When treating the CLCN film with SO₂ gas, the carboxylic salt converted to the acid and the film was not able to respond to the humidity change anymore. The mechanism of the SO₂ gas-gated humidity responsiveness of the CLCN film was characterized. It was found that the acidic gas caused changes of pH, resulting in the conversion of the salt to acid and alteration of the surface property. The influence of concentration of SO₂ gas and pH on humidity responsiveness of the CLCN film was investigated. We hope that this method provides inspirations for the design and fabrication of visualized pH and acidic gas detectors.

Hydrogen-Bonded Supramolecular Liquid Crystal Polymers: Smart Materials with Stimuli-Responsive, Self-Healing, and Recyclable Properties

Hydrogen-bonded liquid crystalline polymers have emerged as promising "smart" supramolecular functional materials with stimuli-responsive, self-healing, and recyclable properties. The hydrogen bonds can either be used as chemically responsive (i.e., pH-responsive) or as dynamic structural (i.e., temperature-responsive) moieties. Responsiveness can be manifested as changes in shape, color, or porosity and as selective binding. The liquid crystalline self-organization gives the materials their unique responsive nanostructures. Typically, the materials used for actuators or optical materials are constructed using linear calamitic (rod-shaped) hydrogen-bonded complexes, while nanoporous materials are constructed from either calamitic or discotic (disk-shaped) complexes. The dynamic structural character of the hydrogen bond moieties can be used to construct self-healing and recyclable supramolecular materials. In this review, recent findings are summarized, and potential future applications are discussed.

4D Printing of Liquid Crystals: What's Right for Me?

Recent years have seen major advances in the developments of both additive manufacturing concepts and responsive materials. When combined as 4D printing, the process can lead to functional materials and devices for use in health, energy generation, sensing, and soft robots. Among responsive materials, liquid crystals, which can deliver programmed, reversible, rapid responses in both air and underwater, are a prime contender for additive manufacturing, given their ease of use and adaptability to many different applications. In this paper, selected works are compared and analyzed to come to a didactical overview of the liquid crystal-additive manufacturing junction. Reading from front to back gives the reader a comprehensive understanding of the options and challenges in the field, while researchers already experienced in either liquid crystals or additive manufacturing are encouraged to scan through the text to see how they can incorporate additive manufacturing or liquid crystals into their own work. The educational text is closed with proposals for future research in this crossover field.

Effect of Crosslinkers on Optical and Mechanical Behavior of Chiral Nematic Liquid Crystal Elastomers

Chiral nematic (N*) liquid crystal elastomers (LCEs) are suitable for fabricating stimuli-responsive materials. As crosslinkers considerably affect the N*LCE network, we investigated the effects of crosslinking units on the physical properties of N*LCEs. The N*LCEs were synthesized with different types of crosslinkers, and the relationship between the N*LC polymeric system and the crosslinking unit was investigated. The N*LCEs emit color by selective reflection, in which the color changes in response to mechanical deformation. The LC-type crosslinker decreases the helical twisting power of the N*LCE by increasing the total molar ratio of the mesogenic compound. The N*LCE exhibits mechano-responsive color changes by coupling the N*LC orientation and the polymer network, where the N*LCEs exhibit different degrees of pitch variation depending on the crosslinker. Moreover, the LC-type crosslinker increases the Young's modulus of N*LCEs, and the long methylene chains increase the breaking strain. An analysis of experimental results verified the effect of the crosslinkers, providing a design rationale for N*LCE materials in mechano-optical sensor applications.

Autonomous Off-Equilibrium Morphing Pathways of a Supramolecular Shape-Memory Polymer

The diverse morphing behaviors of living creatures arise from their unlimited pathways. In contrast, the equilibrium-driven morphing pathways of common synthetic shape-shifting materials are very limited. For a shape-memory polymer (SMP), its recovery from the temporary shape(s) to the permanent shape typically requires external stimulation and follows a single fixed route. Herein, a covalently crosslinked SMP is designed with ample ureidopyrimidinone (UPy) supramolecular moieties in the network. The UPy units endow the SMP with strong time-temperature dependency, which is explored as a mechanism for spatio-temporal programming of autonomous shape-shifting pathways. In particular, the use of digitally controlled photothermal heating provides versatility in control via an off-equilibrium mechanism. In addition, cooling/heating across its glass transition introduces a locking/unlocking mechanism for its temporal morphing. The benefits of these unique features are demonstrated by multi-shape-transformation, an "invisible"-color-based clock, a time-temperature indicator, and sequence-programmable 4D printing.

Preparation and DFT Study for New Three-Ring Supramolecular H-Bonded Induced Liquid Crystal Complexes

Supramolecular three-ring Schiff base novel liquid crystal complexes have been prepared and investigated. Schiff bases of para-substituted aniline derivatives and para-pyridine carbaldehyde have been prepared and then mixed in equimolar quantities with para-alkoxy benzoic acids. On one side, the alkoxy chain length varies from 8 to 16 carbon atoms. On the other side, terminal small compact groups substituting aniline with various polarities are used. Hydrogen-bonding interaction was elucidated by FTIR spectroscopy. The mesomorphic thermal and optical characteristics of the samples were obtained by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). All samples exhibit enantiotropic mesophases. Experimental results obtained for the induced mesophases were correlated with density functional theory (DFT) theoretical calculations. The results revealed that both the polar compact groups' polarity and the alkoxy chain lengths contribute strongly to mesomorphic characteristics and thermal stabilities of the mesophases. Surprisingly, the observed values of enthalpy changes associated with the crystalline mesomorphic transitions lie in the range of 2.2-12.5 kJ/mol. However, the enthalpy changes corresponding to the mesomorphic-isotropic transitions vary from 0.9 to 13.9 kJ/mol, depending on the polarity of para-attached groups to the aniline moiety.

Pyridine-Based Three-Ring Bent-Shape Supramolecular Hydrogen Bond-Induced Liquid Crystalline Complexes: Preparation and Density Functional Theory Investigation

A series of new supramolecular three-ring bent-shape Schiff base liquid crystal (LC) complexes were prepared and studied. On one side, two alkoxy chain lengths of the carboxylic acids were used, namely eight and sixteen carbons. Moreover, on the other side, terminal small compact groups, which substituted aniline, with different polarities were utilized. Furthermore, the hydrogen-bonding interactions in the formed complexes were elucidated by Fourier-transform infrared (FT–IR) spectroscopy. The mesomorphic thermal and optical characteristics of the samples were determined by differential thermal analysis (DSC) and polarized optical microscopy (POM). The complexes exhibited enantiotropic and dimorphic mesophase behaviors. The results indicate that the polarity of the compact groups and the lengths of the alkoxy chains greatly impacted the mesomorphic characteristics and thermal stabilities of the mesophases. The observed values of the enthalpy changes (ΔH) associated with the crystalline smectic-A (TCr-SmA) transitions were extremely small compared with the conventional values that characterize supramolecular hydrogen-bonded liquid crystalline complexes. ΔH, which corresponded to the nematic isotropic transitions (TN-I), varied from 0.13 to 9.54 kJ/mol depending mainly on the polarity of the groups that were para-attached to the aniline moiety. Finally, the theoretical results obtained by density functional theory (DFT) calculations were discussed. The DFT geometrical structures showed non-coplanar structures. The mesomorphic range was correlated with the calculated dipole moment, polarizability and the aspect ratios of the investigated compounds.

Smart Supramolecular Self-Assembled Nanosystem: Stimulus-Responsive Hydrogen-Bonded Liquid Crystals

In a liquid crystal (LC) state, specific orientations and alignments of LC molecules produce outstanding anisotropy in structure and properties, followed by diverse optoelectronic functions. Besides organic LC molecules, other nonclassical components, including inorganic nanomaterials, are capable of self-assembling into oriented supramolecular LC mesophases by non-covalent interactions. Particularly, huge differences in size, shape, structure and properties within these components gives LC supramolecules higher anisotropy and feasibility. Therefore, hydrogen bonds have been viewed as the best and the most common option for supramolecular LCs, owing to their high selectivity and directionality. In this review, we summarize the newest advances in self-assembled structure, stimulus-responsive capability and application of supramolecular hydrogen-bonded LC nanosystems, to provide novel and immense potential for advancing LC technology.

Direct Laser Writing of Four-Dimensional Structural Color Microactuators Using a Photonic Photoresist

With the advent of direct laser writing using two-photon polymerization, the generation of high-resolution three-dimensional microstructures has increased dramatically. However, the development of stimuli-responsive photoresists to create four-dimensional (4D) microstructures remains a challenge. Herein, we present a supramolecular cholesteric liquid crystalline photonic photoresist for the fabrication of 4D photonic microactuators, such as pillars, flowers, and butterflies, with submicron resolution. These micron-sized features display structural color and shape changes triggered by a variation of humidity or temperature. These findings serve as a roadmap for the design and creation of high-resolution 4D photonic microactuators.