Surface-Controlled Orientational Transitions in Elastically Strained Films of Liquid Crystal That Are Triggered by Vapors of Toluene

Optically responsive dry cholesteric liquid crystal marbles

Dry liquid crystal marbles are structures that consist of cholesteric liquid crystal (CLC) droplets prepared by the mixture of chiral-doped thermotropic LCs encapsulated by cellulose nanocrystals (CNCs) that have been dried under ambient conditions. The characterizations revealed that CLC droplets were successfully encapsulated by self-standing CNC shells and responsive to the external gaseous stimulus. The dry LC marbles offer several advantages over previously reported LC-based gas sensors, such as fast response against minor external stimuli, and ease of handling, which make them particularly attractive for practical applications in sensing. We demonstrate the use of these marbles for detecting toluene vapor, a common industrial solvent and pollutant, which we also use to understand the response characteristics. The dry CLC marbles exhibit a significant response to toluene vapor with a detection limit below 500 ppm, attributed to the change of pitch size of the helical structure of CLC droplets induced by the toluene vapor. The CNC-capsulated CLC droplets were stable in emulsion for up to two weeks, and their dried form exhibited a sensitive response upon toluene exposure. The real-time experiments revealed that the LC marbles can be used multiple times without a significant loss of sensitivity, where 90 % of the maximum response was observed at 13.3 ± 4.7 s. These dry LC marbles can also be utilized in other areas, including drug delivery, optical devices, and biosensors.

Pre-compressed polymer cholesteric liquid crystal based optical fiber VOC sensor with high stability and a wide detection range

As the concentration of VOC gases is very high in organic chemical reactions, in order to ensure the safety and accuracy of the experiment, it is very important to develop a gas sensor with a wide detection range. Exploring the mechanism and law of photonic bandgap (PBG) shift after absorption of volatile organic compounds (VOCs) vapors are two basic premises of resolving the PCLC for gas detection with a wide measurement range and stability. Herein, the PCLC films doped with different concentrations of polymer are used for acetone vapor detection, and the shift law of the PBG position is analyzed. As the increase of the detected gas concentration, the intractable problem is that the PBG position of PCLC exhibits red- and blue-shifts successively. Particularly, the pre-compressed technique is highly important for development of a high-performance PCLC based fiber probe, which is crucial for effectively solving the bottleneck problem mentioned. It enables detection of a wide range of acetone vapor concentration from 0 ppm to 50×10⁴ ppm, and the corresponding mean sensitivity of 0.23 pm/ppm. In addition, the thermal crosstalk is generally negligible at temperature below 40°C. Therefore, it is a breakthrough that the described technique not only effectively enhances the stability and robustness of the PCLC fiber probe for VOC vapor detection, but also improves its sensitivity and detection range. The pre-compressed technique provides a novel avenue for fabrication of other PCLC-based devices.

State-of-the-Art Development in Liquid Crystal Biochemical Sensors

As an emerging stimuli-responsive material, liquid crystal (LC) has attracted great attentions beyond display applications, especially in the area of biochemical sensors. Its high sensitivity and fast response to various biological or chemical analytes make it possible to fabricate a simple, real-time, label-free, and cost-effective LC-based detection platform. Advancements have been achieved in the development of LC-based sensors, both in fundamental research and practical applications. This paper briefly reviews the state-of-the-art research on LC sensors in the biochemical field, from basic properties of LC material to the detection mechanisms of LC sensors that are categorized into LC-solid, LC–aqueous, and LC droplet platforms. In addition, various analytes detected by LCs are presented as a proof of the application value, including metal ions, nucleic acids, proteins, glucose, and some toxic chemical substances. Furthermore, a machine-learning-assisted LC sensing platform is realized to provide a foundation for device intelligence and automatization. It is believed that a portable, convenient, and user-friendly LC-based biochemical sensing device will be achieved in the future.

Quantitative volatile organic compound sensing with liquid crystal core fibers

Polymer fibers with liquid crystals (LCs) in the core have potential as autonomous sensors of airborne volatile organic compounds (VOCs), with a high surface-to-volume ratio enabling fast and sensitive response and an attractive non-woven textile form factor. We demonstrate their ability to continuously and quantitatively measure the concentration of toluene, cyclohexane, and isopropanol as representative VOCs, via the impact of each VOC on the LC birefringence. The response is fully reversible and repeatable over several cycles, the response time can be as low as seconds, and high sensitivity is achieved when the operating temperature is near the LC-isotropic transition temperature. We propose that a broad operating temperature range can be realized by combining fibers with different LC mixtures, yielding autonomous VOC sensors suitable for integration in apparel or in furniture that can compete with existing consumer-grade electronic VOC sensors in terms of sensitivity and response speed.

Multifunctional sensors based on liquid crystals scaffolded in nematic polymer networks

Stimuli-responsive liquid crystal (LC) materials have attracted great attention due to their unique characteristics and anisotropic properties. They are not only important for fundamental studies, but also have many potential applications in the electro-optical and biochemical fields. Herein, the interference color obtained from a nematic polymer network-stabilized liquid crystal (PNLC) system is demonstrated to reflect the environmental conditions, including temperature and the presence of volatile organic vapors. The polymerization of LC monomers forms a stable network to template the LCs, while still maintaining the dynamic nature and thermal tunability of LCs. Via adjusting the concentration of LC monomer, a wide temperature sensing range can be achieved between 36 °C and 100 °C with visible color. The same sensor can be used to detect concentration profiles of toluene vapor in a microchannel with a limit of detection of 2300 ppm. This stimuli-responsive PNLC system is expected to be potentially useful for many other naked-eye sensing applications.

Naked-eye color change as a result of temperature change or VOC exposure was demonstrated in a nematic polymer network-stabilized liquid crystal (PNLC) system.

Bottom-Up Assembled Photonic Crystals for Structure-Enabled Label-Free Sensing

Photonic crystals (PhCs) display photonic stop bands (PSBs) and at the edges of these PSBs transport light with reduced velocity, enabling the PhCs to confine and manipulate incident light with enhanced light-matter interaction. Intense research has been devoted to leveraging the optical properties of PhCs for the development of optical sensors for bioassays, diagnosis, and environmental monitoring. These applications have furthermore benefited from the inherently large surface area of PhCs, giving rise to high analyte adsorption and the wide range of options for structural variations of the PhCs leading to enhanced light-matter interaction. Here, we focus on bottom-up assembled PhCs and review the significant advances that have been made in their use as label-free sensors. We describe their potential for point-of-care devices and in the review include their structural design, constituent materials, fabrication strategy, and sensing working principles. We thereby classify them according to five sensing principles: sensing of refractive index variations, sensing by lattice spacing variations, enhanced fluorescence spectroscopy, surface-enhanced Raman spectroscopy, and configuration transitions.

Applications of liquid crystals in biosensing

Liquid crystals (LCs), as a promising branch of highly-sensitive, quick-response, and low-cost materials, are widely applied to the detection of weak external stimuli and have attracted significant attention. Over the past decade, many research groups have been devoted to developing LC-based biosensors due to their self-assembly potential and functional diversity. In this paper, recent investigations on the design and application of LC-based biosensors are reviewed, based on the phenomenon that the orientation of LCs can be directly influenced by the interactions between biomolecules and LC molecules. The sensing principle of LC-based biosensors, as well as their signal detection by probing interfacial interactions, is described to convert, amplify, and quantify the information from targets into optical and electrical parameters. Furthermore, commonly-used LC biosensing targets are introduced, including glucose, proteins, enzymes, nucleic acids, cells, microorganisms, ions, and other micromolecules that are critical to human health. Due to their self-assembly potential, chemical diversity, and high sensitivity, it has been reported that tunable stimuli-responsive LC biosensors show bright perspectives and high superiorities in biological applications. Finally, challenges and future prospects are discussed for the fabrication and application of LC biosensors to both enhance their performance and to realize their promise in the biosensing industry.

Design of Chemoresponsive Soft Matter Using Hydrogen-Bonded Liquid Crystals

Soft matter that undergoes programmed macroscopic responses to molecular analytes has potential utility in a range of health and safety-related contexts. In this study, we report the design of a nematic liquid crystal (LC) composition that forms through dimerization of carboxylic acids and responds to the presence of vapors of organoamines by undergoing a visually distinct phase transition to an isotropic phase. Specifically, we screened mixtures of two carboxylic acids, 4-butylbenzoic acid and trans-4-pentylcyclohexanecarboxylic acid, and found select compositions that exhibited a nematic phase from 30.6 to 111.7 °C during heating and 110.6 to 3.1 °C during cooling. The metastable nematic phase formed at ambient temperatures was found to be long-lived (>5 days), thus enabling the use of the LC as a chemoresponsive optical material. By comparing experimental infrared (IR) spectra of the LC phase with vibrational frequencies calculated using density functional theory (DFT), we show that it is possible to distinguish between the presence of monomers, homodimers and heterodimers in the mixture, leading us to conclude that a one-to-one heterodimer is the dominant species within this LC composition. Further support for this conclusion is obtained by using differential scanning calorimetry. Exposure of the LC to 12 ppm triethylamine (TEA) triggers a phase transition to an isotropic phase, which we show by IR spectroscopy to be driven by an acid-base reaction, leading to the formation of ammonium carboxylate salts. We characterized the dynamics of the phase transition and found that it proceeds via a characteristic spatiotemporal pathway involving the nucleation, growth, and coalescence of isotropic domains, thus amplifying the atomic-scale acid-base reaction into an information-rich optical output. In contrast to TEA, we determined via both experiment and computation that neither hydrogen bonding donor or acceptor molecules, such as water, dimethyl methylphosphonate, ethylene oxide or formaldehyde, disrupt the heterodimers formed in the LC, hinting that the phase transition (including spatial-temporal characteristics of the pathway) induced in this class of hydrogen bonded LC may offer the basis of a facile and chemically selective way of reporting the presence of volatile amines. This proposal is supported by exploratory experiments in which we show that it is possible to trigger a phase transition in the LC by exposure to volatile amines emitted from rotting fish. Overall, these results provide new principles for the design of chemoresponsive soft matter based on hydrogen bonded LCs that may find use as the basis of low-cost visual indicators of chemical environments.

Liquid crystal-based sensing platform for detection of Pb2+ assisted by DNAzyme and rolling circle amplification

Lead ions (Pb²⁺) are one of the most widespread heavy metal contaminants that pose detrimental impact on environment and human health. We demonstrate a highly sensitive and specific liquid crystal (LC)-based sensing platform for detecting Pb²⁺ assisted by DNAzyme and rolling circle amplification (RCA). Magnetic beads (MBs) are functionalized with DNA duplexes of the catalytic strands (DNAzymes) and the substrate strands. In the presence of Pb²⁺, the substrate strands are disassembled due to activation of the DNAzyme, which allows initiation of DNA RCA on MBs. The amplified DNA strands can disrupt arrangement of octadecy trimethyl ammonium bromide monolayers (OTAB), thereby inducing planar orientation of LC molecules at the interface of aqueous and LCs. Thus, LCs exhibit bright appearance. In contrast, RCA cannot be triggered in the absence of Pb²⁺. Therefore, LC molecules adopt perpendicular orientation at the interface, which induces the dark morphology of LCs. The limit of detection reaches as low as 16.7 pM. It is an improvement of more than two orders of magnitude compared to that of previously reported LC-based sensing approaches. This approach also shows excellent performance in monitoring Pb²⁺ in tap water and lake water.

Optical fiber sensor based on a cholesteric liquid crystal film for mixed VOC sensing

This paper proposes a novel cholesteric liquid crystal (CLC) film-based dual-probe fiber sensor to monitor volatile organic compound (VOC) gas. The sensor consists of a 2×2 multimode fiber coupler, in which the two output fiber ends are covered by two types of CLC films (CLCF) with different pitches. It can be observed that the reflection peak wavelengths of CLCs shift to the red side as the VOC gas concentration increases. The sensitivities of the two CLCFs are 8.435 nm·L/mmol and 14.867 nm·L/mmol to acetone, 14.586 nm·L/mmol and 29.303 nm·L/mmol to ethanol, respectively. In addition, the dependence of the peak wavelength shift of CLCF on the total concentration of the acetone and ethanol mixed gas at different mixing ratios is measured. The linear relationships between the peak shift of CLCFs, the total mixed gas concentration and acetone/ethanol ratio are calculated using the least-squares method. Therefore, this proposed dual-probe fiber optic sensor can distinguish the concentrations of acetone and ethanol in a mixed gas of acetone and ethanol.

Chameleon skin-inspired polymeric particles for the detection of toluene vapor

Inspired by the structural coloring in nature, especially the crystalline ordering and responsive characteristics of those found in chameleon skins, artificial photonic materials for sensor applications were fabricated. Cholesteric liquid crystals (CLCs) were employed in the templated synthesis of polymeric particles with periodic structures that allow visible light to undergo Bragg reflection and their response was tested against volatile organic compounds (VOCs). We demonstrate that the particles were responsive against toluene with detection limits on the order of 100 ppm. Such sensitivity was shown to be achieved due to the critical steps followed during the CLC-templated synthesis of particles that resulted in the storage of elastic energy in the anisotropic glassy polymer network. In addition, the design of particle-assisted sensor chips that allow easy integration into wearable optical devices for reliable, continuous and online tracking of VOC concentrations is presented. These results proved that sensors developed from the CLC-templated particles can be used multiple times without a significant loss of sensitivity and offered rapid, sensitive and battery-free detection.

Structural and Optical Response of Polymer-Stabilized Blue Phase Liquid Crystal Films to Volatile Organic Compounds

Engineering useful mechanical properties into stimuli-responsive soft materials without compromising their responsiveness is, in many cases, an unresolved challenge. For example, polymer networks formed within blue-phase liquid crystals (BPs) have been shown to form mechanically robust films, but the impact of polymer networks on the response of these soft materials to chemical stimuli has not been explored. Here, we report on the response of polymer-stabilized BPs (PSBPs) to volatile organic compounds (VOCs, using toluene as a model compound) and compare the response to BPs without polymer stabilization and to polymerized nematic and cholesteric phases. We find that PSBPs generate an optical response to toluene vapor (change in reflection intensity under crossed polars) that is sixfold greater in sensitivity than the polymerized nematic or cholesteric phases and with a limit of detection (140 ± 10 ppm at 25 °C) that is relevant to the measurement of permissible exposure limits for humans. Additionally, when compared to BPs that have not been polymerized, PSBPs respond to a broader range of toluene vapor concentrations (5000 vs <1000 ppm) over a wider temperature interval (25-45 vs 45-53 °C). We place these experimental observations into the context of a simple thermodynamic model to explore how the PSBP response reflects the effect of toluene on competing contributions of double-twisted LC cylinders, disclinations, and polymer network to the free energy that controls the PSBP lattice spacing. Overall, we conclude that the mechanical and thermal stability of PSBPs, when combined with their optical responsiveness to toluene, make this class of self-supporting LCs a promising one as the basis of passive and compact (e.g., wearable) sensors for VOCs.

Strain-enhanced sensitivity of polymeric sensors templated from cholesteric liquid crystals

Detection of volatile organic compounds (VOCs) is an important issue due to their harmful impact on human health. In this study, we aimed at enhancing the sensitivity of the anisotropic polymeric films templated from cholesteric liquid crystals (CLCs) in the identification of VOCs at concentrations on the order of 100 ppm. To increase sensitivity, we introduced negative strain to the films in the direction parallel to the helical axis and evaluated its effect on the sensitivity. Specifically, we used LC mixtures of reactive [4-(3-acryloyoxypropyloxy)benzoic acid 2-methyl-1,4-phenylene ester (RM257)], nonreactive E7 mesogen and chiral dopant [4-((1-methylheptyloxycarbonyl)phenyl-4-hexyloxybenzoate) (S-811)] to synthesize CLC-templated polymeric films with programmed strain profiles using a curved wedge cell, and measured their response against a range of toluene vapor concentrations. Based on the obtained results, we demonstrated a relationship between the negative strain in the cholesteric pitch and the sensitivity of the sensor based on spacial responses evaluated from the change in coloring of the film. Our results showed that negative strain helps to increase the sensitivity of the sensors up to 15 times compared to their unstrained counterparts. Moreover, 90% of the equilibrium response is achieved in less than one minute of exposure which offers rapid diagnosis of VOCs. Our tests for the reversibility of the sensors showed that the CLC-templated polymeric films can be used multiple times without a significant loss of sensitivity.

Seeing the Unseen: The Role of Liquid Crystals in Gas-Sensing Technologies

Fast, real-time detection of gases and volatile organic compounds (VOCs) is an emerging research field relevant to most aspects of modern society, from households to health facilities, industrial units, and military environments. Sensor features such as high sensitivity, selectivity, fast response, and low energy consumption are essential. Liquid crystal (LC)-based sensors fulfill these requirements due to their chemical diversity, inherent self-assembly potential, and reversible molecular order, resulting in tunable stimuliresponsive soft materials. Sensing platforms utilizing thermotropic uniaxial systems-nematic and smectic-that exploit not only interfacial phenomena, but also changes in the LC bulk, are demonstrated. Special focus is given to the different interaction mechanisms and tuned selectivity toward gas and VOC analytes. Furthermore, the different experimental methods used to transduce the presence of chemical analytes into macroscopic signals are discussed and detailed examples are provided. Future perspectives and trends in the field, in particular the opportunities for LC-based advanced materials in artificial olfaction, are also discussed.

Monitoring the two-dimensional concentration profile of toluene vapors by using polymer-stabilized nematic liquid crystals in microchannels

Many volatile organic compounds (VOCs) are hazardous to human health and their concentrations need to be monitored over a large area. However, most chemical sensors can only be used for point detection. In this paper, we report a method to obtain a two-dimensional (2D) concentration profile of toluene vapor by using polymer-stabilized liquid crystals (PSLCs). After the PSLC sample is exposed to toluene vapor ranging from 9300 to 2800 ppm, the PSLC changes its interference color according to different local toluene concentrations. By using the interference color, we can obtain a 2D concentration profile of toluene inside the PSLC and study the diffusion of toluene inside the PSLC in great detail. We also determine the diffusion coefficient of dissolved toluene inside the PSLC to be 1.01 × 10^-6 cm² s^-1.

Construction of a Liquid Crystal-Based Sensing Platform for Sensitive and Selective Detection of l-Phenylalanine Based on Alkaline Phosphatase

The detection of l-phenylalanine (l-Phe) has become one of the most pressing issues concerning diagnosis and treatment of phenylketonuria in neonates; however, a simple and robust methodology is yet to be developed. Here, the application of novel liquid crystals (LCs)-sensing platform for sensitive, selective, and label-free detection of l-Phe was reported at the first time. We devised a strategy to fabricate the sodium monododecyl phosphate (SMP)-decorated LC sensing platform with the appearance of dark. Then, a dark to bright (D-B) optical images alteration of LCs was observed after transferring alkaline phosphatase (ALP) to the interface, owing to cleavage of SMP induced by ALP. LCs remained dark images after the SMP-decorated interface in contact with the pre-incubated ALP and l-Phe. Such optical appearance resulted from the inhibition of ALP by l-Phe, which was further verified by the isothermal titration calorimetry (ITC). The strategy was applied to sensing l-Phe, which have been proven to allow for sensitively and selectively differentiation of l-Phe from interfering compounds with similar aromatic groups, as well as seven other essential amino acids. More importantly, the detection limit of l-Phe reached 1 pg/mL in urine samples, further demonstrating its value in the practical applications. Results obtained in this study clearly demonstrated the superiority of LCs toward the l-Phe detection, which can pave a way for the development of high performance and robust probes for l-Phe detection in clinical applications.

Liquid Crystals with Interfacial Ordering that Enhances Responsiveness to Chemical Targets

The development of stimuli-responsive materials suitable for use in wearable sensors is a key unresolved challenge. Liquid crystals (LCs) are particularly promising, as they do not require power, are light-weight, and can be tuned to respond to a range of targeted chemical stimuli. Here, an advance is reported in the design of LCs for chemical sensors with the discovery of LCs that assume parallel orientations at free surfaces and yet retain their chemoresponsiveness. The resulting LC-based sensors are more sensitive and exhibit faster responses than previous LC sensor designs.

Thermotropic Liquid Crystal-Assisted Chemical and Biological Sensors

In this review article, we analyze recent progress in the application of liquid crystal-assisted advanced functional materials for sensing biological and chemical analytes. Multiple research groups demonstrate substantial interest in liquid crystal (LC) sensing platforms, generating an increasing number of scientific articles. We review trends in implementing LC sensing techniques and identify common problems related to the stability and reliability of the sensing materials as well as to experimental set-ups. Finally, we suggest possible means of bridging scientific findings to viable and attractive LC sensor platforms.

Switchable Plasmonic Film Using Nanoconfined Liquid Crystals

Structural coloration using plasmonic particles has received substantial attention due to its robust, permanent, and scalable characteristics across the full color range. In this study, a plasmonic structure based on a porous anodic aluminum oxide (AAO) film coated with a metallic film was fabricated. Colors were varied by changing the refractive index, which was achieved with a convolution with nanopores of AAO film and an infiltrated liquid crystal (LC) material. LC molecules confined in the porous AAO film were uniformly aligned, and they exhibited pore-size-dependent colors because of the specific refractive index. The thermal phase transition of the LC material in the nanopores changed the effective refractive index, switching the reflected colors, and the LC-infiltrated AAO remained stable over a month. We believe LC materials can extend the use of rigid conventional plasmonic structures from simple sensor applications to multifunctional uses such as color printing, writing pens, and displays.

Tunable Gas Sensing Gels by Cooperative Assembly

The cooperative assembly of biopolymers and small molecules can yield functional materials with precisely tunable properties. Here, the fabrication, characterization, and use of multicomponent hybrid gels as selective gas sensors are reported. The gels are composed of liquid crystal droplets self-assembled in the presence of ionic liquids, which further coassemble with biopolymers to form stable matrices. Each individual component can be varied and acts cooperatively to tune gels' structure and function. The unique molecular environment in hybrid gels is explored for supramolecular recognition of volatile compounds. Gels with distinct compositions are used as optical and electrical gas sensors, yielding a combinatorial response conceptually mimicking olfactory biological systems, and tested to distinguish volatile organic compounds and to quantify ethanol in automotive fuel. The gel response is rapid, reversible, and reproducible. These robust, versatile, modular, pliant electro-optical soft materials possess new possibilities in sensing triggered by chemical and physical stimuli.

Liquid crystals in micron-scale droplets, shells and fibers

The extraordinary responsiveness and large diversity of self-assembled structures of liquid crystals are well documented and they have been extensively used in devices like displays. For long, this application route strongly influenced academic research, which frequently focused on the performance of liquid crystals in display-like geometries, typically between flat, rigid substrates of glass or similar solids. Today a new trend is clearly visible, where liquid crystals confined within curved, often soft and flexible, interfaces are in focus. Innovation in microfluidic technology has opened for high-throughput production of liquid crystal droplets or shells with exquisite monodispersity, and modern characterization methods allow detailed analysis of complex director arrangements. The introduction of electrospinning in liquid crystal research has enabled encapsulation in optically transparent polymeric cylinders with very small radius, allowing studies of confinement effects that were not easily accessible before. It also opened the prospect of functionalizing textile fibers with liquid crystals in the core, triggering activities that target wearable devices with true textile form factor for seamless integration in clothing. Together, these developments have brought issues center stage that might previously have been considered esoteric, like the interaction of topological defects on spherical surfaces, saddle-splay curvature-induced spontaneous chiral symmetry breaking, or the non-trivial shape changes of curved liquid crystal elastomers with non-uniform director fields that undergo a phase transition to an isotropic state. The new research thrusts are motivated equally by the intriguing soft matter physics showcased by liquid crystals in these unconventional geometries, and by the many novel application opportunities that arise when we can reproducibly manufacture these systems on a commercial scale. This review attempts to summarize the current understanding of liquid crystals in spherical and cylindrical geometry, the state of the art of producing such samples, as well as the perspectives for innovative applications that have been put forward.