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

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.

A Review of Developments in Polymer Stabilized Liquid Crystals

Polymer-stabilized liquid crystals (PSLCs) are multi-functional materials consisting of polymer networks in a continuous phase of liquid crystals (LCs), of which polymer networks provide anchoring energy to align the LCs. A number of improvements are detailed, including polymer-stabilized nematic liquid crystals (PSNLCs), polymer-stabilized cholesteric liquid crystals (PSCLCs), polymer-stabilized blue phase liquid crystals (PSBPLCs), polymer-stabilized smectic liquid crystals (PSSLCs), polymer-stabilized ferroelectric liquid crystals (PSFLCs), and polymer-stabilized antiferroelectric liquid crystals (PSAFLCs) in this review. Polymer stabilization has achieved multiple functionalities for LCs; in smart windows, a sufficiently strong electric field allows the LCs to reorient and enables switching from a scattering (transparent) state to a transparent (scattering) state. For broadband reflectors, the reflection bandwidth of LCs is manually tuned by electric fields, light, magnetic fields, or temperature. PSBPLCs open a new way for next-generation displays, spatial light modulators, sensors, lasers, lenses, and photonics applications. Polymer networks in PSFLCs or PSAFLCs enhance their grayscale memories utilized in flexible displays and energy-saving smart cards. At the end, the remaining challenges and research opportunities of PSLCs are discussed.

Experimental and density functional study of the light-assisted gas-sensing performance of a TiO2-CoFe2O4 heterojunction

Toluene gas as a solvent is widely present in industrial production and indoor decoration, and can seriously harm human health even at low concentrations. Furthermore, toluene can be used as a typical biomarker for disease diagnosis. Therefore, the detection of toluene gas is very important. Herein, a hydrothermal method was used to successfully prepare a TiO₂-CoFe₂O₄ heterostructure for detecting toluene gas. The ultraviolet (UV)-visible diffuse reflectance spectra and photoluminescence spectra showed that the bandgap of the heterojunction was considerably shorter than those of pure TiO₂ and CoFe₂O₄, and the recombination of electron-hole pairs was inhibited. At the same time, the response value of the TiO₂-CoFe₂O₄ heterojunction was 10.5 for 20 ppm toluene at 219 °C, which was much better than those of pure TiO₂ and CoFe₂O₄. Moreover, its response value further increased under UV irradiation. In addition, density functional theory (DFT) was innovatively employed in this study to explain in detail how the heterojunction and UV irradiation can improve gas sensitivity through the calculation of the material energy band, adsorption energy, etc. This work provides a good reference for the preparation of high-efficiency and high-sensitivity gas sensors.

Electrospun Core-Sheath Fibers with a Uniformly Aligned Polymer Network Liquid Crystal (PNLC)

Electrospun polymer-liquid crystal (PLC) fibers have potential applications such as wearable sensors and adaptive textiles because of their rapid response and high flexibility. However, existing PLC fibers only have a narrow responsive range and poor resistance to heat and chemicals. Herein, a new type of PLC fiber is prepared by using a coaxial electrospinning process. The core solution is 4'-pentyl-4-biphenylcarbonitrile (5CB), and the sheath solution is a mixture containing 13 wt % PVP and 10 wt % reactive mesogen (RM). After UV exposure of the fibers, 5CB in the core and RM diffusing from the core are cross-linked into an LC polymer. The fibers have a highly uniform morphology with an average diameter of 3.2 ± 0.5 μm, and mesogens inside the fibers align unidirectionally with the long axis of the fibers. The fibers show a broad phase-transition temperature range between 13.5 and 155.5 °C and have a response time of less than 10 s. The temperature range can also be controlled by adjusting components in the electrospun fibers and UV exposure time. The core-sheath fibers prepared in such a manner exhibit excellent heat and chemical resistance with reversible optical responses. Moreover, when the fibers are exposed to volatile organic compounds (VOCs) such as toluene, the fibers show a rapid optical response to toluene vapor within 25 s. This study demonstrates that the fibers are potentially useful for preparing flexible temperature and chemical sensors.

Multifold Enhanced Raman Detection of Organic Molecules as Environmental Water Pollutants

Organic molecules, including the benzene series, have been identified as pollutants in environmental water. Due to their very low solubility, they have very small concentrations in water, and they are difficult to be detected by conventional techniques. In particular, there is a lack of real-time, accurate, and rapid detection methods for such molecules in water. However, they are detrimental to human health in many aspects. Toluene has been an important indicator of such environmental pollution detections. In this work, we propose a 3D SERS scheme consisting of a hollow fiber that is coated on the inner wall with densely arranged silver nanoparticles, which supplies multifold Raman enhancement by the plasmonic microcavity. Strong confinement of excitation laser energy and strongly enhanced Raman signals with the bidirectional collection are utilized to achieve high-sensitivity detection of toluene molecules in water. Raman signal with a reasonable signal-to-noise ratio has been measured for a concentration of 0.53 mg/L, indicating a detection limit even lower than this value for such a Raman spectroscopic technique. The corresponding enhancement factor is higher than 6 × 10³ with respect to the available systems. Thus, this device not only enables direct trace detection and real-time monitoring of the water-polluting status by organic molecules but also supplies a practical approach for biological sensing.

A highly sensitive fluorescent sensor for ammonia detection based on aggregation-induced emission luminogen-doped liquid crystals

As a toxic substance, ammonia can cause serious irritation to the human respiratory system and lungs. Although many detection techniques have been reported, most of them have drawbacks, such as expensive devices and complex and time-consuming fabrication processes. Thus, it is important to develop a simple method for ammonia detection. In this paper, we demonstrate a highly sensitive fluorescent sensor for ammonia detection based on aggregation-induced emission luminogen-doped liquid crystals without the use of polarizers. The homeotropic orientation of the liquid crystals on a modified substrate can be disturbed by ammonia, resulting in the fluorescence intensity change of an aggregation-induced emission luminogen. This aggregation-induced emission luminogen-doped liquid crystal-based fluorescent sensor for ammonia detection exhibited a low detection limit of 5.4 ppm, which is 3 times lower than previously reported liquid crystal-based optical sensors. The detection range is also broad from 0 ppm to 1600 ppm. Meanwhile, this sensor can be applied to detect aqueous ammonia with a low limit of detection of 1.8 ppm. The proposed fluorescent sensor for ammonia detection based on an aggregation-induced emission luminogen-doped liquid crystal is highly sensitive, highly selective, simple, and low cost with wide potential applications in chemical and biological fields. This strategy of designing a liquid crystal fluorescent sensor provides an inspiring stage for other toxic chemical substrates by changing specific decorated molecules.

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.

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.