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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. [ABSTRACT FROM AUTHOR]

Antimony is highly toxic and a key water pollutant, which needs to be monitored closely. To date, however, most analytical methods for antimony detection are quite limited because they are complicated, expensive, and not suitable for real-time monitoring of antimony. In this study, a label-free and rapid method for antimony ions (Sb3+) detection is developed based on liquid crystals and a 10-mer poly-adenine oligonucleotide as a specific recognition probe for the first time. The working principle is based on the binding of the oligonucleotide to Sb3+, which weakens the interaction between the oligonucleotide and cationic surfactants. As a result, the event induces a planar-to-homeotropic orientational change of liquid crystals and a bright-to-dark optical change under crossed polars. This liquid crystal-based optical sensor exhibits a rapid response to Sb3+ in 10 s, a detection range between 20 nM and 5 μM, and a detection limit at 6.7 nM calculated from 10-mins assay time. It also shows good selectivity against other metal ions including Ag+, Cd2+, Cu2+, Fe3+, K+, Mg2+, Mn2+, Na+, Pb2+, and Zn2+. Moreover, this system can be used to detect Sb3+ in aqueous solutions with different pH or ionic strengths. This simple, fast, and low-cost liquid crystal-based sensing approach with high sensitivity and selectivity has a high potential for detecting Sb3+ in natural environments and industrial wastewater. [Display omitted] • A liquid crystal sensor by using a probe Poly-A oligonucleotide for Sb3+ detection • The addition of Sb3+ weakens the interaction between Poly-A and surfactants • The sensor has a low detection limit at 6.7 nM and response time at 10 s • The system shows good selectivity and detection ability in different environments [ABSTRACT FROM AUTHOR]

Abnormal protease activities are associated with cancers, vascular diseases and Alzheimer diseases. Therefore, detection of protease activities has become increasingly important in recent years. Herein, we report a semi-quantitative, liquid crystal (LC)-based protease assay for naked-eye detection of protease activity. In this assay, casein molecules are cleaved by proteases into small peptide fragments, which can be quantified either by using Lowry’s method or using LC. In the latter, peptide fragments adsorb on a solid surface and disrupt LC to produce a bright spot for naked-eye detection. The bright spot is observed only when the surface-adsorbed peptide density exceeds a critical value. In the assay, a major challenge is how to separate undigested casein and remaining protease from peptide fragments to prevent their interferences with LC. To overcome this issue, trichloroacetic acid (TCA) is added to precipitate casein and proteases. By using the assay, we are able to detect 10 ng/mL of protease (activity 7.23 U/mg protease, R 2  = 0.991) or 6.5 μg/mL of peptide fragments. Finally, a continuous protease assay is developed to minimize manual sampling and reduce errors in kinetic studies. [ABSTRACT FROM AUTHOR]

We reported an oligopeptide immobilization strategy for improving stability and sensitivity of liquid-crystal (LC) based protease assay. In this strategy, oligopeptides are immobilized by using N -terminal cysteine or free amine groups on lysine residues to react with surface aldehyde groups and form multiple anchoring points. Our results show that the number of anchoring points is an important factor for improving stability of the immobilized oligopeptides. Oligopeptides with multiple anchoring points are very stable on the surface. They can only be cleaved from the surface by using proteases such as trypsin or α-chymotrypsin. This phenomenon provides a principle for developing an assay for the detection of proteases. On the other hand, oligopeptides with a single anchoring point cannot form stable linkage on the surface, and they can be easily washed off during rinsing steps. By immobilizing oligopeptides with different surface densities, concentrations of trypsin and α-chymotrypsin can be estimated simply by counting the number of bright LC spots. The higher the concentrations of trypsin and α-chymotrypsin, the less LC spots can be observed. In addition, the limit of detection (LOD) of the assay can be lowered by allowing a very large oligopeptide fragment to desorb from the surface after the cleavage. This task can be accomplished by immobilizing an oligopeptide with all anchoring points located at both ends while the cleavage sites are located near the anchoring points. Based on the principle, optimized LODs for trypsin and α-chymotrypsin can reach 1 and 0.1 ng/mL, respectively. [ABSTRACT FROM AUTHOR]

Abstract: We report a biosensor for detecting organophosphates (OPs) by using liquid crystal (LC). The mechanism of the biosensor is based on detection of minute pH changes during the enzymatic hydrolysis of OPs. To hydrolyze OPs, paraoxonase 1 (PON1) is immobilized on a copper grid which also hosts pH-sensitive LC, 4-cyano-4′-pentylbiphenyl (5CB) doped with 0.3% of 4′-pentyl-biphenyl-4-carboxylic acid (PBA). Proximity of enzyme and LC ensures that H+ can be detected by the pH-sensitive LC before it is neutralized by buffer. Thus, the presence of OPs can be reported as color change in LC when they undergo enzymatic hydrolysis. For paraoxon detection, we can reach a limit of detection (LOD) of 1μM by using the naked eye. This type of LC-based biosensor also shows high specificity and does not respond to imidacloprid and ampicillin. [Copyright &y& Elsevier]

Abstract: A liquid crystal (LC)-based optical sensor has been developed to detect vaporous butylamine in air. This LC sensor doped with lauric aldehyde (LA) shows fast and distinct bright-to-dark optical response to butylamine vapor. For example, when the LA doping concentration is 0.1wt%, the LC shows a rapid bright-to-dark optical response within 2min after it is exposed to 10ppmv of butylamine. This optical response is attributed to an orientational transition of LC triggered by a reaction between LA and butylamine. This LC sensor also exhibits reversibility after the sensor is exposed to open air. In addition to primary amines (such as butylamine and octylamine), this LC-based sensor responds to secondary amines (such as diisopropylamine), but the detection limit is 200ppmv, which is much higher than butylamine. [Copyright &y& Elsevier]

Abstract: Conventional immunoassays are often fluorescence- or enzyme-based and require additional readout systems. Here, we report a liquid crystal (LC)-based immunoassay that features a new signal transduction mechanism, whereby the test results appear as colorful spots and can be viewed with the naked eye directly. By using the different colors of LC spots, we can estimate the range of anti-hepatitis B surface antigen concentrations between 300 and 15nM. Because additional labeling process and readout systems are not required in this immunoassay, it is more suitable for point-of-care diagnosis of hepatitis B in resource-limited regions. [Copyright &y& Elsevier]

Vanillin is an aromatic organic compound that is widely used in the food and cosmetics industries. The development of a simple and rapid method for the naked-eye detection of vanillin is essential for monitoring the vanillin production process. In this study, liquid crystal 5CB (4-cyano-4'-pentylbiphenyl) is doped with acetone and used as a sensing platform to detect vanillin in alkaline solutions with high specificity for the first time. Before the addition of vanillin, the acetone-doped 5CB adopts a homeotropic orientation and appears dark due to the formation of an electrical double layer (EDL) at the 5CB/water interface. In the presence of vanillin, it forms a complex with acetone inside 5CB, disrupting the EDL and the homeotropic orientation of 5CB. The event leads to a dark-to-bright transition which can be used to quantify the concentration of vanillin in a range between 0.1 and 25 mg/L. The limit of detection (LOD) for the vanillin sensor is 0.06 mg/L. As a proof of concept, the sensor is employed to monitor the vanillin concentration during oxidation of coniferyl alcohol. In the presence of 5000 mg/L of H 2 O 2 and 1 mg/L iron tetraamidomacrocyclic ligand (Fe-TAML), 1750 mg/L of coniferyl alcohol can be converted to 393.0 mg/L of vanillin after a 30-min reaction at room temperature. This study demonstrates potential applications of the acetone-doped liquid crystal sensors for monitoring the production of vanillin under various conditions. [Display omitted] • An acetone-doped liquid crystal sensor for vanillin detection is developed. • The limit of detection (LOD) for vanillin is 0.06 mg/L with high specificity. • Coniferyl alcohol can be oxidized to vanillin by using H 2 O 2 and a catalyst. • The sensor can be used to monitor the production of vanillin from coniferyl alcohol. [ABSTRACT FROM AUTHOR]

Abstract: We report the use of liquid crystal 4-cyano-4′-biphenylcarbonitrile (5CB) doped with copper perchlorate as an optical sensor for detecting dimethyl methylphosphonate (DMMP) vapor. Our results show that after doping 5CB with copper perchlorate (0.05–50mM), orientations of 5CB supported on plain glass slides gradually change from planar to homeotropic over time, and the time requires for turning homeotropic orientation increases with the decreasing concentration of copper perchlorate. When we expose a film of homeotropically oriented 5CB doped with 0.1mM of copper perchlorate to 0.003ppm of DMMP vapor, the optical appearance of 5CB changes from dark to bright in 2.5min. In contrast, no response can be observed when the same film is exposed to 4500ppm of water, 1000ppm of ethanol or 1000ppm of hexane. The optical response of 5CB to DMMP is fully reversible. Because DMMP is a commonly used simulant for chemical warfare agent, this work demonstrates the feasibility of using 5CB doped with copper perchlorate as a real-time gas sensor for detecting chemical warfare agents. [Copyright &y& Elsevier]

Abstract: We report a liquid crystal (LC)-based sensor for real-time and label-free identification of phospholipase-like toxins. Beta-bungarotoxin exhibits Ca2+-dependent phospholipase A2 activity whereas alpha-bungarotoxin and myotoxin II do not exhibit any phospholipase activity. The sensor can selectively identify beta-bungarotoxin, when it hydrolyzes a phospholipid monolayer self-assembled at aqueous–LC interface, through orientational responses of LCs. As a result, optical signals that reflect the spatial and temporal distribution of phospholipids during the hydrolysis can therefore be generated in a real-time manner. The sensor is very sensitive and requires less than 5pg of beta-bungarotoxin for the detection. When phospholipase A2 inhibitors are introduced together with beta-bungarotoxin, no orientational response of LCs can be observed. In addition, the regeneration of the sensor can be done without affecting the sensing performance. This work demonstrates a simple and cost-effective LC-based sensor for identifying phospholipase-like toxins and for screening compound libraries to find potential toxin inhibitors. [Copyright &y& Elsevier]

Abstract: This paper reports a novel liquid-crystal based method for the optical detection of multiple glycine oligomers simultaneously. The detection principle is based on the disruption of the orientations of a nematic liquid crystal 4-cyano-4′-pentylbiphenyl (5CB) supported on a surface patterned with multiple glycine oligomers. When glycine oligomers with different molecular lengths are immobilized on aldehyde-decorated surfaces under optimized conditions, they form monolayers with different thicknesses corresponding to their molecular lengths. As a result, the orientations of 5CB supported on the surfaces are disturbed when the thickness exceeds 0.5±0.1nm. Because liquid crystals are birefringent materials, the disruption of their orientations can cause optical signals visible to the naked eye. This liquid-crystal based detection scheme for glycine oligomers is able to detect 10μM of triglycine, tetraglycine and pentaglycine, 100μM of diglycine, and 1mM of glycine with a sample size as little as 2μL. It also provides a good spatial resolution over a large area and is sufficiently simple. [Copyright &y& Elsevier]