Liquid crystal-based detection of DNA hybridization using surface immobilized single-stranded DNA

Stable liquid crystal-infiltrated photonic crystal sensing film for facile detection of streptomycin

Liquid crystal (LC) biosensors have attracted interest due to their simplicity and ability to visualize results. However, their inherent instability including the fluidity of LCs and the complexity of operation, limits their potential as reliable and user-friendly detection tools. To enhance their practical applicability, a stable and facile LC-infiltrated photonic crystal (LCP) sensing film is developed through optimization of film substrate preparation and investigation of the response mechanism. The reflection peak of the LCP film, which is modulated by changes in LC orientation within the film, can be recorded using a fiber-optic spectrometer or observed visually. Molecular dynamics simulations, integrated with experimental data, were employed to improve LC induction efficiency and increase signal strength. This approach inherently improves the stability and sensitivity of LC biosensors, expanding their potential for use in compact devices. A triple-helix molecular conformational switch is introduced to establish a versatile and specific detection platform. When streptomycin was chosen as a model analyte, the LCP film exhibited a linear range from 5 nM to 10 μM, with a detection limit of 0.40 nM and a relative standard deviation of 2.19%, indicating high precision and reliability. Its practical application was further confirmed with food samples, highlighting its potential for at-home testing of antibiotic residues.

Polarizer-Free Dye-Doped Liquid Crystal Sensors with High Precision

The surface-induced ordering of liquid crystals (LC) has been harnessed to detect a wide range of chemical and biological stimuli. In most sensor designs, the information-rich response of the LC is transduced from an analyte-triggered change in the out-of-plane orientation of the LC. Quantifying the out-of-plane LC orientation, however, is often complicated by simultaneous changes in the in-plane orientation of the LC when using polarized light for transduction. Here we introduce a sensing approach that combines a dichroic dye-doped LC (DDLC) with unpolarized light and a photodiode to achieve precise quantification of analyte-driven changes in the out-of-plane orientations of LCs. We benchmark the performance of the new methodology against polarizer-based approaches using a model amphiphilic analyte in aqueous solution and show that the DDLC provides a substantial reduction in the coefficient of variation (300% to less than 5%), an enhanced analytical sensitivity (0.16 to 3.73 μM-1), and an expanded dynamic range. In addition, when used to sense concentration gradients of analytes, the new approach distinguishes differences as small as 0.03 μM/μm over a dynamic range of 2 μM/μm, significantly outperforming conventional polarizer-based approaches that detect differences of 0.3 μM/μm over a dynamic range of 0.6 μM/μm. Overall, we conclude that the improved sensing performance and simpler implementation (no polarizers) of the DDLC approach, as compared to conventional LC sensors based on crossed-polars, will facilitate the deployment of LC sensors in diverse contexts, including the development of high-throughput screens for chemical formulations.

Label-Free Liquid Crystal Aptamer Sensors Based on Single-Stranded Nucleic Acid π-Structures for Detecting cTnI

Cardiac troponin I (cTnI) is a highly sensitive and important serological marker for clinical diagnosis of myocardial injury. Its rapid detection is crucial for the early diagnosis of cardiovascular diseases such as acute myocardial infarction. In this study, based on nucleic acid molecular hybridization and aptamer-specific binding to target molecules, a label-free liquid crystal aptamer sensor based on single-stranded nucleic acid π-structures was developed and applied for the quantitative detection of cTnI. The CP1 and CP2 oligonucleotide chains, complementary to the bases at both ends of the aptamer, are covalently bonded to the sensor substrate via APTES and GA-mediated molecules. The aptamer forms a π-structure with CP1 and CP2 through nucleic acid hybridization, serving as a target molecule capture probe. When cTnI is present in the system, cTnI and the complementary oligonucleotide chains competitively bind with the aptamer, causing the breakdown of the π-structure within the sensor. This reinstates the long-range ordered alignment of the 5CB liquid crystal molecules within the sensor, enabling quantitative measurement of cTnI through variations in optical images. Experimental results show that within the range of 0.01 to 25 ng/mL for cTnI concentration, there is a linear correlation between the brightness area coverage (Br) in the polarized light microscopy images of the sensor and the logarithm of the cTnI concentration, with a correlation coefficient (r). The detection limit is 5.16 pg/mL. This method is label-free, simple to operate, and low-cost, with good specificity and a low detection limit, achieving cTnI detection in serum samples.

Liquid Crystal Materials for Biomedical Applications

Liquid crystal is a state of matter being intermediate between solid and liquid. Liquid crystal materials exhibit both orientational order and fluidity. While liquid crystals have long been highly recognized in the display industry, in recent decades, liquid crystals provide new opportunities into the cross-field of material science and biomedicine due to their biocompatibility, multifunctionality, and responsiveness. In this review, the latest achievements of liquid crystal materials applied in biomedical fields are summarized. The start is made by introducing the basic concepts of liquid crystals, and then shifting to the components of liquid crystals as well as functional materials derived therefrom. After that, the ongoing and foreseeable applications of liquid crystal materials in the biomedical field with emphasis put on several cutting-edge aspects, including drug delivery, bioimaging, tissue engineering, implantable devices, biosensing, and wearable devices are discussed. It is hoped that this review will stimulate ingenious ideas for the future generation of liquid crystal-based drug development, artificial implants, disease diagnosis, health status monitoring, and beyond.

Detection of alkali and alkaline earth metal ions using birefringence of hyperswollen lamellar phase

Chemical sensors have targeted various substances. Most sensors electrically amplify signals. Here, we propose a visual detection system that uses a hyperswollen lamellar phase and detects targets in a solution without electric amplification. Amphiphiles with an oligo(ethylene glycol) chain can catch alkali and alkaline earth metal ions and amplify to macroscopic birefringence.

A label-free liquid crystal-assisted aptasensor for trace level detection of tobramycin in milk and chicken egg samples

Developed herein is an aptasensing array based on liquid crystal (LC) for monitoring of tobramycin (TOB) antibiotic. The direction of LC molecules from vertical to a random status was induced by the conformational changes of the specific aptamer due to its selective interaction with the target. The dark view of the aptasensing platform changed to colorful through observation by a polarized light microscope that clarifies the TOB presence. The aptasensor is especially able to determine TOB in the linear concentration range of 0.005-600 pM with a limit of detection (LOD) as 0.0021 pM. The TOB values can be determined successfully in the milk and chicken egg samples that highlights the potential applicability of the designed aptasensor. The proposed sensing approach is facile, operator-independent, label-free, and ultra-sensitive, making it novel for developing real-time portable sensing devices for future.

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.

An ultra-sensitive dual-responsive aptasensor with combination of liquid crystal and intercalating dye molecules: A food toxin case study

Herein, a label-free aptasensor was designed through forming a double-stranded DNA skeleton on the glass substrate for ultrasensitive quantification of ochratoxin A (OTA) as a case study. The function fundament of the dual-responsive aptasensor was the perturbation of the vertical alignment of the liquid crystals (LCs) and intercalation of the SYBR Green I (SGI) dye molecules between the base pairs of the double-stranded DNA structure. The presence of OTA decomposed the double-stranded structure of DNA by releasing the OTA-specific aptamer from the sensing platform that induced an apparent alteration of the optical and fluorescent responses. The aptasensor specifically detected the ultra-low levels of OTA as 47.0E-9 pM (0.047 aM) and 34.0E-3 pM (34 fM) based on the polarized and fluorescent responses, respectively. The aptasensor monitored OTA in the coffee and grape drink samples. The aptasensor provides promising insight for manufacturing real-time, cost-effective, and portable sensing devices for food control usage.

Partial induced reorientation of 5CB in a liquid crystal microarray and a signal-on sensing assay for the detection of aflatoxin B1

Herein, a signal-on liquid crystal microarray (LCM) sensor is designed for the first time with a micro-spectral optical sensing signal. Depending on the change of the orientation of the LC molecules in the LCM films and the intensity of the spectral peaks of the PhCs, the signal-on LCM biosensor achieves the detection of AFB1 and the Partial Response Mechanism (PSM) of the LCM films is discovered.

Overview of Liquid Crystal Biosensors: From Basic Theory to Advanced Applications

Liquid crystals (LCs), as the remarkable optical materials possessing stimuli-responsive property and optical modulation property simultaneously, have been utilized to fabricate a wide variety of optical devices. Integrating the LCs and receptors together, LC biosensors aimed at detecting various biomolecules have been extensively explored. Compared with the traditional biosensing technologies, the LC biosensors are simple, visualized, and efficient. Owning to the irreplaceable superiorities, the research enthusiasm for the LC biosensors is rapidly rising. As a result, it is necessary to overview the development of the LC biosensors to guide future work. This article reviews the basic theory and advanced applications of LC biosensors. We first discuss different mesophases and geometries employed to fabricate LC biosensors, after which we introduce various detecting mechanisms involved in biomolecular detection. We then focus on diverse detection targets such as proteins, enzymes, nucleic acids, glucose, cholesterol, bile acids, and lipopolysaccharides. For each of these targets, the development history and state-of-the-art work are exhibited in detail. Finally, the current challenges and potential development directions of the LC biosensors are introduced briefly.

Control of Liquid Crystal Microarray Optical Signals Using a Microspectral Mode Based on Photonic Crystal Structures

Herein, a novel liquid crystal microarray (LCM) film with optical regulation ability is first constructed by combining liquid crystals (LC) and the highly ordered microporous structure of inverse opal photonic crystals (IOPhCs). The LCM films are fabricated by infiltrating LC molecules into the LC polymer with the structure of IOPhCs, and their properties are very different from those without the LC. Interestingly, the optical property of LCM films can be controlled by changing the orientation of LC molecules, which varies with the interfacial force. In combination with polarization images, spectral reflection peak, circular dichroism spectra, potential difference, and fluorescence images of LCM films, the mechanism of this change is investigated. It is found that the exposed basic group of single-stranded DNA is the key to the change of the optical property of LC microarrays. Meanwhile, the optical signals of LC microarrays based on the PhCs provide a novel LC signal mode for an LC sensing system (microspectral signal mode), and it can be recorded by a fiber-optic spectrometer, which is a great improvement on LC sensing signals. Therefore, the LC microarray sensing signal can be used for accurate analysis of targets by the change of the reflection peak intensity of PhCs. When the LC molecules are induced by different aptamers, the LC microarray sensing interface can be further used for the determination of different targets, such as cocaine and Hg²⁺. The research on LCM films is of significant value for the development of LC sensing technology and also shows great application prospects in biochemical sensing fields.

A novel liquid crystal-based aptasensor for ultra-low detection of ochratoxin a using a π-shaped DNA structure: Promising for future on-site detection test strips

Ochratoxin A (OTA) as the most dangerous mycotoxin is produced by Aspergillus Ochraceus and Penicillium verrucosum. OTA can be found in beverages and foodstuffs that induces the teratogenic, nephrotoxic, carcinogenic, and immunosuppressive effects on humans. Hence, developing highly sensitive methods for its detection is of great importance. Herein, a novel aptasensor was designed for the label-free monitoring of the ultra-low OTA levels by a combination of the superiority of aptamers and long-range orientational order of liquid crystals (LCs). The aptasensing strategy was based on the conformational switch of the immobilized π-shaped DNA structure on the glass substrate in presence of the target. A shift in the orientation of LCs from random to homeotropic state led to the apparent alteration of the optical appearance of the aptasensor platform from bright to dark. The LC-based aptasensor especially detects OTA at the ultra-trace level as low as 0.63 aM with comparable selectivity. The aptasensor could detect OTA successfully in the grape juice, coffee, and human serum samples. The LC-based aptasensor paves a way for developing portable and real-time sensing probes with high performance for food safety control and clinical application.

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.

Detection of bleomycin and its hydrolase by the cationic surfactant-doped liquid crystal-based sensing platform

Bleomycin (BLM) is a broadly used antibiotic to treat different types of cancer. It can be hydrolyzed by bleomycin hydrolase (BLMH), which eventually influences the anti-tumor efficacy of BLM. Therefore, it is particularly important to detect BLM and BLMH. Herein, we demonstrated highly sensitive detection of BLM and BLMH by a simple and convenient liquid crystal (LC)-based sensing platform for the first time. 5CB (a nematic LC) doped with the cationic surfactant OTAB was working as the sensing platform. When the OTAB-laden 5CB interface was in contact with an aqueous solution of ssDNA, LCs displayed a bright image due to disruption of the arrangement of OTAB monolayers by ssDNA, indicating the planar orientation of LCs at the aqueous/LC interface. When BLM·Fe(II) and ssDNA were both present in the aqueous solution, ssDNA underwent irreversible cleavage, which prevented disruption of the arrangement of OTAB monolayers. Accordingly, LCs showed a dark image, suggesting the homeotropic orientation of LCs at the aqueous/LC interface. However, when BLM·Fe(II) was enzymatically hydrolyzed by BLMH, LCs remained the bright image. This approach showed high sensitivity for the detection of BLM and BLMH with the limits of detection of 0.2 nM and 0.3 ng/mL, respectively. Besides, the detection of BLM and BLMH was successfully achieved in human serum. This method has the advantages of high sensitivity, robust stability, simple operation, low cost, and easy detection through naked eyes, which makes it a potential candidate for applications in clinical analysis.

An ultrasensitive platform for PCB77 detection: New strategy for liquid crystal-based aptasensor fabrication

Polychlorinated biphenyls (PCBs) are considered persistent bio-accumulative toxicants which threats global food safety and environmental health. Traditional analytical techniques for detection of PCBs are time-consuming and they do not satisfy urgent need for rapid and accurate monitoring of these persistent pollutants. Biosensor technology may be promising in this respect. Here we demonstrate a novel liquid crystal (LC)-based aptasensing platform as a promising label-free and rapid biosensor for PCB77 detection. This novel molecular strategy utilize triple-helix molecular conformational switch which is mediated formation of duplex on sensing platform in presence of target. Duplex forming leads to optical change from dark to bright in a liquid crystal based aptasensor. The limit of quantification of the LC-aptasensor to PCB77 is 1.5 × 10^-5 μg/L with comparable selectivity. Besides, we also demonstrated that this system is able to detect PCB77 in tap water, environmental water and milk. This strategy has potential for label-free and portable detection of different targets without any aptamer sequence length restrictions.

Bio-electrostatic sensitive droplet lasers for molecular detection

Electrostatics plays a critical function in most biomolecules, therefore monitoring molecular electrostatic interactions at the biointerface can provide the basis in diagnosis and biomedical science. Herein we report a bioelectrostatic responsive microlaser based on liquid crystal (LC) droplets and explored its application for the ultrasensitive detection of negatively charged biomolecules. A whispering gallery mode (WGM) laser from positively charged LC microdroplets was designed as the optical resonator, in which the lasing wavelength shift was employed as the sensing parameter. We verified that molecular electrostatic changes at the biointerface of the droplet trigger a wavelength shift in laser spectra. Compared to a conventional polarized optical microscope, a significantly improved sensitivity and dynamic range by four orders of magnitude were achieved. Our results helped discover that the surface-to-volume ratio plays a critical role in the detection sensitivity in WGM laser-based microsensors. Finally, bovine serum albumin and specific biosensing were exploited to demonstrate the potential applications of microlasers with a detection limit in the order of 1 pM, thus offering new alternatives for ultrasensitive label-free biosensing and monitoring of molecular interactions.

Preparation of DNA-functionalized surfaces for simultaneous homeotropic orientation of liquid crystals and optical recognition of analytes: application to the determination of progesterone

The work describes a simplified method for the preparation of liquid crystal (LC) bioassay using DNA-based capture molecules and having lower detection limits. The capture DNA probes of the stem-loop structure were immobilized on the surface of a glass slide. A homeotropic orientation of LC molecules can be obtained with the proper surface coverage of capture DNA probes. In the presence of analytes (specifically shown here for the progesterone as a model analyte), the molecular binding between capture DNA probes and progesterone opens the loop of the capture DNA probes. The opened sequence is then amenable to hybridization with a reporter DNA probe that is immobilized on gold nanoparticles. This changes the surface microstructure, disrupts the orientation of LC molecules, and results in an enhanced optical response, expressed as the average grey value of the images. This new kind of surface treatment for simultaneous recognition of target molecules and homeotropic anchoring of LCs reduces the number of preparation steps and makes the process of LC bioassay easier. This method has a detection limit as low as 0.1 pmol·L^-1 of progesterone. Graphical abstract Schematic presentation of the liquid crystal-based DNA assay. DMOAP: Dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride; TEA: Triethoxsilylbutyraldehyde; 5CB: 4-cyano-4'-pentylbiphenyl; P4: progesterone.

Detection of sulfadimethoxine using optical images of liquid crystals

A label-free method for sulfadimethoxine (SDM) detection using an aptamer-based liquid crystal biosensor is developed. The sensor probe is fabricated by immobilizing amine-functionalized aptamers onto the glass slide decorating mixed self-assembled layers of triethoxysilylbutyraldehyde (TEA) and N,N-dimethyl-n-octadecyl-3-aminopropyltrimethoxysilylchloride (DMOAP). Liquid crystals (LCs) are supported on the surface and serve as response elements, which assume the homeotropic alignment and cause a dark optical appearance under crossed polarizers. In the presence of SDM, the formation of SDM-aptamer compounds induces a notable change in the topographical structure of the surface, which disturbs the original homeotropic orientation of LCs and results in a bright optical appearance. A detection limit of 10 μg L-1 is obtained, which is far lower than the maximum residue limit (100 μg L-1 in China). This facile method shows good specificity for SDM detection and may have great potential for detecting other small molecules.

Rapid detection of pathogens using lyotropic liquid crystals

Lyotropic liquid crystals play an important role in many biological environments, such as micelles, liposomes, and phospholipid bilayers of cell membranes. In this work, we explore the performance of lyotropic liquid crystals as biosensors for macromolecules, proteins and whole microorganisms in hydrophilic media, i.e., the natural media where these specimens exist. The aim is to detect specific targets employing simple, unpowered sensors that can be used in the field, with minimum additional equipment. A number of different structures have been explored. The novelty in this work is the inclusion of a new optical effect, flow enhanced amplification, that allows for the semiquantitative detection of microscopic targets in lyotropic liquid crystal cells using the naked eye only.

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.

A liquid crystal based method for detection of urease activity and heavy metal ions by using stimulus-responsive surfactant-encapsulated phosphotungstate clusters

A liquid crystal (LC) based method is described for the sensitive determination of the activity of urease and of heavy metal ions which acts as inhibitors. Stimulus-responsive surfactant-encapsulated phosphotungstate clusters (SECs) were fabricated and deposited onto octadecyltrichlorosilane-coated glass. A copper TEM grid filled with LCs was placed on the substrate to construct the LC optical cell. Upon addition of water to the LC interface, the optical appearance of LCs on the glass undergoes a bright-to-dark shift due to an orientational transition of the LCs from a planar to a homeotropic state. However, the LCs display a bright appearance if they are pretreated with an aqueous solution containing urea and urease. This is caused by the disassemby of the SECs from the glass surface due to an increase of the pH value that is induced by the enzymatic hydrolysis of urea by urease. The method is highly sensitive and can detect urease activities as low as 0.03 mU/mL. It can also be applied to the determination of heavy metal ions which exert an inhibitory effect on the activity of urease. For example, Cu(II) can be quantified via urease inhibition in 1 nM concentration. Graphical abstract Schematic presentation of a liquid crystal-based sensor for detection of urease and heavy metal ions by using stimulus-responsive surfactant-encapsulated phosphotungstate clusters.

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.