Liquid Crystal Biosensor Research Articles

Potential liquid crystal-based biosensor depending on the interaction between liquid crystals and proteins

Liquid crystal biosensors serve an essential role to detect biomolecules and chemical events as an effective, simple and early detection tool. The detection of Human serum albumin protein by a room temperature liquid crystal 4́-pentyl-4-biphenylcarbonitrile has been investigated using multiple techniques such as Polarizing optical microscope, Raman spectroscopy and molecular docking. The dynamics of director transfigurations of the liquid crystal molecule in the presence of protein through interactions are reported in the study. The change in the alignment of liquid crystal molecules during the nematic phase is observed under a polarizing optical microscope. The interactions through which the liquid crystal molecules bind with protein is depicted from the docking analysis. The residues in the active sites confirm their presence from docking studies. The spectral behaviour has been investigated by temperature-dependent Raman spectroscopy. The findings from Raman spectra for the interaction between these compounds correlates with the residues confirmed from molecular docking analysis.

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.

Simple, rapid and sensitive detection of Parkinson's disease related alpha-synuclein using a DNA aptamer assisted liquid crystal biosensor.

Alpha-synuclein (αS) has been proposed as a potential biomarker for the diagnosis of Parkinson's disease (PD). However, the detection of αS using a simple, rapid and sensitive approach is still challenging. Herein, we construct a new type of biosensor for the detection of αS, combining the stimuli-responsiveness of liquid crystals (LCs) and the specific interaction of a DNA aptamer with proteins. In principle, the positively charged surfactant hexadecyltrimethylammonium bromide (CTAB) binds with the negatively charged DNA aptamer via electrostatic interactions; in the presence of αS, the DNA aptamer specifically binds with αS and releases CTAB, which is an amphiphilic molecule and subsequently assembles at the LC-aqueous interface, resulting in a homeotropic alignment of LCs with a dark optical signal. In the absence of αS, CTAB binds with the DNA aptamer without affecting the alignment of LCs, which shows planar anchoring with a bright optical signal. The response time of LCs towards αS is rapid and can be down to minutes. The LC biosensor established here has a good specificity for αS and can recognize αS even from a mixture of proteins. The LC biosensor also exhibits high sensitivity with a limit of detection of αS as low as 10 pM, which is comparable to that of the enzyme-linked immunosorbent assay. This work provides a new strategy for the detection of αS in a simple, rapid and sensitive manner, possessing promising potentials towards early diagnosis and clinical applications.

Detection of Cecropin B by liquid-crystal biosensor based on AuNPs signal amplification

ABSTRACT A novel biosensor was prepared by using gold nanoparticles (AuNPs) signal amplification technology combined with liquid crystal biosensor to detect Cecropin B (CB). The surface of the substrate was modified with (3-aminopropyl)-triethoxysilane/N,N-dimethyl-N-octadecyl-(3-aminopropyl)-trimethoxysilyl chloride (APTES/DMOAP), glutaraldehyde (GA) as crosslinking, the AuNPs and Cecropin B antibody (anti-CB) complex could be fixed on the basal surface. CB and anti-CB can bind to make the liquid crystal (LC) changes in a vertical orientation, so as to achieve the purpose of testing. The volume of AuNPs is much larger than that of CB and anti-CB, thus AuNPs can play the role of signal amplification. Compared to the experiments without nano-gold amplification, our method has a lower detection limit. Finally, the limit of CB detection could be as low as 1.49 × 10−2 ng/mL. This method has the advantages of good specificity, high sensitivity, low detection limit and simple operation.

Label-Free Multi-Microfluidic Immunoassays with Liquid Crystals on Polydimethylsiloxane Biosensing Chips.

We developed a new format for liquid crystal (LC)-based multi-microfluidic immunoassays, hosted on a polydimethylsiloxane substrate. In this design, the orientations of the LCs were strongly affected by the interface between the four microchannel walls and surrounding LCs. When the alignment layer was coated inside a microchannel, the LCs oriented homeotropically and appeared dark under crossed polarizers. After antigens bound to the immobilized antibodies on the alignment layer were coated onto the channel walls, the light intensity of the LC molecules changed from dark to bright because of disruption of the LCs. By employing pressure-driven flow, binding of the antigen/antibody could be detected by optical signals in a sequential order. The multi-microfluidic LC biosensor was tested by detecting bovine serum albumin (BSA) and an immunocomplex of BSA antigen/antibody pairs, a protein standard commonly used in labs. We show that this multi-microfluidic immunoassay was able to detect BSA and antigen/antibody BSA pairs with a naked-eye detection limitation of −0.01 µg/mL. Based on this new immunoassay design, a simple and robust device for LC-based label-free microfluidic immunodetection was demonstrated.

Application and Technique of Liquid Crystal-Based Biosensors.

Liquid crystal biosensors are based on changes in the orientation of liquid crystal molecules induced by specific bonding events of biomolecules. These biosensors are expected to serve as a promising system to detect biomolecules, biomolecular activity, and even small chemical molecules because they are inexpensive, sensitive, simple, effective, and portable. Herein, we introduce the principle and fabrication of liquid crystal biosensors and review the research progress in signal-amplified technology for liquid crystal sensing and its application in the detection of viruses, bacteria, proteins, nucleic acids, and small chemical molecules. In addition, the current theoretical and practical issues related to liquid crystal biosensors were investigated.

Detection of acetylcholinesterase and its inhibitors by liquid crystal biosensor based on whispering gallery mode

In this work, a liquid crystal (LC) biosensor based on whispering gallery mode (WGM) lasing is reported and demonstrated for real-time and high-sensitive detection of acetylcholinesterase (AChE) and its inhibitors. Benefit from the double amplification by the WGM resonance and the 5CB molecules, small changes in biological processes are significantly enhanced in WGM spectral responses. The spectral responses provide direct information about the molecular adsorption/desorption at the LC/aqueous solution interface and can be used as an indicator of the enzymatic reaction. The limit of detection achieved was as low as 0.1 pg/mL for fenobucarb and 1 pg/mL for dimethoate, which is considerably lower than the standard levels of pesticides specified for water quality standards. Results indicate that this versatile method holds prospects for application in real-time and high-sensitive monitoring of biochemical reactions and can serve as an alternative solution to compensate for missing capabilities in conventional POM observations.

A novel, label-free liquid crystal biosensor for Parkinson's disease related alpha-synuclein.

A liquid crystal biosensor based on DNA aptamer for sensitive detection of Parkinson's Disease (PD) related alpha-synuclein was developed. This LC biosensor is constructed using a simple and label free method, and it not only enables early PD diagnosis, but also provides a general platform for detection based on DNA aptamer.

Sensing of cocaine using polarized optical microscopy by exploiting the conformational changes of an aptamer at the water/liquid crystal interface.

Liquid crystals (LCs) have the ability to transduce and amplify a molecular stimulus into optical signals due to their elastic and birefringence properties. An aptamer-based LC sensor for cocaine is described here. 3-Morpholinopropanesulfonic acid with amphipathic structure was used to establish recognition sites at a water/LC interface for the detection of cocaine. The cocaine-binding aptamer is formed at the interface. The conformation of the aptamer undergoes a change on binding cocaine, and this triggers the LCs anchoring transition from homeotropic to planar. Binding can also be detected by polarized optical microscopy. The fluorescence spectroscopy and circular dichroism results are used to prove that the conformation of aptamer changed from a hairpin structure to a special three-way junction structure on binding of cocaine at the interface. The assay works in the 1nM to 10μM cocaine concentration range and is specific. Graphical abstract Schematic representation of aptamer-based liquid crystal (LC) biosensor for the detection of cocaine. In this interface biosensing system, after the aptamer binding with cocaine, the conformation of aptamer at the aqueous/LC interface was changed from a hairpin structure to a special three-way junction structure. This triggered the Liquid crystals (LCs) anchoring transition from homeotropic to planar and the sign-on optical signal could be obtained by polarizing optical microscope (POM) in real-time.

Label-free, color-indicating, and sensitive biosensors of cholesteric liquid crystals on a single vertically aligned substrate.

Biosensors based on liquid crystal (LC) materials can be made by employing the sensitive interfacial effect between LC molecules and alignment layers on substrates. In the past, the optical texture observation method was used in the LC biosensor field. However, the method is limited by a complicated fabrication process and quantitative reproducibility of results that bv evidence that both the reliability and accuracy of LC biosensors need to be improved. In this report, we demonstrate that cholesteric LC (CLC) cells in which one substrate is coated with a vertically aligned layer can be used as a new sensing technology. The chirality of the single vertically anchored (SVA)/CLC biosensor was tested by detecting bovine serum albumin (BSA), a protein standard commonly used in the lab. The colors and corresponding spectrum of the SVA/CLC biosensor changed with the BSA concentrations. A detection limit of 1 ng/ml was observed for the SVA/CLC biosensor. The linear optical properties of the SVA/CLC biosensor produced cheap, inexpensive, and color-indicating detection of biomolecules, and may promote the technology of point-of-care devices for disease-related biomarker detection.

Fiber Mach-Zehnder-interferometer-based liquid crystal biosensor for detecting enzymatic reactions of penicillinase.

A novel, to the best of our knowledge, liquid crystal (LC) biosensor, based on an optical fiber Mach-Zehnder interferometer (MZI), is proposed. The proposed optical fiber MZI consists of two single-mode fibers and a tapered photonic crystal fiber (PCF). The PCF is coated with 4'-pentyl-biphenyl-4-carboxylic acid (PBA)-doped 4-cyano-4'-pentylbiphenyl (5CB). Being a pH-sensitive material, PBA can manipulate LC molecules to different orientations according to their pH values. When the orientation of LC molecules changes with varying pH, the effective refractive index of the cladding modes also is accordingly affected. Enzymatic reactions of penicillinase can release H+, which causes the decrease of the pH. Therefore, the enzymatic reactions of penicillinase can be sensed by monitoring the peak shift in the interference spectrum. The effects of the tapered diameter on the sensitivity of the sensor were experimentally investigated as well.

Gold nanoparticle-mediated signal amplification of liquid crystal biosensors for dopamine.

A unique design strategy was developed for the detection of dopamine using a newly synthesized amphiphile containing boronic acid head group at the aqueous-liquid crystal (LC) interface. The optical signal of LC for the detection of dopamine was highly amplified in the presence of functionalized gold nanoparticles.

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.

Determination of alkaline phosphatase activity and of carcinoembryonic antigen by using a multicolor liquid crystal biosensor based on the controlled growth of silver nanoparticles

An ultrasensitive liquid crystal biosensor is described for multicolor visualization of the activity of alkaline phosphatase (ALP) based on the controlled growth of silver nanoparticles. The enzymatic product is accumulated on the surface of the LC sensing film by means of silver deposition, and the birefringent signal (observed with a polarizing microscope) is strongly enhanced as a result. The presence of AuNPs also enhances the sensitivity by about 4 orders of magnitude. The bright spots in polarized optical microscopy (POM) images increase with increasing activities of ALP. The signal intensities of the spots are then calculated by using Photoshop software and by multiplying the average brightness of the spots by the pixel value. The detection limit for ALP is 1.2 nU·mL-1, which is 5-7 orders of magnitude lower than other colorimetric or fluorometric methods. The method was applied to a highly sensitive immunoassay for the carcinoembryonic antigen (CEA) by integrating immunomagnetic separation. The immunoassay was applied to the analysis of complex samples without tedious sample pretreatment, and a detection limit as low as 0.35pg·mL-1 of CEA was achieved. The method has attractive features in that it provides an ultrasensitive multicolor visualization approach for enzymes such as ALP, but also paves the way to a new kind of immunoassay coupled to immunomagnetic separation. Graphical abstract A signal enhanced liquid crystal (LC)-based multicolor immunosensor is describedthat is based on immunomagnetic separation and biometallization. Alkaline phosphatase (ALP) and carcinoembryonic antigen (CEA) can be easily visualized by bare eyes using the polarized optical microscopy (POM) images of LCs.

Sensitive and selective method for detecting cysteine based on optical properties of liquid crystal

We demonstrate a novel strategy to detect cysteine using a liquid crystal (LC) biosensor at an aqueous interface. We synthesized a novel biosurfactant, which was effectively exploited to design an optical biosensor based on treated LC. Lauroyl-doped 5CB was used as a cysteine indicator based on the reaction between lauroyl chloride and cysteine at the aqueous/LC interface. Following the reaction, the orientation of LCs succeeded in a four-petal shape, as observed using a polarizing optical microscope where the optical signals received (as derived from the LCs orientation changes) were attributed to the newly formed biosurfactant. Additionally, the prepared biosensor has a LOD (limit of detection) of cysteine at the picomolar level (pM), better than that LOD which can be achieved by conventional methods These results suggest that use of a LC biosensor is an appropriate approach for cysteine detection, particularly for sensing low levels of cysteine in biological environments.

Label-free liquid crystal biosensor for cecropin B detection

A label-free liquid crystal (LC) biosensor based on orientation changes of LC molecules was reported for the detection of cecropin B. The homeotropic-to-tilted alignment transition of LC molecules, induced by the specific binding event between cecropin B and anti-cecropin B antibody immobilized via glutaraldehyde, could result in obvious change of the optical appearance from a dark to a bright response and as a result, the detection limit of cecropin B was as low as 50 ng/mL. The average gray-scale intensities (GIs) of optical appearances were calculated to quantitatively analyse cecropin B concentrations. This study offers a simple, highly sensitive and specific, lable-free method for cecropin B detection.

Series of In Situ Photoinduced Polymer Graftings for Sensitive Detection of Protein Biomarkers via Cascade Amplification of Liquid Crystal Signals.

Developing of new polymeric materials for the sensitive and rapid detection of trace protein biomarkers has attracted increasing attention in biomedical fields. Herein, series of in situ photoinduced polymer graftings were developed for sensitive detection of protein biomarkers by using featured cascade amplification of liquid crystal (LC) signals. The limit-of-detection (LOD) for native bovine serum albumin (BSA) molecules is around 10 μg/mL in a LC biosensor before signal amplification. Upon the cascade amplification using surface-grafted polymers, poly[poly(ethylene glycol) methacrylate] grafting ( s-P(PEGMA)) exhibits superior amplification ability (104-fold lower than native BSA) than the other two graftings of poly(2-hydroxyethyl methacrylate) ( s-PHEMA) and poly(methacrylic acid) ( s-PMAA; 102-fold lower than native BSA). The contact angles of water and LC on the s-P(PEGMA) grafting show significant difference in comparison with s-PHEMA and s-PMAA graftings ( p < 0.05), implying interfacial energies of the grafted polymers may dictate the orientational transition of LCs. The clinical urine samples collected from the patients with proteinuria were also used to confirm the feasibility of the polymer-amplified LC sensors for practical protein assays. The present work reveals that in situ photoinduced polymer grafting is one promising method to amplify the signals of LC biosensors for the rapid and sensitive detection of trace protein biomarkers.

Detection of pulmonary surfactant protein A by using an aptamer-based liquid crystal biosensor

An aptamer-based liquid crystal biosensor for the detection of pulmonary surfactant protein A has been presented.

Liquid-crystal droplets functionalized with a non-enzymatic moiety for glucose sensing

3-Aminophenyl boronic acid (APBA)-decorated 4-cyano-4′-pentylbiphenyl (5CB) microdroplets were utilized for glucose detection with high specificity and sensitivity. APBA decoration was performed by firstly coating poly(acrylic acid) (PAA) on the 5CB droplet with amphiphilic poly(4-cyanobiphenyl-4′-undecylacrylate-b-acrylicacid) (LCP-b-PAA) and performing covalent bonding of APBA to the PAA chains by EDC coupling between the acrylic groups of PAA and the amine groups of APBA. The binding events between APBA and glucose were translated by configurational change (from radial to bipolar) of the 5CB droplets, observed under a polarized optical microscope with crossed polarizers. The liquid crystal (LC) droplet-based non-enzymatic (NE) biosensor for glucose detection exhibited high sensitivity (detection limit of 0.05mM) even in complex serum sample, high selectivity against cholesterol, uric acid, and acetaminophen, and good stability for 30 d. This NE-based glucose LC biosensor is cost-effective and highly stable compared to enzyme-based LC glucose biosensors and thus, could replace such enzyme-based biosensors.

Label-free liquid crystal biosensor for L-histidine: A DNAzyme-based platform for small molecule assay

We have developed a novel DNAzyme-based liquid crystal (LC) biosensor with high sensitivity for L-histidine, which is based on L-histidine-mediated formation of DNA duplexes by cleaving DNAzyme using L-histidine, resulting in a remarkable optical signal. Firstly, an optimal amount of capture probe is bound to the glass slide, which changes the surface topology as little as possible and shows a zero-background for the sensing system. When the DNAzyme molecule is cleaved by the target, L-histidine, a partial substrate strand is produced, which in turn can hybridize with the capture probe, forming a DNA duplex. The DNA duplexes induce LC molecules to undergo a homeotropic-to-tiled transition, obtaining a remarkable optical signal. The results show that the DNAzyme-based LC biosensor is highly sensitive to L-histidine with a detection limit of 50nM. Compared with previously reported multi-step amplified methods, this newly designed assay system for L-histidine has no amplified procedures with comparable sensitivity. This method is an unprecedented example of DNAzyme-based LC biosensor for small molecules, which has potential to offer a DNAzyme-based LC model used in various targets.