Liquid Crystal Biosensor Research Articles

Gold nanoparticle based signal enhancement liquid crystal biosensors for tyrosine assays

A novel liquid crystal (LC) biosensor based on seedless production of gold nanoparticles (AuNPs) has been developed for the detection of l-tyrosine (Tyr). In this study, tyrosinase (TR) stimulated the biocatalyzed hydroxylation of Tyr to l-DOPA, which mediated the generation and growth of AuNPs. The large-size of AuNPs greatly changed the surface topology of LC sensing interface, the corresponding images were regulated under MATLAB, which allows the quantitative detection of Tyr. Under the optimum conditions, the limit of Tyr detection could be as low as 2×10−7mol/L. Results also showed that phenylalanine (Phe), tryptophan (Trp) and threonine (Thr) caused no interference. This method provides a simple, label-free and sensitive strategy for determination of Tyr.

Liquid Crystal Biosensor Based on Cd<sup>2+</sup> Inducing the Bending of PS-Oligo for the Detection of Cadmium

In this study, a novel liquid crystal (LC) biosensor was developed for the highly sensitive and selective detection of Cd2+ based on Cd2+ inducing the bending of PS-oligo. This strategy makes use of the DNA conformational change to enhance the disruption of orientation of LC leading to an amplified optical signal. DNA containing-SH was bound on the glass slide of the LC cell modified with the DMOAP/APTES. The DMOAP can effectively induce the homeotropic alignment of LC. In the presence of Cd2+, Cd2+ can induce DNA to bend and become a 2 nm spherical structure, which can greatly disrupt the orientational arrangement of LC, resulting in the correspond changes of the optical image of LC cell birefringent under the polarizing microscope. When the Cd2+ concentration is low to 0.1 nM, the optical signal of LC biosensor has an obvious change. But in the absence of Cd2+, there is no orientational response of LC and the optical image under the polarizing microscope is still a uniform dark background. Thus, this LC sensing method has a sensitive and clear distinction between positive and negative results and offers a highly sensitive detection of Cd2+ with a low detection limit down to 0.1 nM.

Acetylcholinesterase liquid crystal biosensor for identification of AChE inhibitors by a reactivator

An improved acetylcholinesterase liquid crystal (LC) biosensor has been developed for the identification of organophosphates (OPs) by using a reactivator. When the acetylcholinesterases (AChEs) inhibited by different kinds of OPs are reactived by a reactivator, the catalytic activity of AChEs can be recovered with different activation efficiency because of the different phosphorylation structures formed in the inhibited AChEs. Accordingly, the reactived AChEs can catalyze the hydrolysis of acetylthiocholine to generate thiocholine product in different degrees, which will result in different catalytic growth of AuNPs and further form distinct orientational response of LCs. Based on such a reactivation mechanism, the AChE LC biosensor with a simple, rapid and visual procedure achieves an obvious identification of three OPs pesticides, methamidophos, trichlorfon and paraoxon, by using a pralidoxime reactivator.

Highly-sensitive liquid crystal biosensor based on DNA dendrimers-mediated optical reorientation

A novel highly-sensitive liquid crystal (LC) biosensing approach based on target-triggering DNA dendrimers was developed for the detection of p53 mutation gene segment at the LC–aqueous interface. In this study, the mutant-type p53 gene segment was the target to trigger the formation of DNA dendrimers from hairpin DNA probes by hybridization chain reaction, and the latter as a ‘signal enhancement element’ further induced the LC reorientation from tilted to homeotropic alignment, resulting in a corresponding optical changes of LC biosensors from birefringent to honeycombed textures or dark framework. The distinct optical reorientational appearances can serve as a characteristic signal to distinguish target concentrations ranging from 0.08nM to 8nM. Moreover, these optical phenomena suggest that the LC reorientation is related to the electric-dipole coupling between the adsorbed DNA and LC molecules, the conformational constraints of DNA and the internal electric field induction upon hybridization. This label-free LC biosensing strategy can open up a new platform for the sensitive detection of specific DNA sequences and enrich the application scope of an LC biosensing technique.

Aptamer-based Liquid Crystal Biosensor for Detection of Platelet-derived Growth Factor BB

Abstract A novel liquid crystal (LC) biosensor was developed for the detection of platelet-derived growth factor BB (PDGF-BB) based on the orientation changes of liquid crystals. One glass slide of the LC cell was first modified with the APTES/DMOAP ((3-aminopropyl)trimethoxysilane/N,N-dimethyl-N-octadecyl(3-aminopropyl) trimethoxysilyl chloride) self-assembled monolayer (SAM) to trigger the homeotropic alignment of LC molecules and thereby produced a black background optical image under the crossed polarized light, and then the specific aptamer of PDGF-BB was immobilized on SAM through glutaraldehyde crosslinking to construct a LC sensing substrate. In the presence of PDGF-BB, the stable triple helix structure of PDGF-BB aptamer was formed by the specific binding event and thus led to the target PDGF-BB was captured to the sensing substrate, making a visible optical change which could be observed under the crossed polarized light via the huge steric effect of the PDGF-BB on disrupting the LC alignment. However, in the absence of PDGF-BB, the low packing density of the PDGF-BB aptamer had little effect on disrupting the ordered alignment to the LC and caused the black background optical image which could also be observed under the crossed polarized light. An obvious optical change was observed when the concentration of target PDGF-BB was 5 nM. The proposed LC biosensing method permits the label-free detection of PDGF-BB with good selectivity and high sensitivity, making them sufficiently simple and particularly useful for low-cost screening bioassay performed away from central laboratories.

Self assembled monolayer based liquid crystal biosensor for free cholesterol detection

A unique cholesterol oxidase (ChOx) liquid crystal (LC) biosensor, based on the disruption of orientation in LCs, is developed for cholesterol detection. A self-assembled monolayer (SAM) of Dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (DMOAP) and (3-Aminopropyl)trimethoxy-silane (APTMS) is prepared on a glass plate by adsorption. The enzyme (ChOx) is immobilized on SAM surface for 12 h before utilizing the film for biosensing purpose. LC based biosensing study is conducted on SAM/ChOx/LC (5CB) cells for cholesterol concentrations ranging from 10 mg/dl to 250 mg/dl. The sensing mechanism has been verified through polarizing optical microscopy, scanning electron microscopy, and spectrometric techniques.

A liquid crystal biosensor for detecting organophosphates through the localized pH changes induced by their hydrolytic products

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.

Liquid Crystal Biosensors

The liquid crystal (LCD) biosensor is a new research area of sensor technology, which integrates the modern biotechnology and advanced electronic sensing technology. The feasibility and the principle of the LCD biosensor were mainly introduced in this paper. The chitosan films exhibited cholesteric phase LCD texture which can be seen by using the polarizing microscope (POM), and there is a main endothermic peak and exothermic peak of chitosan LCD solution in the DSC heating curve and cooling curve respectively. Chitosan maybe will be used as a sensing material by using its LCD characteristics. The development prospects are predicted.

Split Aptamer-Based Liquid Crystal Biosensor for ATP Assay

A novel liquid crystal (LC) biosensor based on the recombination of split aptamer chip was developed for the detection of adenosine triphosphate (ATP). One glass slide of the LC cell is first modified with the TEA/DMOAP mixed self-assembled monolayer (SAM) to ensure the homeotropic alignment of LC molecules and the black background of LC cell optical image under the crossed polarized light. The single-strand ATP aptamer is split into two fragments in this method. One of which, as capture probe, is covalently immobilized on the SAM, while another is used as the detection probe. In the presence of ATP, those two fragments will be combined with each other and exhibit the same role as the ATP aptamer. This binding event leads to a great enhancement in the optical signal of the LC biosensor due to the space size from small to big, which can effectively disrupt the orientational arrangement of LCs, resulting in the corresponding changes of optical images under the crossed polarized light. But in the absence of ATP, those two fragments can not be combined with each other, and as a result, there is no orientational response of LCs and the optical image under the crossed polarized light is still black. So the LC-based imaging method has a sensitive and clear distinction between positive and negative results. On the basis of such an inhibition mechanism the LC biosensor can be used as an effective way to realize the detection of ATP. When the concen- tration of analyte ATP below a critical value (10 nmol/L), there is no clear change in the optical image. The results showed that the detection limit of ATP is 10 nmol/L. This study provides a simple and sensitive ATP LC biosensing approach and offers effective strategies for the development of small molecules LC biosensors. Keywords liquid crystal; self-assembled film; biosensor; ATP; aptamer

Gold nanoparticle based signal enhancement liquid crystal biosensors for DNA hybridization assays

A novel signal enhanced liquid crystal biosensor based on using AuNPs for highly sensitive DNA detection has been developed. This biosensor not only significantly decreases the detection limit, but also offers a simple detection process and shows a good selectivity to distinguish perfectly matched target DNA from two-base mismatched DNA.

Acetylcholinesterase Liquid Crystal Biosensor Based on Modulated Growth of Gold Nanoparticles for Amplified Detection of Acetylcholine and Inhibitor

A novel acetylcholinesterase (AChE) liquid crystal (LC) biosensor based on enzymatic growth of gold nanoparticles (Au NPs) has been developed for amplified detection of acetylcholine (ACh) and AChE inhibitor. In this method, AChE mediates the hydrolysis of acetylthiocholine (ATCl) to form thiocholine, and the latter further reduces AuCl(4)(-) to Au NPs without Au nanoseeds. This process, termed biometallization, leads to a great enhancement in the optical signal of the LC biosensor due to the large size of Au NPs, which can greatly disrupt the orientational arrangement of LCs. On the other hand, the hydrolysis of ATCl is inhibited in the presence of ACh or organophosphate pesticides (OPs, a AChE inhibitor), which will decrease the catalytic growth of Au NPs and, as a result, reduce the orientational response of LCs. On the basis of such an inhibition mechanism, the AChE LC biosensor can be used as an effective way to realize the detection of ACh and AChE inhibitors. The results showed that the AChE LC biosensor was highly sensitive to ACh with a detection limit of 15 μmol/L and OPs with a detection limit of 0.3 nmol/L. This study provides a simple and sensitive AChE LC biosensing approach and offers effective signal enhanced strategies for the development of enzyme LC biosensors.

Three-dimensional structure and multistable optical switching of triple-twisted particle-like excitations in anisotropic fluids

Control of structures in soft materials with long-range order forms the basis for applications such as displays, liquid-crystal biosensors, tunable lenses, distributed feedback lasers, muscle-like actuators and beam-steering devices. Bistable, tristable and multistable switching of well-defined structures of molecular alignment is of special interest for all of these applications. Here we describe the facile optical creation and multistable switching of localized configurations in the molecular orientation field of a chiral nematic anisotropic fluid. These localized chiro-elastic particle-like excitations--dubbed 'triple-twist torons'--are generated by vortex laser beams and embed the localized three-dimensional (3D) twist into a uniform background. Confocal polarizing microscopy and computer simulations reveal their equilibrium internal structures, manifesting both skyrmion-like and Hopf fibration features. Robust generation of torons at predetermined locations combined with both optical and electrical reversible switching can lead to new ways of multistable structuring of complex photonic architectures in soft materials.

Computational modelling of nematic phase ordering by film and droplet growth over heterogeneous substrates

This paper presents a computational study of defect nucleation associated with the kinetics of the isotropic‐to‐nematic phase ordering transition over heterogeneous substrates, as it occurs in new liquid crystal biosensor devices, based on the Landau–de Gennes model for rod‐like thermotropic nematic liquid crystals. Two regimes are identified due to interfacial tension inequalities: (i) nematic surface film nucleation and growth normal to the heterogeneous substrate, and (ii) nematic surface droplet nucleation and growth. The former, known as wetting regime, leads to interfacial defect shedding at the moving nematic‐isotropic interface. The latter droplet regime, involves a moving contact line, and exhibits two texturing mechanisms that also lead to interfacial defect shedding: (a) small and large contact angles of drops spreading over a heterogeneous substrate, and (b) small drops with large curvature growing over homogeneous patches of the substrate. The numerical results are consistent with qualitative defect nucleation models based on the kinematics of the isotropic–nematic interface and the substrate–nematic–isotropic contact line. The results extend current understanding of phase ordering over heterogeneous substrates by elucidating generic defect nucleation processes at moving interfaces and moving contact lines.

Optical modeling of liquid crystal biosensors

Optical simulations of a liquid crystal biosensor device are performed using an integrated optical/textural model based on the equations of nematodynamics and two optical methods: the Berreman optical matrix method [J. Opt. Soc. Am. 62, 502 (1972)] and the discretization of the Maxwell equations based on the finite difference time domain (FDTD) method. Testing the two optical methods with liquid crystal films of different degrees of orientational heterogeneities demonstrates that only the FDTD method is suitable to model this device. Basic substrate-induced texturing process due to protein adsorption gives rise to an orientation correlation function that is nearly linear with the transmitted light intensity, providing a basis to calibrate the device. The sensitivity of transmitted light to film thickness, protein surface coverage, and wavelength is established. A crossover incident light wavelength close to lambda(co) approximately 500 nm is found, such that when lambda>lambda(co) thinner films are more sensitive to the amount of protein surface coverage, while for lambda<lambda(co) the reverse holds. In addition it is found that for all wavelengths the sensitivity increases with the amount of protein coverage. The integrated device model based on FDTD optical simulations in conjunction with the Landau-de Gennes nematodynamics model provides a rational basis for further progress in liquid crystal biosensor devices.

Regulation of the biological properties of medical polymer materials with the use of a gas-discharge plasma and vacuum ultraviolet radiation

The mechanisms of the interaction of a gas-discharge plasma and vacuum ultraviolet (VUV) radiation with medical polymers were considered. Various techniques were proposed for the use of plasma-chemical and photochemical processes for enhancing the biocompatibility of medical polymers. The plasma-chemical and photochemical processes for the surface functionalization and regulation of the biological characteristics of medical polymers by the immobilization of proteins, biologically active compounds, and liquid-crystal biosensors were described. The relationships of the surface processes that occur in the VUV photolysis of polymers with the kinetics of protein adsorption and the adhesion of blood cell components were analyzed.

Quenched disorder in a liquid-crystal biosensor: Adsorbed nanoparticles at confining walls

We analyze the response of a nematic liquid-crystal film, confined between parallel walls, to the presence of nanoscopic particles adsorbed at the walls. This is done for a variety of patterns of adsorption (random and periodic) and operational conditions of the system that can be controlled in experimental liquid-crystal-based devices. We compute simulated optical textures and the total optical output of the sensor between crossed polars, as well as the correlation function for the liquid-crystal tensor order parameter; we use these observables to discuss the gradual destruction of the original uniform orientation. For large concentrations of particles adsorbed in random patterns, the liquid crystal at the center of the sensor adopts a multidomain state, characterized by a small correlation length of the tensor order parameter, and also by a loss of optical anisotropy under observation through crossed polars. In contrast, for particles adsorbed in periodic patterns, the nematic at the center of the cell can remain in a monodomain orientation state, provided the patterns in opposite walls are synchronized.