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

Analyte-mediated formation and growth of nanoparticles for the development of chemical sensors and biosensors

The cornerstone of nanomaterial-based sensing systems is the synthesis of nanoparticles with appropriate surface functionalization that ensures their stability and determines their reactivity with organic or inorganic analytes. To accomplish these requirements, various compounds are used as additives or growth factors to regulate the properties of the synthesized nanoparticles and their reactivity with the target analytes. A different rationale is to use the target analytes as additives or growth agents to control the formation and properties of nanoparticles. The main difference is that the analyte recognition event occurs before or during the formation of nanoparticles and it is based on the reactivity of the analytes with the precursor materials of the nanoparticles (e.g., metal ions, reducing agents, and coatings). The transition from the ionic (or molecular) state of the precursor materials to ordered nanostructured assemblies is used for sensing and signal transduction for the qualitative detection and the quantitative determination of the target analytes, respectively. This review focuses on assays that are based on analyte-mediated regulation of nanoparticles' formation and differentiate them from standard nanoparticle-based assays which rely on pre-synthesized nanoparticles. Firstly, the principles of analyte-mediated nanomaterial sensors are described and then they are discussed with emphasis on the sensing strategies, the signal transduction mechanisms, and their applications. Finally, the main advantages, as well as the limitations of this approach, are discussed and compared with assays that rely on pre-synthesized nanoparticles in order to highlight the major advances accomplished with this type of nano-sensors and elucidate challenges and opportunities for further evolving new nano-sensing strategies.

Technological Advancements in Bio‐recognition using Liquid Crystals: Techniques, Applications, and Performance

The application of liquid crystal (LC) materials has undergone a modern-day renaissance from its classical use in electronics industry as display devices to new-fangled techniques for optically detecting biological and chemical analytes. This review article deals with the emergence of LC materials as invaluable material for their use as label-free sensing elements in the development of optical, electro-optical and electrochemical biosensors. The property of LC molecules to change their orientation on perturbation by any external stimuli or on interaction with bioanalytes or chemical species has been utilized by many researches for the fabrication of high sensitive LC-biosensors. In this review article we categorized LC-biosensor based on biomolecular reaction mechanism viz. enzymatic, nucleotides and immunoreaction in conjunction with operating principle at different LC interface namely LC-solid, LC-aqueous and LC-droplets. Based on bimolecular reaction mechanism, the application of LC has been delineated with recent progress made in designing of LC-interface for the detection of bio and chemical analytes of proteins, virus, bacteria, clinically relevant compounds, heavy metal ions and environmental pollutants. The review briefly describes the experimental set-ups, sensitivity, specificity, limit of detection and linear range of various viable and conspicuous LC-based biosensor platforms with associated advantages and disadvantages therein.

Simple and Label-Free Detection of Carboxylesterase and Its Inhibitors Using a Liquid Crystal Droplet Sensing Platform

In this study, we developed a liquid crystal (LC) droplet-based sensing platform for the detection of carboxylesterase (CES) and its inhibitors. The LC droplet patterns in contact with myristoylcholine chloride (Myr) exhibited dark cross appearances, corresponding to homeotropic anchoring of the LCs at the aqueous/LC interface. However, in the presence of CES, Myr was hydrolyzed; therefore, the optical images of the LC patterns changed to bright fan-shaped textures, corresponding to a planar orientation of LCs at the interface. In contrast, the presence of CES inhibitors, such as benzil, inhibits the hydrolysis of Myr; as a result, the LC patterns exhibit dark cross textures. This principle led to the development of an LC droplet-based sensing method with a detection limit of 2.8 U/L and 10 μM, for CES detection and its inhibitor, respectively. The developed biosensor not only enables simple and label-free detection of CES but also shows high promise for the detection of CES inhibitors.

A self-oriented beacon liquid crystal assay for kanamycin detection with AuNPs signal enhancement

The authors report a self-oriented beacon liquid crystal (LC) biosensor for kanamycin (Kana) detection with gold nanoparticle (AuNPs) signal enhancement. In this study, an assay was proposed for Kana detection using the aptamer as a self-oriented beacon. Without an additional orientation agent, the Kana aptamer was used as a self-oriented beacon both as an orientation agent for the LCs and as a signal recognition probe for biological molecules. Gold nanoparticles are blended with desired concentrations of the target molecules, which can greatly improve the performance of the biosensor. In the presence of Kana, AuNPs-Kana-aptamer conjugates will form on the sensing interface of the biosensor, which can remarkably destroy the orientated arrangement of the LCs, resulting in changes in the corresponding polarized images of the LCs. The limit of Kana detection is as low as 0.1 pmol L^-1. It is important to note that the self-oriented beacons are immobilized on the assembled film of the glass slides for the specific recognition of Kana, simultaneously allowing the homeotropic orientation of the LCs. This study also provides a mechanism for the self-orientation beacon and liquid crystal biosensing.

Hierarchical engineering of Mn2O3/carbon nanostructured electrodes for sensitive screening of acetylcholine in biological samples

Enzymeless electrochemical sensors have received considerable interest for the direct, sensitive, and selective monitoring of biomolecules in a complex biological environment.

Enzymeless copper microspheres@carbon sensor design for sensitive and selective acetylcholine screening in human serum

Follow up of neuronal disorders, such as Alzheimer's and Parkinson's diseases using simple, sensitive, and selective assays is urgently needed in clinical and research investigation. Here, we designed a sensitive and selective enzymeless electrochemical acetylcholine sensor that can be used in human fluid samples. The designed electrode consisted of a micro spherical construction of Cu-metal decorated by a thin layer of carbon (CuMS@C). A simple and one-pot synthesis approach was used for Cu-metal controller formation with a spherical like structures. The spherical like structure was formed with rough outer surface texture, circular build up, homogeneous formation, micrometric spheres size (0.5 -1 µm), and internal hollow structure. The formation of a thin layer of carbon materials on the surface of CuMS sustained the catalytic activity of Cu atoms and enriched negatively charge of the surface. CuMS@C acted as a highly active mediator surface that consisted of Cu metal as a highly active catalyst and carbons as protecting, charge transport, and attractive surface. Therefore, the CuMS@C surface morphology and composition played a key role in various aspects such as facilitated ACh diffusion/loading, increased the interface surface area, and enhanced the catalytic activity. The CuMS@C acted as an electroactive catalyst for ACh electrooxidation and current production, due to the losing of two electrons. The fabricated CuMS@C could be a highly sensitive and selective enzymeless sensor for detecting ACh with a detection limit of 0.1 µM and a wide linear range of 0.01 - 0.8 mM. The designed ACh sensor assay based on CuMS@C exhibited fast sensing property as well as sensitivity, selectivity, stability, and reusability for detecting ACh in human serum samples. This work presents the design of a highly active electrode surface for direct detection of ACh and further clinical investigation of ACh levels.

Nanoparticle-based colorimetric sensors to detect neurodegenerative disease biomarkers

Neurodegenerative disorders (NDDs) are progressive, incurable health conditions that primarily affect brain cells, and result in loss of brain mass and impaired function. Current sensing technologies for NDD detection are limited by high cost, long sample preparation, and/or require skilled personnel. To overcome these limitations, optical sensors, specifically colorimetric sensors, have garnered increasing attention towards the development of a cost-effective, simple, and rapid alternative approach. In this review, we evaluate colorimetric sensing strategies of NDD biomarkers (e.g. proteins, neurotransmitters, bio-thiols, and sulfide), address the limitations and challenges of optical sensor technologies, and provide our outlook on the future of this field.

A portable digital optical kanamycin sensor developed by surface-anchored liquid crystal droplets

There is an increase in demand to develop simple, convenient, and low-cost approaches for rapid and label-free detection of antibiotics. Herein, we propose a new principle for the detection of kanamycin using the surface-anchored liquid crystal (LC) droplets. The optical images of the LC droplets uniformly change from four-clover, uniformly dark, and dark cross appearance gradually with the increase of surfactant concentration. The detection of kanamycin is fulfilled with the aid of a cationic surfactant cetyltrimethylammonium bromide (CTAB) and a kanamycin aptamer. The LC droplets show uniformly dark appearance and four-clover appearance in the presence of the aqueous solutions of CTAB and CTAB/aptamer complex, respectively. However, the specific binding of kanamycin to its aptamer can release the CTAB, which induces the uniformly dark appearance of the LC droplets. A portable device is built to measure the optical luminance of the LC droplets. This system can detect kanamycin with a concentration below 0.1 ng/mL (~0.17 nM) and also allows the detection of kanamycin in real samples such as milk and honey. Therefore, it is very promising in the development of new types of LC-based sensors by the surface-anchored LC droplets assisted with a portable optical device.

A Single-Substrate Biosensor with Spin-Coated Liquid Crystal Film for Simple, Sensitive and Label-Free Protein Detection

A liquid crystal (LC)-based single-substrate biosensor was developed by spin-coating an LC thin film on a dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (DMOAP)-decorated glass slide. Compared with the conventional sandwiched cell configuration, the simplified procedure for the preparation of an LC film allows the film thickness to be precisely controlled by adjusting the spin rate, thus eliminating personal errors involved in LC cell assembly. The limit of detection (LOD) for bovine serum albumin (BSA) was lowered from 10-5 g/mL with a 4.2-μm-thick sandwiched cell of the commercial LC E7 to 10-7 g/mL with a 4.2-μm-thick spin-coated E7 film and further to 10-8 g/mL by reducing the E7 film thickness to 3.4 μm. Moreover, by exploiting the LC film of the highly birefringent nematic LC HDN in the immunodetection of the cancer biomarker CA125, an LOD comparable to that determined with a sandwiched HDN cell was achieved at 10-8 g/mL CA125 using a capture antibody concentration an order of magnitude lower than that in the LC cell. Our results suggest that employing spin-coated LC film instead of conventional sandwiched LC cell provides a more reliable, reproducible, and cost-effective single-substrate platform, allowing simple fabrication of an LC-based biosensor for sensitive and label-free protein detection and immunoassay.

Enzyme inhibition methods based on Au nanomaterials for rapid detection of organophosphorus pesticides in agricultural and environmental samples: A review

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The review systematically and completely collated the enzyme inhibition method based on Au nanomaterials for organophosphorus pesticide detection method in the last 20 years.

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The significance of the optical properties of Au nanomaterials is outlined with different shapes, sizes, and surface modifiers in enzyme inhibition methods.

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The principles, classification and application of enzyme inhibition methods based on Au nanomaterials are comprehensively summarized from a new perspective in agricultural and environmental samples, including colorimetric method, fluorometric method, electrochemical biosensor method.

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Unlike traditional enzyme inhibition method, the merits of enzyme inhibition method based on Au nanomaterials were elaborated in this review.

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Combined with the research progress of enzyme inhibition method, this review predicts the future research direction of enzyme inhibition method, providing a theoretical reference for researchers.

Background

Organophosphorus pesticides (OPs), as insecticides or acaricides, are widely used in agricultural products to ensure agricultural production. However, widespread use of OPs leads to environmental contamination and significant negative consequences on biodiversity, food security, and water resources. Therefore, developing a sensitive and rapid method to determine OPs residues in different matrices is necessary. Originally, the enzyme inhibition methods are often used as preliminary screens of OPs in crops. Many studies on the characteristic of Au nanomaterials have constantly been emerging in the past decade. Combined with anisotropic Au nanomaterials, enzyme inhibition methods have the advantages of high sensitivity, durability, and high stability.

Aim of Review

This review aims to summarize the principles and strategies of gold (Au) nanomaterials in enzyme inhibition methods, including colorimetric (dispersion, particle size of Au nanomaterials) and fluorometric (fluorescence energy transfer, internal filtration effect) detection, and electrochemical sensing system (shape of Au nanomaterials, Au nanomaterials combined with other nanomaterials). The application of enzyme inhibition in agricultural products and research progress was also outlined. Next, this review illustrates the advantages of Au nanomaterial-based enzyme inhibition methods compared with conventional enzyme inhibition methods. The detection limits and linear range of colorimetric and fluorometric detection and electrochemical biosensors have also been provided. At last, key perspectives, trends, gaps, and future research directions are proposed.

Key Scientific Concepts of Review

Herein, we introduced the technology of enzyme inhibition method based on Au nanomaterials for onsite and infield rapid detection of organophosphorus pesticide.

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.

Screening of Xanthine Oxidase Inhibitors by Liquid Crystal-Based Assay Assisted with Enzyme Catalysis-Induced Aptamer Release

Small-molecule drugs play an important role in the treatment of various diseases. The screening of enzyme inhibitors is one of the most important means in developing therapeutic drugs. Herein, we demonstrate a liquid crystal (LC)-based screening assay assisted with enzyme catalysis-induced aptamer release for screening xanthine oxidase (XOD) inhibitors. The oxidation of xanthine by XOD prevents the specific binding of xanthine and its aptamer, which induces a bright image of LCs. However, when XOD is inhibited, xanthine specifically binds to the aptamer. Correspondingly, LCs display a dark image. Three compounds are identified as potent XOD inhibitors by screening a small library of triazole derivatives using this method. Molecular docking verifies the occupation of the active site by the inhibitor, which also exhibits excellent biocompatibility to HEK293 cells and HeLa cells. This strategy takes advantages of the unique aptamer-target binding, specific enzymatic reaction, and simple LC-based screening assay, which allows high-throughput and label-free screening of inhibitors with high sensitivity and remarkable accuracy. Overall, this study provides a competent and promising approach to facilitate the screening of enzyme inhibitors using the LC-based assay assisted with the enzyme catalysis-induced aptamer release.

Biothiol modulated growth and aggregation of gold nanoparticles and their determination in biological fluids using digital photometry

This work describes a novel and easy to use method for the determination of biologically important thiols that relies on their ability to inhibit the catalytic enlargement of AuNP seeds in the presence of ACl₄^- ions and trigger their aggregation. UV-vis spectroscopic monitoring of the plasmon resonance bands of the formed AuNPs showed that the spectral and color transitions depend both on the concentration and the structure of biothiols. The colorimetric changes induced by biothiols were quantified in the concentration range from 5 to 300 μM in the RGB color system with digital photometry using a commercially available flatbed scanner as detector. On the basis of these results, the applicability of the method was tested to the determination of glutathione in red blood cells and cysteine in blood plasma with satisfactory recoveries (88.7-96.5%), low detection limits (1.0 μM), good selectivity against major biomolecules under physiologically relevant conditions and satisfactory reproducibility (<8%). The method requires minimum technical expertise, is easy to use and is performed without scientific equipment, holding promise as a simple assay of biothiol testing even by non-experts.

An acetylcholinesterase-based biosensor for the detection of pesticides using liquid crystals confined in microcapillaries

Here, we demonstrate a capillary-sensing platform based on liquid crystals (LCs) confined in microcapillaries for simple and sensitive detection of acetylcholinesterase (AChE) and its inhibitors. LC droplets were formed through sequential injection of LCs and an aqueous solution into trichloro(octyl)silane (OTS)-treated microcapillaries. When the confined LC droplets make contact with a cationic surfactant solution, myristoylcholine chloride (Myr), the formation of a Myr monolayer at the aqueous/LC interface induces a horizontal orientation of the LCs at the interface along the microcapillary, producing an optical LC droplet texture of a four-petal shape. On the other hand, AChE can catalyze the hydrolysis of Myr into choline and myristic acid. The hydrolyzed Myr is unable to form a monolayer at the aqueous/LC interface, and therefore the confined LC droplets exhibit two bright-lined optical images when in contact with the pre-incubated mixture of Myr and AChE, corresponding to the homeotropic orientation of LCs at the interface. However, in the presence of AChE-inhibiting pesticides, such as fenobucarb and malathion, the activity of AChE is inhibited, and thus, the enzymatic hydrolysis of Myr cannot occur. As a result, the confined LC droplets present the four petal-shaped optical images when in contact with the pre-incubated mixture of Myr, AChE, and pesticides. Based on this principle, an LC-based microcapillary sensor was developed and utilized for the detection of pesticides. Using this sensing platform, fenobucarb and malathion were detected at limits of 5 pg/mL and 2.5 pg/mL, respectively. Moreover, the proposed biosensor was successfully applied to the determination of pesticides in real river water. Therefore, this LC-based microcapillary sensor is a promising platform for simple, rapid, and label-free detection of pesticides with very high sensitivity.

One-Pot Synthesis of Thiol-Modified Liquid Crystals Conjugated Fluorescent Gold Nanoclusters

Gold nanoclusters (AuNCs) and liquid crystals (LCs) have shown great potential in nanobiotechnology applications due to their unique optical and structural properties. Herein, the hardcore of the 4-cyano biphenyl group for commonly used LCs of 4-cyano-4'-pentylbiphenyl (5CB) was utilized to synthesize 4'-(2-mercaptoethyl)-(1,1'-biphenyl)-4-carbonitrile (TAT-12) based on Suzuki coupling and Appel reaction. The structural and optical properties of thiol-modified TAT-12 LCs were demonstrated by nuclear magnetic resonance (NMR) spectroscopy, ultraviolet-visible (UV-vis) spectroscopy and differential scanning calorimetry (DSC). By one-pot synthesis, thiol-modified TAT-12 LCs were used as the ligands to prepare fluorescent gold nanoclusters (AuNCs@TAT-12) according to the Au-S bond between AuNCs and TAT-12. The spectra of UV-vis absorption and X-ray photoelectron spectroscopy (XPS) of AuNCs@TAT-12 indicated that the core of gold of AuNCs@TAT-12 exhibited high gold oxidation states. The fluorescence of AuNCs@TAT-12 was observed with a maximum intensity at ~352 nm coming from TAT-12 on AuNCs@TAT-12 and the fluorescence quantum yield of AuNCs@TAT-12 was calculated to be 10.1%. Furthermore, the fluorescence with a maximum intensity at ~448 nm was attributed to a ligand-metal charge transfer between the ligands of TAT-12 LCs and the core of AuNCs. The image of transmission electron microscopy (TEM) further demonstrated an approximately spherical shape of AuNCs@TAT-12 with an average size of 2.3 nm. A combination of UV-vis absorption spectra, XPS spectra, fluorescence spectra and TEM image, fluorescent AuNCs@TAT-12 were successfully synthesized via one-pot synthesis. Our work provides a practical approach to the synthesis of LCs conjugated AuNCs for future applications in nanobiotechnology.

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.

Acetylcholinesterase-catalyzed silver deposition for ultrasensitive electrochemical biosensing of organophosphorus pesticides

This work reports, for the first time, acetylcholinesterase-catalyzed silver deposition for sensitive electrochemical detection of organophosphorus pesticides.

A liquid-crystal-based immunosensor for the detection of cardiac troponin I

Cardiac troponin I (cTnI) is one of the most sensitive and specific markers of myocardial cell injury. In this study, a label-free biosensor that utilizes the birefringence property of liquid crystal (LC) for the detection of cTnI is demonstrated.

Perspectives in Liquid-Crystal-Aided Nanotechnology and Nanoscience

The research field of liquid crystals and their applications is recently changing from being largely focused on display applications and optical shutter elements in various fields, to quite novel and diverse applications in the area of nanotechnology and nanoscience. Functional nanoparticles have recently been used to a significant extent to modify the physical properties of liquid crystals by the addition of ferroelectric and magnetic particles of different shapes, such as arbitrary and spherical, rods, wires and discs. Also, particles influencing optical properties are increasingly popular, such as quantum dots, plasmonic, semiconductors and metamaterials. The self-organization of liquid crystals is exploited to order templates and orient nanoparticles. Similarly, nanoparticles such as rods, nanotubes and graphene oxide are shown to form lyotropic liquid crystal phases in the presence of isotropic host solvents. These effects lead to a wealth of novel applications, many of which will be reviewed in this publication.

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.

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

A new design was developed for detection of dopamine using a boronic acid based amphiphile at aqueous–liquid crystal interface. The detection was highly enhanced in presence of gold nanoparticles.

Fabrication of Liquid-Crystal-Based Optical Sensing Platform for Detection of Hydrogen Peroxide and Blood Glucose

Rapid and accurate determination of H₂O₂ is of great importance in practical applications. In this study, we demonstrate construction of liquid-crystal (LC)-based sensing platforms for sensitive and real-time detection of H₂O₂ with high accuracy for the first time. Single-stranded DNA (ssDNA) adsorbed onto the surface of nanoceria is released to the aqueous solution in the presence of H₂O₂, which disrupts arrangement of the self-assembled cationic surfactant monolayer decorated at the aqueous/LC interface. Thus, the orientation of LCs changes from a homeotropic to planar state, leading to change in the optical response from dark-to-bright appearance. As H₂O₂ can be produced during oxidation of glucose by glucose oxidase (GOx), detection of glucose is also fulfilled by employing the H₂O₂ sensing platform. Our system can detect H₂O₂ and glucose with concentrations as low as 28.9 nM and 0.52 μM, respectively. It shows high promise of using LC-based sensors for the detection of H₂O₂ and its relevant biomarkers in practical applications.

A novel logic gate based on liquid-crystals responding to the DNA conformational transition

Described herein is a novel liquid crystal (LC)-based DNA logic gate constructed via employing the reorientation of LCs triggered by metal-ion-mediated DNA probe conformational changes.

Conjugation of Nanomaterials and Nematic Liquid Crystals for Futuristic Applications and Biosensors

The established role of nematic liquid crystals (NLCs) in the recent rapid development of displays has motivated researchers to modulate the electro-optical properties of LCs. Furthermore, adding nanomaterials into NLCs has led to enhancements of the properties of NLCs, like reduced threshold of the operating voltage, variation in pretilt angle, reduced switching time, etc. These enhanced properties, due to interfacial dynamics, are enabling wider applications of NLCs and nanomaterials. The recent literature of nanomaterial-doped NLCs is rich with various kinds of nanomaterials in a variety of NLCs. The light has been focused on the most widely used and studied gold nanoparticles in NLCs. The intrinsic inherent property of easy excitation of surface plasmons polaritons (SPP) is the mediating interaction of NLC electric dipoles and the polarization of charges in the GNP surface. The concepts and methods for the application of metal nanomaterials as dopants in NLCs are discussed for future applications, especially biosensors. The biosensing application of NLCs alone has already been proven in the literature. However, it is always desirable to further enhance the detection efficiency and selectivity, which have been achieved by the conjugation of GNPs and nickel nanoparticles with NLCs and their compatibility with biological materials. This aspect of future application of nanoparticles and NLC makes the point more selective to be included in the present manuscript.

Spotlight on Biomimetic Systems Based on Lyotropic Liquid Crystal

The behavior of lyotropic biomimetic systems in drug delivery was reviewed. These behaviors are influenced by drug properties, the initial water content, type of lyotropic liquid crystals (LLC), swell ability, drug loading rate, the presence of ions with higher or less kosmotropic or chaotropic force, and the electrostatic interaction between the drug and the lipid bilayers. The in vivo interaction between LCC-drugs, and the impact on the bioavailability of drugs, was reviewed. The LLC with a different architecture can be formed by the self-assembly of lipids in aqueous medium, and can be tuned by the structures and physical properties of the emulsion. These LLC lamellar phase, cubic phase, and hexagonal phase, possess fascinating viscoelastic properties, which make them useful as a dispersion technology, and a highly ordered, thermodynamically stable internal nanostructure, thereby offering the potential as a sustained drug release matrix for drug delivery. In addition, the biodegradable and biocompatible nature of lipids demonstrates a minimum toxicity and thus, they are used for various routes of administration. This review is not intended to provide a comprehensive overview, but focuses on the advantages over non modified conventional materials and LLC biomimetic properties.

A novel enzyme-mediated gold nanoparticle synthesis and its application forin situdetection of horseradish peroxidase inhibitor phenylhydrazine

Biocatalyzed gold nanoparticle synthesis for thein situdetection of horseradish peroxidase inhibitor (phenylhydrazine) has been demonstrated.

Thermotropic liquid crystal films for biosensors and beyond

Recent results on structural properties and possible bio-sensing applications of planar liquid crystal films are reviewed.

Liquid crystals based sensing platform-technological aspects

In bulk phase, liquid crystalline molecules are organized due to non-covalent interactions and due to delicate nature of the present forces; this organization can easily be disrupted by any small external stimuli. This delicate nature of force balance in liquid crystals organization forms the basis of Liquid-crystals based sensing scheme which has been exploited by many researchers for the optical visualization and sensing of many biological interactions as well as detection of number of analytes. In this review, we present not only an overview of the state of the art in liquid crystals based sensing scheme but also highlight its limitations. The approaches described below revolve around possibilities and limitations of key components of such sensing platform including bottom substrates, alignments layers, nature and type of liquid crystals, sensing compartments, various interfaces etc. This review also highlights potential materials to not only improve performance of the sensing scheme but also to bridge the gap between science and technology of liquid crystals based sensing scheme.

Liquid-Crystal Biosensor Based on Nickel-Nanosphere-Induced Homeotropic Alignment for the Amplified Detection of Thrombin

A new liquid-crystal (LC)-based sensor operated by nickel nanosphere (NiNS)-induced homeotropic alignment for the label-free monitoring of thrombin was reported. When doped with NiNSs, a uniform vertical orientation of 4-cyano-4'-pentylbiphenyl (5CB) was easily obtained. A sandwich system of aptamer/thrombin/aptamer-functionalized gold nanoparticles (AuNPs) was fabricated, and AuNPs-aptamer conjugation caused the disruption of the 5CB orientation, leading to an obvious change of the optical appearance from a dark to a bright response to thrombin concentrations from 0.1 to 100 nM. This design also allowed quantitative detection of the thrombin concentration. This distinctive and sensitive thrombin LC sensor provides a new principle for building LC-sensing systems.

Recent advances in biosensors based on enzyme inhibition

Enzyme inhibitors like drugs and pollutants are closely correlated to human and environmental health, thus their monitoring is of paramount importance in analytical chemistry. Enzymatic biosensors represent cost-effective, miniaturized and easy to use devices; particularly biosensors based on enzyme inhibition are useful analytical tools for fast screening and monitoring of inhibitors. The present review will highlight the research carried out in the last 9 years (2006-2014) on biosensors based on enzyme inhibition. We underpin the recent advances focused on the investigation in new theoretical approachs and in the evaluation of biosensor performances for reversible and irreversible inhibitors. The use of nanomaterials and microfluidic systems as well as the applications of the various biosensors in real samples is critically reviewed, demonstrating that such biosensors allow the development of useful devices for a fast and reliable alarm system.

A cationic surfactant-decorated liquid crystal sensing platform for simple and sensitive detection of acetylcholinesterase and its inhibitor

In this paper, construction of the liquid crystal (LC)-based sensing platform for simple and sensitive detection of acetylcholinesterase (AChE) and its inhibitor using a cationic surfactant-decorated LC interface was demonstrated. A change of the optical images of LCs from bright to dark appearance was observed when the cationic surfactant, myristoylcholine chloride (Myr), was transferred onto the aqueous/LC interface, due to the formation of a stable surfactant monolayer at the interface. A dark-to-bright change of the optical appearance was then observed when AChE was transferred onto the Myr-decorated LC interface. The sensitivity of this new type of LC-based sensor is 3 orders of magnitude higher in the serum albumin solution than that only in the buffer solution. Noteworthy is that the AChE LC sensor shows a very high sensitivity for the detection of the enzyme inhibitor, which is around 1 fM. The constructed low-cost LC-based sensor is quite simple and convenient, showing high promise for label-free detection of AChE and its inhibitors.

Colorimetric detection of microcystin-LR based on disassembly of orient-aggregated gold nanoparticle dimers

Recently we demonstrated oriented formation of gold nanoparticle (AuNP) dimers for ultrasensitive sensing oligonucleotides (J. Am. Chem. Soc. 2013, 135, 12338). Herein, we investigate the reverse process of this sensing mechanism using target analytes to disassemble the orient-aggregated AuNP dimers. This enables us to expand the analytes from oligonucleotides to other molecules, e.g. highly sensitive and selective determination of microcystin-LR (MC-LR) is selected for a demonstration in this work. Aptamers specific to the target molecules are used as linkers to prepare the AuNP dimers. In the presence of the target molecule, the aptamer changes its structure to bind the target molecule. Thus the pre-formed AuNP dimers are disassembled. As a result, the solution color is changed from blue to red. This sensing design retains the advantages of the previously developed sensors based on target molecules guided formation of AuNP dimers, e.g. the overwhelming sensitivity and stability comparing with those non-oriented sensors based on the formation of large aggregates, with the additional advantages as follows: 1) the target molecules are expanded from oligonucleotides to arbitrary molecules that can specifically bind to aptamers; 2) the color change is completed within 5 min, while the previous sensor based on the formation of AuNP dimers cost ~1 hour to obtain stable responses.