Flow based immuno/bioassay and trends in micro-immuno/biosensors

Pulsatile Flow in Microfluidic Systems

This review describes the current knowledge and applications of pulsatile flow in microfluidic systems. Elements of fluid dynamics at low Reynolds number are first described in the context of pulsatile flow. Then the practical applications in microfluidic processes are presented: the methods to generate a pulsatile flow, the generation of emulsion droplets through harmonic flow rate perturbation, the applications in mixing and particle separation, and the benefits of pulsatile flow for clog mitigation. The second part of the review is devoted to pulsatile flow in biological applications. Pulsatile flows can be used for mimicking physiological systems, to alter or enhance cell cultures, and for bioassay automation. Pulsatile flows offer unique advantages over a steady flow, especially in microfluidic systems, but also require some new physical insights and more rigorous investigation to fully benefit future applications.

Chemiluminescence based immunoassay for the detection of heroin and its metabolites

Introduction: Continuous use of opiates causes drug-related illnesses, which poses an alarming situation to develop sensitive detection platform. In this study, a highly sensitive and reliable chemiluminescence immunoassay (CI) has been developed for the detection of heroin and its major metabolites in spiked urine samples. Methods: To develop robust immunoassay, monoacetyl morphine-bovine serum albumin (MAM-BSA) conjugate was synthesized and characterized thoroughly by physicochemical techniques. The anti-MAM antibodies were developed, labeled with horseradish peroxidase (HRP) and immunoassay was developed to detect the presence of target drug in spiked urine samples. Results: A competitive CI was developed, where heroin, MAM, morphine, and codeine concentration were ranged from 0-1000 ng/ mL in spiked urine samples and limit of detection were 80, 95, 90, 75 pg/ mL. Conclusion: The developed CI is highly sensitive, specific, point of care, cost-effective and can be used as a routine technique for quantitative analysis for screening of narcotic drugs.

Electrochemical Flow-ELISA for Rapid and Sensitive Determination of Microcystin-LR Using Automated Sequential Injection System

An amperometric immunoanalysis system based on monoclonal antibodies immobilized on Sepharose beads and packed into a micro-immunocolumn was developed for the quantification of microcystin-LR. Microcystin-LR (MCLR) was used as a reference microcystin variant. Inside the immunocolumn, free microcystins and microcystin-horseradish peroxidase (tracer) were sequentially captured by the immobilized antibodies, and the detection was performed electrochemically using Super AquaBlue ELISA substrate 2,2'-azinobis(3-ethylbenzothiazoline-sulfonic acid) (ABTS). The ABTS^●+ generated by enzymatic oxidation of ABTS was electrochemically determined at a carbon working electrode by applying a reduction potential set at 0.4 V versus Ag/AgCl reference electrode. The peak current intensity was inversely proportional to the amount of analyte bound to the immunocolumn. The amperometric flow-ELISA system, which was automatically controlled through the CapSenze^TM (Lund, Sweden) computer software, enabled determination of MCLR as low as 0.01 µg/L. The assay time was very short (20 min for one assay cycle). In addition, the electrochemical signals were not significantly affected by possible interferences which could be present in the real samples. Along with the simplicity of automation, this makes the developed method a promising tool for use in water quality assessment.

Polymeric cantilever integrated with PDMS/graphene composite strain sensor

This paper describes the mechanical and electrical characteristics of a polydimethylsiloxane (PDMS) cantilever integrated with a high-sensitivity strain sensor. The strain sensor is fabricated using PDMS and graphene flakes that are uniformly distributed in the PDMS. In order to prepare PDMS/graphene composite with uniform resistance, a tetrahydrofuran solution is used to decrease the viscosity of a PDMS base polymer solution. A horn-type sonicator is then used to mix the base polymer with graphene flakes. Low viscosity of the base polymer solution improves the reliability and reproducibility of the PDMS/graphene composite for strain sensor applications. After dicing the composite into the desired sensor shape, a tensile test is performed. The experimental results show that the composite with a concentration of 30 wt.% exhibits a linear response up to a strain rate of 9%. The graphene concentration of the prepared materials affects the gauge factor, which at 20% graphene concentration reaches about 50, and with increasing graphene concentration to 30% decreases to 9. Furthermore, photolithography, PDMS casting, and a stencil process are used to fabricate a PDMS cantilever with an integrated strain sensor. The change in resistance of the integrated PDMS/graphene sensor is characterized with respect to the displacement of the cantilever of within 500 μm. The experimental results confirmed that the prepared PDMS/graphene based sensor has the potential for high-sensitive biosensor applications.

Simultaneous detection of E. coli K12 and S. aureus Using a Continuous Flow Multijunction Biosensor

Rapid detection and identification of potentially harmful bacteria is ideal for food manufacturers to prevent foodborne illness outbreaks. Continuous monitoring method of foodborne pathogens levels and trends in food gives real-time results. Therefore, the objectives of this study were to fabricate and characterize the continuous flow multijunction biosensor for simultaneous detection of Escherichia coli K12 and Staphylococcus aureus. Junction biosensors were fabricated using gold plated tungsten wires coated with polyethylenimine and single walled carbon nanotubes. Each junction was functionalized with streptavidin and biotinylated antibodies specific to E. coli K12 and S. aureus. Then, single or 2 biosensors for each targeted analyte were connected to tubing, perpendicular to the flow direction. Pure serial diluted samples of E. coli K12 and S. aureus and microbial cocktail samples were continuously pumped at a 0.0167 mL/s into the detection zone. Changes in the electric current by biorecognition reactions between antibody and antigens were calculated. The developed junction sensor coupled with the fluidic channel showed the enhancement of the electric signal responses for detection of E. coli K12, compared to the stationary sensor. A linear regression was observed for both the E. coli and S. aureus functionalized array sensors in the detection range of 10(2) to 10(5) CFU/mL. Multiplexed detection of bacteria at the sensing levels as low as 10(2) CFU/mL for E. coli K12 and S. aureus was achieved within 2 min. Therefore, the continuous flow multijunction biosensor shows potential for rapid and continuous multiplexed detection of foodborne pathogens.

Salivary diagnostics using a portable point-of-service platform: a review

Clinical diagnostics can be improved by faster and more accessible disease detection. Our laboratory has developed a point-of-service (POS) device capable of rapid, sensitive, automated, and multiplexed biomarker detection that uses human saliva instead of other biofluids. Here, we review the technology that led to the development of this POS device. This POS technology can advance clinical diagnostics by saving time because of faster diagnosis, saving money because of a shorter hospital stay, and ultimately improving clinical care.

A single-step enzyme immunoassay capillary sensor composed of functional multilayer coatings for the diagnosis of marker proteins

A single-step, easy-to-use enzyme immunoassay capillary sensor, composed of substrate-immobilized hydrophobic coating, hydrogel coating, and soluble coating containing an enzyme-labeled antibody, was developed.

Development of spiropyran-based electrochemical sensor via simultaneous photochemical and target-activatable electron transfer

In traditional electrochemical sensors, the electrochemical signal transduction of the redox-active material is usually controlled by the analytical target. Due to non-specific interaction between the redox mediator and the target, false signal by single stimulus may not be avoided. To address this issue, we have developed a new electrochemical sensor that uses a functional spiropyran, an important class of photo and thermochromic compounds, as both recognition receptor and latent redox mediator, to realize simultaneous photochemical and target-modulated electron transfer. As a proof of principle, β-galactosidase was chosen as a model target. The new synthesized spiropyran probe, SP-β-gal, undergoes reversibly structural isomerization to form merocyanine under UV light irradiation. After the glycosidic bond being cleaved by β-galactosidase, the opened merocyanine of SP-β-gal forms redox-active 2-(2.5-dihydroxystyryl)-1.3.3-trimethyl-3H-indolium, and thus produces a pair of reversible redox current peaks under the electrochemical scanning. To amplify the detection signal, SP-β-gal was self-assembled with single-walled carbon nanotubes (SWCNTs) on the surface of glass carbon electrode. Kinetics experiments confirm that the probe is an ideal candidate for the determination of different concentrations of β-galactosidase digestion kinetics. Further, the SP-β-gal/SWCNTs-modified electrode is chemically stable in complex biological fluids. It was successfully applied to monitor β-galactosidase activity in the 10% calf thymus. This work represents not only a significant step forward in the further development of low-dimensional carbon nanomaterials/small organic molecular probes-based electrochemical biosensors, but also a new platform which may be extended to the assay of other enzyme such as β-D-glycosidase and so on by translating the biorecognition into electrochemical signal responses.

An automated integrated platform for rapid and sensitive multiplexed protein profiling using human saliva samples

During the last decade, saliva has emerged as a potentially ideal diagnostic biofluid for noninvasive testing. In this paper, we present an automated, integrated platform useable by minimally trained personnel in the field for the diagnosis of respiratory diseases using human saliva as a sample specimen. In this platform, a saliva sample is loaded onto a disposable microfluidic chip containing all the necessary reagents and components required for saliva analysis. The chip is then inserted into the automated analyzer, the SDReader, where multiple potential protein biomarkers for respiratory diseases are measured simultaneously using a microsphere-based array via fluorescence sandwich immunoassays. The results are read optically, and the images are analyzed by a custom-designed algorithm. The fully automated assay requires as little as 10 μL of saliva sample, and the results are reported in 70 min. The performance of the platform was characterized by testing protein standard solutions, and the results were comparable to those from the 3.5 h lab bench assay that we have previously reported. The device was also deployed in two clinical environments where 273 human saliva samples collected from different subjects were successfully tested, demonstrating the device's potential to assist clinicians with the diagnosis of respiratory diseases by providing timely protein biomarker profiling information. This platform, which combines noninvasive sample collection and fully automated analysis, can also be utilized in point-of-care diagnostics.

Lab-on-a-Chip Magneto-Immunoassays: How to Ensure Contact between Superparamagnetic Beads and the Sensor Surface

Lab-on-a-chip immuno assays utilizing superparamagnetic beads as labels suffer from the fact that the majority of beads pass the sensing area without contacting the sensor surface. Different solutions, employing magnetic forces, ultrasonic standing waves, or hydrodynamic effects have been found over the past decades. The first category uses magnetic forces, created by on-chip conducting lines to attract beads towards the sensor surface. Modifications of the magnetic landscape allow for additional transport and separation of different bead species. The hydrodynamic approach uses changes in the channel geometry to enhance the capture volume. In acoustofluidics, ultrasonic standing waves force µm-sized particles onto a surface through radiation forces. As these approaches have their disadvantages, a new sensor concept that circumvents these problems is suggested. This concept is based on the granular giant magnetoresistance (GMR) effect that can be found in gels containing magnetic nanoparticles. The proposed design could be realized in the shape of paper-based test strips printed with gel-based GMR sensors.

Current development of microfluidic immunosensing approaches for mycotoxin detection via capillary electromigration and lateral flow technology

Mycotoxin contamination in the food chain has caused serious health issues in humans and animals. Thus, a rapid on-site and lab-independent detection method for mycotoxins, such as aflatoxins (AFTs), is desirable. Microfluidic chip based immunosensor technology is one of the most promising methods for fast mycotoxin assays. In this review, we cover the major microfluidic immunosensors used for mycotoxin analysis, via flow-through (capillary electromigration) and lateral flow technology. Sample preparation from different matrices of agricultural products and foodstuffs is summarized. The choice of materials, fabrication strategies, and detection methods for microfluidic immunosensors are further discussed in detail. The sensors application in mycotoxin determination is also outlined. Finally, future challenges and opportunities are discussed.

Bacterial cytoplasm production of an EGFP-labeled single-chain Fv antibody specific for the HER2 human receptor

The human epidermal growth factor receptor 2 (HER2) is the main diagnostic marker of breast and ovary cancers. Here, to obtain a rapid and sensitive immunodiagnostic tool a single-chain antibody (scFv800E6) specific for the HER2 was fused to the N-terminus of the enhanced green fluorescent protein (EGFP) by a flexible linker. The soluble production of the novel scFv800E6-EGFP protein in the cytoplasm of Escherichia coli was investigated at different induction temperatures (25, 30 and 37°C); the intrinsic fluorescent properties and the binding activity to HER2 positive tumour cells of the fusion protein were analysed. Western blotting and fluorescence analysis of SDS-PAGE revealed the presence of two scFv800E6-EGFP forms, with different mobility and optical properties, their ratio depending on the induction temperature. The fluorescent form maintained the optical fluorescence properties of EGFP and exhibited a binding activity to the HER2-expressing cells comparable to that of the non-fused scFv800E6. In addition, to provide an insight into the effect of the induction temperature on the molecular structure, the folding of the fusion protein was assessed at atomic level by performing molecular dynamics simulations of the homology-derived model of scFv800E6-EGFP at 300 K and 310 K. The comparison of the data collected at these two temperatures revealed that the higher temperature affects specific structural elements. To improve the production of the soluble and functional scFv800E6-EGFP protein, "in silico" results could be utilised for ad hoc design of the molecular structure.

Accelerated immunoassays based on magnetic particle dynamics in a rotating capillary tube with stationary magnetic field

A rapid and simple magnetic particle-based immunoassay has been demonstrated in a capillary mixing system. Antibody-coated micrometer size superparamagnetic polystyrene (SPP) particles were used in an assay for rabbit IgG in a sandwich (noncompetitive) format. The kinetics of the assay was compared between a plate-based system and a single capillary tube. The interaction between the antigen (R-IgG) and the antibody (anti-R-IgG) that was carried by the SPP particles in a rotating capillary was tested under a stationary magnetic field. Competing magnetic and viscous drag forces helped to enhance the interaction between the analyte and the capture antibodies on the particles. The dimensionless Mason number (Mn) was employed to characterize the magnetic particle dynamics; a previously determined critical Mason number (Mn(c)) was employed as a guide to the appropriate experimental conditions of magnetic field strength and rotational speed of the capillary. The advantage of the rotating capillary system included a short assay time and a reduced reactive volume (20 μL). The results show that the immunoassay kinetics were improved by the formation of chains of the SPP particles for the conditions that corresponded to the critical Mason number.

Multiplex competitive microbead-based flow cytometric immunoassay using quantum dot fluorescent labels

In answer to the ever-increasing need to perform the simultaneous analysis of environmental hazards, microcarrier-based multiplex technologies show great promise. Further integration with biofunctionalized quantum dots (QDs) creates new opportunities to extend the capabilities of multicolor flow cytometry with their unique fluorescence properties. Here, we have developed a competitive microbead-based flow cytometric immunoassay using QDs fluorescent labels for simultaneous detection of two analytes, bringing the benefits of sensitive, rapid and easy-of-manipulation analytical tool for environmental contaminants. As model target compounds, the cyanobacterial toxin microcystin-LR and the polycyclic aromatic hydrocarbon compound benzo[a]pyrene were selected. The assay was carried out in two steps: the competitive immunological reaction of multiple targets using their exclusive sensing elements of QD/antibody detection probes and antigen-coated microsphere, and the subsequent flow cytometric analysis. The fluorescence of the QD-encoded microsphere was thus found to be inversely proportional to target analyte concentration. Under optimized conditions, the proposed assay performed well within 30 min for the identification and quantitative analysis of the two environmental contaminants. For microcystin-LR and benzo[a]pyrene, dose-response curves with IC(50) values of 5 μg L(-1) and 1.1 μg L(-1) and dynamic ranges of 0.52-30 μg L(-1) and 0.13-10 μg L(-1) were obtained, respectively. Recovery was 92.6-106.5% for 5 types of water samples like bottled water, tap water, surface water and seawater using only filtration as sample pretreatment.

Automated flow-through amperometric immunosensor for highly sensitive and on-line detection of okadaic acid in mussel sample

An electrochemical immunosensor for okadaic acid (OA) detection has been developed, and used in an indirect competitive immunoassay format under automated flow conditions. The biosensor was fabricated by injecting OA modified magnetic beads onto screen printed carbon electrode (SPCE) in the flow system. The OA present in the sample competed with the immobilized OA to bind with anti-okadaic acid monoclonal antibody (anti-OA-MAb). The secondary alkaline phosphatase labeled antibody was used to perform electrochemical detection. The current response obtained from the labeled alkaline phosphatase to 1-naphthyl phosphate decreased proportionally to the concentration of free OA in the sample. The calculated limit of detection (LOD) was 0.15 μg/L with a linear range of 0.19-25 μg/L. The good recoveries percentages validated the immunosensor application for real mussel samples. The developed system automatically controlled the incubation, washing and current measurement steps, showing its potential use for OA determination in field analysis.

Flow injection/sequential injection analysis systems: potential use as tools for rapid liver diseases biomarker study

Flow injection/sequential injection analysis (FIA/SIA) systems are suitable for carrying out automatic wet chemical/biochemical reactions with reduced volume and time consumption. Various parts of the system such as pump, valve, and reactor may be built or adapted from available materials. Therefore the systems can be at lower cost as compared to other instrumentation-based analysis systems. Their applications for determination of biomarkers for liver diseases have been demonstrated in various formats of operation but only a few and limited types of biomarkers have been used as model analytes. This paper summarizes these applications for different types of reactions as a guide for using flow-based systems in more biomarker and/or multibiomarker studies.

Flow-based systems for rapid and high-precision enzyme kinetics studies

Enzyme kinetics studies normally focus on the initial rate of enzymatic reaction. However, the manual operation of steps of the conventional enzyme kinetics method has some drawbacks. Errors can result from the imprecise time control and time necessary for manual changing the reaction cuvettes into and out of the detector. By using the automatic flow-based analytical systems, enzyme kinetics studies can be carried out at real-time initial rate avoiding the potential errors inherent in manual operation. Flow-based systems have been developed to provide rapid, low-volume, and high-precision analyses that effectively replace the many tedious and high volume requirements of conventional wet chemistry analyses. This article presents various arrangements of flow-based techniques and their potential use in future enzyme kinetics applications.

Bead-based competitive fluorescence immunoassay for sensitive and rapid diagnosis of cyanotoxin risk in drinking water

Due to the increased occurrence of cyanobacterial blooms and their toxins in drinking water sources, effective management based on a sensitive and rapid analytical method is in high demand for security of safe water sources and environmental human health. Here, a competitive fluorescence immunoassay of microcystin-LR (MCYST-LR) is developed in an attempt to improve the sensitivity, analysis time, and ease-of-manipulation of analysis. To serve this aim, a bead-based suspension assay was introduced based on two major sensing elements: an antibody-conjugated quantum dot (QD) detection probe and an antigen-immobilized magnetic bead (MB) competitor. The assay was composed of three steps: the competitive immunological reaction of QD detection probes against analytes and MB competitors, magnetic separation and washing, and the optical signal generation of QDs. The fluorescence intensity was found to be inversely proportional to the MCYST-LR concentration. Under optimized conditions, the proposed assay performed well for the identification and quantitative analysis of MCYST-LR (within 30 min in the range of 0.42-25 μg/L, with a limit of detection of 0.03 μg/L). It is thus expected that this enhanced assay can contribute both to the sensitive and rapid diagnosis of cyanotoxin risk in drinking water and effective management procedures.

Reactive semiconductor nanocrystals for chemoselective biolabeling and multiplexed analysis

Effective biological application of nanocrystalline semiconductor quantum dots continues to be hampered by the lack of easily implemented and widely applicable labeling chemistries. Here, we introduce two new orthogonal nanocrystal bioconjugation chemistries that overcome many of the labeling issues associated with currently utilized approaches. These chemistries specifically target either (1) the ubiquitous amines found on proteins or (2) thiols present in either antibody hinge regions or recombinantly introduced into other proteins to facilitate site-specific labeling. The amine chemistry incorporates aniline-catalyzed hydrazone bond formation, while the sulfhydryl chemistry utilizes nanocrystals displaying surface activated maleimide groups. Both reactive chemistries are rapidly implemented, yielding purified nanocrystal-protein bioconjugates in as little as 3 h. Following initial characterization of the nanocrystal materials, the wide applicability and strong multiplexing potential of these chemistries are demonstrated in an array of applications including immunoassays, immunolabeling in both cellular and tissue samples, in vivo cellular uptake, and flow cytometry. Side-by-side comparison of the immunolabeled cells suggested a functional equivalence between results generated with the amine and thiol-labeled antibody-nanocrystal bioconjugates in that format. Three-color labeling was achieved in the cellular uptake format, with no significant toxicity observed while simultaneous five-color labeling of different epitopes was demonstrated for the immunolabeled tissue sample. Novel labeling applications are also facilitated by these chemistries, as highlighted by the ability to directly label cellular membranes in adherent cell cultures with the thiol-reactive chemistry.

Heterogeneous immunoassays in microfluidic format using fluorescence detection with integrated amorphous silicon photodiodes

Miniaturization of immunoassays through microfluidic technology has the potential to decrease the time and the quantity of reactants required for analysis, together with the potential of achieving multiplexing and portability. A lab-on-chip system incorporating a thin-film amorphous silicon (a-Si:H) photodiode microfabricated on a glass substrate with a thin-film amorphous silicon-carbon alloy directly deposited above the photodiode and acting as a fluorescence filter is integrated with a polydimethylsiloxane-based microfluidic network for the direct detection of antibody-antigen molecular recognition reactions using fluorescence. The model immunoassay used consists of primary antibody adsorption to the microchannel walls followed by its recognition by a secondary antibody labeled with a fluorescent quantum-dot tag. The conditions for the flow-through analysis in the microfluidic format were defined and the total assay time was 30 min. Specific molecular recognition was quantitatively detected. The measurements made with the a-Si:H photodiode are consistent with that obtained with a fluorescence microscope and both show a linear dependence on the antibody concentration in the nanomolar-micromolar range.

Chemical-vapor-deposition-based polymer substrates for spatially resolved analysis of protein binding by imaging ellipsometry

Biomolecular interactions between proteins and synthetic surfaces impact diverse biomedical fields. Simple, quantitative, label-free technologies for the analysis of protein adsorption and binding of biomolecules are thus needed. Here, we report the use of a novel type of substrate, poly-p-xylylene coatings prepared by chemical vapor deposition (CVD) polymerization, for surface plasmon resonance enhanced ellipsometry (SPREE) studies and assess the reactive coatings as spatially resolved biomolecular sensing arrays. Prior to use in binding studies, reactive coatings were fully characterized by Fourier transform infrared spectroscopy, electrochemical impedance spectroscopy, and ellipsometry. As a result, the chemical structure, thickness, and homogeneous coverage of the substrate surface were confirmed for a series of CVD-coated samples. Subsequent SPREE imaging and fluorescence microscopy indicated that the synthetic substrates supported detectable binding of a cascade of biomolecules. Moreover, analysis revealed a useful thickness range for CVD films in the assessment of protein and/or antigen-antibody binding via SPREE imaging. With a variety of functionalized end groups available for biomolecule immobilization and ease of patterning, CVD thin films are useful substrates for spatially resolved, quantitative binding arrays.

Suspended, micron-scale corner cube retroreflectors as ultra-bright optical labels

Corner cube retroreflectors are objects with three mutually perpendicular reflective surfaces that return light directly to its source and are therefore extremely bright and easy to detect. In this work, we have fabricated suspended corner cube retroreflectors, 5 microns in size, consisting of a transparent epoxy core and three surfaces coated with gold as ultra-bright labels for use in a rapid, low-labor diagnostic platform. The authors have demonstrated that individual cubes are easily imaged using low-cost, low numerical aperture objectives in suspension and that they remain suspended over long periods of time. Moreover, we have demonstrated that the gold outer surfaces can be decorated with proteins, and that individual cubes can be bound to magnetic sample preparation particles bearing antibodies which recognize these proteins. The bound cubes can be imaged and tracked as they move through solution in response to an external magnetic field, and we have, as such, demonstrated the principle of the new biosensing approach.