Development of a novel electrokinetically driven microfluidic immunoassay for the detection of Helicobacter pylori

A microfluidic plasma separation device combined with a surface plasmon resonance biosensor for biomarker detection in whole blood

Biomarker detection in whole blood enables understanding of the cause, progression, relapse or outcome of treatment of a disease. Conventional biomarker detection techniques, such as enzyme-linked immunosorbent assay, polymerase chain reaction, and immunofluorescence, require long assay time, costly laboratory instruments, large reagent volume and sample pre-processing. Hence, there is an unmet need for reliable capture and detection of biomarkers in unprocessed blood which are adaptable to point-of-care (POC) testing. Here, we present a simple, low-cost, and rapid protein detection device from whole blood samples which has the potential to be employed in a POC setting. The platform consists of two components: a plasma separation device that extracts plasma from whole blood without the application of any external active forces and a SPR sensor chip that uses a label-free optical technique for the detection of biomarkers in the extracted plasma. We have demonstrated the detection of IgG and IgM biomolecules in unprocessed blood at concentrations lower than the physiological value within 15 min. The proposed technique has the potential for improving the diagnosis and screening of many diseases, including cancer, influenza, human immunodeficiency virus, and SARS-Cov2 at POC.

Microfluidic immunoassay for detection of serological antibodies: A potential tool for rapid evaluation of immunity against SARS-CoV-2

In December 2019, coronavirus disease 2019 became a pandemic affecting more than 200 countries and territories. Millions of lives are still affected because of mandatory quarantines, which hamstring economies and induce panic. Immunology plays a major role in the modern field of medicine, especially against virulent infectious diseases. In this field, neutralizing antibodies are heavily studied because they reflect the level of infection and individuals' immune status, which are essential when considering resumption of work, flight travel, and border entry control. More importantly, it also allows evaluating the antiviral vaccine efficacy as vaccines are still known for being the ultimate intervention method to inhibit the rapid spread of virulent infectious diseases. In this Review, we first introduce the host immune response after the infection of SARS-CoV-2 and discuss the latest results using conventional immunoassays. Next, as an enabling platform for detection with sufficient sensitivity while saving analysis time and sample size, the progress of microfluidic-based immunoassays is discussed and compared based on surface modification, microfluidic kinetics, signal output, signal amplification, sample matrix, and the detection of anti-SARS-CoV-2 antibodies. Based on the overall comparison, this Review concludes by proposing the future integration of visual quantitative signals on microfluidic devices as a more suitable approach for general use and large-scale surveillance.

A novel fluorescent DNA sensor for ultrasensitive detection of Helicobacter pylori

In this work, a novel fluorescent DNA sensor for ultrasensitive detection of Helicobacter pylori (H. pylori) DNA was developed. This strategy took advantage of DNA hybridization between single-stranded DNA (ssDNA, which had been designed as an aptamer specific for H. pylori DNA) and the complementary target H. pylori DNA, and the feature that ssDNA bound to graphene oxide (GO) with significantly higher affinity than double-stranded DNA (dsDNA). ssDNA were firstly covalent conjugated with CuInS₂ quantum dots (QDs) by reaction between the carboxy group of QDs and amino group modified ssDNA, forming ssDNA-QDs genosensor. In the absence of the complementary target H. pylori DNA, GO could adsorb ssDNA-QDs DNA sensor and efficiently quench the fluorescence of ssDNA-QDs. While the complementary target H. pylori DNA was introduced, the ssDNA-QDs preferentially bound with the H. pylori DNA. The formation of dsDNA would alter the conformation of ssDNA and disturb the interaction between ssDNA and GO. Thus, the dsDNA-QDs/GO system exhibited a stronger fluorescence emission than that of the ssDNA-QDs/GO system. Under the optimized conditions, a linear correlation was established between the fluorescence intensity ratio I/I₀ and the concentration of H. pylori DNA in the range of 1.25-875pmolL^-1 with a detection limit of 0.46pmolL^-1. The proposed method was applied to the determination of H. pylori DNA sequence in milk samples with satisfactory results.

Rapid Detection of Food Allergens by Microfluidics ELISA-Based Optical Sensor

The risks associated with the presence of hidden allergens in food have increased the need for rapid, sensitive, and reliable methods for tracing food allergens in commodities. Conventional enzyme immunosorbent assay (ELISA) has usually been performed in a centralized lab, requiring considerable time and sample/reagent consumption and expensive detection instruments. In this study, a microfluidic ELISA platform combined with a custom-designed optical sensor was developed for the quantitative analysis of the proteins wheat gluten and Ara h 1. The developed microfluidic ELISA biosensor reduced the total assay time from hours (up to 3.5 h) to 15-20 min and decreased sample/reagent consumption to 5-10 μL, compared to a few hundred microliters in commercial ELISA kits, with superior sensitivity. The quantitative capability of the presented biosensor is a distinctive advantage over the commercially available rapid methods such as lateral flow devices (LFD) and dipstick tests. The developed microfluidic biosensor demonstrates the potential for sensitive and less-expensive on-site determination for rapidly detecting food allergens in a complex sample system.

Reagents in microfluidics: an 'in' and 'out' challenge

Microfluidic devices are excellent at downscaling chemical and biochemical reactions and thereby can make reactions faster, better and more efficient. It is therefore understandable that we are seeing these devices being developed and used for many applications and research areas. However, microfluidic devices are more complex than test tubes or microtitre plates and the integration of reagents into them is a real challenge. This review looks at state-of-the-art methods and strategies for integrating various classes of reagents inside microfluidics and similarly surveys how reagents can be released inside microfluidics. The number of methods used for integrating and releasing reagents is surprisingly large and involves reagents in dry and liquid forms, directly-integrated reagents or reagents linked to carriers, as well as active, passive and hybrid release methods. We also made a brief excursion into the field of drug release and delivery. With this review, we hope to provide a large number of examples of integrating and releasing reagents that can be used by developers and users of microfluidics for their specific needs.

Increased density and coverage uniformity of viruses on a sensor surface by using U-type, T-type, and W-type microfluidic devices

Microorganisms, molecules, or viruses in the fluidic environment are usually at considerably low Reynolds numbers because of small diameters. The viscous forces of molecules and viruses dominate at considerably low Reynolds numbers. This study developed three microfluidic devices, that is, T type, U type, and W type devices, to control the flow movement, which can increase the adhesion density of viruses on the surface of the sensor. The linker 11-mercaptoundecanoic acid (11-MUA) and Turnip yellow mosaic virus (TYMV) were used in this study and measured by a confocal microscope. Fluorescent intensity and coverage of 11-MUA and TYMV were used to identify the adhesion density quantitatively. Results indicate that 11-MUA layers and TYMV disperse randomly by the dipping method. Attachment tests for T-, U-, and W-type devices demonstrated average fluorescence intensities of 1.56, 2.18, and 2.67, respectively, and average fluorescence coverage of 1.31, 1.87, and 2.55 times those of dipping techniques, respectively. The T-type device produced the lowest fluorescence coverage uniformity (10%-80%), whereas the W-type device produced the highest fluorescence coverage uniformity (80%-90%). Fluorescence intensity correlates positively with flow within a specified flow range; however, the exact relationship between fluorescence intensity and flow requires further study. Attachment tests for TYMV virus samples indicated that the W-type device produced an average fluorescence intensity of 3.59 and average fluorescence coverage of 19.13 times greater than those achieved through dipping techniques. Traditional immersion methods achieved fluorescence coverage of 0%-10%, whereas that of the W-type device reached 70%-90%.

Microfabrication and applications of opto-microfluidic sensors

A review of research activities on opto-microfluidic sensors carried out by the research groups in Canada is presented. After a brief introduction of this exciting research field, detailed discussion is focused on different techniques for the fabrication of opto-microfluidic sensors, and various applications of these devices for bioanalysis, chemical detection, and optical measurement. Our current research on femtosecond laser microfabrication of optofluidic devices is introduced and some experimental results are elaborated. The research on opto-microfluidics provides highly sensitive opto-microfluidic sensors for practical applications with significant advantages of portability, efficiency, sensitivity, versatility, and low cost.

Microfluidic-based biosensors toward point-of-care detection of nucleic acids and proteins

This article reviews state-of-the-art microfluidic biosensors of nucleic acids and proteins for point-of-care (POC) diagnostics. Microfluidics is capable of analyzing small sample volumes (10^-9-10^-18 l) and minimizing costly reagent consumption as well as automating sample preparation and reducing processing time. The merger of microfluidics and advanced biosensor technologies offers new promises for POC diagnostics, including high-throughput analysis, portability and disposability. However, this merger also imposes technological challenges on biosensors, such as high sensitivity and selectivity requirements with sample volumes orders of magnitude smaller than those of conventional practices, false response errors due to non-specific adsorption, and integrability with other necessary modules. There have been many prior review articles on microfluidic-based biosensors, and this review focuses on the recent progress in last 5 years. Herein, we review general technologies of DNA and protein biosensors. Then, recent advances on the coupling of the biosensors to microfluidics are highlighted. Finally, we discuss the key challenges and potential solutions for transforming microfluidic biosensors into POC diagnostic applications.

Integrated electrokinetic sample focusing and surface plasmon resonance imaging system for measuring biomolecular interactions

Label-free biomolecular binding measurement methods, such as surface plasmon resonance (SPR), are becoming increasingly more important for the estimation of real-time binding kinetics. Recent advances in surface plasmon resonance imaging (iSPR) are emerging for label-free microarray-based assay applications, where multiple biomolecular interactions can be measured simultaneously. However, conventional iSPR microarray systems rely on protein printing techniques for ligand immobilization to the gold imaging surface and external pumps for analyte transport. In this article, we present an integrated microfluidics and iSPR platform that uses only electrokinetic transport and guiding of ligands and analytes and, therefore, requires only electrical inputs for sample transport. An important advantage of this new approach, compared to conventional systems, is the ability to direct a single analyte to a specific ligand location in the microarray, which can facilitate analysis parallelization. Additionally, this simple approach does not require complicated microfluidic channel arrangements, external pumps, or valves. As a demonstration, kinetics and affinity have been extracted from measured binding responses of human IgG and goat antihuman IgG using a simple 1:1 model and compared to responses measured with conventional pressure driven analyte transport. The measured results indicate similar binding kinetics and affinity between the electrokinetic and pressure-driven sample manipulation methods and no cross contamination to adjacent measurement locations has been observed.

Parallel microfluidic surface plasmon resonance imaging arrays

Surface plasmon resonance imaging (SPRi) is a label-free technique used for the quantitation of binding affinities and concentrations for a wide variety of target molecules. Although SPRi is capable of determining binding constants for multiple ligands in parallel, current commercial instruments are limited to a single analyte stream on multiple ligand spots. Measurement of binding kinetics requires the serial introduction of different analyte concentrations; such repeated experiments are conducted manually and are therefore time-intensive. To address these challenges, we have developed an integrated microfluidic array using soft lithography techniques for high-throughput SPRi-based detection and determination of binding affinities of antibodies against protein targets. The device consists of 264 element-addressable chambers isolated by microvalves. The resulting 700 pL chamber volumes, combined with a serial dilution network for simultaneous interrogation of up to six different analyte concentrations, allow for further speeding detection times. To test for device performance, human alpha-thrombin was immobilized on the sensor surface and anti-human alpha-thrombin IgG was injected across the surface at different concentrations. The equilibrium dissociation constant was determined to be 5.0 +/- 1.9 nM, which agrees well with values reported in the literature. The interrogation of multiple ligands to multiple analytes in a single device was also investigated and samples were recovered with no cross-contamination. Since each chamber can be addressed independently, this array is capable of interrogating binding events from up to 264 different immobilized ligands against multiple analytes in a single experiment. The development of high-throughput protein analytic measurements is a critical technology for systems approaches to biology and medicine.

Lateral-driven continuous dielectrophoretic microseparators for blood cells suspended in a highly conductive medium

This paper presents lateral-driven continuous dielectrophoretic (DEP) microseparators for separating red and white blood cells suspended in highly conductive dilute whole blood. The continuous microseparators enable the separation of blood cells based on the lateral DEP force generated by a planar interdigitated electrode array placed at an angle to the direction of flow. The simplified line charge model that we developed for the theoretical analysis was verified by comparing it with simulated and measured results. Experimental results showed that the divergent type of microseparator can continuously separate out 87.0% of the red blood cells (RBCs) and 92.1% of the white blood cells (WBCs) from dilute whole blood within 5 min simply by using a 2 MHz, 3 Vp-p AC voltage to create a gradient electric field in a medium that conducts at 17 mS cm(-1). Under the same conditions, the convergent type of microseparator could separate out 93.6% of the RBCs and 76.9% of the WBCs. We have shown that our lateral-driven continuous DEP microseparator design is practical for the continuous separation of blood cells without the need to control the conductivity of the suspension medium, overcoming critical drawbacks of DEP microseparators.

PDMS and its suitability for analytical microfluidic devices

Poly(dimethylsiloxane) also known as PDMS is used in a wide range of biomedical applications. These range from implants through catheters to soft contact lenses. Therefore, it is understandable that PDMS has been extensively tested for these purposes. In past years, the microfluidics has moved from predominantly silicon and glass structures towards polymers due to their ease of manufacturing and moderate cost. PDMS has gained a lot of attention in various analytical applications. However, the testing of its suitability for such applications has not been as thorough as in the biomedical applications, perhaps relying on the experiments from that field. Microfluidic PDMS structures are more and more popular in various analytical devices. Such devices consume less reagents and can work with lower sample volumes. On the other hand, the surface-to-sample-volume ratio becomes larger. That increases the influence of material properties on the actual measurement. Some of the challenges include adsorption, diffusion, surface roughness, permeability and elasticity of PDMS, which are discussed in this paper.

Electrochemical immunoassay on a microfluidic device with sequential injection and flushing functions

An integrated microfluidic device with injecting, flushing, and sensing functions was realized using valves that operate based on direct electrowetting. The device consisted of two substrates: a glass substrate with driving and sensing electrodes and a poly(dimethylsiloxane) (PDMS) substrate. Microfluidic transport was achieved using the spontaneous movement of solutions in hydrophilic flow channels formed with a dry-film photoresist layer. The injection and flushing of solutions were controlled by gold working electrodes, which functioned as valves. The valves were formed either in the channels or in a through-hole in the glass substrate. To demonstrate the system's applicability to an immunoassay, the detection of immobilized antigens was performed as a partial simulation of a sandwich immunoassay. Human alpha-fetoprotein (AFP) or an anti-human AFP antibody was immobilized on a platinum working electrode in the chamber using a plasma-polymerized film (PPF). By applying a potential to the injection valves, necessary solutions were injected one by one through the channels into a reaction chamber at the center of the chip and incubated for reasonable periods of time. The solutions were then flushed through the flushing valve and absorbed in a filter paper placed under the device. After incubation with the corresponding antibodies labeled with glucose oxidase (GOD), electrochemical detection was conducted. In both cases, the obtained current depended on the amount of immobilized antigen. The calibration curves were sigmoidal, and the detection limit was 0.1 ng. The developed microfluidic system could potentially be a fundamental component for a micro immunoassay of the next generation.

Disposable microfluidic ELISA for the rapid determination of folic acid content in food products

A micro-analytical system for rapid and quantitative analysis by inhibition immunoassay is presented and applied to the detection of folic acid. Eight polymer microchannels of 65-nL volume each and containing microelectrodes are embedded in a cartridge so that they can be operated simultaneously. All fluidic steps as well as the amperometric detection in the channels are operated by an instrument and software developed in-house. The fluidic steps of the immunoassay occur through hydrodynamic loading of the different solutions through the channels. The speed and duration of the flow and incubation parameters can thus be adapted to the biological and testing requirements. The effectiveness of the system was demonstrated by analysing folic acid concentrations in real infant formula samples within 5 min. In an effort to get a fully monitored assay, each fluidic step is monitored thanks to continuous amperometric detection of oxygen in the microchannel.

Highly efficient dynamic modification of plastic microfluidic devices using proteins in microchip capillary electrophoresis

New dynamic coating agents were investigated for the manipulation of electroosmotic flow (EOF) in poly(methylmethacrylate) (PMMA) microchips. Blocking proteins designed for enzyme-linked immunosorbent assay (ELISA) applications (e.g. Block Ace and UltraBlock), and egg-white lysozyme were proposed in this study. The EOF could be enhanced, suppressed or its direction could be reversed, depending on the buffer pH and the charge on the proteins. The coating procedure is simple, requiring only filling of the microchannels with a coating solution, followed by a rinse with a running buffer solution prior to analysis. One major advantage of this method is that it is not necessary to add the coating agent to the running buffer solution. Block Ace and UltraBlock coatings were stable for at least five runs in a given microchannel without the need to condition the coating between runs other than replenishing the buffer solution after each run, i.e. the RSD values of EOF (n=5) were less than 4.3%, and there was no significant change in the EOF after 5 runs. The reproducibility of the coating procedures was found from the channel-to-channel RSD values of the EOF, and were less than 5.0% when using HEPES-Na buffer (pH 7.4) as the running buffer. Several examples of electrophoretic separations of amino acids and biogenic amines derivatized with 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F) are demonstrated in this paper. The dynamic coating method has the potential for a broad range of applications in microchip capillary electrophoresis (microchip CE) separations.

Protein-sensing assay formats and devices

Proteins are used as biocatalysts, therapeutic or diagnostic agents, and as such they are biotechnological products. Moreover, they are biomarkers for health states, diseases or toxic or other adverse effects, and the intracellular protein network is essential for the adaptation of an organism to its environment. Thus, there is a strong need for analytical methods for protein determination, which allow not only to indicate the presence of a protein, but also its concentration, covalent modification and activity, and corresponding developments of new methods experienced strong support. Among those methods only those were considered here, which are based on affinity reactions between an immobilized capture agent, such as an antibody or a receptor, and the target protein. Immobilization methods range from adsorption on hydrophobic materials, in membranes or gels to covalent binding and bioaffinity reactions, such as the oriented immobilization of antibodies on protein A/G layers. The applicability of the various methods is dependent on physical and chemical properties of the immobilization substrate and of the capture agent, i.e. the presence of surface charges, hydrophobic areas or functional groups for chemical coupling. The choice of the immobilization substrate is influenced by the combination of the assay and detection principle, which meets best the practical requirements. Assay formats range from direct, label-free one-step detection of the affinity reaction between the capture agent and the target protein to multi-step procedures, such as an enzyme-tracer-based sandwich assays. Each approach has its particular advantages and disadvantages with respect to the complexity of the assay, i.e. number of required reagents and of incubation steps, the possible degree of automation, assay time, availability of suitable reagents, required sample volume, sensitivity and specificity, including the possibility to determine several proteins simultaneously. No general recommendation for the "best choice" was given in this contribution, but examples were chosen, which illustrate the potential of the different systems.

Miniaturized immunoassay microfluidic system with electrokinetic control

A portable heterogeneous immunoassay system is presented in this paper. It consists of a poly(dimethylsiloxane) (PDMS) based microfluidic chip as the immunoreactor, a miniaturized programmable high voltage sequencer as the power supply and the flow controller, and a laser-optical fiber fluorescence detection module as the signal reader. The operation of this immunoassay system is automatic. The sequential reagent dispensing and washing processes are controlled by the programmable sequencer. The reagent consumption was only 12 microL and the assay time was only 26 min. The detection limit for Escherichia coli O157:H7 bacterial antigen with this miniaturized system was 0.3 ng/muL, lower than that obtained using fluorescence microscope in previous studies.