Immunosensors: Antibody-based biosensors

Synergistic applications of quantum dots and magnetic nanomaterials in pathogen detection: a comprehensive review

The rapid and accurate detection of pathogens is crucial for effective disease prevention and management in healthcare, food safety, and environmental monitoring. While conventional pathogen detection methods like culture-based techniques and PCR are sensitive and selective, they are often time-consuming, require skilled operators, and are not suitable for point-of-care or on-site testing. To address these limitations, innovative sensor technologies have emerged that leverage the unique properties of nanomaterials. Quantum dots (QDs) and magnetic nanomaterials are two classes of nanomaterials that have shown particular promise for pathogen sensing. This review comprehensively examines the synergistic applications of QDs and magnetic nanomaterials for detecting bacteria, viruses, phages, and parasites.

Flux-Type versus Concentration-Type Sensors in Transdermal Measurements

New transdermal biosensors measure analytes that diffuse from the bloodstream through the skin, making it important to reduce the system response time and understand measurement output. While highly customized models have been created for specific sensors, a generalized model for transdermal sensor systems is lacking. Here, a simple one-dimensional diffusion model was used to characterize the measurement system and classify biosensors as either flux types or concentration types. Results showed that flux-type sensors have significantly faster response times than concentration sensors. Furthermore, flux sensors do not measure concentration, but rather have an output measurement that is proportional to skin permeability. These findings should lead to an improved understanding of transdermal measurements and their relation to blood analyte concentration. In the realm of alcohol research, where the majority of commercially available sensors are flux types, our work advocates toward moving away from transdermal alcohol concentration as a metric, and instead suggests embracing transdermal alcohol flux as a more suitable alternative.

A review of graphene quantum dots and their potential biomedical applications

Today, nanobiotechnology is a pioneering technology in biomedicine. Every day, new nanomaterials are synthesized with elevated physiochemical properties for better diagnosis and treatment of diseases. One advancing class of materials is the Graphene family. Among different kinds of graphene derivatives, graphene quantum dots (GQDs) show fantastic optical, electrical, and electrochemical features originating from their unique quantum confinement effect. Due to the distinct properties of GQD, including large surface-to-volume ratio, low cytotoxicity, and easy functionalization, this nanomaterial has gone popular in biomedical field. Herein, a short overview of different strategies developed for GQD synthesis and functionalization is discussed. In the following, the most recent progress of GQD based nanomaterials in different biomedical fields, including bio-imaging, drug/gene delivery, antimicrobial, tissue engineering, and biosensors, are reviewed.

Electroanalytical overview: utilising micro- and nano-dimensional sized materials in electrochemical-based biosensing platforms

Research into electrochemical biosensors represents a significant portion of the large interdisciplinary field of biosensing. The drive to develop reliable, sensitive, and selective biosensing platforms for key environmental and medical biomarkers is ever expanding due to the current climate. This push for the detection of vital biomarkers at lower concentrations, with increased reliability, has necessitated the utilisation of micro- and nano-dimensional materials. There is a wide variety of nanomaterials available for exploration, all having unique sets of properties that help to enhance the performance of biosensors. In recent years, a large portion of research has focussed on combining these different materials to utilise the different properties in one sensor platform. This research has allowed biosensors to reach new levels of sensitivity, but we note that there is room for improvement in the reporting of this field. Numerous examples are published that report improvements in the biosensor performance through the mixing of multiple materials, but there is little discussion presented on why each nanomaterial is chosen and whether they synergise well together to warrant the inherent increase in production time and cost. Research into micro-nano materials is vital for the continued development of improved biosensing platforms, and further exploration into understanding their individual and synergistic properties will continue to push the area forward. It will continue to provide solutions for the global sensing requirements through the development of novel materials with beneficial properties, improved incorporation strategies for the materials, the combination of synergetic materials, and the reduction in cost of production of these nanomaterials.

Ratiometric Electrochemistry: Improving the Robustness, Reproducibility and Reliability of Biosensors

Electrochemical biosensors are an increasingly attractive option for the development of a novel analyte detection method, especially when integration within a point-of-use device is the overall objective. In this context, accuracy and sensitivity are not compromised when working with opaque samples as the electrical readout signal can be directly read by a device without the need for any signal transduction. However, electrochemical detection can be susceptible to substantial signal drift and increased signal error. This is most apparent when analysing complex mixtures and when using small, single-use, screen-printed electrodes. Over recent years, analytical scientists have taken inspiration from self-referencing ratiometric fluorescence methods to counteract these problems and have begun to develop ratiometric electrochemical protocols to improve sensor accuracy and reliability. This review will provide coverage of key developments in ratiometric electrochemical (bio)sensors, highlighting innovative assay design, and the experiments performed that challenge assay robustness and reliability.

Flexible electrochemical biosensors for healthcare monitoring

As the interest in wearable devices has increased recently, increasing biosensor flexibility has begun to attract considerable attention. Among the various types of biosensors, electrochemical biosensors are uniquely suited for the development of such flexible biosensors due to their many advantages, including their fast response, inherent miniaturization, convenient operation, and portability. Therefore, many studies on flexible electrochemical biosensors have been conducted in recent years to achieve non-invasive and real-time monitoring of body fluids such as tears, sweat, and saliva. To achieve this, various substrates, novel nanomaterials, and detection techniques have been utilized to develop conductive flexible platforms that can be applied to create flexible electrochemical biosensors. In this review, we discussed recently reported flexible electrochemical biosensors and divided them into specific categories including materials for flexible substrate, fabrication techniques for flexible biosensor development, and recently developed flexible electrochemical biosensors to externally monitor target molecules, thereby providing a means to noninvasively examine cells and body fluid samples. In conclusion, this review will discuss the materials, methods, recent studies, and perspectives on flexible electrochemical biosensors for healthcare monitoring and wearable biosensing systems.

Interactions between Biomedical Micro-/Nano-Motors and the Immune Molecules, Immune Cells, and the Immune System: Challenges and Opportunities

Mobile micro- and nano-motors (MNMs) emerge as revolutionary platforms for biomedical applications, including drug delivery, biosensing, non-invasive surgery, and cancer therapy. While for applications in biomedical fields and practical clinical translation, the interactions of these untethered tiny machines with the immune system is an essential issue to be considered. This review highlights the recent approaches of surpassing immune barriers to prevent foreign motors from triggering immune responses. In addition to trials focusing on the function preservation of MNMs, examples of versatile MNMs working with the immune components (immune molecules, immune cells and the whole system) to achieve cancer immunotherapy, immunoassay, and detoxification are outlined. The immune interference part provides researchers an idea about what is the limit presented by the immune components. The coworking part suggests ways to bypass or even utilize the limit. With interdisciplinary cooperation of nanoengineering, materials science, and immunology field, the rationally designed functional MNMs are expected to provide novel opportunities for the biomedical field.

Label-free anti-Müllerian hormone sensor based on polyaniline micellar modified electrode

A label-free electrochemical immunosensor based on polyaniline (PANI) micellar electrode was firstly fabricated for direct AMH detection. To control the size regularity of PANI, a micelle-based method using ammonium peroxydisulfate (APS) as a reducing agent was employed in the polymerization process. The Anti-AMH antibodies were readily immobilized onto PANI via peptide bond to enhance the sensor specificity and sensitivity. This sensor was applied for the detection of AMH, an ovarian response indicator in female related to residual eggs during a woman's monthly cycle. The sensor performances were systematically investigated by differential pulse voltammetry. The anodic peak current decreases with the increase of AMH concentration owing to blocking of electron transfer by AMH. Under the optimal conditions, this sensor offers high sensitivity with a low detection limit of 0.1 ng mL^-1 and a wide linear range of 0.1-4 ng mL^-1, which is sensitive enough to indicate the ability to produce eggs during a woman's monthly cycle. Furthermore, this system requires lower sample volume (5 μL), while offers the simple fabrication with low cost and no synthetic challenge and faster analysis compared with a standard ELISA. Ultimately, this sensor was successfully applied for the detection of AMH in human serum with satisfactory results. Thus, it might be an alternative tool for AMH screening in clinical setting.

Recent advances in graphene-based nanobiosensors for salivary biomarker detection

As biosensing research is rapidly advancing due to significant developments in materials, chemistry, and electronics, researchers strive to build cutting-edge biomedical devices capable of detecting health-monitoring biomarkers with high sensitivity and specificity. Biosensors using nanomaterials are highly promising because of the wide detection range, fast response time, system miniaturization, and enhanced sensitivity. In the recent development of biosensors and electronics, graphene has rapidly gained popularity due to its superior electrical, biochemical, and mechanical properties. For biomarker detection, human saliva offers easy access with a large variety of analytes, making it a promising candidate for its use in point-of-care (POC) devices. Here, we report a comprehensive review that summarizes the most recent graphene-based nanobiosensors and oral bioelectronics for salivary biomarker detection. We discuss the details of structural designs of graphene electronics, use cases of salivary biomarkers, the performance of existing sensors, and applications in health monitoring. This review also describes current challenges in materials and systems and future directions of the graphene bioelectronics for clinical POC applications. Collectively, the main contribution of this paper is to deliver an extensive review of the graphene-enabled biosensors and oral electronics and their successful applications in human salivary biomarker detection.

Minimally Invasive Platforms in Biosensing

The interaction of sensing components with body fluids is a basic requirement for clinical diagnostics; a variety of novel platforms have recently been developed for invasive and non-invasive sensing. In this manuscript, recent advancements related to minimally invasive platform for biosensing are reviewed. Many approaches have been utilized for generating minimally invasive platforms that require a small volume of body fluid; for example, the use of small-scale needles known as microneedles for minimally invasive detection has been demonstrated. The use of capillary action in microneedle-assisted biosensing may facilitate the detection of analytes in body fluids. This review considers recent innovations in the structure and performance of minimally invasive sensos.

Capturing transient antibody conformations with DNA origami epitopes

Revealing antibody-antigen interactions at the single-molecule level will deepen our understanding of immunology. However, structural determination under crystal or cryogenic conditions does not provide temporal resolution for resolving transient, physiologically or pathologically relevant functional antibody-antigen complexes. Here, we develop a triangular DNA origami framework with site-specifically anchored and spatially organized artificial epitopes to capture transient conformations of immunoglobulin Gs (IgGs) at room temperature. The DNA origami epitopes (DOEs) allows programmed spatial distribution of epitope spikes, which enables direct imaging of functional complexes with atomic force microscopy (AFM). We establish the critical dependence of the IgG avidity on the lateral distance of epitopes within 3–20 nm at the single-molecule level. High-speed AFM imaging of transient conformations further provides structural and dynamic evidence for the IgG avidity from monovalent to bivalent in a single event, which sheds light on various applications including virus neutralization, diagnostic detection and cancer immunotherapy.

Understanding antibody-antigen interactions is important to deepen the understanding of immunology. Here, the authors report on the application of DNA origami structures for the controlled presentation of antigens to study antibody binding behaviours at room temperature.

Highly Sensitive Biosensors Based on Biomolecules and Functional Nanomaterials Depending on the Types of Nanomaterials: A Perspective Review

Biosensors are very important for detecting target molecules with high accuracy, selectivity, and signal-to-noise ratio. Biosensors developed using biomolecules such as enzymes or nucleic acids which were used as the probes for detecting the target molecules were studied widely due to their advantages. For example, enzymes can react with certain molecules rapidly and selectively, and nucleic acids can bind to their complementary sequences delicately in nanoscale. In addition, biomolecules can be immobilized and conjugated with other materials by surface modification through the recombination or introduction of chemical linkers. However, these biosensors have some essential limitations because of instability and low signal strength derived from the detector biomolecules. Functional nanomaterials offer a solution to overcome these limitations of biomolecules by hybridization with or replacing the biomolecules. Functional nanomaterials can give advantages for developing biosensors including the increment of electrochemical signals, retention of activity of biomolecules for a long-term period, and extension of investigating tools by using its unique plasmonic and optical properties. Up to now, various nanomaterials were synthesized and reported, from widely used gold nanoparticles to novel nanomaterials that are either carbon-based or transition-metal dichalcogenide (TMD)-based. These nanomaterials were utilized either by themselves or by hybridization with other nanomaterials to develop highly sensitive biosensors. In this review, highly sensitive biosensors developed from excellent novel nanomaterials are discussed through a selective overview of recently reported researches. We also suggest creative breakthroughs for the development of next-generation biosensors using the novel nanomaterials for detecting harmful target molecules with high sensitivity.

Wearable sensors for monitoring the physiological and biochemical profile of the athlete

Athletes are continually seeking new technologies and therapies to gain a competitive edge to maximize their health and performance. Athletes have gravitated toward the use of wearable sensors to monitor their training and recovery. Wearable technologies currently utilized by sports teams monitor both the internal and external workload of athletes. However, there remains an unmet medical need by the sports community to gain further insight into the internal workload of the athlete to tailor recovery protocols to each athlete. The ability to monitor biomarkers from saliva or sweat in a noninvasive and continuous manner remain the next technological gap for sports medical personnel to tailor hydration and recovery protocols per the athlete. The emergence of flexible and stretchable electronics coupled with the ability to quantify biochemical analytes and physiological parameters have enabled the detection of key markers indicative of performance and stress, as reviewed in this paper.

Clinical translation of biomedical sensors for sports medicine

The digital health field has seen a surge in product development over the last decade, with product introductions ranging from wrist monitors, epidermal electronics, electronic pills and smart garments, much of these precipitated through the commercialisation and commoditisation of sensor technology. The emergence of wearable technology has recently garnered heightened interest by physicians and the general public. The convenient use of wireless technology to track and monitor physiological parameters, such as heart rate, distance, sleep and stress, has emerged to become relevant to patient care and human performance assessment. However, collecting data is not enough to inform clinical decision-making. It is essential to translate the acquired data into information relevant to clinicians. Our experiences tell us that team competencies must mirror the interdisciplinary technology itself. Thus, an interdisciplinary team blending expertise from engineering, medicine, and nursing is believed to be essential in translating wearable technology into the field. This review discusses the application of wearable sensors to monitor human performance assessment in domains necessitating accurate, reliable, and timely transmission of acquired bio-metric and bio-vital data. A key result disseminating from our investigations is the need to develop predictive models based off of the data acquired from wearable devices to necessitate the development of athlete-centred treatment plans to expedite the return-to-play time and to maximise performance.

Engineering Biosensors with Dual Programmable Dynamic Ranges

Although extensively used in all fields of chemistry, molecular recognition still suffers from a significant limitation: host-guest binding displays a fixed, hyperbolic dose-response curve, which limits its usefulness in many applications. Here we take advantage of the high programmability of DNA chemistry and propose a universal strategy to engineer biorecognition-based sensors with dual programmable dynamic ranges. Using DNA aptamers as our model recognition element and electrochemistry as our readout signal, we first designed a dual signaling "signal-on" and "signal-off" adenosine triphosphate (ATP) sensor composed of a ferrocene-labeled ATP aptamer in complex to a complementary, electrode-bound, methylene-blue labeled DNA. Using this simple "dimeric" sensor, we show that we can easily (1) tune the dynamic range of this dual-signaling sensor through base mutations on the electrode-bound DNA, (2) extend the dynamic range of this sensor by 2 orders of magnitude by using a combination of electrode-bound strands with varying affinity for the aptamers, (3) create an ultrasensitive dual signaling sensor by employing a sequestration strategy in which a nonsignaling, high affinity "depletant" DNA aptamer is added to the sensor surface, and (4) engineer a sensor that simultaneously provides extended and ultrasensitive readouts. These strategies, applicable to a wide range of biosensors and chemical systems, should broaden the application of molecular recognition in various fields of chemistry.

Biosensors for the Detection of Environmental and Urban Pollutions

Release of harmful pollutants such as heavy metals, pesticides, and pharmaceuticals to the environment is a global concern. Rapid and reproducible detection of these pollutants is thus necessary. Biosensors are the sensitive and high specific tools for detection of environmental pollutants. Broad range various types of biosensors have been fabricated for this purpose. This review focuses on the feature and application of biosensors developed for environmental and urban pollutants detection. J. Cell. Biochem. 119: 207-212, 2018. © 2017 Wiley Periodicals, Inc.

Aptamer-based sandwich-type biosensors

Sandwich-type biosensor platforms have drawn lots of attentions due to its superior features, compared to other platforms, in terms of its stable and reproducible responses and easy enhancement in the detection sensitivity. The sandwich-type assays can be developed by utilizing a pair of receptors, which bind to the different sites of the same target. In this mini-review paper, the sandwich-type biosensors using either pairs of aptamers or aptamer-antibody pairs are reviewed in terms of its targets and platforms, the schematic designs, and their analytical performance.

A new strategy for label-free electrochemical immunoassay based on "gate-effect" of β-cyclodextrin modified electrode

A novel label-free electrochemical immunoassay was developed for prostate-specific antigen (PSA) detection via using β-cyclodextrin (β-CD) assembled layer created gates for the electron transfer of probe. To construct the sensor, a gold electrode was self-assembled with monoclonal anti-PSA antibody labeled 6-mercapto-β-cyclodextrin. Interspaces among β-CD molecules in the layer were automatically formed on gold electrode, which act as the channel of the electron transfer of [Fe(CN)6](3-/4-) probe. When PSA bind with anti-PSA, it can block these channels on the electrode surface due to their steric hindrance effect, resulting in the decrease in redox current of the probe. Through such a gate-controlled effect, ultra trace amount of PSA may make the currents change greatly after the immunoreaction, which enhanced the signal-to-noise ratio to achieve the amplification effect. By evaluating the logarithm of PSA concentrations, the immunosensor had a good linear response to the current changes with a detection limit of 0.3 pg/mL (S/N = 3) when PSA concentration ranged from 1.0 pg/mL to 1.0 ng/mL. The label-free immunosensor exhibited satisfactory performances in sensitivity, repeatability as well as specificity.

Detection of orexin A neuropeptide in biological fluids using a zinc oxide field effect transistor

Biomarkers which are indicative of acute physiological and emotional states are studied in a number of different areas in cognitive neuroscience. Currently, many cognitive studies are conducted based on programmed tasks followed by timed biofluid sampling, central laboratory processing, and followed by data analysis. In this work, we present a sensor platform capable of rapid biomarker detection specific for detecting neuropeptide orexin A, found in blood and saliva and known as an indicator of fatigue and cognitive performance. A peptide recognition element that selectively binds to orexin A was designed, characterized, and functionalized onto a zinc oxide field effect transistor to enable rapid detection. The detection limit using the sensor platform was sub-picomolar in water, and picomolar to nanomolar levels in saliva and serum. The transistor and recognition element sensor platform can be easily expanded, allowing for multiple biomarkers to be detected simultaneously, lending itself to complex biomarker analysis applicable to rapid feedback for neuroscience research and physiological monitoring.

Bioconjugation of protein-repellent zwitterionic polymer brushes grafted from silicon nitride

A new method for attaching antibodies to protein-repellent zwitterionic polymer brushes aimed at recognizing microorganisms while preventing the nonspecific adsorption of proteins is presented. The poly(sulfobetaine methacrylate) (SBMA) brushes were grafted from α-bromo isobutyryl initiator-functionalized silicon nitride (Si(x)N(4), x ≥ 3) surfaces via controlled atom-transfer radical polymerization (ATRP). A trifunctional tris(2-aminoethyl)amine linker was reacted with the terminal alkylbromide of polySBMA chains. N-Hydroxysuccinimide (NHS) functionalization was achieved by reacting the resultant amine-terminated polySBMA brush with bifunctional suberic acid bis(N-hydroxysuccinimide ester). Anti-Salmonella antibodies were subsequently immobilized onto polySBMA-grafted Si(x)N(4) surfaces through these NHS linkers. The protein-repellent properties of the polySBMA-grafted surface after antibody attachment were evaluated by exposing the surfaces to Alexa Fluor 488-labeled fibrinogen (FIB) solution (0.1 g·L(-1)) for 1 h at room temperature. Confocal laser scanning microscopy (CLSM) images revealed the minimal adsorption of FIB onto the antibody-coated polySBMA in comparison with that of antibody-coated epoxide monolayers and also bare Si(x)N(4) surfaces. Subsequently, the interaction of antibodies immobilized onto polySBMA with SYTO9-stained Salmonella solution without using blocking solution was examined by CLSM. The fluorescent images showed that antibody-coated polySBMA efficiently captured Salmonella with only low background noise as compared to antibody-coated monolayers lacking the polymer brush. Finally, the antibody-coated polySBMA surfaces were exposed to a mixture of Alexa Fluor 647-labeled FIB and Salmonella without the prior use of a blocking solution to evaluate the ability of the surfaces to capture bacteria while simultaneously repelling proteins. The fluorescent images showed the capture of Salmonella with no adsorption of FIB as compared to antibody-coated epoxide surfaces, demonstrating the potential of the zwitterionic layer in preventing the nonspecific adsorption of the proteins during the detection of bacteria in complex matrices.

The preparation of reagentless electrochemical immunosensor based on a nano-gold and chitosan hybrid film for human chorionic gonadotrophin

A novel amperometric immunosensor for determination of human serum chorionic gonadotrophin (hCG) was constructed by immobilizing hCG antibody with nano-gold and chitosan (CHIT) hybrid film electrochemical co-deposited on a glassy carbon electrode. The fabricated procedures and electrochemical behaviors of proteins on such an interface were characterized with electrochemical impedance, cyclic voltammetry and chronoamperometry. The effect of incubation time on the electrochemical behavior of immunosensor was studied and other experimental conditions such as mass ratio of CHIT and HAuCl4, depositing time of hybrid film, the pH, and temperature were studied too. The current response of the immunosensor is in linear relationship with concentration of hCG. The resulting immunosensor offers an excellent amperometric response for hCG ranging from 0.20 to 100 mIU mL(-1) with a detection limit of 0.1 mIU mL(-1). A satisfactory determination of hCG in serum samples can be obtained by this immunosensor.

Feasibility studies on si-based biosensors

The aim of this paper is to summarize the efforts carried out so far in the fabrication of Si-based biosensors by a team of researchers in Catania, Italy. This work was born as a collaboration between the Catania section of the Microelectronic and Microsystem Institute (IMM) of the CNR, the Surfaces and Interfaces laboratory (SUPERLAB) of the Consorzio Catania Ricerche and two departments at the University of Catania: the Biomedical Science and the Biological Chemistry and Molecular Biology Departments. The first goal of our study was the definition and optimization of an immobilization protocol capable of bonding the biological sensing element on a Si-based surface via covalent chemical bonds. We chose SiO(2) as the anchoring surface due to its biocompatibility and extensive presence in microelectronic devices. The immobilization protocol was tested and optimized, introducing a new step, oxide activation, using techniques compatible with microelectronic processing. The importance of the added step is described by the experimental results. We also tested different biological molecule concentrations in the immobilization solutions and the effects on the immobilized layer. Finally a MOS-like structure was designed and fabricated to test an electrical transduction mechanism. The results obtained so far and the possible evolution of the research field are described in this review paper.

Biosensor analyses of serum autoantibodies: application to antiphospholipid syndrome and systemic lupus erythematosus

Autoimmune disorders are rare human diseases characterized by the presence of circulating autoantibodies that bind the body's own structural compounds as target antigens. The detection of autoantibodies is important for the diagnostic process. Immunofluorescence and immunoassay methods do not allow a reliable characterization of binding characteristics. Therefore, novel analytical techniques should be considered. This review describes the application of surface plasmon resonance biosensor systems for the diagnosis of autoimmune disorders. The covalent attachment of native antigens to the sensor chip is a suitable method for obtaining highly reproducible analyses of autoantibodies, allowing the evaluation of kinetic rate and affinity constants, and it may enable the identification of disease-relevant autoantibodies linked to disease progression. The autoantibody microarray is another future-oriented technique. Patterns of differential antigen recognition should allow early diagnosis. This is due to the fact that a broad range of autoreactive B cell responses in autoimmune disorders can only be mirrored by including a sufficient number of antigens in a microarray format.

A reagentless amperometric immunosensor for alpha-1-fetoprotein based on gold nanowires and ZnO nanorods modified electrode

A novel strategy for the preparation of reagentless immunosensor for rapid determination of alpha-1-fetoprotein (AFP) in human serum has been developed. The immunosensor was prepared by immobilizing alpha-1-fetoprotein antibody (AFP Ab) onto the glassy carbon electrode modified by gold nanowires (Au NWs) and ZnO nanorods (ZnO NRs) composite film. Gold nanowires and ZnO nanorods were produced by an electrodeposition strategy using nanopore polycarbonate (PC) membrane. A sandwich immunoassay format was employed to detect AFP with horseradish peroxidase (HRP)-labeled AFP as tracer. The morphology of the Au NWs and ZnO NRs composite film has been investigated by scanning electron microscopy (SEM) and energy disperse spectroscopy (EDS) analysis. The resulting immunosensor offered an excellent amperometric response for AFP ranging from 0.5 to 160.0ngmL(-1) with a detection limit of 0.1ngmL(-1). Because of the combination of the biocompatibility of ZnO NRs and the direct electron-transfer of Au NWs between HRP and electrode, the proposed immunosensor displayed a direct electrochemical response of HRP to the reduction of H(2)O(2) with high sensitivity, quick response, good repeatability and long-term stability.

Amperometric immunosensor based on toluidine blue/nano-Au through electrostatic interaction for determination of carcinoembryonic antigen

A new current amplified immunosensor for the determination of carcinoembryonic antigen (CEA) was demonstrated in this work. The electrode architecture was fabricated by positively charged toluidine blue (TB) coated on negatively charged poly-sulfanilic acid (PSAA) modified glassy carbon electrode (GCE) surface through electrostatic interactions to form a TB/PSAA film, which provided an interface containing amine groups to assemble gold nanoparticles (nano-Au) for immobilization of carcinoembryonic antibody (anti-CEA) and horseradish peroxidase (HRP) instead of bovine serum albumin (BSA) to block sites against non-specific binding. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were employed to characterize the electrochemical properties of the modified processes. The CVs reduction current of the immunosensor charged linearly in two concentration ranges of CEA from 0.5 to 5.0 and 5.0 to 120.0 ng/ml in presence of 0.3mM H2O2 in analyte solution, and the detection limit was 0.2 ng/ml at three times background noise. The proposed method is economical, efficient and potentially attractive for clinical immunoassays.

Comparison of random and oriented immobilisation of antibody fragments on mixed self-assembled monolayers

The sensitivity of immunosensors is strongly dependent on the amount of immobilised antibodies and their remaining antigen binding properties. The use of smaller and well-oriented antibody fragments as bioreceptor molecules influences the final immunosensor signal. The aim of this study was to compare the immunosensor responses of different immobilised antibody fragments, such as F(ab')2 and Fab', with their parental IgG. In addition, we evaluated the oriented versus the random covalent immobilisation method of the Fab' fragments. First, an optimisation of cleavage protocol to generate these F(ab')2 and Fab' fragments was performed. Subsequently, we pursued a study with limited denaturation effects during immobilisation of the bioreceptor molecules and with reduced steric hindrance during antigen binding using mixed self-assembled monolayers (SAM) of thiols as the chemical linking layer. The Surface Plasmon Resonance technique was used to evaluate the degree of immobilisation of the antibody fragments and their parental IgGs on the mixed SAMs and the binding signals of their specific antigens. In this study, we demonstrate that for a particular antibody/antigen system (anti-hIgG/hIgG), the optimised fragmentation protocol in combination with an oriented immobilisation of Fab' fragments on mixed SAMs leads to a >2-fold increase of the antigen binding signals compared to randomly covalent immobilised full-length antibodies.

Photoimmobilization of Proteins for Affinity Capture Combined with MALDI TOF MS Analysis

Affinity capture surfaces can be prepared in a number of ways. A method of obtaining such surfaces through UV-activated immobilization of binding proteins using a benzophenone derivative is reported. Photoimmobilized protein G was used to selectively capture and preconcentrate bovine IgG from a mixture with BSA, and the affinity of photoattached concanavalin A toward ovalbumin was compared with that of commercially available concanavalin A on agarose beads. The results of the capture after tryptic digestion were analyzed by MALDI TOF MS. Immobilized trypsin was also prepared through photoimmobilization and later used to digest hemoglobin. Immobilized enzyme digestion resulted in more partial cleavages than solution-phase digestion. More methionine and tryptophan oxidation was also observed. Photoimmobilization was shown to be a quick and easy way of immobilizing ligands on surfaces.

Biosensors in clinical chemistry

Biosensors are analytical devices composed of a recognition element of biological origin and a physico-chemical transducer. The biological element is capable of sensing the presence, activity or concentration of a chemical analyte in solution. The sensing takes place either as a binding event or a biocatalytical event. These interactions produce a measurable change in a solution property, which the transducer converts into a quantifiable electrical signal. Present-day applications of biosensors to clinical chemistry are reviewed, including basic and applied research, commercial applications and fabrication techniques. Recognition elements include enzymes as biocatalytic recognition elements and immunoagents and DNA segments as affinity ligand recognition elements, coupled to electrochemical and optical modes of transduction. The future will include biosensors based on synthetic recognition elements to allow broad applicability to different classes of analytes and modes of transduction extending lower limits of sensitivity. Microfabrication will permit biosensors to be constructed as arrays and incorporated into lab-on-a-chip devices.

Deriving topological constraints from functional data for the design of reagentless fluorescent immunosensors

The possibility of obtaining, from any antibody, a fluorescent conjugate which responds to the binding of the antigen by a variation of fluorescence, would be of great interest in the micro- and nano-analytical sciences. This possibility was explored with antibody mAb4E11, which is directed against the dengue virus and for which no structural data is available. Three rules of design were developed to identify residues of the antibody to which a fluorophore could be chemically coupled, after changing them to cysteine by mutagenesis. (i) The target residue belonged to the hypervariable loops of the antibody. (ii) It was adjacent, along the amino acid sequence of the antibody, to a residue which was functionally important for the interaction with the antigen. (iii) It was not important in itself for the interaction with the antigen. Eight conjugates between a single chain variable fragment of mAb4E11 and an environment-sensitive fluorophore were constructed. Three of them showed an increase in their fluorescence intensity by 1.5-2.8-fold on antigen binding, without loss of affinity. This increase allowed the titration of the antigen in serum above a threshold concentration of 10nM. Experiments of quenching with potassium iodide suggested that the fluorescence variation was due to a shielding of the fluorescent group from the solvent by the binding of the antigen, and that therefore its mechanism is general.

Knowledge-based design of reagentless fluorescent biosensors from recombinant antibodies

The possibility of obtaining from any antibody a fluorescent conjugate which responds to the binding of the antigen by a variation of its fluorescence, would be of great interest in the analytical sciences and for the construction of protein chips. This possibility was explored with antibody mAbD1.3 directed against hen egg white lysozyme. Rules of design were developed to identify the residues of the antibody to which a fluorophore could be chemically coupled, after changing them to cysteine by mutagenesis. These rules were based on: the target residue belonging to a topological neighbourhood of the antigen in the structure of the complex between antibody and antigen; its absence of functional importance for the interaction with the antigen; and its solvent accessibility in the structure of the free antibody. Seventeen conjugates between the single-chain variable fragment scFv of mAbD1.3 and an environment-sensitive fluorophore were constructed. For six of the ten residues which fully satisfied the design rules, the relative variation of the fluorescence intensity between the free and bound states of the conjugate was comprised between 12 and 75% (in non-optimal buffer), and the affinity of the conjugate for lysozyme remained unchanged relative to the parental scFv. In contrast, such results were true for only one of the seven residues which failed to satisfy one of the rules and were used as controls. One of the conjugates was studied in more detail. Its fluorescence increased proportionally to the concentration of lysozyme in a nanomolar range, up to 90% in a defined buffer, and 40% in serum. This increase was specific for hen egg lysozyme and it was not observed with a closely related protein, turkey egg lysozyme. The residues which gave operational conjugates (six in V(L) and one in V(H)), were located in the immediate vicinity of residues which are functionally important, along the sequence of FvD1.3. The results suggest rules of design for constructing antigen-sensitive fluorescent conjugates from any antibody, in the absence of structural data.

On-line post-capillary affinity detection of immunoglobulin G for capillary zone electrophoresis

To address the quality issues of antibody manufacturing, post-capillary affinity detection of immunoglobulin G (IgG) is developed for capillary zone electrophoresis. In analogy to a two-dimensional separation system, capillary zone electrophoresis (CZE), as the first dimension, resolves IgG variants based on their differences in molecular structure. IgG variants separated by CZE are discriminated against other serum and cellular proteins by affinity complex formation with protein A binding fragment in a post-capillary reactor. The analytical power of post-capillary affinity detection is demonstrated for rapid and selective heterogeneity analysis of human IgG subclasses and monoclonal antibodies in complex sample matrices. By comparing with pre-capillary formation of affinity complexes between IgG and protein A, post-capillary affinity detection clearly exhibit greater resolving power for examining IgG microheterogeneity. Affinity complex formation prior to CZE analysis, however, has the advantage of lower detection limits. Detection limits suffer with post-capillary affinity detection because of the high fluorescence background contributed by the fluorescently labeled protein A in the post-capillary reactor, and the need to determine a small change in the background level upon complex formation.