Regeneration of ethyl parathion antibodies for repeated use in immunosensor: a study on dissociation of antigens from antibodies

Recent progress in biosensor regeneration techniques

Biosensors are widely used in various applications, from medical diagnostics to environmental monitoring. Their widespread and continuous use necessitates regeneration methods to ensure cost-effectiveness and sustainability. In the realm of advancing human-centric bioelectronics for continuous monitoring, employing these sensors for real-time, in situ detection of biomarkers presents a considerable challenge. This mini-review examines diverse strategies utilized for the regeneration of biosensors, categorizing them based on their underlying mechanisms and discussing representative works. We explore methods ranging from surface engineering/re-functionalization, chemical treatments, allosteric regulation of bioreceptors, to manipulations of electric/magnetic fields, highlighting their working principles and exemplary studies. The advantages of each method, such as simplicity, high regeneration efficiency, and versatility, are discussed alongside their challenges, including degradation over cycles, limited applicability, and potential damage to sensors. As the demand for continuous and real-time biosensing escalates, the development of efficient and reliable regeneration strategies becomes essential. This mini-review offers an overview of the current landscape of biosensor regeneration, aiming to guide future research and innovations in this area.

Prefab Hollow Glass Microsphere-Based Immunosensor with Liquid Crystal Sensitization for Acute Myocardial Infarction Biomarker Detection

Quantitative detection of cardiac troponin biomarkers in blood is an important method for clinical diagnosis of acute myocardial infarction (AMI). In this work, a whispering gallery mode (WGM) microcavity immunosensor based on a prefab hollow glass microsphere (HGMS) with liquid crystal (LC) sensitization was proposed and experimentally demonstrated for label-free cardiac troponin I-C (cTnI-C) complex detection. The proposed fiber-optic immunosensor has a simple structure; the tiny modified HGMS serves as the key sensing element and the microsample reservoir simultaneously. A sensitive LC layer with cTnI-C recognition ability was deposited on the inner wall of the HGMS microcavity. The arrangement of LC molecules is affected by the cTnI-C antigen-antibody binding in the HGMS, and the small change of the surface refractive index caused by the binding can be amplified owing to the birefringence property of LC. Using the annular waveguide of the HGMS, the WGMs were easily excited by the coupling scanning laser with a microfiber, and an all-fiber cTnI-C immunosensor can be achieved by measuring the resonant wavelength shift of the WGM spectrum. Moreover, the dynamic processes of the cTnI-C antigen-antibody binding and unbinding was revealed by monitoring the wavelength shift continuously. The proposed immunosensor with a spherical microcavity can be a cost-effective tool for AMI diagnosis.

N protein-based ultrasensitive SARS-CoV-2 antibody detection in seconds via 3D nanoprinted, microarchitected array electrodes

Rapid detection of antibodies to SARS-CoV-2 is critical for COVID-19 diagnostics, epidemiological research, and studies related to vaccine evaluation. It is known that the nucleocapsid (N) is the most abundant protein of SARS-CoV-2 and can serve as an excellent biomarker due to its strong immunogenicity. This paper reports a rapid and ultrasensitive 3D biosensor for quantification of COVID-19 antibodies in seconds via electrochemical transduction. This sensor consists of an array of three-dimensional micro-length-scale electrode architecture that is fabricated by aerosol jet 3D printing, which is an additive manufacturing technique. The micropillar array is coated with N proteins via an intermediate layer of nano-graphene and is integrated into a microfluidic channel to complete an electrochemical cell that uses antibody-antigen interaction to detect the antibodies to the N protein. Due to the structural innovation in the electrode geometry, the sensing is achieved in seconds, and the sensor shows an excellent limit of detection of 13 fm and an optimal detection range of 100 fm to 1 nm. Furthermore, the sensor can be regenerated at least 10 times, which reduces the cost per test. This work provides a powerful platform for rapid screening of antibodies to SARS-CoV-2 after infection or vaccination.

Fiber-integrated WGM optofluidic chip enhanced by microwave photonic analyzer for cardiac biomarker detection with ultra-high resolution

Cardiac troponin I (cTnI) plays an important role in emergency diagnosis of cardiovascular diseases, which exists predominately in the form of cardiac troponin I-C (cTnI-C) complex. We proposed a fiber-integrated optofluidic chip immunosensor with time-delay-dispersion based microwave photonic analyzer (MPA) for cTnI-C detection. The whispering gallery mode (WGM) fiber probe was fabricated by embedding a polydopamine functionalized hollow glass microsphere (HGMS) into the etched capillary-fiber structure, and the WGMs could be excited through the efficient coupling between the thin-wall capillary and the HGMS. The reflective WGM optofluidic chip functioned as a wavelength tuner to construct fiber ring laser cavity, whose laser output wavelength was cTnI-C concentration-dependent. The tiny wavelength variation of sensing laser was converted into a radio-frequency (RF) response, which was retrieved by measuring the change of RF-domain free spectrum range (FSR) in time-delay-dispersion based MPA, and the quantitative detection of cTnI-C complex can be achieved with high resolution. Experimental results show that this immunosensor had a limit of detection (LOD) of 0.59 ng/mL, and a detection resolution of 1.2 fg/mL. The relative resolving power was 10²-10⁴-fold higher than that of others optical fiber cTnI biosensors. The proposed fiber-integrated optofluidic chip provides an innovative lab-on-chip diagnostic tool for myocardial damage.

Sensing of COVID-19 Antibodies in Seconds via Aerosol Jet Nanoprinted Reduced-Graphene-Oxide-Coated 3D Electrodes

Rapid diagnosis is critical for the treatment and prevention of diseases. An advanced nanomaterial-based biosensing platform that detects COVID-19 antibodies within seconds is reported. The biosensing platform is created by 3D nanoprinting of three-dimensional electrodes, coating the electrodes by nanoflakes of reduced-graphene-oxide (rGO), and immobilizing specific viral antigens on the rGO nanoflakes. The electrode is then integrated with a microfluidic device and used in a standard electrochemical cell. When antibodies are introduced on the electrode surface, they selectively bind with the antigens, changing the impedance of the electrical circuit which is detected via impedance spectroscopy. Antibodies to SARS-CoV-2 spike S1 protein and its receptor-binding-domain (RBD) are detected at a limit-of-detection of 2.8 × 10-15 and 16.9 × 10-15 m, respectively, and read by a smartphone-based user interface. The sensor can be regenerated within a minute by introducing a low-pH chemistry that elutes the antibodies from the antigens, allowing successive sensing of test samples using the same sensor. Sensing of S1 and RBD antibodies is specific, which cross-reacts neither with other antibodies such as RBD, S1, and nucleocapsid antibody nor with proteins such as interleukin-6. The proposed sensing platform could also be useful to detect biomarkers for other infectious agents such as Ebola, HIV, and Zika.

Detection of a novel glycodelin biomarker using electrochemical immunosensor for endometriosis

Endometriosis is one of the important issues in women worldwide, which decreases the quality of women's lives in their reproductive age. The diagnosis of endometriosis is carried out by the invasive procedure, which is expensive and painful. In the last few decades, researchers have given more attention to constructing a suitable biomarker-based biosensor for semi/non-invasive diagnosis of endometriosis. As a result, glycodelin (GLY) was found as a promising biomarker because of its selectivity and sensitivity. To the best of our knowledge, it was the first study that reported the detection of GLY biomarker using an electrochemical immunosensor. Briefly, a label-free electrochemical immunosensing platform was constructed through in-situ surface modification of cysteamine layer and immobilisation of antibody (anti-GLY) with help of glutaraldehyde. The interaction between antigen and antibody was measured using square wave voltammetry (SWV). The SWV signal could decrease proportionally with the increasing GLY concentration ranging from 1 to 1000 ng mL^-1 (R² = 0.9981) and a detection limit (LOD) of 0.43 ng mL^-1. Moreover, an immunosensor could exhibit high sensitivity, selectivity, long-term stability, reproducibility and regeneration. Accuracy of the immunosensor was compared with enzyme-linked immunosorbent assay (ELISA), and satisfying results were obtained. The detection of GLY biomarker may be a new possibility for endometriosis diagnosis.

Aptamers vs. antibodies as capture probes in optical porous silicon biosensors

Over the past decade aptamers have emerged as a promising class of bioreceptors for biosensing applications with significant advantages over conventional antibodies. However, experimental studies comparing aptasensors and immunosensors, under equivalent conditions, are limited and the results are inconclusive, in terms of benefits and limitations of each bioreceptor type. In the present work, the performance of aptamer and antibody bioreceptors for the detection of a his-tagged protein, used as a model target, is compared. The bioreceptors are immobilized onto a nanostructured porous silicon (PSi) thin film, used as the optical transducer, and the target protein is detected in a real-time and label-free format by reflective interferometric Fourier transform spectroscopy. For the antibodies, random-oriented immobilization onto the PSi nanostructure results in a poor biosensing performance. Contrary, Fc-oriented immobilization of the antibodies shows a similar biosensing performance to that exhibited by the aptamer-based biosensor, in terms of binding rate, dynamic detection range, limit of detection and selectivity. The aptasensor outperforms in terms of its reusability and storability, while the immunosensor could not be regenerated for subsequent experiments.

Flexible and Printed Electrochemical Immunosensor Coated with Oxygen Plasma Treated SWCNTs for Histamine Detection

Heterocyclic amine histamine is a well-known foodborne toxicant (mostly linked to "scombroid poisoning") synthesized from the microbial decarboxylation of amino acid histidine. In this work, we report the fabrication of a flexible screen-printed immunosensor based on a silver electrode coated with single-walled carbon nanotubes (SWCNTs) for the detection of histamine directly in fish samples. Biosensors were realized by first spray depositing SWCNTs on the working electrodes and by subsequently treating them with oxygen plasma to reduce the unwanted effects related to their hydrophobicity. Next, anti-histamine antibodies were directly immobilized on the treated SWCNTs. Histamine was detected using the typical reaction of histamine and histamine-labeled with horseradish peroxidase (HRP) competing to bind with anti-histamine antibodies. The developed immunosensor shows a wide linear detection range from 0.005 to 50 ng/mL for histamine samples, with a coefficient of determination as high as 98.05%. Average recoveries in fish samples were observed from 96.00% to 104.7%. The biosensor also shows good selectivity (less than 3% relative response for cadaverine, putrescine, and tyramine), reproducibility, mechanical and time stability, being a promising analytical tool for the analysis of histamine, as well as of other food hazards.

Regeneration of Arrayed Gold Microelectrodes Equipped for a Real-Time Cell Analyzer

The label-free cell-based assay is advantageous for biochemical study because of it does not require the use of experimental animals. Due to its ability to provide more dynamic information about cells under physiological conditions than classical biochemical assays, this label-free real-time cell assay based on the electric impedance principle is attracting more attention during the past decade. However, its practical utilization may be limited due to the relatively expensive cost of measurement, in which costly consumable disposable gold microchips are used for the cell analyzer. In this protocol, we have developed a general strategy to regenerate arrayed gold microelectrodes equipped for a commercial label-free cell analyzer. The regeneration process includes trypsin digestion, rinsing with ethanol and water, and a spinning step. The proposed method has been tested and shown to be effective for the regeneration and repeated usage of commercial electronic plates at least three times, which will help researchers save on the high running cost of real-time cell assays.

Recent Advances Incorporating Superparamagnetic Nanoparticles into Immunoassays

Superparamagnetic nanoparticles (SPMNPs) have attracted interest for various biomedical applications due to their unique magnetic behavior, excellent biocompatibility, easy surface modification, and low cost. Their unique magnetic properties, superparamagnetism, and magnetophoretic mobility have led to their inclusion in immunoassays to enhance biosensor sensitivity and allow for rapid detection of various analytes. In this review, we describe SPMNP characteristics valuable for incorporation into biosensors, including the use of SPMNPs to increase detection capabilities of surface plasmon resonance and giant magneto-resistive biosensors. The current status of SPMNP-based immunoassays to improve the sensitivity of rapid diagnostic tests is reviewed, and suggested strategies for the successful adoption of SPMNPs for immunoassays are presented.

A novel general and efficient technique for dissociating antigen in circulating immune complexes

Circulating immune complexes (CICs) are produced during the immune response. It is more clinically important to establish a general and efficient CICs dissociation technique for the detection of antigens for CICs other than the detection of free antigens in the serum. Polyethylene glycol (PEG) two-precipitation separation and glycine-HCl as a buffer system were employed to develop a general and efficient buffer dissociation technique to separate CICs from serum and dissociate antigens from CICs. The measurement value of new PEG two-precipitation separation technique was higher than traditional PEG precipitation separation technique. There were slight differences in the dissociation conditions of HCV Core-IC, HIV P24-IC, Ins-IC and TG-IC as compared to HBsAg-IC. The detection of antigens in HBsAg-IC, HCV Core-IC, HIV P24-IC, Ins-IC and TG-IC with this technique was superior to that with HCl Dissociation, Trypsin Digestion or Immune Complex Transfer technique. PEG two-precipitation dissociation technique may reduce macromolecular protein and the adhesion of free antigens during the co-precipitation, which increases the efficiency of separation and precipitation of CICs. This technique also avoids the damage of reagents to antigens, assuring the repeatability, reliability and validity. Thus, this technique is application in samples negative or positive for free antigens.

Biosensor Regeneration: A Review of Common Techniques and Outcomes

Biosensors are ideally portable, low-cost tools for the rapid detection of pathogens, proteins, and other analytes. The global biosensor market is currently worth over 10 billion dollars annually and is a burgeoning field of interdisciplinary research that is hailed as a potential revolution in consumer, healthcare, and industrial testing. A key barrier to the widespread adoption of biosensors, however, is their cost. Although many systems have been validated in the laboratory setting and biosensors for a range of analytes are proven at the concept level, many have yet to make a strong commercial case for their acceptance. Though it is true with the development of cheaper electrodes, circuits, and components that there is a downward pressure on costs, there is also an emerging trend toward the development of multianalyte biosensors that is pushing in the other direction. One way to reduce the cost that is suitable for certain systems is to enable their reuse, thus reducing the cost per test. Regenerating biosensors is a technique that can often be used in conjunction with existing systems in order to reduce costs and accelerate the commercialization process. This article discusses the merits and drawbacks of regeneration schemes that have been proven in various biosensor systems and indicates parameters for successful regeneration based on a systematic review of the literature. It also outlines some of the difficulties encountered when considering the role of regeneration at the point of use. A brief meta-analysis has been included in this review to develop a working definition for biosensor regeneration, and using this analysis only ∼60% of the reported studies analyzed were deemed a success. This highlights the variation within the field and the need to normalize regeneration as a standard process across the field by establishing a consensus term.

Sensitive detection of atrazine in tap water using TELISA

Sensitive detection of atrazine in tap water using a universal platform: novel TELISA.

Towards maintenance-free biosensors for hundreds of bind/release cycles

A single aptamer bioreceptor layer was formed using a common streptavidin-biotin immobilization strategy and employed for 100-365 bind/release cycles. Chemically induced aptamer unfolding and release of its bound target was accomplished using alkaline solutions with high salt concentrations or deionized (DI) water. The use of DI water scavenged from the ambient atmosphere represents a first step towards maintenance-free biosensors that do not require the storage of liquid reagents. The aptamer binding affinity was determined by surface plasmon resonance and found to be almost constant over 100-365 bind/release cycles with a variation of less than 5% relative standard deviation. This reversible operation of biosensors based on immobilized aptamers without storage of liquid reagents introduces a conceptually new perspective in biosensing. Such new biosensing capability will be important for distributed sensor networks, sensors in resource-limited settings, and wearable sensor applications.

A rapid method to regenerate piezoelectric microcantilever sensors (PEMS)

Piezoelectric microcantilever sensors (PEMS) can be sensitive tools for the detection of proteins and cells in biological fluids. However, currently available PEMS can only be used a single time or must be completely stripped and refunctionalized prior to subsequent uses. Here we report the successful use of an alternative regeneration protocol employing high salt concentrations to remove the target, leaving the functional probe immobilized on the microcantilever surface. Our model system employed the extracellular domain (ECD) of recombinant human Epidermal Growth Factor Receptor (EGFR) as the probe and anti-human EGFR polyclonal antibodies as the target. We report that high concentrations of MgCl₂ dissociated polyclonal antibodies specifically bound to EGFR ECD immobilized on the sensor surface without affecting its bioactivity. This simple regeneration protocol both minimized the time required to re-conjugate the probe and preserved the density of probe immobilized on PEMS surface, yielding identical biosensor sensitivity over a series of assays.

Tumor delivery of antisense oligomer using trastuzumab within a streptavidin nanoparticle

PURPOSE

Trastuzumab (Herceptin™) is often internalized following binding to Her2+ tumor cells. The objective of this study was to investigate whether trastuzumab can be used as a specific carrier to deliver antisense oligomers into Her2+ tumor cells both in vitro and in vivo.

METHODS

A biotinylated MORF oligomer antisense to RhoC mRNA and its biotinylated sense control were labeled with either lissamine for fluorescence detection or 99mTc for radioactivity detection and were linked to biotinylated trastuzumab via streptavidin. The nanoparticles were studied in SUM190 (RhoC+, Her2+) study and SUM149 (RhoC+, Her2−) control cells in culture and as xenografts in mice.

RESULTS

As evidence of unimpaired Her2+ binding of trastuzumab within the nanoparticle, accumulations were clearly higher in SUM190 compared to SUM149 cells and, by whole-body imaging, targeting of SUM190 tumor was similar to that expected for a radiolabeled trastuzumab. As evidence of internalization, fluorescence microscopy images of cells grown in culture and obtained from xenografts showed uniform cytoplasm distribution of the lissamine-MORF. An invasion assay showed decreased RhoC expression in SUM190 cells when incubated with the antisense MORF nanoparticles at only 100 nM.

CONCLUSION

Both in cell culture and in animals, the nanoparticle with trastuzumab as specific carrier greatly improved tumor delivery of the antisense oligomer against RhoC mRNA into tumor cells overexpressing Her2 and may be of general utility.

P-glycoprotein inhibition by membrane cholesterol modulation

P-glycoprotein (Pgp) is a transmembrane protein that actively exports lipophilic chemotherapeutics from the cells causing multidrug resistance. Pgp molecules are partially localized in TX-100-resistant rafts, and the activity of the transporter is highly sensitive to the presence of cholesterol. To better understand these relationships, the influence of membrane cholesterol content on Pgp function, as measured via calcein accumulation, was studied in correlation with changes elicited in membrane structure. Membrane cholesterol was modulated by heptakis(2,6-di-O-methyl)-beta-cyclodextrin (DIMEB) and the cholesterol inclusion complex of DIMEB (Chol-DIMEB). Changes in membrane cholesterol level were reflected by alterations in the overall lipid packing as measured by Merocyanine 540 (MC540) staining and were also accompanied by changes in the raft association of the pump. DIMEB and Chol-DIMEB treatments have also lead to increased permeability of the cell membrane in both directions, raising the possibility that the effects on pumping efficiency reflect leakage of ATP also from the non-permeabilized cells. However, the treatments did not influence the intracellular ATP levels of the non-permeabilized cells. Our data suggest that Pgp inhibition by cyclodextrin treatments arises through modulation of its membrane microenvironment, rather than as a result of concomitant cytotoxicity.

Comparison of electrochemical immunosensors based on gold nano materials and immunoblot techniques for detection of histidine-tagged proteins in culture medium

In this work, the direct electrochemical determination of poly-histidine tagged proteins using immunosensor based on anti-His (C-term) antibody immobilized on gold electrodes modified with 1,6-hexanedithiol, gold colloid particles or gold nanorods is described. The recombinant histidine-tagged silk proteinase inhibitor protein (rSPI2-His(6)) expressed in Pichia system selected as antigen for this immonosensor. An electrochemical impedance spectroscopy was used as label free detection technique for immune conjugation. The gold nanorods modified electrode layer showed better analytical response than gold nano particles. The linear calibration range was observed between 10 pg/ml and 1 ng/ml with limit of detection 5 pg/ml (S/N=3). Up to four successive assay cycles with retentive sensitivity were achieved for the immunosensors regenerated with 0.2M glycine-HCl buffer, pH 2.8. The performance of this immnosensor were compared with immuoblotting techniques.

Label-free flow-enhanced specific detection of Bacillus anthracis using a piezoelectric microcantilever sensor

Differentiation between species of similar biological structure is of critical importance in biosensing applications. Here, we report specific detection of Bacillus anthracis (BA) spores from that of close relatives, such as B. thuringiensis (BT), B. cereus (BC), and B. subtilis (BS) by varying the flow speed of the sampling liquid over the surface of a piezoelectric microcantilever sensor (PEMS). Spore binding to the anti-BA spore IgG coated PEMS surface is determined by monitoring the resonance frequency change in the sensor's impedance vs. frequency spectrum. Flow increases the resonance frequency shift at lower flow rates until the impingement force from the flow overcomes the binding strength of the antigen and decreases the resonance frequency shift at higher flow rates. We showed that the change from increasing to decreasing resonance frequency shift occurred at a lower fluid flow speed for BT, BC, and BS spores than for BA spores. This trend reduces the cross reactivity ratio of BC, BS, and BT to the anti-BA spore IgG immobilized PEMS from around 0.4 at low flow velocities to less than 0.05 at 3.8 mm s(-1). This cross reactivity ratio of 0.05 was essentially negligible considering the experimental uncertainty. The use of the same flow that is used for detection to further distinguish the specific binding (BA to anti-BA spore antibody) from nonspecific binding (BT, BC, and BS to anti-BA spore antibody) is unique and has great potential in the detection of general biological species.

Amperometric immunosensor for detecting Schistosoma mansoni antibody

An immunosensor for detecting the antibody anti-apyrase of Schistosoma mansoni based on rigid composite materials, containing graphite powder and epoxy resins, developed in this work, is described. A surface modification strategy for the use of oxidized graphite in the detection of antibody-antigen interaction was developed. This modification strategy is based on silanization of conductive composite. First, the graphite powder-epoxy resin was treated with concentrated hydrogen peroxide to improve surface hydroxyl groups and to form a hydrophilic layer. Second, 3- aminopropyltriethoxysilane was subsequently used to functionalize the treated surface to form amino groups, which were further activated with glutaraldehyde to introduce a layer of aldehyde groups. Contact angle microscopy and scanning electron microscopy were used as a qualitative analysis of the deposition of silane on the surface of the sensor. The effectiveness of the modification strategy was validated by amperometric immunoassays of S. mansoni. Amperometric signals related to concentrations of this immobilized protein were observed, and the effects of pH and incubation times were analyzed. This surface modification strategy provides a platform on which proteins can be directly immobilized for immunosensor and protein array applications.

Aspects of recent development of immunosensors

This chapter focuses on the recent developments in the field of immunosensors. Immunosensors incorporate the specific immunochemical reaction with the modern transducers including electrochemical (potentiometric, conductometric, capacitative, impedance, amperometric), optical (fluorescence, luminescence, refractive index), and microgravimetric transducers. These immunosensor devices with dramatic improvements in the sensitivity and selectivity possess the abilities to investigate the reaction dynamics of antibody–antigen binding and the potential to revolutionize conventional immunoassay techniques. With the rapid development of immunological reagents and detection equipments, immunosensors have allowed an increasing range of analytes to be identified and quantified and in particular, simple-to-use, inexpensive, and reliable immunosensing systems have been developed for areas such as outpatient monitoring, large screening programs, and remote environmental surveillance. Immunosensors with lowered detection limits and increased sensitivities have been developed in various fields, particularly in clinical analysis. A noticeable development trend is also observed in the development of immunosensors combining with other techniques such as flow injection analysis (FIA) or capillary electrophoretic (CE) analysis, which complement and improve the present immunoassay methods. Belov et al. have proposed a novel immunophenotyping method for leukemias which uses a cluster of differentiation antibody microarray, and a microarray of enzyme-linked immunosorbent assay has been developed for autoimmune diagnosis of systematic rheumatic disease. Development of microfluidic immunosensor systems for proteomics and drug discovery have also been reported in recent years where the microfluidic system integrates multiple processes in a single device to improve analytical performance by reducing the reagent consumption and the analysis time, and increasing reliability and sensitivity through automation.

Improving immunobinding using oriented immobilization of an oxidized antibody

Recent technical advances in biorecognition engineering and microparticle fabrication enabled us to develop a single-step purification process using magnetic particles (MPs). The process is simple, efficacious, easy to automate, and economical. The method immobilizes the ligand molecule in a particular orientation on commercial MPs that have surface carboxyl groups. Mouse IgG and anti-mouse IgG antibody were the model capture and ligand molecules for this study. The immunobinding efficacy of anti-mouse IgG antibody using "oriented immobilization" was compared with the efficacy of a conventional amine-coupling system that results in random orientation and of another standard method, the biotin-streptavidin system. The oriented immobilization was accomplished by oxidizing the sugar moiety in the CH(2) domain of the antibody's Fc and covalently conjugating the moiety to the hydrazine-coated MP. The specific binding affinity of the oriented immobilization process was about 2.5 times that of the amine-coupling system, and selectivity from a binary mixture was about 2 times greater for the oriented immobilization method. Results were nearly identical for the biotin-streptavidin system and the oriented immobilization system, matching the calculated binding stoichiometry between mouse IgG and anti-mouse IgG antibody. The binding improvement over the amine-coupling system shown by assay was confirmed by a separate surface plasmon resonance experiment. In summary, the oriented immobilization method was as effective as the streptavidin-biotin system, yet simpler and cost-effective.

Quantitative evaluation of sensitivity and selectivity of multiplex nanoSPR biosensor assays

A new functionalization procedure was developed to replace cyltrimethylammoniumbromide coating on gold nanorods (GNRs) fabricated through seed-mediated growth with chemically active alkanethiols; antibodies were then attached to the GNRs to yield gold nanorod molecular probes (GNrMPs). The functionalization procedure was shown to minimize nonspecific binding. Multiplex sensing was demonstrated for three targets (goat anti-human IgG, goat anti-rabbit IgG, and goat anti-mouse IgG) through the distinct response of the plasmon spectra of GNrMPs to binding events. Quantification of the plasmonic binding events and estimation of ligand binding kinetics tethered to these nanoscale structures was also demonstrated through a mathematical approach. Evaluation of the experimental and theoretical data yields an affinity constant K(a) = 1.34 x 10(7) M(-1), which was in agreement with the IgG-antiIgG binding affinity reported in the literature. The GNrMP sensors were found to be highly specific and sensitive with the dynamic response in the range between 10(-9) M and 10(-6) M. The limit of detection of GNrMPs was found to be in the low nanomolar range, and is a function of the binding affinity: for a higher probe-target affinity pair, the limit of detection can be expected to reach femto molar levels. This technique can play a key role in developing tunable sensors for sensitive and precise monitoring of biological interactions.

Cell killing by antibody-drug conjugates

Antibody-drug conjugates (ADCs) are designed to specifically bind to and kill cells expressing their target antigens. In addition to the obvious requirement of the presence of the target antigen on the cell surface, several other factors contribute to the sensitivity of target cells to the action of ADCs. These include (i) the rate of internalization of the ADC, (ii) its proteolytic degradation in late endosomes and lysosomes and the subsequent release of cytotoxic drug, and (iii) the intracellular concentration of the released drug. In addition to killing antigen-expressing cells, some ADCs were found to kill bystander cells irrespective of their antigen expression. This review summarizes the current knowledge of the mechanisms of killing of antigen-expressing and bystander cells by antibody-drug conjugates.

Multiplex biosensor using gold nanorods

Gold nanorods (GNRs) with different aspect ratios were fabricated through seed-mediated growth and surface activation by alkanethiols for the attachment of antibodies to yield gold nanorod molecular probes (GNrMPs). Multiplex sensing was demonstrated by the distinct response of the plasmon spectra of the GNrMPs to binding events of three targets (goat anti-human IgG1 Fab, rabbit anti-mouse IgG1 Fab, rabbit anti-sheep IgG (H+L)). Plasmonic sensors are highly specific and sensitive and can be used to monitor refractive index changes caused by molecular interactions in their immediate vicinity with potential to achieve single-particle biosensing. This technique can play a key role in developing novel optical biosensors for both in vivo and in vitro detection and single-receptor kinetics.

In situ detection of Bacillus anthracis spores using fully submersible, self-exciting, self-sensing PMN-PT/Sn piezoelectric microcantilevers

In this study, we have demonstrated in situ, all-electrical detection of Bacillus anthracis (BA) spores using lead magnesium niobate-lead titanate/tin (PMN-PT/Sn) piezoelectric microcantilever sensors (PEMS) fabricated from PMN-PT freestanding films and electrically insulated with methyltrimethoxysilane (MTMS) coatings on the tin surface. Antibody specific to BA spore surface antigen was immobilized on the platinum electrode of the PMN-PT layer. In phosphate-buffered saline (PBS) solution, the PMN-PT/Sn PEMS exhibited quality (Q) values ranging from 50 to 75. The detection was carried out in a closed-loop flow cell with a liquid volume of 0.8 ml and a flow rate of 1 ml min(-1). It was shown that one sensor, "PEMS-A" (500 microm long, 800 microm wide, with a 22 microm thick PMN-PT layer, a 20 microm thick tin layer and a 1 +/- 0.5 x 10(-12) g Hz(-1) mass detection sensitivity) exhibited resonance frequency shifts of 2100 +/- 200, 1100 +/- 100 and 700 +/- 100 Hz at concentrations of 20,000, 2000, and 200 spores ml(-1) or 16,000, 1600, and 160 total spores, respectively. Additionally, "PEMS-B" (350 microm long, 800 microm wide, with an 8 microm thick PMN-PT layer, a 6 microm thick tin layer and a 2 +/- 1 x 10(-13) g Hz(-1) mass detection sensitivity) exhibited resonance frequency shifts of 2400 +/- 200, 1500 +/- 200, 500 +/- 150 and 200 +/- 100 Hz at concentrations of 20,000, 2000, 100, and 45 spores ml(-1) or 16,000, 1600, 80, and 36 total spores, respectively.

Multiplexed detection of mycotoxins in foods with a regenerable array

The occurrence of different mycotoxins in cereal products calls for the development of a rapid, sensitive, and reliable detection method that is capable of analyzing samples for multiple toxins simultaneously. In this study, we report the development and application of a multiplexed competitive assay for the simultaneous detection of ochratoxin A (OTA) and deoxynivalenol (DON) in spiked barley, cornmeal, and wheat, as well as in naturally contaminated maize samples. Fluoroimmunoassays were performed with the Naval Research Laboratory array biosensor, by both a manual and an automated version of the system. This system employs evanescent-wave fluorescence excitation to probe binding events as they occur on the surface of a waveguide. Methanolic extracts of the samples were diluted threefold with buffer containing a mixture of fluorescent antibodies and were then passed over the arrays of mycotoxins immobilized on a waveguide. Fluorescent signals of the surface-bound antibody-antigen complexes decreased with increasing concentrations of free mycotoxins in the extract. After sample analysis was completed, surfaces were regenerated with 6 M guanidine hydrochloride in 50 mM glycine, pH 2.0. The limits of detection determined by the manual biosensor system were as follows: 1, 180, and 65 ng/g for DON and 1, 60, and 85 ng/g for OTA in cornmeal, wheat, and barley, respectively. The limits of detection in cornmeal determined with the automated array biosensor were 15 and 150 ng/g for OTA and DON, respectively.

Optical immunosensors for environmental monitoring: How far have we come?

Immunosensing has proved to be a very interesting research area. This review discusses what has actually been achieved in the field of optical immunosensing for environmental screening, and what still needs to be done. The review is presented from a practical point of view. In terms of the basic design of the immunosensor, there is a trend towards decreasing assay time; indeed, this has been reduced from 15-20 minutes to less than 5 minutes. Another goal is to simplify the manifold, and label-free approaches combining indirect assay formats and the detection of antibody binding are popular. Rapid displacement assays have also been investigated thoroughly. In terms of some important features of immunosensing devices, the reusability of the sensing element has been studied in great depth, and working lifetimes of more than five hundred assays can now be found for all assay formats. Multianalyte assays are now being investigated, and current systems are able to monitor 2-3 target compounds, although this number is set to increase greatly (to >30) in the near future. In this sense, an increasing number of publications can be found on microarrays intended for multianalyte determinations. The application of immunosensing to real situations is the main challenge. Immunosensors are barely commercialized and are yet to be established as research or routine tools, due to a lack of validated protocols for a wide range of sample matrices. Regarding compounds considered as analytes, some significant pollutants such as dioxins or pharmaceuticals are rarely chosen as targets, although the current tendency is towards a broader spectrum of analytes. New immunoreagents should be raised for these compounds, for use in immunosensors that can be used as screening tools.

Immunosensing based on site-directed immobilization of antibody fragments and polymers that reduce nonspecific binding

Antibody Fab'-fragments can be directly coupled onto gold, and the space between the fragments can be filled with protein repellent disulfide bearing polymers. Coupling of the antibody Fab'-fragments, and thus both the amount of nonspecific binding and antigen binding but also the ability to regenerate the layer, is dependent on the immobilization procedure. First, the immobilization has taken place by coupling the Fab'-fragments to the surface and thereafter attaching the polymer in the remaining space between the antibodies. Second, the Fab'-fragments have been added after the surface has been coated by polymer. Third, the Fab'-fragments and polymer have been added onto the surface from the same solution. Up to 80% of the antigen could be removed during regeneration, if proper concentrations of polymer and Fab'-fragments were immobilized onto the gold surface. Only about 60% of the antigen could be removed, when the fragments were coupled directly onto a clean Au surface before the polymer or if low concentrations of polymer were attached onto gold before the Fab'-fragments. The first immobilization method, however, showed the highest response to antigen.