Site-directed antibody immobilization techniques for immunosensors

Single-step label-free nanowell immunoassay accurately quantifies serum stress hormones within minutes

A non-faradaic label-free cortisol sensing platform is presented using a nanowell array design, in which the two probe electrodes are integrated within the nanowell structure. Rapid and low volume (≤5 μl) sensing was realized through functionalizing nanoscale volume wells with antibodies and monitoring the real-time binding events. A 28-well plate biochip was built on a glass substrate by sequential deposition, patterning, and etching steps to create a stack nanowell array sensor with an electrode gap of 40 nm. Sensor response for cortisol concentrations between 1 and 15 μg/dl in buffer solution was recorded, and a limit of detection of 0.5 μg/dl was achieved. Last, 65 human serum samples were collected to compare the response from human serum samples with results from the standard enzyme-linked immunosorbent assay (ELISA). These results confirm that nanowell array sensors could be a promising platform for point-of-care testing, where real-time, laboratory-quality diagnostic results are essential.

Metal Nanoparticle and Quantum Dot Tags for Signal Amplification in Electrochemical Immunosensors for Biomarker Detection

With the increasing importance of healthcare and clinical diagnosis, as well as the growing demand for highly sensitive analytical instruments, immunosensors have received considerable attention. In this review, electrochemical immunosensor signal amplification strategies using metal nanoparticles (MNPs) and quantum dots (Qdots) as tags are overviewed, focusing on recent developments in the ultrasensitive detection of biomarkers. MNPs and Qdots can be used separately or in combination with other nanostructures, while performing the function of nanocarriers, electroactive labels, or catalysts. Thus, different functions of MNPs and Qdots as well as recent advances in electrochemical signal amplification are discussed. Additionally, the methods most often used for antibody immobilization on nanoparticles, immunoassay formats, and electrochemical methods for indirect biomarker detection are overviewed.

Advances in Molecularly Imprinted Polymers Based Affinity Sensors (Review)

Recent challenges in biomedical diagnostics show that the development of rapid affinity sensors is very important issue. Therefore, in this review we are aiming to outline the most important directions of affinity sensors where polymer-based semiconducting materials are applied. Progress in formation and development of such materials is overviewed and discussed. Some applicability aspects of conducting polymers in the design of affinity sensors are presented. The main attention is focused on bioanalytical application of conducting polymers such as polypyrrole, polyaniline, polythiophene and poly(3,4-ethylenedioxythiophene) ortho-phenylenediamine. In addition, some other polymers and inorganic materials that are suitable for molecular imprinting technology are also overviewed. Polymerization techniques, which are the most suitable for the development of composite structures suitable for affinity sensors are presented. Analytical signal transduction methods applied in affinity sensors based on polymer-based semiconducting materials are discussed. In this review the most attention is focused on the development and application of molecularly imprinted polymer-based structures, which can replace antibodies, receptors, and many others expensive affinity reagents. The applicability of electrochromic polymers in affinity sensor design is envisaged. Sufficient biocompatibility of some conducting polymers enables to apply them as "stealth coatings" in the future implantable affinity-sensors. Some new perspectives and trends in analytical application of polymer-based semiconducting materials are highlighted.

Conducting Polymers in the Design of Biosensors and Biofuel Cells

Fast and sensitive determination of biologically active compounds is very important in biomedical diagnostics, the food and beverage industry, and environmental analysis. In this review, the most promising directions in analytical application of conducting polymers (CPs) are outlined. Up to now polyaniline, polypyrrole, polythiophene, and poly(3,4-ethylenedioxythiophene) are the most frequently used CPs in the design of sensors and biosensors; therefore, in this review, main attention is paid to these conducting polymers. The most popular polymerization methods applied for the formation of conducting polymer layers are discussed. The applicability of polypyrrole-based functional layers in the design of electrochemical biosensors and biofuel cells is highlighted. Some signal transduction mechanisms in CP-based sensors and biosensors are discussed. Biocompatibility-related aspects of some conducting polymers are overviewed and some insights into the application of CP-based coatings for the design of implantable sensors and biofuel cells are addressed. New trends and perspectives in the development of sensors based on CPs and their composites with other materials are discussed.

Antibody CD133 Biofunctionalization of Ammonium Acryloyldimethyltaurate and Vinylpyrrolidone Co-Polymer-Based Coating of the Vascular Implants

Current vascular stents, such as drug eluting stents (DES), have some serious drawbacks, like in stent restenosis and thrombosis. Therefore, other solutions are sought to overcome these post-implantations complications. These include the strategy of biofunctionalization of the stent surface with antibodies that facilitate adhesion of endothelial cells (ECs) or endothelial progenitor cells (EPCs). Rapid re-endothelialization of the surface minimizes the risk of possible complications. In this study, we proposed ammonium acryloyldimethyltaurate/vinylpyrrolidone co-polymer-based surface (AVC), which was mercaptosilanized in order to expose free thiol groups. The presence of free thiol groups allowed for the covalent attachment of CD133 antibodies by disulfide bridges formation between mercaptosilanized surface and cysteine of the protein molecule thiol groups. Various examinations were performed in order to validate the procedure, including attenuated total reflection–Fourier transform infrared spectroscopy (ATR-FTIR) and Fourier transform Raman spectroscopy (FT-Raman), atomic force microscopy (AFM) and scanning electron microscopy (SEM). By means of ATR-FTIR spectroscopy presence of the CD133 antibody within coating was confirmed. In vitro studies proved good biocompatibility for blood cells without induction of hemolytic response. Thus, proposed biofunctionalized CD133 antibody AVC surface has shown sufficient stability for adapting as cardiovascular implant coating and biocompatibility. According to conducted in vitro studies, the modified surface can be further tested for applications in various biological systems.

Progress and Insights in the Application of MXenes as New 2D Nano-Materials Suitable for Biosensors and Biofuel Cell Design

Recent progress in the application of new 2D-materials-MXenes-in the design of biosensors, biofuel cells and bioelectronics is overviewed and some advances in this area are foreseen. Recent developments in the formation of a relatively new class of 2D metallically conducting MXenes opens a new avenue for the design of conducting composites with metallic conductivity and advanced sensing properties. Advantageous properties of MXenes suitable for biosensing applications are discussed. Frontiers and new insights in the area of application of MXenes in sensorics, biosensorics and in the design of some wearable electronic devices are outlined. Some disadvantages and challenges in the application of MXene based structures are critically discussed.

Nanostructured functional peptide films and their application in C-reactive protein immunosensors

Peptides with an active redox molecule are incorporated into nanostructured films for electrochemical biosensors with stable and controllable physicochemical properties. In this study, we synthesized three ferrocene (Fc)-containing peptides with the sequence Fc-Glu-(Ala)_n-Cys-NH₂, which could form self-assembled monolayers on gold and be attached to antibodies. The peptide with two alanines (n = 2) yielded the immunosensor with the highest performance in detecting C-reactive protein (CRP), a biomarker of inflammation. Using electrochemical impedance-derived capacitive spectroscopy, the limit of detection was 240 pM with a dynamic range that included clinically relevant CRP concentrations. With a combination of electrochemical methods and polarization-modulated infrared reflection-absorption spectroscopy, we identified the chemical groups involved in the antibody-CRP interaction, and were able to relate the highest performance for the peptide with n = 2 to chain length and efficient packing in the organized films. These strategies to design peptides and methods to fabricate the immunosensors are generic, and can be applied to other types of biosensors, including in low cost platforms for point-of-care diagnostics.

Antibody immobilization techniques in mass sensitive immunosensor: enhanced sensitivity through limited mass load

Background:

Biosensors are analytical devices that include a sample-delivery approach between a biological recognition element and a transducer required to convert the physicochemical change produced from the interaction of biological molecules-receptor interaction into signal. The immunosensor is a special type of biosensors that includes an antibody as a biorecognition element to detect analyte as antigens. In mass-sensitive sensors, antigen-antibody interactions can be specified by measuring the frequency change and most commonly knowns are surface acoustic wave, bulk acoustic wave, quartz crystal microbalance and microcantilevers.

Methods:

Different methods for antibody immobilization including functionalization of the transducer surface with specific groups have been reported for antibody immobilization. This stage affects the limit of detection and overall performance. In this review, perspectives on immobilization strategies of mass sensitive immunosensors according to transducer types will be presented. The choice of immobilization methods and their impact on performance in terms of capture molecule loading, orientation and signal improvement is will also be discussed.

Results:

One of the most critical point during configuration of the biorecognition layer is to improve the sensitivity. Therefore, we initially focused on comparisons of the antibody immobilization strategies in the biorecognition layer in terms of mass load level and high sensitivity.

Conclusion:

The lack of significant data on the mass accumulations up to the functionalization and antibody immobilization steps, which are the basis of immusensor production, has been identified. However, mass sensitive immunosensors have the potential to become more common and effective analytical devices for many application areas.

Broad-spectrum electrochemical immunosensor based on one-step electrodeposition of AuNP-Abs and Prussian blue nanocomposite for organophosphorus pesticide detection

Broad-spectrum antibodies can effectively recognize substances with similar structures and have broad application prospects in field rapid detection. In this study, broad-spectrum antibodies (Abs) against organophosphorus pesticides (OPs) were used as sensitive recognition elements, which could effectively recognize most OPs. Gold nanoparticles (AuNPs) have good biocompatibility. It combined with Abs to form a gold-labeled probe (AuNPs-Abs), which enhances the effective binding of antibodies to nanomaterials. Prussian blue (PB) was added to electrodeposition solution to enhance the conductivity, resulting in superior electrochemical performance. The AuNP-Abs-PB composite film was prepared by electrodeposition on the electrode surface to improve the anti-interference ability and stability of the immunosensor. Under the optimal experimental conditions, the immunosensor had a wide detection range (IC₂₀-IC₈₀: 1.82 × 10^-3-3.29 × 10⁴ ng/mL) and high sensitivity. Most importantly, it was simple to be prepared and could be used to detect multiple OPs.

Measuring protein biomarker concentrations using antibody tagged magnetic nanoparticles

Under physiological conditions biomarker concentrations tend to rise and fall over time e.g. for inflammation. Ex vivo measurements provide a snapshot in time of biomarker concentrations, which is useful, but limited. Approaching real time monitoring of biomarker concentration(s) using a wearable, implantable or injectable in vivo sensor is therefore an appealing target. As an early step towards developing an in vivo biomarker sensor, antibody (AB) tagged magnetic nanoparticles (NPs) are used here to demonstrate the in vitro measurement of ~5 distinct biomarkers with high specificity and sensitivity. In previous work, aptamers were used to target a given biomarker in vitro and generate magnetic clusters that exhibit a characteristic rotational signature quite different from free NPs. Here the method is expanded to detect a much wider range of biomarkers using polyclonal ABs attached to the surface of the NPs. Commercial ABs exist for a wide range of targets allowing accurate and specific concentration measurements for most significant biomarkers. We show sufficient detection sensitivity, using an in-house spectrometer to measure the rotational signatures of the NPs, to assess physiological concentrations of hormones, cytokines and other signaling molecules. Detection limits for biomarkers drawn mainly from pain and inflammation targets were: 10 pM for mouse Granzyme B (mGZM-B), 40 pM for mouse interferon-gamma (mIFN-γ), 7 pM for mouse interleukin-6 (mIL-6), 40 pM for rat interleukin-6 (rIL-6), 40 pM for mouse vascular endothelial growth factor (mVEGF) and 250 pM for rat calcitonin gene related peptide (rCGRP). Much lower detection limits are certainly possible using improved spectrometers and nanoparticles.

Hybrid Tamm-surface plasmon polariton mode for highly sensitive detection of protein interactions

The total internal reflection ellipsometry (TIRE) method was used for the excitation and study of the sensitivity properties of the hybrid Tamm plasmon polariton - surface plasmon polariton (TPP-SPP) and single surface plasmon resonance (SPR) modes of the GCSF receptor immobilization. Additionally, the optimized sensitivity of the hybrid TPP-SPP mode was investigated and compared with the single SPR mode when the BSA proteins formed a layer on the gold surface. The dispersion relations for the hybrid TPP-SPP and single SPR modes were used to explain the enhanced sensitivity of the ellipsometric parameters for the hybrid TPP-SPP mode over the conventional SPR. The SPP component (δΔ_h-SPP/δλ=53.9°/nm) of the hybrid TPP-SPP mode was about 6.4 times more sensitive than single SPR (δΔ_SPR/δλ=8.4°/nm) for the BSA protein layer on the gold film. It was found that the sensitivity of the hybrid plasmonic mode can be made controllable by using the strong coupling effect between the TPP and SPP components. The strong coupling regime reduces absorption and scattering losses of the metal for the SPP component in the hybrid TPP-SPP mode and, as a result, narrows the plasmonic resonance.

Toward a nanopaper-based and solid phase immunoassay using FRET for the rapid detection of bacteria

In this study, we propose a novel sensitive solid-based immunosensor developed on a plasmonic nanopaper platform for the detection of Escherichia coli (E. coli) bacteria. This plasmonic nanopaper that comprises of carboxylated bacterial cellulose (CBC) impregnated with gold nanoparticles (AuNP-CBC), employed as a quencher and a sustainable functionalized platform to be conjugated with protein A. Thus, the conjugated protein A allows the aligned linkage of EAb-QD (anti-E. coli conjugated quantum dot) and EAb-AF (anti-E. coli conjugated Alexa Fluor 488). Interestingly, once E. coli was captured by the AuNP-CBC/EAb-QD or AuNP-CBC/EAb-AF, the energy transfer from the QD or Alexa Fluor fluorophores is triggered due to the conformational change in the antibody structure and this, in turn, causes a decrease in the distance between fluorophores and the quencher nanopaper and, therefore diminishing their photoluminescence. The immunosensors performed successfully to recognize E. coli at concentrations as low as 50 CFU mL⁻¹ in the standard buffer. The examined functionality of the immunosensors in a real matrix such as chicken extract and lettuce juice demonstrated a highly efficient response while QD is the main fluorophore with a limit of detection around 100 CFU mL⁻¹.

Electrochemical label free sensing of human IgG - Protein A interaction

A novel and rapid Electrochemical Immunosensing platform was developed for the direct sensing of antibody human immuno globulin gamma (IgG) interaction with virulence factor of S. aureus, staphylococcal protein A (SpA) in the presence of electroactive redox couple ferri/ferro cyanide (K₃/K₄[Fe(CN)₆]). The receptor SpA was attached to BioPE-DOTAP binary lipid bilayer tethered on alkane thiol molecular cushions. Atomic force microscopy (AFM), High-resolution transmission electron microscope (TEM), Fourier-transform infrared spectroscopy (FT-IR), dynamic light scattering (DLS) techniques were used to study the molecular interactions. The AFM images showed array like formation of BioPE-DOTAP on the monolayer surface. The IgG sensor showed a linear range from 10^-21 M to 10^-16 M.

Electrochemical Immunosensing Platform for the Determination of the 20S Proteasome Using an Aminophenylboronic/Poly-indole-6-carboxylic Acid-Modified Electrode

The first electrochemical immunosensor for the determination of the 20S proteasome (P20S) was developed, entailing the immobilization of an antibody on an aminophenylboronic/poly-indole-6-carboxylic acid-modified electrode. The proposed electrochemical bioplatform is a simple and feasible analytical tool applicable for the determination of P20S in human plasma, considering its high clinical and biological relevance. Cyclic voltammetry, electrochemical impedance spectroscopy, and square wave voltammetry (SWV) were used to determine the optimal step-by-step process to obtain the electrochemical immunosensor. The interaction of P20S with the recognition layer of the immobilized antibody on the nanostructured surface took place by incubating the electrode in a P20S solution at 20 °C for 2 h. Using SWV as an electro-analytical technique, this immunosensor can quantify P20S. The current was linear with the P20S concentration within two dynamic concentration ranges from 20.0 to 80.0 and 80.0 to 200.0 ng·mL^-1 (r² = 0.992 and 0.98, respectively) with a limit of detection and quantification of 6 and 18 ng·mL^-1, respectively. Moreover, the immunosensor showed considerable repeatability and reproducibility, when the determination was done in human serum, which confirms that it is a promising alternative for direct detection of P20S in biological fluids with minimal interference.

Controlling orientation, conformation, and biorecognition of proteins on silane monolayers, conjugate polymers, and thermo-responsive polymer brushes: investigations using TOF-SIMS and principal component analysis

Control over orientation and conformation of surface-immobilized proteins, determining their biological activity, plays a critical role in biointerface engineering. Specific protein state can be achieved with adjusted surface preparation and immobilization conditions through different types of protein-surface and protein-protein interactions, as outlined in this work. Time-of-flight secondary ion mass spectroscopy, combining surface sensitivity with excellent chemical specificity enhanced by multivariate data analysis, is the most suited surface analysis method to provide information about protein state. This work highlights recent applications of the multivariate principal component analysis of TOF-SIMS spectra to trace orientation and conformation changes of various proteins (antibody, bovine serum albumin, and streptavidin) immobilized by adsorption, specific binding, and covalent attachment on different surfaces, including self-assembled monolayers on silicon, solution-deposited polythiophenes, and thermo-responsive polymer brushes. Multivariate TOF-SIMS results correlate well with AFM data and binding assays for antibody-antigen and streptavidin-biotin recognition. Additionally, several novel extensions of the multivariate TOF-SIMS method are discussed.

Graphical abstract

Core-Shell Magnetic Nanoparticles for Highly Sensitive Magnetoelastic Immunosensor

A magnetoelastic (ME) biosensor for wireless detection of analytes in liquid is described. The ME biosensor was tested against human IgG in the range 0-20 μg∙mL-1. The sensing elements, anti-human IgG produced in goat, were immobilized on the surface of the sensor by using a recently introduced photochemical immobilization technique (PIT), whereas a new amplification protocol exploiting gold coated magnetic nanoparticles (core-shell nanoparticles) is demonstrated to significantly enhance the sensitivity. The gold nanoflowers grown on the magnetic core allowed us to tether anti-human IgG to the nanoparticles to exploit the sandwich detection scheme. The experimental results show that the 6 mm × 1 mm × 30 μm ME biosensor with an amplification protocol that uses magnetic nanoparticles has a limit of detection (LOD) lower than 1 nM, works well in water, and has a rapid response time of few minutes. Therefore, the ME biosensor is very promising for real-time wireless detection of pathogens in liquids and for real life diagnostic purpose.

The application of antibody-aptamer hybrid biosensors in clinical diagnostics and environmental analysis

The growing number of various diseases and the increase of environmental contamination are the causes for the development of novel methods for their detection. The possibility of the application of affinity-based biosensors for such purposes seems particularly promising as they provide high selectivity and low detection limits. Recently, the usage of hybrid antibody-aptamer sandwich constructs was shown to be more advantageous in terms of working parameters in comparison to aptamer-based and immune-based biosensors. This review is focused on the usage of hybrid antibody-aptamer receptor layers for the determination of clinically and environmentally important target molecules. In this work, antibodies and aptamer molecules are characterized and the methods of their immobilization as well as analytical signal generation are shown. This is followed by the critical presentation of examples of hybrid sandwich biosensors that have been elaborated in the past 12 years.

Functional Molecular Interfaces for Impedance-Based Diagnostics

In seeking to develop and optimize reagentless electroanalytical assays, a consideration of the transducing interface features lies key to any subsequent sensitivity and selectivity. This review briefly summarizes some of the most commonly used receptive interfaces that have been employed within the development of impedimetric molecular sensors. We discuss the use of high surface area carbon, nanoparticles, and a range of bioreceptors that can subsequently be integrated. The review spans the most commonly utilized biorecognition elements, such as antibodies, antibody fragments, aptamers, and nucleic acids, and touches on some novel emerging alternatives such as nanofragments, molecularly imprinted polymers, and bacteriophages. Reference is made to the immobilization chemistries available along with a consideration of both optimal packing density and recognition probe orientation. We also discuss assay-relevant mechanistic details and applications in real sample analysis.

Immunoassay of interferon gamma by quartz crystal microbalance biosensor

Interferon gamma (IFNγ) is a cytokine and an immunochemical marker that can be used for revealing of infectious diseases and especially for distinguishing of viral and some types of bacterial infections. Blood tests for IFNγ are typically based on immunoassays like Enzyme-Linked Immunosorbent Assay (ELISA). In this paper, a biosensor working on the principle of quartz crystal microbalance (QCM) was developed as an alternative to the standard analytical methods for IFNγ. The biosensor contained antibodies against IFNγ immobilized on QCM and also on gold nanoparticles. A sandwich containing QCM, gold nanoparticles and IFNγ was formed and formation of the sandwich caused decrease of oscillation frequency. The assay exerted limit of detection 5.7 pg/ml for a sample sized 50 μl and one measuring cycle was finished within 90 min. The assay by biosensor fully correlated to standard ELISA. In a conclusion, the biosensor appears to be a fully applicable analytical tool for a simple assay of IFNγ. Overall simplicity and no special requirement on staff and equipment are the major advantages of the here presented assay.

Biosensor surface functionalization by a simple photochemical immobilization of antibodies: experimental characterization by mass spectrometry and surface enhanced Raman spectroscopy

Surface functionalization is a key step in biosensing since it is the basis of an effective analyte recognition. Among all the bioreceptors, antibodies (Abs) play a key role thanks to their superior specificity, although the available immobilization strategies suffer from several drawbacks. When gold is the interacting surface, the recently introduced Photochemical Immobilization Technique (PIT) has been shown to be a quick, easy-to-use and very effective method to tether Abs oriented upright by means of thiols produced via tryptophan mediated disulphide bridge reduction. Although the molecular mechanism of this process is quite well identified, the detailed morphology of the immobilized antibodies is still elusive due to inherent difficulties related to the microscopy imaging of Abs. The combination of Mass Spectrometry, Surface-Enhanced Raman Spectroscopy and Ellman's assay demonstrates that Abs irradiated under the conditions in which PIT is realized show only two effective disulphide bridges available for binding. They are located in the constant region of the immunoglobulin light chain so that the most likely position Ab assumes is side-on, i.e. with one Fab (i.e. the antigen binding portion of the antibody) exposed to the solution. This is not a limitation of the recognition efficiency in view of the intrinsic flexibility of the Ab structure, which makes the free Fab able to sway in the solution, a feature of great importance in many biosensing applications.

Magnetic-Based Enrichment of Rare Cells from High Concentrated Blood Samples

Here, we tested two magnetic-bead based systems for the enrichment and detection of rare tumor cells in concentrated blood products. For that, the defined numbers of cells from three pancreatic cancer cell lines were spiked in 108 peripheral blood mononuclear cells (PBMNCs) concentrated in 1 mL, mimicking diagnostic leukapheresis (DLA) samples, and samples were processed for circulating tumor cells (CTC) enrichment with the IsoFlux or the KingFisher systems, using different types of magnetic beads from the respective technology providers. Beads were conjugated with different anti-EpCAM and MUC-1 antibodies. Recovered cells were enumerated and documented by fluorescent microscopy. For the IsoFlux system, best performance was obtained with IsoFlux CTC enrichment kit, but these beads compromised the subsequent immunofluorescence staining. For the KingFisher system, best recoveries were obtained using Dynabeads Biotin Binder beads. These beads also allowed one to capture CTCs with different antibodies and the subsequent immunofluorescence staining. KingFisher instrument allowed a single and streamlined protocol for the enrichment and staining of CTCs that further prevented cell loss at the enrichment/staining interface. Both IsoFlux and KingFisher systems allowed the enrichment of cell line cells from the mimicked-DLA samples. However, in this particular experimental setting, the recovery rates obtained with the KingFisher system were globally higher, the system was more cost-effective, and it allowed higher throughput.

Guided mode resonance sensor for the parallel detection of multiple protein biomarkers in human urine with high sensitivity

The rising cost of global healthcare provision and new approaches to managing disease are driving the development of low-cost biosensing modalities, such as label-free photonic methods based on dielectric resonances. Here, we use the combined sensing and imaging capability of a guided mode resonance (GMR) sensor to detect multiple biomarkers (troponin, procalcitonin and C-Reactive Protein) in parallel in undiluted urine samples. A key requirement of such a biosensor is the simple and direct functionalization with suitable antibodies to ensure the disease-specific detection of protein biomarkers. Here, antibodies were immobilized using a succinimidyl-[(N-maleimidopropionamido)-hexaethyleneglycol] ester (SM(PEG)6) spacer. The polyethylene glycol (PEG) chemistry enables low detection limits of 10 pg mL-1 or better for all protein biomarkers, while minimizing non-specific binding compared to more commonly used strategies such as (3-Aminopropyl)triethoxysilane (APTES) or dextran. Our approach supports the vision of a simple yet highly sensitive diagnostic platform that could be used for pre-screening patients for a wide range of diseases at point-of-care, thereby relieving the pressure on overstretched healthcare services.

Electrochemical immunosensor towards invasion-associated protein p60: An alternative strategy for Listeria monocytogenes screening in food

This work reports the development of an electrochemical immunosensor for rapid, specific and decentralized detection of the invasion-associated protein p60 secreted by Listeria monocytogenes, a life-threatening foodborne pathogen. A disposable screen-printed electrode was used as transducer surface and monoclonal and polyclonal antibodies that specifically recognize Listeria monocytogenes p60 protein and Listeria spp. p60 proteins, respectively, were used as the sandwich immuno-pair. The reaction was detected with the aid of an additional secondary antibody conjugated with the enzyme reporter (alkaline phosphatase) and using 3-indoxyl phosphate/silver ions as the mixture substrate. The analytical signal was acquired through the voltammetric stripping of the enzymatically deposited silver, which was directly correlated to p60 concentration in the sample. In optimized conditions, a limit of detection and quantification of 1.5 ng mL^-1 and 5.1 ng mL^-1 were achieved, respectively, in a useful time (<3 h). As proof-of-concept, the proposed immunosensor was successfully applied to spiked milk samples, demonstrating to be a suitable device for further use in real sample detection of Listeria monocytogenes in food products.

A Label-Free Immunosensor Based on Graphene Oxide/Fe3O4/Prussian Blue Nanocomposites for the Electrochemical Determination of HBsAg

In this article, a highly sensitive label-free immunosensor based on a graphene oxide (GO)/Fe3O4/Prussian blue (PB) nanocomposite modified electrode was developed for the determination of human hepatitis B surface antigen (HBsAg). In this electrochemical immunoassay system, PB was used as a redox probe, while GO/Fe3O4/PB nanocomposites and AuNPs were prepared and coated on screen-printed electrodes to enhance the detection sensitivity and to immobilize the hepatitis B surface antibody (HBsAb). The immunosensor was fabricated based on the principle that the decrease in peak currents of PB is proportional to the concentration of HBsAg captured on the modified immunosensor. The experimental results revealed that the immunosensor exhibited a sensitive response to HBsAg in the range of 0.5 pg·mL-1 to 200 ng·mL-1, and with a low detection limit of 0.166 pg·mL-1 (S/N = 3). Furthermore, the proposed immunosensor was used to detect several clinical serum samples with acceptable results, and it also showed good reproducibility, selectivity and stability, which may have a promising potential application in clinical immunoassays.

Stepwise Preparation of a Polymer Comprising Protein Building Blocks on a Solid Support for Immunosensing Platform

In immunosensing, immobilization of the antibody on the sensing platform significantly influences the performance of the sensor. Herein, we propose a novel antibody-immobilization method based on a protein-polymer chain containing multiple copies of an antibody-binding protein, the Z-domain. In our approach, the Z-domain-containing polymer is prepared on the surface of the sensing platform with a biotinylation reaction from the archaeon Sulfolobus tokodaii. Biotinylation from S. tokodaii has a unique property by which biotin protein ligase (BPL) forms an extremely stable complex with its biotinylated substrate protein (BCCP). Here, we employed two types of engineered proteins: one was the fusion protein of BCCP with the Z-domain (BZB), in which BCCP was genetically attached to the N- and C-termini of the Z-domain; the other was a BPL dimer prepared by connecting two BPL molecules with a cross-linking reagent. We applied these two engineered proteins alternately onto the BPL-modified solid support of the surface plasmon resonance sensor chip, and succeeded in growing polymer chains comprising multiple units of BZB and the BPL dimer. The antibody-binding capability of the Z-domain-containing polymer thus prepared is adjustable by controlling the number of cycles of protein addition and the surface density of the polymer on the solid support.

Perspectives, Tendencies, and Guidelines in Affinity-Based Strategies for the Recovery and Purification of PEGylated Proteins

In recent years, the effective purification of PEGylated therapeutic proteins from reaction media has received particular attention. Although several techniques have been used, affinity-based strategies have been scarcely explored despite the fact that, after PEGylation, marked changes in the molecular affinity parameters of the modified molecules are observed. With this in mind, future contributions in the bioseparation of these polymer-protein conjugates are expected to exploit affinity in chromatographic and nonchromatographic techniques which will surely derive in the integration of different operations. However, this will only occur as novel ligands which are simultaneously found. As it will be mentioned, these novel ligands may be screened or designed. In both cases, computer-aided tools will support their identification or development. Additionally, ligand discovery by high-throughput screening (HTS) is believed to become a fast, economic, and informative technology that will aid in the mass production of ligands along with genetic engineering and related technologies. Therefore, besides analyzing the state of the art in affinity separation strategies for PEGylated molecules, this review proposes a basic guideline for the selection of adequate ligands to provide information and prospective on the future of affinity operations in solving this particular bioengineering problem.

A Novel Surface Modification and Immobilization Method of Anti-CD25 Antibody on Nonwoven Fabric Filter Removing Regulatory T Cells Selectively

Anti-CD25 antibodies were immobilized on polypropylene (PP) nonwoven fabrics to specifically remove mouse regulatory T cells (Tregs) from mouse spleen cells. PP fibers were coated with peptide nanosheets, which were prepared by self-assembling of a mixture of X-poly(sarcosine)-b-(l-Leu-Aib)₆ (X: glycolic acid or a phenylboronic acid) and Y-poly(sarcosine)-b-(d-Leu-Aib)₆ (Y: glycolic acid or diazirine derivative). Anti-CD25 antibodies were immobilized by covalent linking between the sugar moiety of the antibody and the phenylboronic acid group on the peptide nanosheet. The removal rate of mouse Tregs from the mouse spleen cells was more than 95% only by passing the filters, while the nonspecific removal rates of other cells were less than 15%. The coating of peptide nanosheets on PP fibers was very effective to provide a suitable environment for the immobilized antibody to interact with the counterpart cells while the coating suppressed nonspecific adsorption of other cells.

de la Fuente J, Moros M. Nanoparticle-based biosensors for detection of extracellular vesicles in liquid biopsies

Selecting the appropriate nanoparticle, functionalization chemistry and sensing methodology can speed up the translation of liquid biopsies into the clinic.

Site-Specific and Covalent Immobilization of His-Tagged Proteins via Surface Vinyl Sulfone-Imidazole Coupling

Site-specific immobilization of proteins on a surface has been a long-lasting challenge in the fields of biosensing and biotechnology because of the need for improving the biological activity of immobilized protein via the maximal exposure of its bioactive domain. Herein, we reported a new site-specific immobilization method for His-tagged proteins onto a vinyl sulfone (VS)-bearing surface in a covalent manner. X-ray photoelectron spectroscopy characterization indicated the specificity of the addition reaction of the imidazole group in histidine on the VS-bearing surface at pH 7.0. Solution-based experiments were carried out to verify the reaction priority of the imidazole residue of histidine with the VS group at neutral conditions. The real-time immobilization process of two His-tagged proteins (HaloTag-6His and anti-HER2 Fab-6His) on surfaces presenting VS, preactivated carboxyl, and NTA groups were studied by quartz crystal microbalance. Compared to the existing methods utilizing covalent (NHS/EDC activated carboxyl) and coordinate (Ni²⁺-NTA) linking, our method offers two significant advantages for protein immobilization: high density and high specificity. The orientation of the two His-tagged proteins on the VS-bearing surface was confirmed by an enzyme-linked assay and an HER2⁺ liposome binding experiment. Our method of site-specific immobilization of His-tagged proteins is efficient and straightforward, which would be helpful to expand the applications of recombinant proteins in enzyme immobilization, biosensor and array fabrication, and drug delivery system.

Construction of Immunomagnetic Particles with High Stability in Stringent Conditions by Site-Directed Immobilization of Multivalent Nanobodies onto Bacterial Magnetic Particles for the Environmental Detection of Tetrabromobisphenol-A

Bacterial magnetic particles (BMPs) are an attractive carrier material for immunoassays because of their nanoscale size, dispersal ability, and membrane-bound structure. Antitetrabromobisphenol-A (TBBPA) nanobodies (Nbs) in the form of monovalence (Nb1), bivalence (Nb2), and trivalence (Nb3) were biotinylated and immobilized onto streptavidin (SA)-derivatized BMPs to construct the complexes of BMP-SA-Biotin-Nb1, -Nb2, and -Nb3, respectively. An increasing order of binding capability of BMP-SA-Biotin-Nb1, -Nb2, and -Nb3 to TBBPA was observed. These complexes showed high resilience to temperature (90 °C), methanol (100%), high pH (12), and strong ionic strength (1.37 M NaCl). A BMP-SA-Biotin-Nb3-based enzyme linked immunosorbent assay (ELISA) for TBBPA dissolved in methanol was developed, showing a half-maximum inhibition concentration (IC₅₀) of 0.42 ng mL^-1. TBBPA residues in landfill leachate, sewage, and sludge samples determined by this assay were in a range of <LOD-1.17 ng mL^-1, <LOD-0.75 ng mL^-1, and <LOD-3.65 ng g^-1 (dw), respectively, correlating well with the results by liquid chromatography tandem mass spectrometry. The BMP-SA-Biotin-Nb3 was reusable at least three times without significant loss of the binding capability. The BMP-SA-Biotin-Nb3-based ELISA, with a total assay time of less than 30 min, is promising for the rapid monitoring of TBBPA in the environment.

Highly-branched Cu2O as well-ordered co-reaction accelerator for amplifying electrochemiluminescence response of gold nanoclusters and procalcitonin analysis based on protein bioactivity maintenance

The point of fabricating ultrasensitive electrochemiluminescence (ECL)-based biosensors should be focused on how to maintain high immune recognition of antigens by antibodies in whole process. That is not effortless due to the structure of the protein can be destroyed root in toxic nanocarriers, excessive cyclic potential and superoxide radicals in coreactant, all of which can lead to reduce the bioactivity of antigen and antibody. In this work, the effect of negative voltage and divers coreactant on protein bioactivity were verified. Based on that, a motivated ECL biosensor with good biocompatibility was fabricated for procalcitonin (PCT) detection using Au nanoclusters (Au NCs) as low-potential cathodic luminophor and K₂S₂O₈ as non-toxic coreactant, respectively. Besides, highly-branched Cu₂O was utilized to catalyze K₂S₂O₈ and produce more radical anion SO₄^•-, which can oxidize Au NCs^•- to generate more high-energy-state Au NCs*, thus doubling the ECL intensity to meet the requirements of trace analysis. In addition, protein A (PA) as specific antibody capturer was employed to bind the Fc region of anti-PCT in an orientated way, further maintaining the physiological activity of antibody. As expected, all strategies undoubtedly practically improved the immune recognition of the biosensor and reduced the detection limit to 2.90 fg/mL.

Optimization of Microparticle Reagents to Collect and Detect Antibody

Bead reagents are used in a large number of assays in bioscience and biotechnology to collect and purify antibodies by immobilization. Bead-based immunoassays offer high-throughput analysis of multiple antibodies in a single sample. Although a variety of antibody-binding moieties on the collection beads have been studied, the physical and material properties of collection beads have not been optimized to isolate specific antibodies over a broad range of concentrations from complex environments containing cells. We present a study of how to optimally use microparticles coated with protein G to collect low concentrations of IgG antibodies from complex solutions. We study the impact of bead material, bead size, incubation time, and protein G density to more efficiently collect antibodies and detect specific antibodies via fluorescent antigen labeling. The minimum detectable limit and the minimum incubation time for antibody collection are used as metrics to evaluate the collection parameters. We found that larger silica beads can capture more antibodies from a low concentration of sample, with a minimum incubation time of 60 min to equilibrium binding, resulting in a minimum detectable concentration of antibodies of 26 nM. We show that simple biophysical optimization of antibody collection reagents can be used to improve the collection of low concentrations of antibodies in complex environments. We demonstrate that the technology may be useful for monitoring antibody secretions from hybridoma cultures.

Photoclick chemistry to create dextran-based nucleic acid microarrays

In the literature, there are reports of the utilization of various hydrogels to create generic platforms for protein microarray applications. Here, a novel strategy was developed to obtain high-performance microarrays. In it, a dextran hydrogel is used to covalently immobilize oligonucleotides and proteins. This method employs aqueous solutions of dextran methacrylate (Dx-MA), which is a biocompatible photopolymerizable monomer. Capture probes are immobilized inside the hydrogel via a light-induced thiol-acrylate coupling reaction at the same time as the dextran polymer is formed. Hydrogel microarrays based on this technique were prepared on different surfaces, such as a Blu-ray Disk and polycarbonate or alkene-functionalized glass slides, and these systems showed high probe-loading capabilities and good biorecognition yields. This methodology presents advantages such as a low cost, a short analysis time, a low limit of detection, and multiplexing capabilities, among others. Confocal fluorescence microscopy analysis demonstrated that in these hydrogel-based microarrays, receptor immobilization and the biorecognition event occurred within the hydrogel and not merely on the surface.

Application of antihelix antibodies in protein structure determination

Antibodies are indispensable tools in protein engineering and structural biology. Antibodies suitable for structural studies should recognize the 3-dimensional (3D) conformations of target proteins. Generating such antibodies and characterizing their complexes with antigens take a significant amount of time and effort. Here, we show that we can expand the application of well-characterized antibodies by "transplanting" the epitopes that they recognize to proteins with completely different structures and sequences. Previously, several antibodies have been shown to recognize the alpha-helical conformation of antigenic peptides. We demonstrate that these antibodies can be made to bind to a variety of unrelated "off-target" proteins by modifying amino acids in the preexisting alpha helices of such proteins. Using X-ray crystallography, we determined the structures of the engineered protein-antibody complexes. All of the antibodies bound to the epitope-transplanted proteins, forming accurately predictable structures. Furthermore, we showed that binding of these antihelix antibodies to the engineered target proteins can modulate their catalytic activities by trapping them in selected functional states. Our method is simple and efficient, and it will have applications in protein X-ray crystallography, electron microscopy, and nanotechnology.

Bioactivity-Protected Electrochemiluminescence Biosensor Using Gold Nanoclusters as the Low-Potential Luminophor and Cu2S Snowflake as Co-reaction Accelerator for Procalcitonin Analysis

The expansion of electrochemiluminescence (ECL) technology to immunoassay at the core of care emphasizes all immune molecules will not be inactivated in the analysis process. That poses a major challenge to ECL-based biosensors due to the deoxynucleotide sequences of an antigen or antibody could be oxidized through a route of excessive cyclic potential. Herein, an ultrasensitive ECL biosensor was developed based on a novel bioactivity-protected sensing strategy utilizing Au nanoclusters (Au NCs) as low-potential luminophor for detection of procalcitonin (PCT). Bovine serum albumin (BSA)-templated Au NCs exhibited a low-potential anodic ECL signal in triethylamine (TEA) solution at 0.87 V, where it is suitable for the survival of immune molecules. Taking advantage of good conductivity and high surface area, a Cu₂S snowflake not only functions as a satisfying substrate for connecting immune molecules but also acts as co-reaction accelerator to produce more cationic radicals TEA^•+, which could improve the ECL intensity needed to meet the requirements of trace analysis. Otherwise, HWRGWVC (HC-7) heptapeptide as specific antibody immobilizer for site-oriented fixation was introduced to further maintain the bioactivity of an antibody. In view of the preceding discussion, the obtained biosensor exhibited ultrahigh immune recognition to targets so that the detection limit was as low as an unprecedented value of 2.36 fg/mL, which will be of great significance to the application and development of a biosensor in the future.

Fabrication of a polypropylene immunoassay platform by photografting reaction

The technology of an immunoassay detection platform is critical to clinical disease diagnoses, especially for developing a medical diagnostic system. A polymer-based immunoassay platform was fabricated on nonwoven fabric polypropylene (PP) using a photografting reaction to graft 2-hydroxyethyl methacrylate (HEMA) and sulfobetaine (SBMA). The antifouling properties of PP-g-P(HEMA-co-SBMA) were investigated by fibrinogen adsorption and platelet adhesion. Carbonyldiimidazole was employed to activate the pendant hydroxyl groups in HEMA moieties and covalently coupled antibody molecules. The detection of the limit of the immunoassay platform was as low as 10 pg/mL. Antibody amount and bioactivity affected the availability of antibody and the sensitivity of immunoassay. The immune efficiency was dependent on the strategies of antibody immobilization. The immune efficiency of Au-g-P(SBMA-co-HEMA) and Au-SH surfaces measured by QCM-D was 165% and 35.7%, respectively. The covalently binding antibody via hydrophilic polymer chains as spacers could retain fragment antigen-binding up orientation, maintain the bioactivity of antibody, and mainly improve the accessibility of antibody molecules via adjusting the conformations of polymer chains when the antibodies recognized the antigens. Therefore, grafting hydrophilic polymers, such as zwitterionic PSBMA and reactive PHEMA onto nonwoven fabric PP, and binding antibody by covalent strategy had the potential to be developed as a commercial immunoassay platform.

Preactivation Crosslinking—An Efficient Method for the Oriented Immobilization of Antibodies

Crosslinking of proteins for their irreversible immobilization on surfaces is a proven and popular method. However, many protocols lead to random orientation and the formation of undefined or even inactive by-products. Most concepts to obtain a more targeted conjugation or immobilization requires the recombinant modification of at least one binding partner, which is often impractical or prohibitively expensive. Here a novel method is presented, which is based on the chemical preactivation of Protein A or G with selected conventional crosslinkers. In a second step, the antibody is added, which is subsequently crosslinked in the Fc part. This leads to an oriented and covalent immobilization of the immunoglobulin with a very high yield. Protocols for Protein A and Protein G with murine and human IgG are presented. This method may be useful for the preparation of columns for affinity chromatography, immunoprecipitation, antibodies conjugated to magnetic particles, permanent and oriented immobilization of antibodies in biosensor systems, microarrays, microtitration plates or any other system, where the loss of antibodies needs to be avoided, and maximum binding capacity is desired. This method is directly applicable even to antibodies in crude cell culture supernatants, raw sera or protein-stabilized antibody preparations without any purification nor enrichment of the IgG. This new method delivered much higher signals as a traditional method and, hence, seems to be preferable in many applications.