Biotinylated steroid derivatives as ligands for biospecific interaction analysis with monoclonal antibodies using immunosensor devices

Fluorescence-based kinetic analysis of miniaturized protein microarrays

Ideal monitoring devices should enjoy a combination of characteristics, e.g. high sensitivity, multiplexing, portability, short time-to-result (TTR). Typically, no device meets all of these demands since some of them are contradictory, to some extent. Herein, we present a miniaturized platform based on fluorescent detection, which is sensitive, readily allows multiplexing, and allows real-time monitoring of the signal, thus allowing extraction of kinetic information as well as drastic reduction of TTR. This is achieved via miniaturization of active spots, integration with microfluidics, and algorithmic approaches. We validate its performance by comparing with evanescent field excitation, which obtains similar results, however without the addition of the necessary complex hardware.

Nanotechnology-Based Surface Plasmon Resonance Affinity Biosensors for In Vitro Diagnostics

In the last decades, in vitro diagnostic devices (IVDDs) became a very important tool in medicine for an early and correct diagnosis, a proper screening of targeted population, and also assessing the efficiency of a specific therapy. In this review, the most recent developments regarding different configurations of surface plasmon resonance affinity biosensors modified by using several nanostructured materials for in vitro diagnostics are critically discussed. Both assembly and performances of the IVDDs tested in biological samples are reported and compared.

Label-free biological and chemical sensors

Highly sensitive, label-free biodetection methods have applications in both the fundamental research and healthcare diagnostics arenas. Therefore, the development of new transduction methods and the improvement of the existing methods will significantly impact these areas. A brief overview of the different types of biosensors and the critical parameters governing their performance will be given. Additionally, a more in-depth discussion of optical devices, surface functionalization methods to increase device specificity, and fluidic techniques to improve sample delivery will be reviewed.

Small molecule immunosensing using surface plasmon resonance

Surface plasmon resonance (SPR) biosensors utilize refractive index changes to sensitively detect mass changes at noble metal sensor surface interfaces. As such, they have been extensively applied to immunoassays of large molecules, where their high mass and use of sandwich immunoassay formats can result in excellent sensitivity. Small molecule immunosensing using SPR is more challenging. It requires antibodies or high-mass or noble metal labels to provide the required signal for ultrasensitive assays. Also, it can suffer from steric hindrance between the small antigen and large antibodies. However, new studies are increasingly meeting these and other challenges to offer highly sensitive small molecule immunosensor technologies through careful consideration of sensor interface design and signal enhancement. This review examines the application of SPR transduction technologies to small molecule immunoassays directed to different classes of small molecule antigens, including the steroid hormones, toxins, drugs and explosives residues. Also considered are the matrix effects resulting from measurement in chemically complex samples, the construction of stable sensor surfaces and the development of multiplexed assays capable of detecting several compounds at once. Assay design approaches are discussed and related to the sensitivities obtained.

Ultrasensitive detection of testosterone using conjugate linker technology in a nanoparticle-enhanced surface plasmon resonance biosensor

A rationally designed oligoethylene glycol linker conjugate to testosterone was synthesised and covalently immobilized on a surface plasmon resonance (SPR) biosensor surface. The sensing surface was stable for more than 330 binding and regeneration cycles allowing a high degree of re-use. This surface was then used in the development of an ultrasensitive immunobiosensor system for testosterone in buffer utilizing both secondary antibody and gold nanoparticle signal enhancement. The mechanism for the increased sensitivity results from increased binding mass and a gold plasmon coupling effect. The addition of a secondary antibody with an attached gold nanoparticle increased the signal sensitivity of the assay 12.5-fold compared with primary antibody alone. In the enhanced format the assay had limits of detection (LOD) of 3.7 pgml(-1) with standard in running buffer, and 15.4 pgml(-1) in a stripped human saliva matrix. This immunobiosensor system has sufficient sensitivity to measure testosterone across the broad physiologically relevant range in male saliva (29-290 pgml(-1)) in under 13 min allowing monitoring of testosterone in near real-time.

Chemical synthesis and biochemical characterization of a biotinylated derivative of 17beta-estradiol with a long side chain covalently attached to its C-7alpha position

High-affinity biotinylated derivatives of 17beta-estradiol (E2) are of value for isolation of various estrogen-binding proteins (including estrogen receptors) and also for studying protein-protein interactions involving these proteins. In this study, we developed a simplified route for the chemical synthesis of a biotinylated derivative of E2 (compound 7) with a side chain attached to its C-7alpha position. Compound 7, i.e., 7alpha-{7-[8-(biotinamido)-octanamido]-heptyl}-estradiol, could be readily synthesized from 6-keto-estradiol-3,17beta-di-tetrahydropyranyl ether (compound 2, which can be prepared from E2), with a final yield of 36%. In vitro receptor-binding assay confirmed that the synthesized affinity ligand has a high binding affinity for both human estrogen receptor alpha and beta. When the affinity ligand (compound 7) was immobilized with avidin on an affinity column, it effectively bound human estrogen receptor alpha, and the receptor protein could be selectively eluted with a biotin-containing buffer. Using the same affinity ligand, prolyl 4-hydroxylase beta-subunit (also known as protein disulfide isomerase) was identified as one of the high-affinity E2-binding proteins in the whole cytosolic protein mixture prepared from MCF-7 human breast cancer cells. Computational molecular modeling analysis showed that compound 7 can bind to human estrogen receptor alpha in a similar manner as ICI-182,780 and raloxifene, and their binding energy values are also similar.

Biophysical and biochemical approach to locating an inhibitor binding site on cholesteryl ester transfer protein

Cholesteryl ester transfer protein (CETP) transfers neutral lipids between different types of plasma lipoprotein. Inhibitors of CETP elevate the fraction of plasma cholesterol associated with high-density lipoproteins and are being developed as new agents for the prevention and treatment of cardiovascular disease. The molecular basis of their function is not yet fully understood. To aid in the study of inhibitor interactions with CETP, a torcetrapib-related compound was coupled to different biotin-terminated spacer groups, and the binding of CETP to the streptavidin-bound conjugates was monitored on agarose beads and in a surface plasmon resonance biosensor. CETP binding was poor with a 2.0 nm spacer arm, but efficient with polyethyleneglycol spacers of 3.5 or 4.6 nm. The conjugate based on a 4.6 nm spacer was used for further biosensor experiments. Soluble inhibitor blocked the binding of CETP to the immobilized drug, as did preincubation with a disulfide-containing covalent inhibitor. To provide a first estimate of the binding site for torcetrapib-like inhibitors, CETP was modified with a disulfide-containing agent that modifies Cys-13 of CETP. Mass spectrometry of the modified protein indicated that a single half-molecule of the disulfide was covalently bound to CETP, and peptide mapping after digestion with pepsin confirmed previous reports based on mutagenesis that Cys-13 was the site of modification. Modified CETP was unable to bind to the biosensor-mounted torcetrapib analog, indicating that the binding site on CETP for torcetrapib is in the lipid-binding pocket near the N-terminus of the protein. The crystal structure of CETP shows that the sulfhydryl group of Cys-13 resides at the bottom of this pocket.

Bisphenol A directly targets tubulin to disrupt spindle organization in embryonic and somatic cells

There is increasing concern that animal and human reproduction may be adversely affected by exposure to xenoestrogens that activate estrogen receptors. There is evidence that one such compound, Bisphenol A (BPA), also induces meiotic and mitotic aneuploidy, suggesting that these kinds of molecules may also have effects on cell division. In an effort to understand how Bisphenol A might disrupt cell division, a phenotypic analysis was carried out using sea urchin eggs, whose early embryonic divisions are independent of zygotic transcription. Fertilized Lytechinus pictus eggs exposed to BPA formed multipolar spindles resulting in failed cytokinesis in a dose-dependent, transcriptionally independent manner. By use of novel biotinylated BPA affinity probes to fractionate cell-free extracts, tubulin was identified as a candidate binding protein by mass spectrometry, and BPA promoted microtubule polymerization and centrosome-based microtubule nucleation in vitro but did not appear to display microtubule-stabilizing activity. Treatment of mammalian cells demonstrated that BPA as well as a series of Bisphenol A derivatives induced ectopic spindle pole formation in the absence of centrosome overduplication. Together, these results suggest a novel mechanism by which Bisphenol A affects the nucleation of microtubules, disrupting the tight spatial control associated with normal chromosome segregation, resulting in aneuploidy.

SPR Imaging of Photo-Cross-Linked Small-Molecule Arrays on Gold

Identification of small-molecule ligands for a protein of interest can facilitate the analysis of the protein's functions in biological systems. Small-molecule microarrays have allowed for rapid detection of such ligand-protein interactions in a high-throughput manner, although a label on a protein is needed to observe these interactions. By combining SPR imaging technology with our recently developed photo-cross-linked small-molecule array platform, we developed a novel platform that allows in situ observation of interactions between photo-cross-linked small molecules on gold surfaces and nonlabeled proteins in solution. Interactions of estrogenic and androgenic substances with estrogen receptor alpha were observed using this platform.

Reduction of nonspecific binding proteins to self-assembled monolayer on gold surface

We developed a gold coated glass chip bearing a poly(ethyleneglycol) (PEG) type compound as hydrophilic spacer for surface plasmon resonance studies, which enabled adequate estimation of K(d) value between FK506 and FKBP12 not only using purified FKBP12 (K(d)=22 nM) but also using Escherichia coli lysate expressing FKBP12 (K(d)=15 nM). These results indicated effectiveness of the PEG spacer for reduction of nonspecific interactions. Chemical stability and simple surface-structure of the novel chip are also attractive.

A new assay format for electrochemical immunosensors: polyelectrolyte-based separation on membrane carriers combined with detection of peroxidase activity by pH-sensitive field-effect transistor

A new rapid immunotechnique combining separation of reactants by filtration through a porous membrane and potentiometric detection of the bound enzyme label by a pH-sensitive field-effect transistor is proposed. The complexes to be detected are formed by the method described earlier in (Anal. Chem. 71 (1999) 3538), including a homogeneous binding of immunoreactants and a polyanion carrier (polymethacrylate) followed by heterogeneous separation on a membrane incorporating an immobilized polycation (poly-N-vinyl-4-ethylpyridinium). The proposed technique for a sensitive detection of peroxidase label is based on the measurement of pH changes in the optimised substrate solution containing o-phenylenediamine, hydrogen peroxide and ascorbic acid. The antigens studied were herbicide atrazine and hormone testosterone. Their specific detection is realised via competitive binding of free and peroxidase-labelled antigens by antibodies integrating with a (staphylococcal protein A-polyanion) conjugate. The total analysis time is 20-25 min. The range of quantitative detection is 0.2-100 ng ml(-1) for atrazine and 5-300 ng ml(-1) for testosterone. Data scatter of replicate tests varies from 3 to 10%. Application of protein A-polyanion conjugate allows to use the proposed protocol for different antigens without additional treatment of specific antisera.

Novel amperometric immunosensor for rapid separation-free immunoassay of carcinoembryonic antigen

A novel immunosensor for rapid separation-free determination of carcinoembryonic antigen (CEA) in human serum is proposed. The immunosensor is prepared by co-immobilizing thionine and horseradish peroxidase (HRP)-labeled CEA antibody on a glassy carbon electrode (GCE) through covalently binding them to GCE with a glutaraldehyde (GA) linkage. The electrochemical behavior of the immobilized thionine displays a surface-controlled electrode process with an average electron transfer rate constant of 4.74+/-2.99 s(-1). It can be used as an electron transfer mediator for enzymatic activity detection of the HRP-labeled antibody to CEA. After the immunosensor is incubated with CEA solution at 23 degrees C for 40 min, the access of activity center of the HRP to thionine is partly inhibited, which leads to a linear decrease in the catalytic efficiency of the HRP to the oxidation of immobilized thionine by H(2)O(2) at -300 mV over two CEA concentration ranges from 0.5 to 3.0 and 3.0 to 167 ng/ml. Under optimal conditions, the detection limit for the CEA immunoassay is 0.1 ng/ml at three times background noise. The immunosensor shows good accuracy and acceptable storage stability, precision and reproducibility with intra-assay CVs of 6.1% and 5.8% at 2.5 and 50 ng/ml CEA, respectively, and an inter-assay CV of 6.3% at 50 ng/ml. This method is economical and shortens the analytical time, making it potentially attractive for clinical immunoassays.

Real time kinetic analysis of the interaction between immunoglobulin G and histidine using quartz crystal microbalance biosensor in solution

Quartz crystal microbalance (QCM) biosensor integrated in a flow injection analysis (FIA) system was used for the investigation of the specific interaction between immunoglobin G (IgG) and histidine. The histidine was immobilized on the gold electrodes of the piezoelectric crystal using appropriate procedures based on self-assembling of the dithiothreitol (DTT). The specific interaction of the immobilized ligand with IgG in solution was followed as a change in the resonant frequency of the modified crystal and studied in real time without any additional labels. With the mass sensitive biosensor system, the differences in affinity of three different species of IgG: human IgG, goat IgG and mouse IgG were easily distinguished and their respective kinetic rate constants (kass and kdiss) and equilibrium association constants (KA) were determined from the curves of frequency versus time. For the interactions, KA were 2.92 x 10(4), 3.23 x 10(4) and 4.08 x 10(4) M(-1) for human IgG, goat IgG and mouse IgG, respectively.

Reagentless Amperometric Immunosensors Based on Direct Electrochemistry of Horseradish Peroxidase for Determination of Carcinoma Antigen-125

A novel strategy for immunoassay and the preparation of reagentless immunosensors was proposed. This strategy was based on the immobilization of antigen and the direct electrochemistry of horseradish peroxidase (HRP) that was labeled to an antibody. A reagentless immunosensor for carcinoma antigen-125 (CA 125) determination was developed. The immunosensor was prepared by immobilizing CA 125 with titania sol-gel on a glassy carbon electrode by the vapor deposition method. The incubation of the immunosensor in phosphate buffer solution (PBS) including HRP-labeled CA 125 antibody led to the formation of a HRP-modified surface. The immobilized HRP displayed its direct electrochemistry with a rate constant of 3.04 +/- 1.21 s(-1). With a competition mechanism, a differential pulse voltammetric determination method for CA 125 was established by the peak current decrease of the immobilized HRP. The current decrease resulted from the competitive binding of the CA 125 in sample solution and the immobilized CA 125 to the limited amount of HRP-labeled CA 125 antibody. Under optimal conditions, the current decrease was proportional to CA 125 concentration ranging from 2 to 14 units mL(-1) with a detection limit of 1.29 units mL(-1) at a current decrease by 10%. The CA 125 immunosensor showed good accuracy and acceptable precision and fabrication reproducibility with intraassay CVs of 8.7 and 5.5% at 8 and 14 units mL(-1) CA 125 concentrations, respectively, and interassay CV of 19.8% at 8 units mL(-1). The storage stability was acceptable in a pH 7.0 PBS at 4 degrees C for 15 days. The proposed method provided a new promising platform for clinical immunoassay.

A versatile planar QCM-based sensor design for nonlabeling biomolecule detection

Despite high theoretical sensitivity, low-cost manufacture, and compactness potentially amenable to lab-on-a-chip use, practical hurdles have stymied the application of the quartz crystal microbalance (QCM) for aqueous applications such as detection of biomolecular interactions. The chief difficulty lies in achieving a sufficiently stable resonance signal in the presence of even minute fluctuations in hydrostatic pressure. In this work, we present a novel versatile planar sensor chip design (QCM chip) for a microliter-scale on-line biosensor. By sealing the quartz resonator along its edges to a flat, solid support, we provide uniform support for the crystal face not exposed to solvent, greatly decreasing deformation of the crystal resonator under hydrostatic pressure. Furthermore, this cassette design obviates the need for direct handling when exchanging the delicate quartz crystal in the flow cell. A prototype 27-MHz sensor signal exhibited very low noise over a range of flow rates up to 100 microL/min. In contrast, signals obtained from a conventional QCM sensor employing an O-ring-based holder were less stable and deteriorated even further with increasing flow rate. Additional control designs with intermediate amounts of unsupported undersurface yielded intermediate levels of stability, consistent with the interpretation that deformation of the crystal resonator under fluctuating hydraulic pressure is the chief source of noise. As a practical demonstration of the design's high effective sensitivity, we readily detected interaction between myoglobin and surface-bound antibody.

Determination of monoenzyme- and bienzyme-stimulated precipitation by a cuvette-based surface plasmon resonance instrument

This paper describes the use of a cuvette-based surface plasmon resonance (SPR) instrument to measure biocatalyzed precipitation reactions. Enzyme-modified SPR sensor disk forms the base of a cuvette, in which the substrate solution is added with stirring. The determination of the substrate concentration relies on the measurement of SPR angle shift (Deltatheta(SPR)) induced by the deposition of the insoluble products without involving in any electrochemical reactions. As examples, horseradish peroxidase (HRP)-modified monoenzyme SPR sensor and HRP-glucose oxidase bienzyme-layered sensor are created to determine hydrogen peroxide and glucose via the catalyzed oxidation of 4-chloro-1-naphthol (4-CN). The deposition of the oxidized 4-CN-insoluble products leads to SPR angle shifts, which are linear to H(2)O(2) and glucose in the concentration ranges of 0.067-7.24 x 10(-5) and 0.7-8.3 x 10(-4) mM, respectively. The SPR sensitivities are greater than those of nonelectrochemical quartz crystal microbalance (QCM) (the parallel results in this study) and compare favorable with those of electrochemical QCM and electrochemical SPR methods. This study opens the field for enhanced SPR measurements by using biocatalyzed precipitation as a signal amplification method.

Immunosensors--principles and applications to clinical chemistry

INTRODUCTION

Immunosensors are affinity ligand-based biosensor solid-state devices in which the immunochemical reaction is coupled to a transducer. The fundamental basis of all immunosensors is the specificity of the molecular recognition of antigens by antibodies to form a stable complex. This is similar to the immunoassay methodology. Immunosensors can be categorized based on the detection principle applied. The main developments are electrochemical, optical, and microgravimetric immunosensors. In contrast to immunoassay, modern transducer technology enables the label-free detection and quantification of the immune complex.

METHODS

The analysis of trace substances in environmental science, pharmaceutical and food industries is a challenge since many of these applications demand a continuous monitoring mode. The use of immunosensors in these applications is most appropriate. Similarly, a series of clinical problems may be solved by continuous monitoring of certain analytes.

CONCLUSIONS

Clinical chemists should take advantage of immunosensors in clinical diagnostics. There are many recent developments in the immunosensor field which have potential impacts. The future role of this technique in intralaboratory, as well as bedside testing, will become even more important as the clinical laboratory is faced with increasing pressure to contain costs.

Survey of the year 2000 commercial optical biosensor literature

We have compiled a comprehensive list of the articles published in the year 2000 that describe work employing commercial optical biosensors. Selected reviews of interest for the general biosensor user are highlighted. Emerging applications in areas of drug discovery, clinical support, food and environment monitoring, and cell membrane biology are emphasized. In addition, the experimental design and data processing steps necessary to achieve high-quality biosensor data are described and examples of well-performed kinetic analysis are provided.