Use of the USDT flow immunosensor for quantitation of benzoylecgonine in urine

Cocaine Detection by a Laser-Induced Immunofluorometric Biosensor

The trafficking of illegal drugs by criminal networks at borders, harbors, or airports is an increasing issue for public health as these routes ensure the main supply of illegal drugs. The prevention of drug smuggling, including the installation of scanners and other analytical devices to detect small traces of drugs within a reasonable time frame, remains a challenge. The presented immunosensor is based on a monolithic affinity column with a large excess of immobilized hapten, which traps fluorescently labeled antibodies as long as the analyte cocaine is absent. In the presence of the drug, some binding sites of the antibody will be blocked, which leads to an immediate breakthrough of the labeled protein, detectable by highly sensitive laser-induced fluorescence with the help of a Peltier-cooled complementary metal-oxide-semiconductor (CMOS) camera. Liquid handling is performed with high-precision syringe pumps and microfluidic chip-based mixing devices and flow cells. The biosensor achieved limits of detection of 7 ppt (23 pM) of cocaine with a response time of 90 s and a total assay time below 3 min. With surface wipe sampling, the biosensor was able to detect 300 pg of cocaine. This immunosensor belongs to the most sensitive and fastest detectors for cocaine and offers near-continuous analyte measurement.

Continuous-flow fluoro-immunosensor for paclitaxel measurement

A fluorescence-based continuous-flow immunosensor for sensitive, precise, accurate and fast determination of paclitaxel was developed. The sensor utilizes anti-paclitaxel antibody immobilized through its Fc region and crosslinked by dimethylpimelimidate to protein A attached covalently onto the silanized inner walls of a glass capillary column followed by saturation of the paclitaxel-binding sites with rhodamine-labeled paclitaxel. The assay is based on the displacement and detection downstream of the rhodamine-labeled paclitaxel, by a flow-through spectrofluorometer, as a result of the competition with paclitaxel introduced as a pulse into the stream of carrier buffer flowing through the system. The peak height of the fluorescence intensity profile of the displaced rhodamine-labeled paclitaxel was directly proportional to the concentration of paclitaxel applied and was a function of the carrier buffer flow rate. The sensitivity of the immunosensor response ranged from 0.31 relative fluorescence units (RFU)/ng/ml at a flow rate 0.1 ml/min to 0.52 RFU/ng/ml at 1 ml/min, while the lower detection limit ranged from 1 ng/ml at 0.1 ml/min to 4 ng/ml at 1 ml/min. The immunosensor response was very reproducible (RSD=4.8%; n=10) and linear up to 100 ng/ml. The assay time ranged from 2 min at 1 ml/min to 8 min at 0.1 ml/min. A technique developed to resaturate the antigen binding sites of the immobilized antibody with rhodamine-labeled paclitaxel was successful in regenerating the capillary column without affecting its performance, thus enhancing the economic viability of the immunosensor. The immunosensor was successfully applied for the determination of paclitaxel in human plasma.

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.

Assessment of an automated solid phase competitive fluoroimmunoassay for benzoylecgonine in untreated urine

A new solid phase fluoroimmunoassay using a fully automated flow fluorometer adapted for urinalysis of drug metabolites is described. Fluorescein-conjugated benzoylecgonine (FL-BE) and monoclonal antibodies (mAb) against benzoylecgonine (BE) were the reagents used for demonstration. The solid phase consisted of anti-BE mAbs immobilized on the surface of polymethyl methacrylate (PMMA) beads. Free BE in solution competed with FL-BE and reduced bead-bound fluorescence in a concentration-dependent manner. The binding of FL-BE to the anti-BE mAb reached steady-state within minutes. FL-BE was not bound by uncoated beads nor beads coated with non-specific proteins or IgG. The signal-to-noise ratio was 33, and the sensitivity of the assay was 2 ng ml(-1) for BE. The effective concentration of BE was 1 to 100 ng ml(-1), with an IC50 value of 12 ng ml(-1). The mAb showed equal affinities for BE, cocaine, and cocaethylene, but a five order-of-magnitude lower affinity for ecgonine and ecgonine methylester. In a double-blind comparison using clinical urine samples, the data from this single-step competitive assay had excellent agreement with results obtained using a fiber-optic biosensor (FOB), and the EMIT assay performed commercially. The assay provided kinetic data rapidly and can be used to detect small analytes for which antibodies and fluorescein conjugates are available. The affinity of the mAb for FL-BE, calculated from kinetic analysis of the time course of the on and off reaction, was 2.25 x 10(-9) M.

Influence of antibody valency in a displacement immunoassay for the quantitation of 2,4-dichlorophenoxyacetic acid

The influence of antibody valency in a displacement immunoassay was investigated by comparing the whole antibody molecule with the corresponding Fab-fragment. The displacement immunoassay for the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) takes advantage of the cross-reactivity of monoclonal anti-2,4-D antibodies and the Fab-fragments toward immobilized 2-methyl-4-chlorophenoxyacetic acid (MCPA). Due to the low affinity of the antibodies toward MCPA (cross-reactivity of approximately 30%), the addition of 2,4-D resulted in a displacement of the antibodies or the fragments. The detection limits obtained with whole anti-2,4-D antibodies and Fab-fragments were 0.1 microg/l and 0.01 microg/l 2,4-D, respectively. The whole antibodies and the Fab-fragments show similarities, such as the cross-reactivity toward MCPA (26% and 33%), and some characteristics of the calibration curve, for example the large detection range and the sensitivity. In contrast to the bivalent antibodies, however, increasing the hapten/protein ratios of the immobilized MCPA-BSA conjugates did not affect the detection limit when using the Fab-fragments. Moreover, kinetic experiments reveal a faster displacement reaction with the Fab-fragments. A disadvantage of using the Fab-fragments is the generation of lower absorbance values in the ELISA.

On-line flow displacement immunoassay for fatty acid-binding protein

In standard displacement flow immunoassays the analyte in the sample creates an active dissociation of labelled antigens (or antigen homologues) from an antigen binding site of an immobilized antibody, after which the labelled substance is measured downstream. Such systems have been described for molecules up to 1 kDa. In this study, we demonstrate displacement in a flow system for the detection of a small protein, cytoplasmic heart-type fatty acid-binding protein (15 kDa), a plasma marker for myocardial injury. The displacement system uses an inverse set-up: enzyme labelled monoclonal antibodies are associated to immobilized antigen, and are displaced by analyte in the sample. The system permits detection of both physiological (2-12 microg l(-1)) and pathological concentrations (12-2000 microg l(-1)) of fatty acid-binding protein in an on-line flow system.

Biosensors in chemical separations

Identification of biomolecules in complex biological mixtures represents a major challenge in biomedical, environmental, and chemical research today. Chemical separations with traditional detection schemes such as absorption, fluorescence, refractive index, conductivity, and electrochemistry have been the standards for definitive identifications of many compounds. In many instances, however, the complexity of the biomixture exceeds the resolution capability of chemical separations. Biosensors based on molecular recognition can dramatically improve the selectivity of and provide biologically relevant information about the components. This review describes how coupling chemical separations with online biosensors solves challenging problems in sample analysis by identifying components that would not normally be detectable by either technique alone. This review also presents examples and principles of combining chemical separations with biosensor detection that uses living systems, whole cells, membrane receptors, enzymes, and immunosensors.

Multianalyte detection using a capillary-based flow immunosensor

A highly sensitive, dual-analyte detection system using capillary-based immunosensors has been designed for explosive detection. This model system consists of two capillaries, one coated with antibodies specific for 2,4,6-trinitrotoluene (TNT) and the other specific for hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) combined into a single device. The fused silica capillaries are prepared by coating anti-TNT and anti-RDX antibodies onto the silanized inner walls using a hetero-bifunctional crosslinker. After immobilization, the antibodies are saturated with a suitable fluorophorelabeled antigen. A "T" connector is used to continuously flow the buffer solution through the individual capillaries. To perform the assay, an aliquot of TNT or RDX or a mixture of the two analytes is injected into the continuous flow stream. In each capillary, the target analyte displaces the fluorophore-labeled antigen from the binding pocket of the antibody. The labeled antigen displaced from either capillary is detected downstream using two portable spectrofluorometers. The limits of detection for TNT and RDX in the multi-analyte formate are 44 fmol (100 microliters of 0.1 ng/ml TNT solution) and 224 fmol (100 microliters of 0.5 ng/ml RDX solution), respectively. The entire assay for both analytes can be performed in less than 3 min.

Immunosensors: electrochemical sensing and other engineering approaches

This article overviews the engineering approaches and the recent trends in the development of alternative immunoassay systems. A brief description of the main principles and limitations of conventional immunoassay is given. Immunosensing approaches overcoming these limitations are discussed. Alternatives to traditional immunoassay systems are discussed in terms of the enhancement of immunointeraction processes and in terms of the various detection principles. Applications of flow-injection techniques to the development of immunosensing systems are presented. Immunosensors are categorized based on the detection principle employed, as immunoelectrodes (electrochemical immunosensors), piezoelectric immunosensors, or as sensors based on optical detection of the immunointeraction. The discussion focuses on electrochemical immunosensors. In conclusion, the engineering issues involved in immunosensor development are outlined and trends towards practical applications are discussed.

Development of a displacement immunoassay by exploiting cross-reactivity of a monoclonal antibody

An enzyme-linked immunosorbent assay-based displacement assay was developed for the determination of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D). Advantage was taken of the cross-reactivity of a monoclonal anti-2,4-D antibody toward 2-methyl-4-chlorophenoxyacetic acid (MCPA). MCPA was conjugated with bovine serum albumin (BSA), immobilized on the surface of a microtiter plate, and saturated with the anti-2,4-D antibody. Due to the low affinity of the antibody toward MCPA (cross-reactivity of approximately 30%), the addition of 2,4-D resulted in a displacement of the antibody. Remaining antibodies were subsequently detected using a peroxidase-labeled goat anti-mouse antibody. The detection limit was as low as 0.1 microgram/liter for 2,4-D, which complies with the European Union Drinking Water Directives. When 2,4-D-BSA was used instead of MCPA-BSA conjugates, no significant displacement of bound antibody was observed.