A novel signal amplification technology based on catalyzed reporter deposition and its application in a Dot-ELISA with ultra high sensitivity

Signal amplification systems in immunoassays: implications for clinical diagnostics

Biomarkers in physiological specimens serve as useful sensors for clinical diagnosis. Accurate detection of specific markers is crucial for the diagnosis of disease, monitoring drug therapy and patient screening. In vitro immunoassays are probably the most common, simple and relatively inexpensive serological tools used in clinical laboratories for the diagnosis and management of disease. Despite continued efforts to improve the performance of immunoassays in the past three decades, there is a need for highly sensitive assays that can detect the lowest levels of disease markers with greater accuracy. This review summarizes recent advances made towards increasing the sensitivity of immunoassays by amplifying detection signals, with implications for the development of highly sensitive diagnostic systems; it also discusses the principles of related methodologies.

Minimizing nonspecific protein adsorption in liquid crystal immunoassays by using surfactants

In this paper, we report the role of surfactants in minimizing nonspecific protein adsorption in liquid crystal (LC)-based immunoassays in which LC is used as a readout system. Among all surfactants tested, only nonionic surfactant such as Tween 20 can effectively reduce the nonspecific protein adsorption, while maintaining the selectivity of the LC-based immunoassay. We also show that to minimize nonspecific protein adsorption, Tween 20 can be added directly into the antibody solution to a final concentration of 0.8 mM. After the addition of Tween 20, better correlations between the antibody concentrations and the interference colors of LCs can therefore be obtained. For example, when Cy3 antibiotin was used, black, yellow, red, and green interference colors correspond to a concentration of 5, 25, 50, and 100 μg/mL, respectively. This feature gives LC immunoassay a unique advantage over the fluorescence-based immunoassay.

Filtration-based tyramide amplification technique—a new simple approach for rapid detection of aflatoxin B1

The catalyzed reporter deposition (CARD) method of signal amplification, also called "tyramide signal amplification", has been used in immunoassays not only to increase sensitivity but also to reduce assay time. The current approach to tyramide amplification in immunoassays involves slow incubation with agitation. In this paper we describe new filtration-based tyramide amplification and substrate visualization techniques. Compared with the standard method, this new approach greatly enhances spot intensities in membrane immunoassay and reduces biotinylated tyramide (B-T) and substrate consumption approximately fiftyfold, without loss of specificity. An improved test device and a cost-effective method for preparation of membranes for Super-CARD amplification have also been developed. The techniques have been used for rapid detection of aflatoxin B(1) (AFB(1)) in a variety of foodstuffs with a detection limit of 12.5 microg kg(-1). The assay procedure involves sequential addition of standards or sample, AFB(1)-horseradish peroxidase (HRP) conjugate, B-T, avidin-HRP, and substrate solution over anti-AFB(1) antibody-spotted zones of the membrane surface. The method saves time, improves reproducibility, eliminates many washing steps and avoids manipulation of the membranes between the different steps, while maintaining the sensitivity of the standard method. Average recoveries from different non-infected food samples spiked with AFB(1) at concentrations from 25 to 100 mg kg(-1) were between 95 and 105%. AFB(1) results obtained on different days for Aspergillus parasiticus infection of corn and groundnut samples correlated well with estimates obtained by HPLC.

Investigation of several parameters influencing signal generation in flow-through membrane-based enzyme immunoassay

Rapid-response analytical tests that can be performed at the point of sampling are based on a visual detection system. The influence of different factors on the signal generation in a membrane-based enzyme immunoassay was investigated. The research was applied to a flow-through immunoassay for the detection of ochratoxin A (OTA). This assay format is a very convenient, simple and fast qualitative screening tool. Conjugates of OTA with horseradish peroxidase (HRP) and alkaline phosphatase (AP) were used as enzyme tracers. A new conjugate OTA-AP has been synthesized in our laboratory and its performance in the assay was compared with that of OTA-HRP. Different substrate systems for HRP and AP were compared. Several reagents, including polymers and surfactants, were tested for their possible effect on signal generation with the use of OTA-HRP conjugate. Polymers such as poly(vinyl alcohol) (PVA) and poly(ethylene glycol) (PEG) 6000 exerted a favourable effect on signal amplification, whereas surfactants negatively affected assay performance. The highest signal amplification (30-70% compared to the standard assay procedure) was achieved using 0.5% PVA in tetramethylbenzidine (TMB) Colorburst substrate solution and phosphate-buffered saline (PBS) for the washing step. It allowed more reliable visual estimation of the results from OTA-HRP assay. Exclusion of the detergent (Tween 20) from the washing solution exerted a favourable effect on assay performance using both enzyme tracers. The assay using OTA-HRP was more susceptible to matrix interferences than the assay with OTA-AP. Signal development in the matrix was better for the OTA-AP assay and visual estimation of the results was easier to perform in this case. For the analysis of spiked wheat samples, OTA-AP conjugate gave a more sensitive, stable and reproducible assay with a cut-off level of 4 microg kg(-1) for OTA. The application of the new OTA-AP conjugate resulted in improved assay performance for the food samples.

Method for homogeneous spotting of antibodies on membranes: application to the sensitive detection of ochratoxin A

Membrane-based dot immunoassays are now widely used in almost every branch of biology and medicine. However, the quality of the immobilized antigen or antibody spots on the membranes was found to be highly operator-dependent and spotting by conventional methods often leads to heterogeneous spot morphologies and deposition inconsistencies. To circumvent these problems, a spotting method has been developed which is based on focussed absorption of an applied antibody solution through an aqueous network of capillary channels formed between the membrane and a wetted absorbent body. The method does not require any equipment for creating vacuum and according to assay requirements highly homogeneous spots of uniform size, in the range of 0.8- to 9-mm diameter, can be obtained by varying the volume of the applied antibody solution. Spot intensities were sufficiently high even at high antibody dilutions. Immobilization of anti-ochratoxin A (anti-OA) antibody by this method gave 2-fold increased sensitivity in a competitive assay of the toxin compared to conventional spotting methods. The calculated CV of the colour intensity for spots of different sizes (0.8 to 9 mm) was between 4.5 and 1%. Application of this spotting technique has been demonstrated for detection of OA in wine and coffee samples with the elimination of matrix interferences in the same immunoassay system. This was achieved by selective removal of nonspecific interfering substances from the sample extract during the assay. The detection limit of OA in wine (1 microg L(-1)) and coffee (2.5 microg kg(-1)) obtained by the present new method is superior to values reported recently. Thus, the present new method will be highly useful for improved performance of membrane-based immunoassays in almost every branch of biology and medicine.

An Analytical Device for On-Site Immunoassay. Demonstration of Its Applicability in Semiquantitative Detection of Aflatoxin B1 in a Batch of Samples with Ultrahigh Sensitivity

A simple analytical device has been developed for performing noninstrumental immunofiltration-based assay on a batch of samples. The device consists of membrane strips, with antibody-immobilized zones, attached to a polyethylene card. A moist filter paper placed between the membrane and the polyethylene card acts as the absorbent body. The device was used to estimate very low concentrations of aflatoxin B1 (AFB1) present in food samples by using an improved catalyzed reporter deposition (Super-CARD) method of signal amplification involving biotinylated tyramine (B-T) and avidin-horseradish peroxidase conjugate. 4-chloro-1-naphthol was used as the substrate for visualization. Semiquantitative results are obtained by visual comparison of the color intensity (inversely related to the analyte concentration) of a sample spot with those of reference standards. Quantitative estimation is possible by densitometric analysis (detection limit 0.25 pg/spot, 0.01 ng mL(-1)). Dilute samples can be assayed by in situ concentration with improved dose-response characteristics. A batch of 12 extracted samples can be analyzed in a single test card within 12 min. Spiked and contaminated samples of groundnut, corn, wheat, cheese, and chilli were analyzed without sample cleanup. The matrix interferences were eliminated by using appropriate dilution of the aqueous methanol extracts. Mean recoveries from different food samples were between 91 and 104%. The values obtained for infected corn and groundnut samples correlated well (R2=0.99) with the estimates by HPLC. The method is well-suited for visual screening of agricultural and food samples for AFB1 under field conditions.

Amplification methods for the immunolocalization of rare molecules in cells and tissues

The needs to precisely assign macromolecules to specific locations and domains within tissues and cells and to reveal antigens which are present in low or even in trace amounts, led to the elaboration of a wide spectrum of immunocytochemical amplification procedures. These arise from the successive improvements of tissue preparation techniques, of antigen retrieval procedures and of immunological or non-immunological detection systems. Improvement of detection systems may be the most active in the development of amplification techniques. Since the early work of Coons, in which by the introduction of the indirect technique has started amplifying the signal, different systems have succeeded in increasing the sensitivity of antigens detection. Indeed, amplification techniques such as the multiple antibody layers, the multiple bridges, the enzyme complexes, the avidin-biotin, the silver intensification, and the numerous variations and combinations among these have increased the sensitivity for the detection of scarce tissue antigens. However, as shown by the recent progress carried out with new approaches such as the catalyzed reporter deposition (CARD) and the enhanced polymer one-step staining (EPOS), more efficient methods are still needed. In electron microscopy, few techniques have reached the resolution afforded by the post-embedding immunogold approach. In spite of this and in order to further increase its sensitivity, new probes and novel approaches are allowing combination of the gold marker with the amplification capacity of enzymes afforded by the CARD technique. Immunogold amplification strategies, such as the multiple incubations with the primary antibody and the use of an anti-protein A antibody have also led to enhanced signals displaying the advantages in terms of resolution and possibilities of quantification inherent to the colloidal gold marker.

A novel signal amplification technology for ELISA based on catalyzed reporter deposition. Demonstration of its applicability for measuring aflatoxin B(1)

In an earlier communication we have described a novel signal amplification technology termed Super-CARD, which is able to significantly improve antigen detection sensitivity in conventional Dot-ELISA by approximately 10(5)-fold. The method utilizes hitherto unreported synthesized electron rich proteins containing multiple phenolic groups which, when immobilized over a solid phase as blocking agent, markedly increases the signal amplification capability of the existing CARD method (Bhattacharya, R., Bhattacharya, D., Dhar, T.K., 1999. A novel signal amplification technology based on catalyzed reporter deposition and its application in a Dot-ELISA with ultra high sensitivity. J. Immunol. Methods 227, 31.). In this paper we describe the utilization of this Super-CARD amplification technique in ELISA and its applicability for the rapid determination of aflatoxin B(1) (AFB(1)) in infected seeds. Using this method under identical conditions, the increase in absorbance over the CARD method was approximately 400%. The limit of detection of AFB(1) by this method was 0.1 pg/well, the sensitivity enhancement being 5-fold over the optimized CARD ELISA. Furthermore, the total incubation time was reduced to 16 min compared to 50 min for the CARD method. Assay specificity was not adversely affected and the amount of AFB(1) measured in seed extracts correlated well with the values obtained by conventional ELISA.