Fluorescence polarisation for immunoreagent characterisation

Use of anchor protein modules in fluorescence polarisation aptamer assay for ochratoxin A determination

A new strategy for sensitive fluorescence polarisation (FP) analysis is proposed which uses aptamer as the receptor and anchor protein modules as the enhancers by including the aptamers in complexes with protein modules. This approach is based on increasing the size differences of bound and unbound fluorophores. The strategy was applied in an ochratoxin A (ОТА) assay with the competitive binding of fluorophore-labelled and free OTA with aptamer-based receptors. We showed that the binding of labelled OTA with aptamer included in complexes with anchors led to higher a FP than binding with free aptamer. This allowed the aptamer concentration to be reduced, thus lowering the limit of detection by a factor of 40, down to 3.6 nM. The assay time was 15 min. To evaluate the applicability of the FP assay with aptamer-anchor complex to real samples, we conducted OTA measurements in spiked white wine. The OTA limit of detection in wine was 2.8 nM (1.1 μg/kg), and the recoveries ranged from 83% to 113%. The study shows that the proposed anchor strategy is efficient for increasing the sensitivity of FP-based aptamer assays.

Determination of traces of several pesticides in sunflower oil using organic phase immuno electrodes (OPIEs)

Testing for traces of different pesticides (triazinic, organophosphates and chlorurates), present in hydrophobic matrices such as sunflower oil was checked using new immunosensors working in organic solvent mixtures (OPIEs). The competitive process took place in an n-hexane-chloroform 75% (V/V) mixture, while the subsequent final enzymatic measurement was performed in decane using tert-butylhydroperoxide as substrate of the enzymatic reaction. A Clark electrode was used as transducer and peroxidase enzyme as marker. A linear response of between about 10 nM and 4 μM was usually obtained in the presence of sunflower oil. Immunosensors show satisfactory selectivity and precision and recovery tests carried out on commercial sunflower oil samples gave excellent results. Lastly, theoretical confirmation of the possibility that immunosensors can act positively in organic solvent mixtures was discussed on the basis of Hill׳s coefficient values.

Rapid screening and identification of dominant B cell epitopes of HBV surface antigen by quantum dot-based fluorescence polarization assay

A method for quickly screening and identifying dominant B cell epitopes was developed using hepatitis B virus (HBV) surface antigen as a target. Eleven amino acid fragments from HBV surface antigen were synthesized by 9-fluorenylmethoxy carbonyl solid-phase peptide synthesis strategy, and then CdTe quantum dots were used to label the N-terminals of all peptides. After optimizing the factors for fluorescence polarization (FP) immunoassay, the antigenicities of synthetic peptides were determined by analyzing the recognition and combination of peptides and standard antibody samples. The results of FP assays confirmed that 10 of 11 synthetic peptides have distinct antigenicities. In order to screen dominant antigenic peptides, the FP assays were carried out to investigate the antibodies against the 10 synthetic peptides of HBV surface antigen respectively in 159 samples of anti-HBV surface antigen-positive antiserum. The results showed that 3 of the 10 antigenic peptides may be immunodominant because the antibodies against them existed more widely among the samples and their antibody titers were higher than those of other peptides. Using three dominant antigenic peptides, 293 serum samples were detected for HBV infection by FP assays; the results showed that the antibody-positive ratio was 51.9% and the sensitivity and specificity were 84.3% and 98.2%, respectively. In conclusion, a quantum dot-based FP assay is a very simple, rapid, and convenient method for determining immunodominant antigenic peptides and has great potential in applications such as epitope mapping, vaccine designing, or clinical disease diagnosis in the future.

Immuno-SLM—a combined sample handling and analytical technique

Immuno-supported liquid membrane (immuno-SLM) extraction is a new technique that makes use of antibody (Ab)-antigen interactions as the "extraction force" to drive the mass transfer in a selective way. In immuno-SLM, anti-analyte (Ag) Abs are introduced into the acceptor phase of the SLM unit to trap the Ag that passes from the flowing donor through the SLM into the stagnant acceptor. The amount of immuno-extracted analyte (AbAg) is quantified by connecting the immuno-SLM unit on-line with a non-competitive heterogeneous fluorescence flow immunoassay (FFIA) that makes use of a fluorescein-labeled analyte tracer that titrates the residual excess of Ab present in the acceptor. A restricted access (RA) column is used for the separation of the two tracer fractions (Ag* and AbAg*) formed, and the eluted AbAg* fraction is measured downstream by a fluorescence detector. Factors influencing the optimum immuno-SLM extraction parameters, i.e., donor flow rate, extraction time and type of Ab, were investigated for immuno extraction of the model analyte atrazine. Immuno-SLM coupled to FFIA (immuno-SLM-FFIA) and FFIA alone were compared in terms of the assay sensitivities obtained and the sample matrix influence. The concentration at the mid-point of the calibration curve (IC(50)) was 16.0+/-1.4 and 36+/-16 microg/l, the limit of detection (LOD) was 2.0+/-1.1 and 20+/-10 microg/l, and the dynamic range was 2-100 and 20-500 microg/l atrazine for immuno-SLM-FFIA and FFIA, respectively. The matrix influence on the FFIA was significant in orange juice and surface water, whereas the influence was minor for immuno-SLM-FFIA with recoveries between 104% and 115% for 5 microg/l atrazine in tap water, orange juice and river water.

Use of L-lysine fluorescence derivatives as tracers to enhance the performance of polarization fluoroimmunoassays. A study using two herbicides as model antigens

Fluorescence polarization immunoassay (FPIA) is a convenient homogeneous assay, the use of which is restricted in environmental analysis by low sensitivity and matrix effects. We selected the herbicides 2,4D and 2,4,5T to synthesize new L-lysine-based fluorescent tracers using solid-phase chemistry. In addition, three different immunogens of 2,4,5T were prepared for immunization and antibody production. The new tracers and antibodies were adapted to FPIA. Tracers with the hapten attached to the alpha-aminogroup of L-lysine and fluorescein to the e-amino group exhibited at least a 5-fold increased sensitivity when compared to the previously reported ethylenediamine-based tracer (2,4D-EDA-F). The isomeric structure (hapten attached to the e-amino and fluorescein to the alpha-amino group) appeared 7.6 times less sensitive, and all other alternative structures exhibited even lower sensitivities. This observation was confirmed against the monoclonal anti-2,4D antibody E2/G2 and polyclonal anti-2,4,5T antibodies. The affinity constant of 2,4D-EDA-F with E2/G2 was 8.1 times higher when compared with the new tracer, suggesting the more specific nature of the L-lysine-based tracer, the use of which leads to a more sensitive assay. This type of tracer could improve performance and lower substantially the detection limits of FPIAs.

Fluorescence polarization immunoassay: detection of antibody to Brucella abortus

Fluorescence polarization immunoassay (FPA) is a homogeneous immunoassay useful for rapid and accurate detection of antibody or antigen. The principle of the assay is that a fluorescent dye (attached to an antigen or an antibody fragment) can be excited by plane-polarized light at the appropriate wavelength. As a rule, a small molecule rotates faster when in solution than a larger molecule. The rotation rate may be assessed by measuring light intensity in the vertical and horizontal planes. Generally, the time it takes a molecule to rotate through a given angle is an indication of its size. When a small molecule that rotates rapidly is bound to a larger molecule, the rotation rate is decreased and this decrease is measured. Because it is a primary antigen-antibody interaction, the rate of reaction is very rapid and usually a result may be obtained in minutes. This technology was applied to the detection of antibody to Brucella abortus in serum and milk, providing for the first time a rapid primary binding assay that is cost effective for use in the field.

A flow immunoassay for studies of human exposure and toxicity in biological samples

This paper describes a heterogeneous competitive flow immunoassay with a high sample throughput which can be used for the screening of smaller analytes in various samples. The method is based on off-line incubation of the analyte (Ag), a fluorescent labelled tracer (Ag*) and the corresponding antibody (Ab). The separation of bound (Ab-Ag*) and free tracer (Ag*) is based on a size exclusion and reversed phase mechanism utilizing a restricted access (RA) column. The column traps the free unbound tracer (Ag*) in its hydrophobic (C18) inner cavity but excludes the large Ab-Ag* complex, which is passed on and measured by the fluorescence detector. The flow immunoassay was developed using the triazine herbicide atrazine as a model compound owing to its human toxicity and widespread use. A sample throughput of 80 samples per hour and a detection limit of 300 pg ml-1 in water were obtained. Urine samples were successfully applied for direct injections into the flow system, while for human plasma samples an additional clean-up step using solid phase extraction was efficiently included where pure extract is obtained with the highly stable and biocompatible extracting column material. The resulting detection limits for atrazine in plasma and water samples using this clean-up and trace enrichment procedure were found to be 2 ng ml-1 and 20 pg ml-1 respectively.