An ultra-sensitive electrochemical enzyme immunoassay for thyroid stimulating hormone in human serum

Progress in Electrochemical Immunosensors with Alkaline Phosphatase as the Signal Label

Electrochemical immunosensors have shown great potential in clinical diagnosis, food safety, environmental protection, and other fields. The feasible and innovative combination of enzyme catalysis and other signal-amplified elements has yielded exciting progress in the development of electrochemical immunosensors. Alkaline phosphatase (ALP) is one of the most popularly used enzyme reporters in bioassays. It has been widely utilized to design electrochemical immunosensors owing to its significant advantages (e.g., high catalytic activity, high turnover number, and excellent substrate specificity). In this work, we summarized the achievements of electrochemical immunosensors with ALP as the signal reporter. We mainly focused on detection principles and signal amplification strategies and briefly discussed the challenges regarding how to further improve the performance of ALP-based immunoassays.

Gold nanoparticle core–europium(iii) chelate fluorophore-doped silica shell hybrid nanocomposites for the lateral flow immunoassay of human thyroid stimulating hormone with a dual signal readout

Simultaneous colorimetric and fluorometric signal detection with hybrid nanocomposite labels.

Detection of TNT-derivatives with recombinant phages

New immunoreagents for detection of TNT-derivatives TNP and TNP-Tris were developed using phage display technique. The monovalent and pentavalent recombinant phages carrying scFv specific for TNT were constructed and compared with each other to define the impact of valency and molecule dimension on antibody binding in immunoassay. Also, the bifunctional phages were generated, which carried TNT-specific scFvs as well as enzyme β-lactamase as a model marker on its surface. The most sensitive recombinant phages were selected and used for detection of TNP-Tris in a competitive ELISA based on immobilized antigen. Preincubation and partial phages saturation with a sample containing antigen allowed competition with immobilized hapten and displacement of free antigen. The phages exposing enzyme were used as immunoreagents for single step detection. The other phages were detected with specific marked antibodies. To date, the results presented in this paper are the first ever published regarding the recombinant phages for the detection of TNT.

Potentiometric enzyme immunoassay using miniaturized anion-selective electrodes for detection

An enzyme-linked immunosorbent assay (ELISA) for prostate specific antigen (PSA) detection in human serum was developed based on the potentiometric detection of 6,8-difluoro-4-methylumbelliferone (DiFMU). The assays were carried out in anti-human PSA capture antibody modified microtiter plates (150 microL volume). After incubation in the PSA-containing serum samples, beta-galactosidase-labeled PSA tracer antibody was added. The beta-galactosidase label catalyzed the hydrolysis of 6,8-difluoro-4-methylumbelliferyl-beta-D-galactopyranoside (DiFMUG) and the resulting DiFMU(-) anion was detected by potentiometric microelectrodes with anion-exchanger membrane. The selectivity of the anion-exchanger electrode is governed by the lipophilicity of the anions in the sample. Since DiFMU(-) is much more lipophilic (log P = 1.83) than any of the inorganic anions normally present in the working buffers and occurs in its anionic form at the physiological pH (pK(a) = 4.19), it was chosen as the species to be detected. The potentiometric ELISA-based method detects PSA in serum with a linear concentration range of 0.1-50 ng/mL. These results confirm the applicability of potentiometric detection in diagnostic PSA assays. Owing to simple methodology and low cost, potentiometric immunoassays seem to offer a feasible alternative to the development of in vitro diagnostic platforms.

An electrochemical enzyme immunoassay for chicken luteinizing hormone: extension of the detection limit by adequate control of the nonspecific adsorption

A noncompetitive heterogeneous enzyme immunoassay for the determination of chicken luteinizing hormone (LH) was equipped with an electrochemical endpoint in order to further enhance its sensitivity. The immunological principle of the original ELISA remained essentially unchanged, except for the fact that the peroxidase label was replaced by alkaline phosphatase, since in the upgraded version of the assay, p-aminophenyl phosphate was to be used as the substrate of alkaline phosphatase. Enzyme-generated p-aminophenol was injected into a flow-injection system and detected amperometrically in a thin-layer flow cell with a glassy carbon electrode at 0.325 V vs Ag/AgCl. A classical problem associated with this type of solid-phase immunoassay is the adsorption of proteins other than the capture antibody to the solid phase. The detection sensitivity is therefore often limited by a large background signal observed in the absence of antigen. In the present study, an experiment was designed to examine in each step of the assay the contribution of each of the potential sources of background current. It was shown that the major contribution to the background current was caused by the nonspecific adsorption of biotinylated secondary antibody. Adsorption of the secondary antibody (biotinylated goat anti-rabbit IgG) to the capture antibody (mouse anti-chicken LHbeta) was clearly a case of specific aspecificity, whereas adsorption to the solid phase itself had to be treated as a nonspecific aspecificity. Addition of 0.25% mouse serum to the secondary antibody as a source of mouse immunoglobulin could overcome the cross-reaction and markedly reduced adsorption to capture antibody. The second part of nonspecific adsorption was eliminated by using combinations of Tween 20 and bovine serum albumin as blocking agents. Controlling the adsorption of the biotinylated secondary antibody in this way decreased the detection limit from 39 pg/ml in the original assay to 2.5 pg/ml in the electrochemical version. This way, the plasma volume of samples containing on the order of 1 ng/ml LH was reduced to less than 10 microl. The linear range was 2.5-625 pg/ml. The method allowed us to measure LH in buffer and in adult and juvenile chicken plasma.

Adsorption of immunoglobulin G on carbon paste electrodes as a basis for the development of immunoelectrochemical devices

The attachment of Immunoglobulin G (IgG) to a carbon paste electrode is investigated in this work. Studies of the immobilization of the immunoglobulin on this electrode were carried out. Alkaline phosphatase (AP) has been used as an enzyme label, linked to the antibody, for the determination of the adsorbed immunoglobulin. Various substrates for this enzyme have been tested and alpha-naphthyl phosphate was found to be the best because of the lower oxidation potential of the enzymatic product, alpha-naphthol, and greater sensitivity under the same experimental conditions. An electrodic pretreatment based on an anodic oxidation of the electrode surface in a phosphate media pH 9 was carried out prior to the adsorption step. This activation is also suitable for the removal of the IgG layer and the regeneration of the electrodic surface after each determination, with the intention of using the same electrode in the subsequent assays. A good reproducibility of the signal was achieved in this way (Relative Standard Deviation, R.S.D. = 3.7%). Using cyclic voltammetry, IgG labelled with AP has been quantified in a range from 2 x 10(-12) to 7 x 10(-11) M and a detection limit of 2.3 x 10(-12) M (signal-to-noise ratio = 3) was found. Finally, human IgG was quantified under non-optimized conditions on the electrodic surface through the reaction with AP labelled IgG using two different immunological designs.

Highly sensitive amperometric enzyme immunoassay for alpha-fetoprotein in human serum

A sandwich amperometric enzyme immunoassay with flow injection for alpha-fetoprotein in human serum has been developed with alkaline phosphatase as the enzyme label. p-Hydroxyphenyl phosphate was used as the substrate for alkaline phosphatase. The hydrolysis product, hydroquinone, was detected by oxidative amperometry in a flow injection system. The amperometric wall jet detector was fitted with a glassy carbon working electrode held at 350 mV vs. Ag/AgCl. The detection limit of hydroquinone in 30 mM borate buffer pH 9.5 was 1.2 x 10(-10) M (linearity range: 10(-9)-5.12 x 10(-6 M). A detection limit for free alkaline phosphatase of 1.2 x 10(-15) M (linearity range: 10(-15)-10(-13) M), or about 36 000 molecules, was observed (same borate buffer and incubations of 10 min at 25 degrees C). These conditions were maintained for the amperometric alpha-fetoprotein immunoassay. For comparison purposes, a photometric detection system was set up, with p-nitrophenyl phosphate as enzyme substrate and the same pair of antibodies and incubation conditions. The detection limit for alpha-fetoprotein obtained by amperometry, 0.07 ng/ml (linearity range = 5-500 ng/ml), was 14 times lower than by photometry. The amperometric enzyme immunoassay correlates well with a commercial colorimetric immunoassay (r = 0.986, slope = 0.967, n = 240).