Potentiometric Immunoassay

Instead of immobilizing the antibody onto the transducer, it is possible to use a bare (amperometric or potentiometric) electrode for probing enzyme immunoassay reactions (42). In this case, the content of the immunoassay reaction vessel is injected to an appropriate flow system containing an electrochemical detector, or the electrode can be inserted into the reaction vessel. Remarkably low (femtomolar) detection limits have been reported in connection with the use of the alkaline phosphatase label (43,44). This enzyme catalyzes the hydrolysis of phosphate esters to liberate easily oxidizable phenolic products. 

The remarkable selectivity that is inherent in the reaction of an antibody with the antigen or hapten against which it was raised is the basis for the extensive use of immunoassay for the rapid analysis of samples in clinical chemistry. Immunochemical reactions offer a means by which the applicability of potentiometric techniques can be broadened. A number of strategies for incorporating immunoassay into the methodology of potentiometry have been explored... 

Immunoassays have been based on the potentiometric measurement of marker ions such tetrapentylammonium ion (TPA ) that are loaded in phospholipid liposomes Complement mediated immunolysis of these loaded vesicles is caused... 

M.Y. Keating and G.A. Rechnitz, Potentiometric enzyme immunoassay for digoxin using polystyrene beads. Anal. Lett. 18, 1-10 (1985). 

T. Fonong and G.A. Rechnitz, Homogeneous potentiometric enzyme immunoassay for human immunoglobulin G. Anal. Chem. 56, 2586-2590 (1984). 

G.B. Sigal, D.G. Hafeman, J.W. Parce and H.M. Mcconnell, Electrical-properties of phospholipid-bilayer membranes measured with a light addressable potentiometric sensor, ACS Symp. Ser., 403 (1989) 46-64. J.D. Olson, P.R. Panfili, R. Armenta, M.B. Femmel, H. Merrick, J. Gumperz, M. Goltz and R.F. Zuk, A silicon sensor-based filtration immunoassay using biotin-mediated capture, J. Immunol. Methods, 134(1) (1990) 71-79. 

J.T. Colston, P. Kumar, E.D. Rael, A.T.C. Tsin, J.J. Valdes and J.P. Chambers, Detection of sub-nanogram quantities of Mojave toxin via enzyme immunoassay with light addressable potentiometric detector, Biosens. Bioelectron., 8(2) (1993) 117-121. 

B2. Boitieux, J. L., Lemay, C., Desmet, G., and Thomas, D., Use of solid phase biochemistry for potentiometric enzyme immunoassay of oestradiol-17fl-preliminary report. Clin. Chim. Acta 113, 175-182 (1981). 

Instead of immobilizing the antibody onto the transducer, it is possible to use a bare (amperometric or potentiometric) electrode for probing enzyme immunoassay reactions (59). In this case, the content of the immunoassay reac-... 

The potentiometric determination of estradiol-1 P in solution provides an example of a solid-phase competitive immunoassay (316). The anti-estradiol-17P antibodies are immobilized on a pig skin gelatin membrane. After incubation with HRP-labeled steroid and estradiol, the membrane is mounted over an iodide-selective electrode to measure the enzymatic activity. The electrode potential is a function of antigen concentration at levels ranging from 57 pM to 9.2 nM. 

An enzyme immunoassay using adenosine deaminase as the enzyme label has been described (318). Potentiometric rate measurements were made with an ammonia gas-sensing electrode. The immunoassay system employed is based on competition between a model haptenic group, dinitrophenyl (DNP) covalently coupled to adenosine deaminase, and free DNP hapten for the available binding sites on the anti-DNP antibody molecules. The detection limit is 50 ng antibody. 

Gebauer, C.R. Rechnitz, G.A. Deaminating enzyme labels for potentiometric enzyme immunoassay. Anal. Biochem. 1982, 124, 338-348. 

Electrochemical immunoassays include a wide variety of devices based on the coupling of immunological reactions with electrochemical transduction. All of them involve the immobilization of an immunoreagent component on the surface of the electrode transducer. Electrochemical detection is based on the direct intrinsic redox behavior either of an analyte species or of some reporter molecule. For the detection no expensive equipment is needed, with the measurement of either a simple current or a voltage charge. Different electrochemical detection strategies are used, but ampero-metric detection is most widely used. Potentiometric and conductometric detection are applied in different assays as well. 

Currently, immunosensors provide a powerful tool for monitoring biospecific interactions in real time or for deriving information about the binding kinetics of an immunoreaction or the structure/function relationships of molecules. Amperometric, potentiometric, conductive, and impedimetric transducers have been applied in direct and indirect electrochemical immunoassays. 

In indirect electrochemical immunoassays, the binding reaction is visualized indirectly via an auxiliary reaction by a labelling compoxmd. Because antibodies and antigens are usually not electrochemically active within the desired potential range, redox-active compounds can be applied as labels for indication. Although the ability to work reagent-free (which can be accomplished with potentiometric and capacitive immunoassays) is not possible with indirect amperometric immunoassays, non-specific binding of molecules other than the labelled immunoreactant will not contribute to the... 

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