Voltammetric immunoassays

The development of electrochemical immunosensors generally involves the immobilization of an immunocomplex on a single electrode, followed by detection via the 

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Voltammetric immunoassay (VIA) (i) Isoenzyme electrodes (ii) Metal chelates Voltammetry Experimental methods may be quite complicated (35)... 

Pneumolysin Sandwich immunoassay with ALP or HRP labelled antibody using 3-indoxyl phosphate (3-IP) as substrate Cyclic voltammetric detection of indigo carmine (Ey 2 =-0.15 V)... 

Immunoassays, electrochemical — A quantitative or qualitative assay based on the highly selective antibody-antigen binding and electrochemical detection. Poten-tiometric, capacitive, and voltammetric methods are used to detect the immunoreaction, either directly without a label or indirectly with a label compound. The majority of electrochemical immunoassays are based on -> voltammetry (-> amperometry) and detection of redox-active or enzyme labels of one of the immunochemical reaction partners. The assay formats are competitive and noncompetitive (see also -> ELISA). 

Zhang, S., Jiao, K., and Chen, H. Investigation of voltammetric enzyme - linked immunoassay based on a new system of OAP-H2O2-HRP. Electroanalysis 1999, 11, 511-516. 

The discussion is restricted to immunoassays based on voltammetric electroanalysis, involving the application of a potential to an electrochemical cell and measurement of the current resulting from an oxidation or reduction at the electrode surface. Immunoassays based on potentiomet-ric electrochemical techniques are also under active development. Additionally, electrochemical sensors in which immunological reactions provide the selectivity are being developed for a variety of purposes Proceedings, 1983 Czaban, 1985). 

P. Fanjul-Bolado, M.B. Gonzalez-Garcia, and A. Costa-Garda, Voltammetric determination of alkaline phosphatase and horseradish peroxidase activity using 3-indoxyl phosphate as substrate application to enzyme immunoassay, Talanta 64(2), 452-457 (2004). 

We have developed nanoparticle label-based electrochemical immunoassay of OP exposure biomarkers, OP-ChE (44). The principle and procedure of this method is shown in Figure 5. Here, zircomia (Zr02) NPs, electrochemically deposited on the electrode surface, which functioned as the capture antibodies in the sandwich immunoassay. The zirconia- NP- modified electrode is then exposed to the sample solution and the ChE-OP in the sample is captured by the Zr02 nanoparticles. The QD-tagged, anti-AChE conjugate is introduced to form the sandwich-like complex on the sensor surface. The captured QDs are then dissolved by a drop of acid to release cadmium ions. This is followed by square wave voltammetric (SWV) [abbreviation already introduced but unexplained] detection of the released cadmium ions at an in situ plated mercury/bismuth film electrode. 

In contrast to direct immunosensors, indirect immunosensors, using as a label an enzyme [307-309] or an ionophore [310], have been more developed. This technique is derived from well established and widely utilized enzyme immunoassays evaluated by photometric methods. Enzyme immunoassay with electrochemical detection (EEIA) represents an important innovation of EIA by the replacement of optical detection with voltammetric or potentiometric measurements. It may also be adapted to flow-through systems [311, 312]. 

This immunosensor-type assay uses ferrocene methanol as co-substrate (redox mediator) of glucose oxidase (GOD). The various steps of the assay, shown in Fig. 8.20, are as follows (1) Covalent immobilization of protein A on graphite-polystyrene screen-printed electrodes (SPEs) (2) Addition of the rabbit IgG to be quantified which is captured specifically by protein A (3) Addition of a biotinylated goat anti-rabbit antibody (4) Addition of avidin-GOD conjugate (5) Addition of glucose and ferrocene methanol (6) Measurement of catalytic current by flow injection immunoassay. The voltammetric current corresponds to the one-electron oxidation of the ferrocenyl group to ferricinium. The electrode can subsequently be regenerated up to 30 times. The feasibility of the assay has been demonstrated for the case of monoclonal mouse anti-human prolactin (PL) with a detection limit of 0.02 pg mL [90]. 

Electrochemical immunoassays were pioneered in the 1980s by Heineman, Halsall, and coworkers.23-29 Early in their studies, they employed voltammetric and amperometric techniques to measure proteins and small biologically important molecules, such as hormones, via competitive and sandwich-type immunoassays. Heineman and Halsall and other researchers explored many different measurement techniques and incorporated microfluidics, - reagent delivery systems, interdigitated electrode arrays, b34-36 magnetic beads ° b33,35-37 various designs to improve... 

Niwa, O., Y. Xu, H.B. Halsall, and W.R. Heineman. 1993. SmaU-volume voltammetric detection of 4-aminophenol with interdigitated array electrodes and its application to electrochemical enzyme immunoassay. Anal. Chem. 65 1559-1563. 

Wei MY, Wen SD, Yang XQ, Guo LH (2009) Development of redox-labeled electrochemical immunoassay for polycyclic aromatic hydrocarbons with controlled surface modification and catalytic voltammetric detection. Biosens Bioelectron 24 2909-2914... 

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