Enzyme competition electrode modeling

Many problems involving competitive reaction kinetics may be treated by invoking the steady-state assumption within the digital simulation this has been done in at least two instances [29-34]. The first of these involves the development of a model for enzyme catalysis in the amperometric enzyme electrode [29-31]. In this model, the enzyme E is considered to be immobilized in a diffusion medium covering an electrode that is operated at a fixed potential such that the product (P) of enzyme catalysis is electroactive under diffusion-controlled conditions. (This model has also served as the basis for the simulation of the voltammetric response of the enzyme electrode [35].) The substrate (S) diffuses through the medium that contains the immobilized enzyme and is catalyzed to form P by straightforward enzyme kinetics ... 

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. 

The homogeneous assay is based on a competitive equilibrium between Ag and Ag for a limited amount of antibody as described (see Fig. 14 and Section 11.2). The assay relies on a decrease in the current signal for the reduction/oxidation of Ag when bound to Ab. Since all of these assays have no enzyme multiplication feature, the detection limits are generally much higher. In the model assay that follows, the free, equilibrium concentration of electroactively labeled estriol is determined by differential pulse polarography at a dropping mercury electrode (Wehmeyer et al., 1982). The peak current from the polarographic reduction of Ag is then proportional to the concentration of Ag present. The separation of Ab Ag from Ag is unnecessary because of the attenuation of the Ab Ag reduction. 

The microplate model for immunoassays has been successfully adapted for electrochemical measurements, with a 96-weU screen-printed carbon microplate system [50], shown in Fig. 8. The weU bottoms form the sensing surfaces with carbon working electrodes and Ag/AgCl counter electrodes. The system employs intermittent pulse amperometry at fixed pulse potentials for detectiOTi. In this report, aflatoxin Bi was the analyte, and a spiked com matrix was chosen for examination of extraction efficiencies and matrix effects in a competitive enzyme-linked... 

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