Figure 5 The reaction cycles leading to the observed current.

The dependence of the signal and background current densities on the concentration of the HRP-labeled antibody is shown in Figure 6. In the absence of antigen, the current is small. At a 1:1 ratio of HRP-labeled antibody to antigen, the signal-to-noise ratio is 20:1, and at a 4:1 ratio, the signal-to-noise ratio is 18:1. Nonspecific interaction of the HRPlabeled antibody fragment with the redox hydrogel or other electrode components can lead to high background signals, result in lower signal-to-noise ratios, and a less sensitive assay. These nonspecific interactions may involve electrostatic attractions or hydrophobic interactions. In almost all immunoassays, blocking agents such as salts, proteins, or surfactants are used to partially block nonspecific adsorption. In this system, low concentrations (0.1 wt%) of BSA, combined with 0.16 M NaCl, pH 7.4 phosphate buffer eliminate nearly all nonspecific adsorption.