Experiment on point of care testing (POCT) for highly sensitive detection of protein requires a very small volume of solution. In this study, we develop a highly sensitive device for protein detection which combine the enzymatic oxidase reaction and highly sensitive detection of H2O2. The detection principle of this device was based on simultaneous oxidation and reduction that occurred on a single electrode. H2O2 produced during enzymatic reaction of immunoassay was oxidized and at the same time reduced the silver. The deposited silver was then detected by coulometry.