An electrochemical sandwich immunoassay method that can be sensitive to a few protein molecules (human immunoglobulin G or human prostate-specific antigen) is reported, based on HAuCl₄–NH₂OH redox reaction to enlarge the size of second antibody labeled gold nanoparticles and in situ microliter-droplet anodic stripping voltammetry analysis with enhanced cathodic preconcentration of gold.