We report an ultrasensitive sandwich-type electrochemical immunoassay of proteins, on the basis of in situ enzymatic formation of CdS quantum dots (QDs), simultaneous chemical dissolution of CdS and cathodic pre-concentration of Cd, and microliter-droplet anodic stripping voltammetry (ASV) detection directly on the immunoelectrode. The antibody 2 labeled with alkaline phosphatase (ALP) and Au nanoframes (Ab(2)-ALP-AuNFs) can catalyze hydrolysis of ascorbic acid 2-phosphate to form ascorbic acid and inorganic phosphate, and the latter can stabilize the formation of CdS QDs on the immunoelectrode through Cd2+-S2- reaction. A beforehand "potential control" and then an injection of 7 mu L 0.1 M aqueous HNO3 lead to dissolution of the CdS QDs for simultaneous electrodeposition of metallic Cd, and ASV directly on the immunoelectrode is then conducted to quantify the antigen analyte. The use of AuNFs can increase the enzyme load and activity for improved signaling. Under optimized conditions, this method is used for ultrasensitive analysis of human immunoglobulin G (IgG) and human cardiopathy biomarker cardiac troponin I (cTnI), giving limits of detection (LODs, S/N = 3) of 1.2 fg mL(-1) for IgG and of 1.1 fg mL(-1) for cTnI (equivalent to 173 molecules in the 6 mu L sample employed for cTnI analysis). The LOD of IgG is ca. 3 orders of magnitude lower than that of the fluorescent method by experimentally recording the emission spectrum of the CdS QDs at lambda(ex) = 290 nm and lambda(em) = 500 nm.