A new and novel signal probe based on silver nanoclusters (Ag NCs)/horseradish peroxidase (HRP) nanocomposite (Ag NCs-HRP) was developed. The probe was utilized for electrochemical immunoassay for the detection of carcinoembryonic antigen (CEA). The Ag NCs was synthesized via chemical reduction in the presence of DNA strand containing CEA aptamer (ATACCAGCTTATTCAATT) and the Ag NCs synthesis template (C-12). After the adsorption of HRP onto Ag NCs surface, the resulted Ag NCs-HRP could bind with CEA and acted as signaling probe. The assay involves the following steps: (1) construction of an immunosensor by immobilizing the antibody against CEA on a glassy carbon electrode; (2) incubate the immunosensor with sample containing CEA; (3) incubate the immunosensor with Ag NCs-HRP; (4) signal amplification via HRP catalyzed deposition of an insulating polymer film, poly(3,3'-dimethoxybenzidine) (PDB) onto electrode surface; (5) measurement via square wave voltammetry in Fe(CN)(6)(3-/4-) solution. The current decreased due to the formation of insulating PDB film is logarithmically related to the concentration of CEA in the 1 pg to 10 ng mL(-1) range, with a detection limit of 0.5 pg mL(-1). The developed nanocluster/enzyme signal probe can certainly be modified for the detection of different other trace targets in bioanalytical fields. (C) 2015 Elsevier Ltd. All rights reserved.