A　novel　and　in　situ　amplified　immunoassay　strategy　with　quadruple　signal　amplification　was　designed　for　highly　efficient　electrochemical　detection　of　low-abundance　proteins　(carcinoembryonic　antigen,　CEA,　as　a　model)　by　using　nanogold-functionalized　DNAzyme　concatamers　with　redox-active　intercalators.　To　construct　such　an　in　situ　amplification　system,　streptavidin-labeled　gold　nanoparticles　(AuNP-SA)　were　initially　used　for　the　labelling　of　initiator　strands　(S0)　and　detection　antibody　(mAb2)　with　a　large　ratio　(mAb2-AuNP-S0),　and　then　two　auxiliary　DNA　strands　S1　and　S2　were　designed　for　in　situ　propagation　of　DNAzyme　concatamers　with　the　hemin/G-quadruplex　format.　The　quadruple　signal　amplification　was　implemented　by　using　the　avidin-biotin　chemistry,　nanogold　labels,　DNA　concatamers,　and　DNAzymes.　In　the　presence　of　target　CEA,　the　sandwiched　immunocomplex　was　formed　between　the　immobilized　primary　antibodies　on　the　electrode　and　the　conjugated　detection　antibodies　on　the　mAb　2-AuNP-S0.　The　carried　S0　initiator　strands　could　progress　a　chain　reaction　of　hybridization　events　between　alternating　S1/S2　DNA　strands　to　form　a　nicked　double-helix.　Upon　addition　of　hemin,　the　hemin-binding　aptamers　could　be　bound　to　form　the　hemin/G-quadruplex-based　DNAzymes.　The　formed　double-helix　DNA　polymers　could　cause　the　intercalation　of　numerous　electroactive　methylene　blue　molecules.　During　the　electrochemical　measurement,　the　formed　DNAzymes　could　catalyze　the　reduction　of　H2O2　in　the　solution　to　amplify　the　electrochemical　signal　of　the　intercalated　methylene　blue.　Under　optimal　conditions,　the　electrochemical　immunoassay　exhibited　a　wide　dynamic　range　of　1.0　fg　mL-1　to　20　ng　mL-1　toward　CEA　standards　with　a　low　detection　limit　of　0.5　fg　mL-1.　Intra-assay　and　inter-assay　coefficients　of　variation　(CV)　were　less　than　8.5%　and　11.5%,　respectively.　No　significant　differences　at　the　0.05　significance　level　were　encountered　in　the　analysis　of　14　clinical　serum　specimens　between　the　developed　immunoassay　and　commercialized　electrochemiluminescent　(ECL)　method　for　detection　of　CEA.　©　2013　American　Chemical　Society.