A　novel　enzyme-free　sandwich　electrochemical　immunoassay　with　an　ultrahigh　sensitivity　was　developed　for　detection　of　alpha-fetoprotein　(AFP,　as　a　model　analyte)　using　carbon　nanotube-enriched　gold　nanoparticles　(CNT-AuNPs)　as　nanolabels/nanocatalysts　on　anti-AFP/glutaraldehyde/thionine-modified　glassy　carbon　electrodes　(GCEs).　The　assays　were　carried　out　in　a　pH　8.0　acetic　acid-buffered　solution　containing　6　mM　p-nitrophenol　(NP)　and　6　mM　NaBH4　after　the　formation　of　the　sandwich-type　immunocomplex.　Initially,　the　NP　molecules　were　reduced　to　p-aminophenol　(AP)　by　the　catalysis　of　the　immobilized　gold-nanoparticle　labels　on　the　CNT-AuNPs　with　the　aid　of　NaBH4,　then　the　generated　AP　molecules　were　electrochemically　oxidized　to　p-quinone　imine　(QI)　by　an　electron　mediator　of　thionine,　and　then　the　oxidized　QI　molecules　were　reduced　back　to　APs　by　NaBH4.　The　redox　cycling　of　AP　and　QI　continuously　increased　the　signaling,　leading　to　a　high　sensitivity.　Compared　with　individual　gold-nanoparticle　labels,　the　immunosensor　using　CNT-AuNPs　as　labels　displayed　a　wider　linear　range　of　8.0　x　10(-7)-2.0　x　10(2)　ng/mL　with　a　lower　detection　limit　(LOD)　of　0.8　fg/mL　AFP　at　a　signal-to-noise　ratio　of　3,　which　was　lower　6　orders　than　that　of　commercially　available　ELISA.　Intra-and　inter-assay　coefficients　of　variation　were　below　10%.　In　addition,　the　assay　was　evaluated　with　clinical　serum　samples,　and　no　significant　differences　at　the　5%　confidence　level　were　encountered　in　the　analysis　of　real　samples　between　the　proposed　immunoassay　and　commercially　available　Roche　2010　Electrochemiluminescent　Automatic　Analyzer　for　determination　of　AFP.　(C)　2010　Elsevier　B.V.　All　rights　reserved.