A　facile　and　sensitive　mediator-free　electrochemical　immunosensor　for　detection　of　alpha-fetoprotein　(AFP)　was　designed　by　using　nanosilver-coated　silica　nanoparticles　(Ag-SiO2)　as　bionanolabels.　To　construct　such　an　electrochemical　immunosensor,　silver　ions/single-stranded　DNA/graphene　nanosheets　were　initially　immobilized　on　a　gold　electrode　in　turn,　then　silver　ions　were　in　situ　reduced　to　silver　nanoparticles　with　the　aid　of　NaBH4,　and　anti-AFP　antibodies　conjugated　to　silver　nanoparticles　were　used.　In　the　presence　of　AFP　analyte,　the　sandwiched　immunocomplex　was　formed　on　the　electrode　surface　by　using　horseradish　peroxidase-anti-AFP　conjugate-labeled　Ag-SiO2　(HRP-anti-AFP-Ag-SiO2)　as　secondary　antibodies.　Compared　with　pure　silver　nanoparticles,　Ag-SiO2　nanocomposites　could　provide　a　large　room　for　the　immobilization　of　HRP-anti-AFP,　and　improve　the　electrochemical　responses　of　the　immunosensor.　Meanwhile,　the　presence　of　highly　conductive　graphene　nanosheets　and　silver　nanoparticles　provided　a　good　pathway　for　electron　transfer.　Under　optimal　conditions,　the　immunosensor　exhibited　good　electrochemical　responses　toward　AFP　ranging　from　0.3　to　200　ng/mL　with　a　detection　limit　CLOD)　of　0.05　ng/mL　(at　3　sigma)　in　pH　6.0　PBS-H2O2　system.　Intra-　and　inter-assay　displayed　good　precisions　with　coefficient　of　variation　below　9.5%.　In　addition,　the　method　was　evaluated　with　23　clinical　serum　samples,　receiving　good　correlation　with　results　from　commercially　available　electrochemiluminescent　analyzer.　(C)　2011　Elsevier　Ltd.　All　rights　reserved.