A　novel　highly　sensitive　and　selective　electrochemical　sensing　system　based　on　DNAzyme　and　rolling　circle　amplification　(RCA)　for　the　determination　of　Pb2+　was　developed　in　the　present　study.　Firstly,　the　DNAzyme　catalytic　strands　were　immobilized　onto　magnetic　beads　surface　and　then　hybridized　with　substrate　strands.　In　the　presence　of　Pb2+,　the　DNAzyme　could　be　activated　to　cleave　the　substrate　strand　into　two　DNA　fragments.　After　RCA　reaction,　a　long　ssDNA　product　with　repeating　sequence　was　obtained.　Subsequently,　CdS　QDs　modified　ssDNA　(CdS　QD-ssDNA)　were　used　as　signaling　probes　to　hybridize　with　the　long　ssDNA　product.　Due　to　the　dramatic　signal　amplification　by　the　numerous　QDs　and　the　low　background　signal　by　magnetic　separation,　ultra-low　level　(7.8　pM)　of　Pb2+　could　be　detected.　Furthermore,　with　the　application　of　Pb2+　dependent　DNAzyme,　the　proposed　sensing　system　exhibited　high　selectivity.　The　proposed　sensing　system　showed　a　promising　potential　for　on-site　testing　and　would　gain　wide　applications　in　real　sample　analysis.　(C)　2012　Elsevier　B.V.　All　rights　reserved.