A　new　enzyme-free　and　ultrasensitive　electrochemical　Pb2+　biosensor　was　developed.　By　coupling　the　DNA-assisted　cascade　of　hybridization　reaction　with　the　quantum　dots　(QDs)　for　signal　amplification,　a　detection　limit　as　low　as　6.1　pM　can　be　obtained　for　Pb2+.　In　this　study,　the　＇8-17＇　DNAzyme　was　used　for　specific　recognition　of　Pb2+.　In　the　presence　of　Pb2+,　the　DNAzyme　was　activated　and　cleaved　the　substrate　strand.　And　then,　the　hybridization　between　the　linker　probe　and　signal　probe　was　initiated,　which　resulted　in　formation　of　a　long　cascade　DNA　structure　as　well　as　assemble　of　numerous　QDs　at　last.　By　the　use　of　magnetic　beads,　the　free　signal　probe　can　be　easily　removed　by　external　magnetic　field.　After　acid　lysis,　a　great　amount　of　redox　cations　can　be　released　from　the　QDs　and　eventually　result　in　significantly　amplified　electrochemical　signals.　This　method　is　highly　sensitive,　selective　and　simple　without　the　participation　of　any　protein　based　enzyme　(nuclease),　thereby　holds　great　potential　for　real　sample　analysis.　©　2014　Elsevier　Ltd.