A　novel　magneto-controlled　electrochemical　DNA　biosensor　is　designed　for　the　ultrasensitive　detection　of　lead　coupling　a　lead-specific　DNAzyme　with　DNA-based　hybridization　chain　reaction　(HCR).　To　construct　such　a　magnetic　lead　sensor,　DNAzyme-based　molecular　beacons,　selective　to　cleavage　in　the　presence　of　Pb2+,　are　initially　immobilized　onto　magnetic　beads,　which　were　used　as　the　recognition　elements.　Upon　addition　of　target　lead,　catalytic　cleavage　of　substrate　DNA　segments　in　the　double-stranded　DNAzymes　results　in　the　capture　of　the　initiator　strands　via　the　conjugated　catalytic　strands　on　magnetic　beads.　The　captured　DNA　initiator　strands　trigger　the　hybridization　chain　reaction　between　two　alternating　hairpin　DNA　structures　labeled　with　ferrocene　to　form　a　nicked　double-helix　on　the　magnetic　beads.　Numerous　ferrocene　molecules　are　formed　on　the　neighboring　probes,　each　of　which　produces　an　electrochemical　signal　within　the　applied　potential.　Under　optimal　conditions,　the　electrochemical　signal　of　the　magnetic　lead　sensor　increases　with　the　increasing　lead　level　in　the　sample,　and　exhibits　a　linear　response　over　a　Pb2+　concentration　range　of　0.1-75　nM　with　a　detection　limit　of　37　pM.　Quantitative　measurement　of　Pb2+　in　the　complex　sample　demonstrates　the　selectivity　of　the　sensor　scheme　and　points　favorably　to　the　application　of　such　technologies　to　the　analysis　of　environmental　samples.　The　unique　combination　of　a　DNAzyme　with　hybridization　chain　reaction　makes　it　possible　to　change　the　DNAzyme　to　select　for　other　compounds　of　interest.　This　work　represents　the　initial　steps　toward　the　creation　of　a　robust　field　sensor　for　lead　in　groundwater　or　drinking　water.　(C)　2013　Elsevier　B.V.　All　rights　reserved.