Horseradish　peroxidase　mimicking　DNAzyme　(HRP-DNAzyme)　attracts　growing　interest　as　an　amplifying　label　for　biorecognition　and　biosensing　events,　especially　for　DNA　detection.　However,　in　the　traditional　designs,　one　target　molecule　can　only　generate　one　HRP-DNAzyme,　which　limits　the　signal　enhancement　and　thus　its　sensitivity.　In　this　article,　we　propose　an　amplified　and　label-free　colorimetric　DNA　detection　strategy　based　on　nicking　endonuclease　(NEase)-assisted　activation　of　HRP-DNAzymes　(NEAA-DNAzymes).　This　new　strategy　relies　on　the　hairpin-DNAzyme　probe　and　NEase-assisted　target　recycling.　In　the　hairpin-DNAzyme　probe,　the　HRP-DNAzyme　sequence　is　protected　in　a　＂caged＂　inactive　structure,　whereas　the　loop　region　includes　the　target　complementary　sequence.　Upon　hybridization　with　target,　the　beacon　is　opened,　resulting　in　the　activation　of　the　HRP-DNAzyme.　Meanwhile,　upon　formation　of　the　duplex,　the　NEase　recognizes　a　specific　nucleotide　sequence　and　cleaves　the　hairpin-DNAzyme　probe　into　two　fragments.　After　nicking,　the　fragments　of　the　hairpin-DNAzyme　probe　spontaneously　dissociate　from　the　target　DNA.　Amplification　is　accomplished　by　another　hairpin-DNAzyme　probe　hybridizing　to　the　released　intact　target　to　continue　the　strand-scission　cycle,　which　results　in　activation　of　numerous　DNAzymes.　The　activated　HRP-DNAzymes　generate　colorimetric　or　chemiluminescence　readout　signals,　thus　providing　the　amplified　detection　of　DNA.　The　detection　limit　of　the　colorimetric　method　is　10　pmol/L,　which　are　three　orders　of　magnitude　lower　than　that　without　NEase.　In　addition,　the　detection　limit　of　the　chemiluminescence　method　is　0.2　pmol/L.　Meanwhile,　this　strategy　also　exhibits　high　discrimination　ability　even　against　single-base　mismatch.　(C)　2011　Elsevier　B.V.　All　rights　reserved.