The　background　signal　is　a　major　factor　that　restricts　the　limit　of　detection　of　biosensors.　Herein,　we　present　a　zero-background　DNA-sensing　approach　that　utilizes　enzyme-guided　gold　nanoparticle　(AuNP)　enlargement.　This　sensing　strategy　is　based　on　the　finding　that　small　nanoparticles　are　invisible　under　a　darkfield　optical　microscope,　thus　completely　eliminating　the　background　signal.　In　the　event　of　target　binding,　Ag　deposition　is　triggered　and　enlarges　the　AuNP　beyond　its　optical　diffraction　limit,　thereby　making　the　invisible　AuNP　visible.　Because　the　plasmon　scattering　of　Ag　is　stronger　than　that　of　Au,　only　a　thin　layer　of　Ag　is　required　to　greatly　enhance　the　scattering　intensity　of　the　AuNPs.　Our　investigation　revealed　that　a　target　DNA　concentration　as　low　as　5.0　x　10(-21)　M　can　transform　the　darkfield　image　of　the　nanoparticle　from　completely　dark　(invisible)　to　a　blue　dot　(visible).　(C)　2015　Elsevier　B.V.　All　rights　reserved.