The　first　reconfigurable　colorimetric　DNA　switches　based　on　target　DNA　binding　are　reported.　This　DNA　binding　actuates　a　change　in　the　interparticle　distance　between　gold　nanoparticle　dimers.　A　significant　spectral　shift　of　68　nm　is　achievable　from　on-off　switching.　The　reconfigurability　is　possible　owing　to　thiol　and　EDC-imidazole　coupling　which　anchors　the　DNA　linkers　to　the　nanoparticles.　The　huge　spectral　shift　allows　the　unaided　eye　to　observe　single　target　biomolecular　binding　event　in　real　time　under　a　darkfield　microscope.　The　limit-of-detection　for　target　molecules　in　PBS　and　human　serum　are　10　-13　M　and　10-11　M　respectively.　An　improved　fabrication　strategy　via　asymmetric　functionalization　is　also　described,　assisted　by　solid　phase　synthesis　which　minimizes　the　formation　of　trimers　and　multimers.　Copyright　©　2013　WILEY-VCH　Verlag　GmbH　&　Co.　KGaA,　Weinheim.