A　novel　signal-on　junction-probe　electrogenerated　chemiluminescence　(ECL)　aptamer　biosensor　has　been　developed　for　the　detection　of　ultratrace　thrombin　based　on　a　structure-switching　ECL-quenching　mechanism.　The　ECL　aptamer　biosensor　comprises　two　main　parts:　an　ECL　substrate　and　an　ECL　intensity　switch.　The　ECL　substrate　was　made　by　modifying　the　complex　of　Au　nanoparticle　and　ruthenium　(II)　tris-bipyridine　(Ru(bpy)32+-AuNPs)　on　the　surface　of　gold　electrode　(GE),　and　the　ECL　intensity　switch　contains　three　probes　designed　according　to　the　＇　junction-probe＇　strategy.　The　first　probe　is　capture　probe　(Cp)　which　was　functionalized　with　a　thiol　group　at　one　end　and　covalently　attached　to　Ru(bpy)32+-AuNPs　modified　GE　through　S-Au　bonding.　The　second　probe　is　aptamer　probe　(Ap),　which　containing　15-base　anti-thrombin　DNA　aptamer.　The　third　one　is　ferrocene-labeled　probe　(Fp),　which　was　functionalized　with　ferrocene　tag　at　one　end.　We　demonstrated　that,　in　the　absence　of　thrombin,　Cp,　Ap　and　Fp　will　hybridize　to　form　a　ternary　＇　Y＇　junction　structure　and　resulted　in　a　quenching　of　ECL　of　Ru(bpy)32+.　Whereas,　in　the　presence　of　thrombin,　the　Ap　prefers　to　form　the　G-quadruplex　aptamer-thrombin　complex　and　lead　to　an　obvious　recovery　of　ECL　of　Ru(bpy)32+,　which　provided　a　sensing　platform　for　the　detection　of　thrombin.　Using　this　reusable　sensing　platform,　a　simple,　rapid　and　selective　signal-on　ECL　aptamer　biosensor　for　the　detection　of　thrombin　with　a　detection　limit　of　8.0×10-15M　has　been　developed.　The　success　in　the　present　biosensor　served　as　a　significant　step　towards　the　development　of　monitoring　ultratrace　thrombin　in　clinical　detection.　©　2010　Elsevier　B.V.