An　amplified　colorimetric　detection　of　Hg2+　is　proposed　by　combining　T-Hg2+-T　base　pairs　and　hybridization　chain　reaction　(HCR).　Two　hairpins　consisting　of　three-fourths　and　one-fourth　of　the　horseradish　peroxidase　(HRP)-mimicking　DNAzyme　in　inactive　configuration　are　used　as　functional　elements.　In　the　presence　of　Hg2+,　one　of　the　hairpins　is　opened　by　an　assistant　probe　with　the　help　of　T-Hg2+-T　base　pairs　and　this　triggers　an　autonomous　cross-opening　of　the　two　hairpins　using　the　strand　displacement　principle,　resulting　in　the　formation　of　DNA　nanowires　consisting　of　numerous　reunited　Q-quadruplex　DNAzyme　units.　The　resulting　catalytically　active　hemin/G-quadruplex　HRP-mimicking　DNAzymes　catalyze　the　oxidation　of　ABTS2-　by　H2O2　into　a　green-colored　cationic　radical　ABTS•+　for　the　colorimetric　readout.　This　＂turn-on＂　sensing　system　enables　the　high　sensitive　and　selective　detection　of　aqueous　Hg2+　with　a　detection　limit　of　9.7pM.　©　2013　Elsevier　B.V.