This　work　designs　a　new　label-free　aptasensor　for　the　colorimetric　determination　of　small　molecules　(adenosine　5′-triphosphate,　ATP)　by　using　visible　gold　nanoparticles　as　the　signal-generation　tags,　based　on　target-triggered　hybridization　chain　reaction　(HCR)　between　two　hairpin　DNA　probes.　The　assay　is　carried　out　referring　to　the　change　in　the　color/absorbance　by　salt-induced　aggregation　of　gold　nanoparticles　after　the　interaction　with　hairpins,　gold　nanoparticles　and　ATP.　To　construct　such　an　assay　system,　two　hairpin　DNA　probes　with　a　short　single-stranded　DNA　at　the　sticky　end　are　utilized　for　interaction　with　gold　nanoparticles.　In　the　absence　of　target　ATP,　the　hairpin　DNA　probes　can　prevent　gold　nanoparticles　from　the　salt-induced　aggregation　through　the　interaction　of　the　single-stranded　DNA　at　the　sticky　end　with　gold　nanoparticles.　Upon　target　ATP　introduction,　the　aptamer-based　hairpin　probe　is　opened　to　expose　a　new　sticky　end　for　the　strand-displacement　reaction　with　another　complementary　hairpin,　thus　resulting　in　the　decreasing　single-stranded　DNA　because　of　the　consumption　of　hairpins.　In　this　case,　gold　nanoparticles　are　uncovered　owing　to　the　formation　of　double-stranded　DNA,　which　causes　their　aggregation　upon　addition　of　the　salt,　thereby　leading　to　the　change　in　the　red-to-blue　color.　Under　the　optimal　conditions,　the　HCR-based　colorimetric　assay　presents　good　visible　color　or　absorbance　responses　for　the　determination　of　target　ATP　at　a　concentration　as　low　as　1.0　nM.　Importantly,　the　methodology　can　be　further　extended　to　quantitatively　or　qualitatively　monitor　other　small　molecules　or　biotoxins　by　changing　the　sequence　of　the　corresponding　aptamer.　©　2016　Elsevier　B.V.