An　enzyme-free　and　label-free　fluorescent　biosensor　is　developed　by　G-quadruplex-based　hybridization　chain　reaction　(HCR)　for　small　molecules,　using　adenosine　triphosphate　(ATP)　as　the　model.　Aptamer　probes　for　the　recognition　of　small　molecules　are　hybridized　with　blocking　probes.　The　G-quadruplex　sequences　are　incorporated　into　one　of　the　two　HCR　hairpin　probes.　In　the　presence　of　small　molecules　(ATP),　the　formation　of　aptamer-ATP　bioaffinity　complexes　induces　the　release　of　blocking　probes;　the　released　blocking　probes　initiate　HCR　and　numerous　G-quadruplexes　along　DNA　nanowires　are　self-assembled　after　the　HCR　process.　Using　N-methyl　mesoporphyrin　IX　(NMM)　as　the　fluorophore,　a　＇turn-on＇　fluorescence　response　can　be　achieved　and　detected　as　low　as　15　μmol　L-1　of　ATP.　This　biosensor　is　applied　to　detect　ATP　in　biologic　samples　with　satisfactory　results.　©　2015　Elsevier　B.V.　All　rights　reserved.