A　new　electrochemical　biosensor　for　the　monitoring　of　ultratrace　terbium　based　on　the　conformational　change　of　DNA　containing　a　single　guanine　(G)-rich　stretch　was　described　here.　The　biosensor　was　fabricated　by　immobilizing　a　thiolated　DNA　containing　a　single　G-rich　stretch　on　the　gold　surface　as　probe　surface.　The　G-rich　DNA　probe　was　found　to　be　capable　of　changing　its　configuration　from　flexible　single-stranded　structures　to　rigid　tetramolecular　G-quadruplex　in　the　presence　of　terbium　III,　which　provided　a　switchable　charge　transport　path　for　the　oxidation　of　[Fe(CN)(6)](4-).　The　switchable　surface　provided　a　sensing　platform　for　the　single-step　and　reagentless　detection　of　Tb3+.　Using　this　reusable　electrochemical　sensing　platform,　a　simple,　rapid,　and　selective　biosensor　for　the　determination　of　ultratrace　terbium　ions　with　a　detection　limit　of　6.0　x　10(-11)　M　has　been　developed.　The　success　in　the　present　biosensor　served　as　a　significant　step　toward　the　development　of　monitoring　ultratrace　Tb3+　in　river　water　or　seawater.　(C)　2009　Elsevier　B.V.　All　rights　reserved.