A　simple,　sensitive　and　reusable　electrochemical　sensor　was　designed　for　determination　of　mercury　(II)　(Hg2+)　by　coupling　target-induced　conformational　switch　of　DNA　hairpins　with　thymine-He2+-thymine　(T-Hg2+-T)　coordination　chemistry.　The　hairpin　probe　consisted　of　a　stem　of　6　base　pairs　enclosing　a　14　nucleotide　(nt)　loop　and　an　additional　12　nt　sticky　end　at　the　3＇　end.　Each　hairpin　was　labeled　with　ferrocene　(Fc)　redox　tag　in　the　middle　of　the　loop,　which　was　immobilized　on　the　electrode　via　self-assembly　of　the　terminal　thiol　moiety　at　the　5＇　end.　In　the　presence　of　target　analyte,　Hg2+-mediated　base　pairs　induced　the　conformational　change　from　the　sticky　end　to　open　the　hairpins,　resulting　in　the　ferrocene　tags　close　to　the　electrode　for　the　increasing　redox　current.　The　strong　coordination　reaction　of　T-Hg2+-T　resulted　in　a　good　repeatability　and　intermediate　precision　down　to　10%.　The　dynamic　concentration　range　spanned　from　5.0　nM　to　1.0　mu　M　Hg2+　with　adetection　limit　of　2.5　nM　at　the　3s(blank)　level.　The　strategy　afforded　exquisite　selectivity　for　Hg2+　against　other　environmentally　related　metal　ions.　Inspiringly,　the　developed　sensor　could　be　reused　by　introduction　of　iodide　(I-).　(C)　2012　Elsevier　B.V.　All　rights　reserved.