A　sensitive　and　selective　electrochemical　Hg2+　sensor　was　developed　based　on　T-Hg2+-T　structure　and　exonuclease　(Exo)　III　-assisted　target　recycling　amplification　at　heated　gold　disk　electrode　(HAuDE).　First,　a　DNA　signal　probe　P1　was　for　the　first　time　designed　and　labeled　with　ferrocene　(Fc)　near　the　attached　SH-5′-end,　so　as　to　shorten　the　distance　between　Fc　and　the　electrode　and　enhance　the　initial　current　of　Fc　compared　with　that　labeled　at　the　3′-end　far　from　the　electrode.　Then　the　signal　amplification　was　achieved　by　Exo　III-assisted　Hg2+　recycling.　Briefly,　the　P1　was　complementary　to　the　assistant　DNA　P2　except　the　T-T　mismatches.　In　the　presence　of　Hg2+,　the　P1　self-assembled　on　the　HAuDE　could　hybridize　with　P2　and　form　DNA　duplex　with　blunt　end　at　the　3′-　terminus,　triggering　Exo　III　to　stepwise　digest　mononucleotides　from　the　3′-terminus　of　P1,　ultimately　liberating　Hg2+　and　P2,　which　could　be　“recycled”　resulting　in　the　digestion　of　a　large　amount　of　P1　and　significantly　decrease　the　amount　of　Fc.　The　electrochemical　signal　difference　before　and　after　digestion　was　proportional　to　the　Hg2+　concentration.　Furthermore,　during　the　digestion　period,　the　Exo　III　activity　could　be　significantly　increased　by　elevating　the　electrode　temperature,　great　improving　the　sensitivity　and　efficiency　for　Hg2+　detection.　A　detection　limit　of　6.2　pM　(S/N　=　3)　could　be　obtained　with　an　electrode　temperature　of　40　°C　during　60　min　digestion　period,　which　was　lower　ca.　two　magnitudes　than　that　at　0　°C　and　one　magnitude　than　that　at　25　°C.　©　2018　Elsevier　B.V.