The　authors　describe　an　electrochemical　assay　for　mercury　ion　(Hg2+)　by　using　a　glassy　carbon　electrode　(GCE)　to　which　a　25-base　thymine　oligomer　was　covalently　immobilized.　On　addition　of　Hg(II)　ion,　it　is　captured　via　T-Hg(II)-T　coordination.　The　T-Hg(II)-T　complex　catalyzes　the　reduction　of　auric　acid　by　hydroxylamine　and　this　leads　to　the　formation　of　gold　amalgam　(AuHg).　The　electrode　is　then　placed　in　an　ammoniacal　solution　of　silver　nitrate.　On　applying　a　deposition　potential　of　-0.12　V　for　60　s,　silver　nanoparticles　are　deposited　on　the　surface　that　serve　as　signal　reporters　to　indicate　the　concentration　of　Hg(II)　in　the　sample　solution.　The　electrode,　if　operated　at　50　mV　(vs.　Ag/AgCl)　enables　Hg(II)　to　be　detected　in　the　0.005　to　80　nM　concentration　range,　with　a　detection　limit　as　low　as　2　pM.　The　method　was　successfully　applied　to　the　determination　of　Hg(II)　in　spiked　river　water.