A　simple,　selective　and　sensitive　turn-on　fluorescent　sensor　for　the　detection　of　mercury(ii)　ion　was　developed　using　Sybr　Green　I　as　the　signal　reporter　and　SWCNTs　as　the　quencher.　Due　to　the　affinity　of　SWCNTs　towards　ssDNA　and　organic　dye,　Sybr　Green　I,　thymine-rich　ssDNA　and　SWCNTs　could　form　a　self-assembly　of　three　components,　resulting　in　fluorescence　quenching.　Upon　addition　of　another　thymine-rich　ssDNA　and　mercury(ii)　ion,　formation　of　dsDNA　via　T-Hg2+-T　base　pairs　enabled　Sybr　Green　I　to　intercalate　into　the　dsDNA,　resulting　in　the　restoration　of　fluorescence.　SWCNTs　were　found　to　reduce　the　background　signal　and　improve　the　analytical　sensitivity.　A　linear　relationship　between　the　fluorescence　intensity　and　the　concentration　of　mercury(ii)　ion　was　observed　in　the　range　of　20-1250　nM　(R　=　0.9985)　with　a　detection　limit　of　7.9　nM.　The　proposed　method　was　applied　to　detect　mercury(ii)　ion　in　tap　water　samples　with　good　results.　©　2011　The　Royal　Society　of　Chemistry.