Hydrogen　sulfide　(H2S)　is　a　dramatically　toxic　gas　molecule;　long　term　exposure　to　H2S　environment　poses　a　serious　threat　to　human　health.　It　is　highly　imperative　to　develop　sensitive　and　selective　methods　for　H2S　detection.　In　this　study,　a　signal-on　fluorescence　biosensor　for　the　H2S　(used　to　represent　the　total　amount　of　H2S,　HS-　and　S2-)　detection　was　developed　based　on　silver-mediated　base　pairs　(C-Ag+-C).　Two　separate　C-rich　single-stranded　DNAs　(ssDNAs)　were　synthesized,　namely,　one　was　labeled　with　fluorescein　(6-carboxyfluorescein,　FAM)　and　the　other　was　labeled　with　quencher　(black　hole　quencher,　BHQ1).　The　two　ssDNAs　were　in　a　flexible　single　strand　owing　to　the　mismatch　of　the　ssDNA,　and　hence,　significant　fluorescence　intensity　was　observed.　In　the　presence　of　Ag+,　DNA　probes　could　selectively　capture　Ag+　to　form　double　stranded　DNA　(dsDNA)　via　the　C-Ag+-C　base　pairs,　which　made　the　quench　of　FAM　by　BHQ1　and　low　fluorescence　signal　was　detected.　With　the　addition　of　H2S　to　the　system,　the　dsDNA　was　decomposed　into　ssDNA　since　Ag+　could　be　released　from　C-Ag+-C　by　the　formation　of　a　more　stable　complex　Ag2S,　which　resulted　in　the　recovery　of　the　fluorescence　intensity.　The　enhanced　fluorescence　intensity　had　a　linear　relationship　with　H2S　concentration　in　the　range　from　0.5　to　20　μM　with　a　detection　limit　(LOD)　of　0.16　μM　(S/N　=　3).　The　proposed　system　was　applied　to　detect　H2S　(used　to　represent　the　total　amount　of　H2S,　HS-　and　S2-)　in　air　near　a　garbage　station　with　satisfying　results.　©　2020　The　Royal　Society　of　Chemistry.