Colorimetric　assay　platforms　for　dissolved　hydrogen　sulfide　(H2S)　have　been　developed　for　more　than　100　years,　but　most　still　suffer　from　relatively　low　sensitivity.　One　promising　route　out　of　this　predicament　relies　on　the　design　of　efficient　signal　amplification　methods.　Herein,　we　rationally　designed　an　unprecedented　H2S-induced　deactivation　of　(gold　core)(ultrathin　platinum　shell)　nanocatalysts　(Au　at　TPt-NCs)　as　a　highly　efficient　signal　amplification　method　for　ultrasensitive　headspace-colorimetric　assay　of　dissolved　H2S.　Upon　target　introduction,　Au　at　TPt-NCs　were　deactivated　to　different　degrees　dependent　on　H2S　levels,　and　the　degrees　could　be　indicated　by　using　a　Au　at　TPt-NCs-triggered　catalytic　system　as　a　signal　amplifier,　thus　paving　a　way　for　H2S　sensing.　The　combination　of　experimental　studies　and　density　functional　theory　(DFT)　studies　revealed　that　the　Au　at　TPt-NCs　with　only　2-monolayer　equivalents　of　Pt　(θPt　=　2)　were　superior　for　H2S-induced　nanocatalyst　deactivation　owing　to　their　enhanced　peroxidase-like　catalytic　activity　and　deactivation　efficiency　stemmed　from　the　unique　synergistic　structural/electronic　effects　between　Au　nanocores　and　ultrathin　Pt　nanoshells.　Importantly,　our　analytical　results　showed　that　the　designed　method　was　indeed　highly　sensitive　for　sensing　H2S　with　a　wide　linear　range　of　10-100　nM,　a　slope　of　0.013　in　the　regression　equation,　and　a　low　detection　limit　of　7.5　nM.　Also　the　selectivity,　reproducibility,　and　precision　were　excellent.　Furthermore,　the　method　was　validated　for　the　analysis　of　H2S-spiked　real　samples,　and　the　recovery　in　all　cases　was　91.6-106.7%.　With　the　merits　of　high　sensitivity　and　selectivity,　simplification,　low　cost,　and　visual　readout　with　the　naked　eye,　the　colorimetric　method　has　the　potential　to　be　utilized　as　an　effective　detection　kit　for　point-of-care　testing.　©　2015　American　Chemical　Society.