Au　at　Ag　core-shell　nanoparticles　(NPs)　were　synthesized　and　coupled　with　copper　ion　(Cu2+)　for　the　colorimetric　sensing　of　iodide　ion　(I-).　This　assay　relies　on　the　fact　that　the　absorption　spectra　and　the　color　of　metallic　core-shell　NPs　are　sensitive　to　their　chemical　ingredient　and　dimensional　core-to-shell　ratio.　When　I-　was　added　to　the　Au　at　Ag　core-shell　NPs-Cu2+　system/solution,　Cu2+　can　oxidize　I-　into　iodine　(I2),　which　can　further　oxidize　silver　shells　to　form　silver　iodide　(AgI).　The　generated　Au　at　AgI　core-shell　NPs　led　to　color　changes　from　yellow　to　purple,　which　was　utilized　for　the　colorimetric　sensing　of　I-.　The　assay　only　took　10　min　with　a　lowest　detectable　concentration　of　0.5　μM,　and　it　exhibited　excellent　selectivity　for　I-　over　other　common　anions　tested.　Furthermore,　Au　at　Ag　core-shell　NPs-Cu2+　was　embedded　into　agarose　gels　as　inexpensive　and　portable　＇test　strips＇,　which　were　successfully　used　for　the　semi-quantitation　of　I-　in　dried　kelps.　©　2015　Elsevier　B.V.