In　this　study,　a　novel　method　for　the　fast,　sensitive　and　selective　detection　of　Cu2+using　gold　nanoparticles　(AuNPs)　was　developed　and　used　in　immunoassays.　In　the　presence　of　L-cysteine,　L-cysteine　can　bind　to　the　surface　of　citrate-stabilized　AuNPs　through　Au-S　bonds.　As　a　result,　aggregation　of　AuNPs　occurs　through　electrostatic　interactions　between　the　cysteinebound　AuNPs.　In　contrast,　in　the　presence　of　Cu2+,　Cu2+　can　catalyze　O2　oxidation　of　cysteine,　leading　to　the　quick　formation　of　disulfide　cystine.　An　increase　in　the　concentration　of　Cu2+　decreased　L-cysteine-induced　AuNPs　aggregation　by　decreasing　the　number　of　free　cysteine　thiol　groups,　and　the　solution　color　changed　from　purple　to　red.　Therefore,　the　concentration　of　Cu2+　can　be　detected　with　the　naked　eye　or　with　ultraviolet　visible　spectroscopy,　and　the　detection　limits　of　Cu2+　were　20　nM　and　10　nM,　respectively.　This　sensitivity　was　approximately　three　orders　of　magnitude　higher　than　that　of　traditional　AuNPs-based　colorimetric　Cu2+　detection　methods.　Because　of　the　high　sensitivity　of　the　proposed　method,　we　further　used　it　with　a　labeled　antibody　in　colorimetric　immunoassays.　The　detection　limit　of　the　cancer　biomarker　α-fetoprotein　was　2　ngml1,　which　is　comparable　to　the　detection　limit　of　the　enzyme-linked　immunosorbent　assay　method.　©　2012　Nature　Japan　K.K.　All　rights　reserved.