This　communication　reports　an　efficient　visual　detection　method　of　Cu2+　by　L-cysteine　functionalized　gold　nanoparticles　in　aqueous　solution.　Upon　exposure　to　Cu2+,　the　gold　nanoparticle　solution　changed　from　red　to　blue,　in　response　to　surface　plasmon　absorption　of　dispersed　and　aggregated　nanoparticles.　This　colorimetric　sensor　allows　a　rapid　quantitative　assay　of　Cu2+　down　to　the　concentration　range　of　10(-5)　M.　Recognition　of　Cu2+　and　formation　of　the　aggregates　are　proposed　to　occur　via　a　2　:　1　sandwich　complex　between　L-cysteine　and　Cu2+.