A　new　ligand　10-mercaptodecyl-1-iminodiacetic　acid　(MDIA)　was　synthesized　and　used　to　modify　gold　nanoparticles　to　provide　a　simple　assay　to　repeatedly　sense　Cu2+　in　the　solution　at　room　temperature.　This　functionalized　gold　nanosensor　was　applied　for　the　detection　of　Cu2+　in　water　samples　with　sensitivity　and　simplicity.　The　chelation/aggregation　process　is　reversible　via　addition　of　a　strong　metal　ion　chelator　such　as　EDTA.　This　simple　and　fast　colorimetric　sensor　is　important　in　the　application　of　copper　ion　detection　in　water　quality　during　the　emergency　and　early　warning　monitoring.