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In the present work, silver nanoparticles (AgNPs) were prepared by a simple and green method using carrageenan as reducing and capping agent. The as-synthesized carrageenan-AgNPs was demonstrated as an effective duel colorimetric sensing for selective and sensitive recognition of Cu²⁺ and S^2−, which could be used to detect these ions with naked eyes. In addition, the possible sensing mechanism was that Cu²⁺ ions caused serious aggregation of carrageenan-AgNPs, which led to the color change of carrageenan-AgNPs. AgNPs were etched by S^2− forming Ag₂S, which played an important role in the determination of S^2− ions. Furthermore, it has been successfully applied to the determination of Cu²⁺ and S^2− in tap water and lake water, showing its great potential for the analysis of environmental water samples.