A new method for visual detection of Cu2+ based on azide- and alkyne-functionalized polydiacetylene (PDA-aa) vesicles has been developed. In the presence of ascorbic acid, Cu2+ can be reduced to Cu+ and catalyze the click reaction between the two functional groups. After incubation with Cu2+ and ascorbic acid, a PDA-aa solution changed its color from blue to red, which was attributed to conformational transition of the conjugated backbone. Other metal ions were explored but only induced negligible color changes. This method provides a naked eye detection of Cu 2+ in water without the aid of complex instruments.