The authors describe colorimetric test strips by using electrospun nanofiber membranes (NFMs) carrying gold/silver core/shell nanoparticles (Au/Ag NPs). The Au/Ag NPs were immobilized on aminated porous polyacrylonitrile NFMs to obtain test strips with a tortuous porous structure and a large surface area (38.6 m(2) g(-1)). The color of the resultant NFMs, measured at a wavelength of 420 nm, is red-shifted when exposed to copper ions (Cu2+) with a color change from yellow to pink to colorless. The effect is due to leaching Au/Ag NPs from the NFMs in the presence of ammonium chloride, thiosulfate and Cu2+ upon which soluble thiosulfate complexes of Ag+, Au3+ and Cu2+ are formed. The effect can be readily seen with bare eyes. Under optimized conditions, this method has a low limit of detection (50 nM at S/N = 3), a fast assay time (3 min), good specificity, and excellent reversibility. The colorimetric test strip was successfully applied to the analysis of Cu2+ in drinking water sample.