A highly luminescent freestanding film composed of the quantum cluster, Au 15, was prepared. We studied the utility of the material for specific metal ion detection. The sensitivity of the red emission of the cluster in the composite to Cu 2+ has been used to make a freestanding metal ion sensor, similar to pH paper. The luminescence of the film was stable when exposed to several other metal ions such as Hg 2+, As 3+, and As 5+. The composite film exhibited visual sensitivity to Cu 2+ up to 1 ppm, which is below the permissible limit (1.3 ppm) in drinking water set by the U.S. environmental protection agency (EPA). The specificity of the film for Cu 2+ sensing may be due to the reduction of Cu 2+ to Cu 1+/Cu 0 by the glutathione ligand or the Au 15 core. Extended stability of the luminescence of the film makes it useful for practical applications. © 2012 American Chemical Society.