A polymeric hydrogel film was made photochemically for the fluorescent detection of copper ions. An eight-branched PEG macromer having a nitrocinnamate (NC) moiety as the pendant group was synthesized and found to form a gel upon exposure to 365-nm irradiation in the absence of any photoinitiators or catalysts. Peptide 1 (Dns-Gly-His-Lys(NC)) was synthesized with Fmoc solid-phase method and contained three essential parts: a dansyl fluorophore for fluorescent signal transduction, a metal binding site for selective recognition of copper ions from aqueous solution, and a NC group for interaction with PEG-NC macromer. The peptide was found to be efficiently immobilized within the hydrogel network during the photo-cross-linking process. The prepared PEG-NC/1 gel film is strongly fluorescent and can be quenched by the presence of copper ions. The quenching process was reversed by acid treatment and an excellent reversibility was obtained. The designed membrane sensor showed a superior selectivity toward copper ions as demonstrated by fluorescence spectroscopy and epifluorescence imaging.