Polydiacetylene-based chemosensors are prepared by assembling liposomes of diacetylene monomers with terminal amine groups on an aldehyde-modified glass substrate, and polymerizing them by irradiation with UV light. Incubation of the immobilized film sensor in a poly(acrylic acid) solution results in a color change from the original blue (solid line in Figure) to the red phase (broken line).