Polymer microfibers encapsulating polydiacetylene (PDA) supramolecules (see figure) are prepared by using the electrospinning technique. Randomly distributed diacetylene monomers are found to self-assemble during fiber formation and enable the generation of blue-colored PDA-containing polymer fibers upon UV irradiation. Micro-patterned images with a single fiber as well as the feasibility of a fiber-based sensor are demonstrated.