We have developed fluorogenic (fluorescent off-on) ion sensors of a new type, based on stimulus-responsive synthetic copolymers incorporating a polarity-sensitive fluorophore. Upon binding a target ion in aqueous solution, these macromolecular fluorogenic sensors change their three-dimensional structure from an open form to a globular form at functional temperatures, and this induces a change of microenvironmental polarity around the polarity-sensitive fluorophore. The fluorophore transforms this environmental polarity change into a fluorescence signal. Our polymeric sensors were prepared by random copolymerization of comonomers selected from four types of comonomers. First, the role of each type of comonomer in the polymeric sensors was examined through the development of a fluorogenic H+ sensor. Next, a fluorogenic K+ ion sensor was developed in order to demonstrate the generality of this "buildup" design concept. Finally, a fluorogenic SO42- sensor working in water was developed for the first time.