A polymeric chemosensor (PNa) containing naphthalimide signal moiety and piperazinyl ring recognition moiety for the detection of proton was synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization, which can guarantee narrow molecular weight distribution high purity, and precise polymeric structure. PNa and its corresponding monomer (Na) showed both fluorescence enhancement in low proton concentration and fluorescence suppression in high proton concentration, which can be observed by both naked eyes and optical responses. Quarternization of the alkylated amine of Na (NaI) can better support the "Off-On-Off" mechanism shown by PNa and Na. In addition, PNa displayed better sensitivity than Na. The effect of proton on thin PNa film was also investigated, and the addition of proton led to a decrease of the fluorescence intensity. (C) 2009 Elsevier Ltd. All rights reserved.