A pyrene based turn-on fluorescent polymeric probe was developed for the tunable detection of nerve agent mimics. Glycidyl methacrylate (GMA) and dimethylacrylamide (DMA) were copolymerized by reversible addition fragmentation chain transfer (RAFT) polymerization to yield poly-(glycidyl methacrylate-co-dimethylacrylamide) [p(GMA-co-DMA)](P1). The subsequent reaction of the secondary amine of 2-(2-((pyren-1-ylmethyl)-amino)-ethoxy)-ethanol with the epoxide unit of P1 yielded P2, which can detect diethyl cyanophosphate (DCNP), a nerve agent mimic, very efficiently by the turn-on fluorescence technique, induced by DCNP promoted intramolecular N-alkylation in both the solution and vapor phase. The lowest detection limit of P2 to DCNP was obtained as 0.1 mM, which is close to the toxicity limit of Tabun. Moreover, the detection of DCNP was successfully controlled by altering the purging of CO2/N-2 gases or tuning the pH of the solution. P2 showed an efficient ON/OFF reversible fluorescence response toward CO2 and N-2 gases, further helping tunable ON/OFF sensing of DCNP. The CO2-tunable detection of DCNP was further correlated to the pH-dependent control of detection sensitivity. At low pH, no change in the fluorescence enhancement was observed upon exposure to DCNP, whereas the fluorescence intensity of the probe remarkably increased at high pH. Thus, these CO2/pH controllable detection properties can offer new insights into the design of new stimuli-responsive polymeric probes with fluorescence turn-on detection of nerve agent mimics.