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Coumarin-containing stimuli-responsive block copolymers were synthesized via atom transfer radical polymerization (ATRP), which exhibited a sensitivity of fluorescence emission intensity to carbon dioxide by a fluorescence quenching mechanism. Poly(7-acryloyloxy 4-methylcoumarin-r-methyl methacrylate)-b-poly(dimethylaminoethyl methacrylate) (P(CMA-r-MMA)-b-PDMAEMA) with different block lengths were synthesized using a coumarin functional ATRP initiator 7-(2bromoisobutyryloxy)-4-methylcoumarin. The block copolymers were self-assembled into a vesicular morphology in an aqueous medium with the poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) block in the inner and outer shells and the P(CMA-r-MMA) block in the core. The hydrophilic fraction calculated from proton nuclear magnetic resonance spectra and transmission electron microscopy and optical microscopy images confirmed the self-assembly of the block copolymers to the vesicular morphology in the aqueous medium. The size of the self-assembled structures was studied in different conditions by dynamic light scattering. Ultraviolet−visible spectroscopy showed the presence of coumarin in the structure of vesicles and also its dimerization during light irradiation. An aqueous colloidal dispersion of the vesicular assemblies was used for sensing CO 2 gas in aqueous media using fluorescence spectroscopy. The wall thickness of the vesicles decreased upon CO 2 bubbling, which was accompanied by an increase of their hydrodynamic radius. Therefore, the distance of coumarin moieties increased and subsequently resulted in a decrease in their aggregation state and fluorescence intensity. This process was also reversibly carried out to increase the pH of the medium by N 2 bubbling. Indeed, the prepared fluorescence chemosensor was used for the detection of CO 2 on the basis of the fluorescence quenching mechanism. The chemosensor formed from the block copolymer having the highest amount of PDMAEMA and 7-acryloyloxy-4-methylcoumarin (CMA) showed the highest reversible CO 2 sensing ability at room temperature.