Detecting fluctuations in carbon dioxide by switching ‘ON’ or ‘OFF’ fluorescence in aqueous environments has often been targeted for efficient monitoring. This switch needs to occur in a drastic and fast way that is visually observable to be effective. CO2 dissolves in water, leading to a reduction in pH, which can be used to trigger a response. Specifically, the modification of fluorescent dyes with pH-responsive units could create a dye that responds to CO2 and switches ‘off’ fluorescence. Here, we developed a water-soluble polymer functionalized with a CO2-responsive pyrene dye. This functional dye contains a tertiary amine conjugated to the aromatic pyrene, which can be protonated at pH ∼ 6.5 or lower. After only 15 s of CO2 bubbling, a significant fluorescence ‘OFF’ response was observed, with a drastic reduction in fluorescence at 480 nm as a consequence of disrupting non-covalent excimer bonds. Moreover, the exposure to atmospheric air results in a recovery of the excimer state and, therefore, the fluorescence, demonstrating its reversible nature.