Fluorescence aqueous detection of CO₂ via protonation of 2-phenyl-4-methylimidazole

Carbon dioxide (CO₂) concentration plays a critical role in human life and production, agricultural growth, and marine environments. However, compared to current CO₂ detection methods, there remains a need to develop a simple, highly sensitive method that is applicable for both gaseous and aqueous-phase CO₂ detection. Herein, we introduce a sensing film that is user-friendly, facile to fabricate, and fluorescently responsive to CO₂. After extensive screening, we selected a 2-phenyl-4-methylimidazole (PMZ)@polymethyl methacrylate (PMMA) film that is suitable for aqueous-phase detection and exhibits high-sensitivity fluorescence response to CO₂. In aqueous media, CO₂ induces a blue shift in the PMZ@PMMA sensing film's fluorescence emission. The sensing film is based on the dissolution of CO₂ in water to form carbonic acid, followed by the protonation of the fluorescent material in the film by the protons from carbonic acid, resulting in the formation of a protonated product (PMZH⁺). The response time of this sensing film is approximately 5 min, and its detection linear range is 1.8%-6.0%, which makes it well-suited for monitoring CO₂ concentrations in brewing process control, agricultural production and storage, life-science incubation, and early-stage safety alarms. The preparation of this sensing film is relatively simple, and the film is easy to store. It provides a theoretical basis and practical platform for CO₂ sensing.