Analyzing the components and contents of environmental gases is of great importance for industrial production, human health, and environmental safety. Carbon dioxide (CO₂) has drawn great concern for its adverse effects on climate change and physical health. Simple, in situ, and selective methods for CO₂ detection are thus highly demanded. This work presents a versatile and easily fabricated paper sensor for the selective detection of ambient CO₂. The sensor consists of two components with a fluorescent molecule ANT-PPh₃ as the sensing element and an ionic liquid [DBUH]⁺[Im]⁻ for the absorption of CO₂. Based on the photo-induced electron transfer (PET) between ANT-PPh₃ and [DBUH]⁺[Im]⁻, the sensor exhibited fluorescence enhancement in the presence of CO₂, and the response process is reversible. The sensor exhibits improved selectivity for CO₂ over SO₂ and NO₂ compared with most reported works, with good stability and fast response to human breath. Without complex instrumentation and gas pre-collection processes, the early notification of high indoor CO₂ concentrations was realized using this sensor. Taking advantage of its intuitive fluorescence changes, specific responsiveness, and convenience, we believe our designed CO₂ sensor could be applied practically in monitoring and indicating high indoor CO₂ concentrations in living scenarios.