An easily fabricated gas sensor based on planar polymer optical waveguides with an integrated zeolite imidazole framework-8 (ZIF-8) thin film is presented for carbon dioxide detection and sensing. The planar optical waveguides are made of polymethylmethacrylate and fabricated by hot embossing, which makes it flexible and cost-efficient. Thin ZIF-8 films are uniformly grown on the waveguides surface through a simple solution method, which is crucial for the envisioned production of metal organic framework-based sensing devices on a large scale. Experimental results show that the produced optical elements exhibit a sensitivity of ≈2.5 μW/5 vol% toward carbon dioxide (CO2) with very rapid response time (≈6 s) and excellent reversibility of adsorption and desorption of the gas molecules. The demonstrated planar polymer sensing devices provide the potential to develop flexible on-chip gas sensors in an inexpensive and reproducible way.