Abstract Different integrated photonic sensors are investigated for the detection in the mid-infrared region of the two gases namely CO 2 and CH 4 . The three studied structures are ridge waveguides, based both on chalcogenide films (ChG) or porous germanium (PGe) and slot waveguides based on ChG. Waveguide dimensions are optimized to obtain the highest power factor between guided light and gas while maintaining a single mode propagation in the mid-infrared wavelength range. The achievable power factor is 1% in case of ChG ridge-waveguide, 45% for PGe-ridge, and 58% in case of ChG-slot. Extremely low limits of detection (LOD), 0.1 ppm for CO 2 at λ = 4.3 μ m and 1.66 ppm for CH 4 at λ = 7.7 μ m are obtained for a ChG slot waveguide, due to the large gas absorption coefficients in the mid-infrared spectral range. For PGe waveguides, low LOD values are also computed: 0.12 ppm for CO 2 at λ = 4.3 μ m and 1.89 ppm for CH 4 at λ = 7.7 μ m. These results show that the proposed structures could achieve competitive performance required for generic spectroscopic detection on a chip for environment and health sensing.