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Two optical waveguide sensors based on SOS (silicon-on-sapphire) for detecting CO₂ are theoretically proposed. The operational wavelength is 4.23 μm, which is the maximum absorption line of CO₂. The power confinement factor (η) value is over 40% and 50%, the propagation loss is 0.98 dB/cm and 2.99 dB/cm, respectively, in the slot waveguide and SWGS (subwavelength grating slot) waveguide. An inverted tapered structure is used for the transition from strip waveguide to slot waveguide and constitutes the sensing absorption region, with the coupling efficiency that can reach more than 90%. When the optimal absorption length of the slot waveguide and SWGS waveguide is 1.02 cm and 0.33 cm, respectively, the maximum sensitivity can reach 6.66 × 10⁻⁵ (ppm⁻¹) and 2.60 × 10⁻⁵ (ppm⁻¹). Furthermore, taking the slot waveguide as an example, spiral and meander structures enable the long-distance sensing path to integrate into a small area.