A new approach to the creation of an active medium of optical gas sensors for the detection of carbon monoxide is proposed. The approach is based on the use of a liquid crystal host doped with iron compounds. Cholesteric liquid crystal CLC-2103L was doped by Fe3O4 magnetite nanoparticles with a concentration of 0.30-0.67% to develop the primary transducer of a carbon monoxide sensor. The proposed gas sensor registers the optical signal in the visible region of the spectrum, corresponding to the optical reflection by the helical structure of a cholesteric liquid crystal (CLC).