New MOS (metal-oxide-semiconductor) field-effect gas sensors capable of detecting carbon monoxide at low concentrations and operating temperatures are fabricated and tested. Acatalytic multi-layer gate system is presented composed of a 60 nm iron oxide nanoparticle film deposited by thermal evaporation and an additional 60 nm platinum surface layer gate electrode film deposited using direct pattern focused ion beam (FIB) deposition. The sensor integrates the catalytic properties of a highly diffusive platinum thin film and the gas absorptive properties of iron oxide, both mechanisms previously shown to independently produce work function changes in an MOS capacitor in the presence of carbon monoxide.