Single-walled carbon nanotubes (SWCNTs) are recognized as versatile materials for the formation of chemiresistive sensors. However, imparting high sensitivity and selectivity to SWCNTs remains a major challenge. Herein, we report a new sensory system that interfaces SWCNTs and catalytic metal nanoparticles (NPs) in a film of a porous ion exchange polymer to produce sensitive and selective sensors. The porous polymer films are based on imidazolium-functionalized triptycene polyether sulfone, and the environment created by this polymer results in the formation of ultrasmall (<5 nm) Au NPs. The polymer serves to suppress the growth of Au NPs while maintaining gas transport. The size control promotes strong interactions between Au NPs and carbon monoxide (CO), and this composition produces a robust, sensitive, and selective CO chemiresistive sensor. We further demonstrate that the modulation of a gate voltage in chemical field-effect transistor sensing devices enhances the performance by promoting the activity of Au NPs. The sensors display an increase of CO response at room temperature in air under the negative gate voltage. Our results demonstrate that the combination of a porous ion exchange matrix, NPs, and gate voltage-modulated chemiresistors can be synergistically tuned to create sensitive and selective sensors for target analytes.