The real-time detection of harmful gases like NO 2 and NH 3 is crucial for smart agriculture and the noninvasive diagnosis of kidney, liver, lung, and heart diseases. However, sensing these gases and understanding the primary sensing mechanisms can be challenging since the nitrogen site of NO 2 or NH 3 is the primary site that binds the sensing crystallite. Herein, we designed and fabricated low-cost sensors for dual NO 2 and NH 3 sensing using a nanodevice-on-chips sensor (NDCS) with mixed-valence Co 3 O 4 (Co 2+ and Co 3+ ). Co 3 O 4 −NDCS detects NO 2 and NH 3 at low power, room temperature, and 70% relative humidity. It also exhibits stable, recoverable, and reproducible sensing performance for NO 2 and NH 3 , comparable to several high-performance sensors. The dual sensing performances are closely related to two distinct Co sites (Co 2+ and Co 3+ ) in the Co 3 O 4 crystal. Co 2+ could host the nitrogen atom of either NO 2 or NH 3 to form Co−N bonding, which is preferable to NO 2 detection. Meanwhile, the N atom could replace one of the six oxygen atoms coordinating with Co 3+ to form Co−N bonding, favorable to NH 3 detection. Looking forward, the strategy proposed here makes it possible to fabricate high-performance gas sensors with the desired nanostructures and known sensing mechanisms. KEYWORDS: microhemisphere/nanoparticles, nanodevice-on-chip, mixed-valence Co 2+ and Co 3+ , dual detection of NO 2 and NH 3 , Co−N bonding, RT and low-power sensing