Gas sensors are being used in diverse applications in security, food safety, environmental monitoring, indoor air quality monitoring, personal healthcare, etc. During the past decade, two-dimensional (2D) nanostructured materials have attracted intense attention because of their unique chemical and physical properties. They have demonstrated promising potential for gas sensing devices because of their large surface-to-volume ratio, high surface sensitivity, and excellent semiconducting properties. Combining 2D nanostructured materials with other dimensional materials also holds great promise for developing high-performance gas sensors. This review presents a comprehensive summary of recent progress in gas sensors with advanced heterostructures based on 2D nanostructured materials. Furthermore, the fundamental sensing mechanisms of different types of gas sensors are systematically discussed, and key device architectures and their performances are summarized. Finally, the challenges and prospects for the future development of each type of gas sensor are addressed to promote better sensing-device architectures.

The human olfactory system comprises olfactory receptor neurons, projection neurons, and interneurons that perform remarkably sophisticated functions, including sensing, filtration, memorization, and forgetting of chemical stimuli for perception. Developing an artificial olfactory system that can mimic these functions has proved to be challenging. Herein, inspired by the neuronal network inside the glomerulus of the olfactory bulb, we present an artificial chemosensory neuronal synapse that can sense chemical stimuli and mimic the functions of excitatory and inhibitory neurotransmitter release in the synapses between olfactory receptor neurons, projection neurons, and interneurons. The proposed device is based on a flexible organic electrochemical transistor gated by the potential generated by the interaction of gas molecules with ions in a chemoreceptive ionogel. The combined use of a chemoreceptive ionogel and an organic semiconductor channel allows for a long retentive memory in response to chemical stimuli. Long-term memorization of the excitatory chemical stimulus can be also erased by applying an inhibitory electrical stimulus due to ion dynamics in the chemoresponsive ionogel gate electrolyte. Applying a simple device design, we were able to mimic the excitatory and inhibitory synaptic functions of chemical synapses in the olfactory system, which can further advance the development of artificial neuronal systems for biomimetic chemosensory applications.