The detection and monitoring of nitrogen dioxide (NO2) play a vital role in the environmental, healthcare, farming, and industrial sectors. However, the development of NO2 gas sensors with simultaneously high sensitivity, reversibility, low detection limit, and excellent selectivity remains challenging. Two-dimensional (2D) layered transition metal oxides has emerged as a new class of van der Waals materials and attracted considerable research in gas sensing research due to its unique structural and electronic properties. In this talk I will introduce our latest progress in the development of highly sensitive NO2 gas sensors with superior selectivity based on layered -MoO3 and its heterostructures. Ultrasensitive NO2 gas sensors based on nanoribbons of MoO3 as well as centimeter-sized MoO3 single crystalline flake have been developed, allowing us to understand the gas sensing properties of layered -MoO3 both in the form of interconnected network of nanoribbons as well as individual single crystalline domain of MoO3. Furthermore, a highly sensitive NO2 gas sensor based MoS2@MoO3 heterostructure was also recently demonstrated by post-sulfurization of MoO3 with a unique sensing behaviour which can be related with the electronic structures at the heterointerface. This work lays the foundation for developing a potential sensing platform based on layered 2D oxides to enable monolithic, scalable and integrable sensing technologies.