<p>Heterostructure materials have gained significant research interest due to their distinct physical properties. Toxic gases severely affect the respiratory system of the human body. We propose an in-plane monolayer heterostructure based on transition metal dichalcogenides (TMD) to investigate the effect of toxic gas adsorption. The toxic gas molecules are adsorbed on top of the chalcogen-atom of the in-plane TMD heterostructure. The electronic structure calculations reflect that the band gap of the proposed host material remains almost unchanged upon the adsorption of gas molecules. The gas adsorption leads to the nearly unaltered valance band and conduction band. This is due to the lack of hybridization between the molecular orbitals of the adsorbate and the host material. The Mulliken population method confirms the charge transfer between the host in-plane TMD heterostructure and the adsorbed gas molecules. Furthermore, it is observed that the adsorption of gas molecules significantly changes the dielectric and optical response of the TMD in-plane heterostructure. Our investigations demonstrate that the proposed material has the potential for toxic gas sensing.</p>