A high-sensitivity CO₂ sensor based on a ZIF-8/GO hybrid membrane-modified cascaded dual-sphere fiber interferometer

Accurate detection of carbon dioxide (CO₂) is important in environmental monitoring and industrial process control. Optical fiber sensors have attracted considerable attention for CO₂ detection owing to their compact size, immunity to electromagnetic interference, and capability for remote sensing. However, achieving high sensitivity and adsorption affinity within practical CO₂ concentration ranges remains challenging. In this work, we propose a fiber interferometric CO₂ sensor based on a cascaded dual-sphere intermodal interferometer (IMI) functionalized with a ZIF-8/graphene oxide (GO) hybrid membrane. The dual-sphere interferometric structure enhances the interaction between cladding modes and the surrounding environment, while the ZIF-8/GO hybrid membrane provides a hierarchical adsorption–diffusion interface that improves CO₂ adsorption affinity and gas transport efficiency. Experimental results show that the sensor exhibits a high sensitivity of −286 pm/% CO₂ within the concentration range of 0.5–4.0%, together with excellent linearity (R² = 0.998), good repeatability, and negligible interference from temperature and humidity variations. Furthermore, the hierarchical micro-mesoporous architecture facilitates efficient gas diffusion, endowing the sensor with favorable response and recovery kinetics for continuous monitoring applications. These results indicate that the proposed sensing strategy provides an effective approach for high-performance optical fiber CO₂ sensing through the integration of an optimized interferometric structure and a hybrid MOF/2D sensing membrane.