MXene-based hybrid materials for CO2 sensing: interfacial mechanisms and environmental applications

The growing need for accurate, fast detection of carbon dioxide (CO 2 ) concentrations within biological systems highlights the constraints of currently employed sensors, especially when operating in a biologically relevant environment, which is highly humid, heterogeneous, and rapidly changing. A recently discovered family of two-dimensional materials, known as MXenes, has shown great promise for novel CO 2 sensors due to their remarkable properties, including high electrical conductivity, tunable surface chemistry, hydrophilic properties, and processability. However, recent studies indicate that sensing performance in MXene-based systems is governed not solely by intrinsic material properties but predominantly by hybrid interfacial interactions and environmental coupling effects. The review starts by discussing the compositional variety of MXenes beyond Ti 3 C 2 T x and describes a broad range of effects achievable by selecting a transition metal, surface functional groups, the presence of defects, and the interlayer composition. The fundamental interactions between MXenes and CO 2 are then analyzed, including physisorption, chemisorption, humidity-assisted bicarbonate formation, proton-coupled electron transfer, and interlayer swelling. Particular attention is given to MXene-based hybrid architectures, such as polymer-functionalized and metal oxide-decorated systems, which enhance sensing performance through interfacial charge transfer. At the device level, we discussed several existing MXene and MXene hybrid sensors, including resistive, capacitive, optical, and flexible devices, and described their key characteristics and trade-offs, such as sensitivity, stability, and susceptibility to humidity and interference from other gases. While the potential of MXene-based CO 2 sensors for plant and agricultural monitoring is highlighted, current evidence is largely derived from laboratory-scale studies performed under controlled environmental conditions. Direct demonstrations in living plant systems and realistic greenhouse environments remain limited, underscoring an important direction for future research. Finally, key challenges related to stability, cross-sensitivity, scalability, and translation from laboratory demonstrations to realistic agricultural environments are discussed, and design strategies for next-generation hybrid sensing systems are proposed.