Rapid quantification of methane in water with parts-per-billion sensitivity using a metal-organic framework-functionalized quartz crystal resonator

Wetlands and water bodies are essential sources of methane emissions, a greenhouse gas that is roughly 25 times more potent than carbon dioxide. However, the biological production, fluxes, and interplay between methane and carbon dioxide due to microbial activity must be better understood. This is primarily attributed to the lack of sensor technology that can provide the required spatial and temporal resolution. Herein, we demonstrate how a porous metal-organic framework material can be used to create a sensor to quantify dissolved methane. The sensor is based on a quartz crystal microbalance, which measures methane adsorption using a quartz resonator functionalised with the material. Combining quartz crystal microbalance and the porous material yields fast response rates and high sensitivity. This is due to a favourable partitioning coefficient between the empty pores of the material and the aqueous phase, promoting rapid migration of dissolved methane into the material. The result is a sensor system that achieves equilibration and response times below 60 seconds with parts-per-billion sensitivity. A fully functioning prototype has been designed, built and evaluated to demonstrate real-life applicability, demonstrating sense response using spiked lake water. The modular nature of metal-organic frameworks opens possibilities for designing materials for selective sensing of other aqueous analytes. Thus, our study showcases the importance of new materials for methane sensing and general environmental monitoring.