Quantification of Methane in Water at Parts Per Billion Sensitivity Using a Metal–Organic Framework-Functionalized Quartz Crystal Resonator

Abstract

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 to provide the required spatial and temporal resolution. Herein, we demonstrate how a porous metal–organic framework material can create a sensor to quantify dissolved methane. The sensor is based on a quartz crystal microbalance, which measures methane adsorption using a quartz resonator functionalized with the material. Combining the quartz crystal microbalance and the nanoporous material yields fast response times and high sensitivity. This is due to a favorable 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 under 60 s with parts per billion sensitivity. The high sensor performance is based on microporous pore size distribution, surface hydrophobicity, and crystallite size, yielding strong synergy. A fully functioning prototype has been designed, built, and evaluated to demonstrate real-life applicability and obtain a response from methane-spiked lake water. The modular nature of metal–organic frameworks opens possibilities for creating materials for selective sensing of other aqueous species. Thus, our study showcases the importance of materials for methane sensing and environmental monitoring in general.