Greenhouse gases emissions such as carbon dioxide and methane which are generated from the processing and use of fossil fuels have created serious environmental concerns. The need to monitor the emissions of these gases in mobile and stationary point sources present novel challenges in sensor design. Surface acoustic SAW devices, which offer high sensitivity, low cost, wireless and passive options as well as tunable selectivity when coated with proper sensing materials, can be a potential platform for monitoring these gases. However, many coating materials including metal oxides and polymers are not suitable for developing room temperature gas sensors due to their poor interaction capacity (physisorption or chemisorption) with the gases. Recently, metal-organic frameworks (MOFs) have drawn a considerable attention in sensing applications for their high sorption capacity at ambient condition and functional tenability. Here, we present on the development of surface acoustic wave (SAW) carbon dioxide and methane sensors by applying the zeolitic imidazolate framework-8 (ZIF-8) MOF as the sensing layer.

The SAW delay lines with the nominal operating frequency of 436 MHz were custom fabricated on Y-Z LiNbO₃ piezoelectric substrates and 9 MHz quartz crystal microbalance (QCM) resonators made of At-cut quartz and gold electrodes were purchased from INFICON. The transducers were coated with 200 nm thick ZIF-8 layer by simple solution-based dip coating method and exposed to various gases at room temperature in a N₂ environment. Relative to pure N₂, the sensors showed a good response to CO₂ and CH₄ while remaining neutral to CO and air. The resonance frequency of the QCM sensor decreased by 217 and 9.2 Hz and the phase of the SAW in the delay line changed by 0.72 and 0.04 radian for pure CO₂ and CH₄, respectively. The responses of the sensors varied linearly with the concentrations of either gas. From a linear fit, the sensitivities of the QCM sensor to CO₂ and CH₄ were evaluated to be 2.18 Hz/vol-% and 0.1 Hz/vol-%, respectively. Similarly, the SAW sensor showed a sensitivity of 0.39 deg/vol-% to CO₂ while its response to various concentrations of CH₄ is under study. Since ZIF-8 is a non-conducting material and its stiffness changes negligibly upon gas adsorption for thin film cases, the mass loading effect can be considered the dominant sensing mechanism for both sensors. When the coated films adsorbed the gas molecules, their mass density increased thereby inducing a decrease in acoustic velocity which was recorded in terms of frequency or phase change. Given that the SAW sensors can be operated in wireless and passive mode, the current study is of great importance in developing passive sensors for distant monitoring of greenhouse gases in the environment and at other emission sites.