In the development of sensing elements, we focused on comparing two distinct capacitor geometries—metal-insulator-metal (MIM) and interdigitated electrode (IDE)—for the detection of volatile organic compounds (VOCs) using a metal-organic framework (MOF), specifically ZIF-8, as an affinity layer. Our findings underscore the advantages of utilizing the MIM configuration over the conventional IDE setup in capacitive VOC sensors. Our experiments demonstrated that capacitors with the MIM configuration, which employs a permeable top electrode and the MOF material ZIF-8, exhibit enhanced sensitivity and selectivity, particularly towards polar VOCs. This improvement is due to the direct measurement of changes in the dielectric properties of the ZIF-8 layer without interference from substrate effects that are prevalent in IDE sensors. The MIM sensors achieved a lower detection limit and faster response times, essential for effective real-time VOC monitoring. Through rigorous testing, we established that the thickness of the top electrode is crucial for optimizing the sensor's performance. Thinner electrodes facilitated faster VOC diffusion into the sensing layer, thus enabling quicker sensor responses without sacrificing sensitivity. We also compared the transduction mechanisms utilized in MOF-based sensors, highlighting that the dielectric (capacitive) approach, when applied in the MIM configuration, offers significant advantages over optical and gravimetric methods in terms of both sensitivity and selectivity. Moreover, we have proven the concept of novel sensor technology that harnesses the kinetic selectivity of adsorption in MOFs, for the detection and monitoring of VOCs. These sensors are uniquely capable of detecting VOCs at sub-ppm concentrations, which is crucial for applications such as environmental monitoring, food freshness assessment, and health diagnostics through breath analysis. The primary innovation lies in the sensor's ability to differentiate VOCs not just by their chemical affinity but by their diffusion rates, which vary significantly even among molecules with similar physical sizes due to subtle differences in their interactions with the MOF structure.
