Enhancing the Sensitivity and Selectivity of Conjugated Polymer Gas Sensors Towards Volatile Amines Using Metal Organic Additives

Organic semiconducting (OSC) materials have seen a rapid growth in interest over the past decade due to their room temperature solution processability, tunable electronic properties, and lightweight flexible nature making them excellent candidates for organic electronic devices. In particular, the vast chemical diversity offered by OSC materials enables them to have unique interactions with various molecules and has made them popular in the field of chemical gas sensors. Previous studies have shown OSC based chemical gas sensors can detect ammonia down to part per billion (ppb) concentrations with short response times but have yet to elucidate the factors that affect their sensing performance. Herein, we demonstrate that using metal phthalocyanine small molecules as additives can improve the sensing performance of conjugated polymer gas sensing films towards ammonia and other volatile amines. Polymer organic field effect transistor (OFET) gas sensors are fabricated through meniscus-guided coating and the additives are directly blended into the polymer solution. By incorporating the additive, an ultra-low limit of detection of 0.05 ppb is achieved for ammonia, with a response time in the order of seconds. We hypothesize that the additives increase heterogeneity in the distribution of binding sites on the sensor surface, which increases the adsorption of amines at low concentrations. We further investigate the selectivity of the sensors by testing a range of amines to uncover the impact of the analyte’s molecular volume and gas phase Lewis basicity. Using a Freundlich isotherm to model the sensing data, we determine the adsorption intensity and capacity of the sensors for each amine. To quantitatively compare the sensing performance, we propose a new parameter defined as the ratio of the analyte’s gas phase basicity to its molecular volume. An exponential relationship was found between the adsorption capacity and the proposed parameter, resulting in an equation that can be used to theoretically predict the capacity of the sensor towards any given analyte. Beyond its quantitative merit, the above relationship also serves as a bridge that connects the molecular-level interactions between the sensor material and analyte, with the macroscale sensing performance of the film.