A critical insight into porous organic polymers (POPs) and its perspectives for next-generation chemiresistive exhaled breath sensing: a state-of-the-art review

In recent years, the development of chemiresistive type exhaled breath sensors is gaining significant attention as they serve as a tool for premature disease diagnosis where the concentration of exhaled breath biomarkers is analyzed in a noninvasive manner employing headspace analysis. From a commercial viewpoint, the existing metal-oxide-based materials used for chemiresistive sensors suffer due to the interference of relative humidity levels (% RH), higher operating temperature, and stability. In particular, the effect of % RH while sensing breath volatiles is more prominent as the human exhaled breath contains more than 95% of RH. In this context, many other materials, especially high surface area polymeric materials, including zeolites-based imidazolate frameworks (ZIFs), metal-organic frameworks (MOFs), and porous organic polymers (POPs), have been investigated for the fabrication of chemiresistive sensors. Among them, POPs constitute next-generation materials in the spotlight for breath-sensing applications. On this note, review articles highlighting the chemiresistive sensing of POPs toward exhaled breath sensing are rare. This motivated us to compile this critical review by organizing the recent advancements and state-of-the-art chemiresistive sensing of POPs, including boronic ester-based, imine linkage based, and porphyrin-based covalent organic frameworks (COFs). Also, the chemiresistive sensing signatures of other recently emerged POPs, namely, heptazine-based organic networks/frameworks (HMPs), covalent triazine-based frameworks (CTFs), and tetrazine-based 1-D polymers, have been emphasized. The major challenges related to the synthesis, optimization of building units, surface modification/functionalization, and inherent properties of POPs for fabricating chemiresistive breath sensors have been extensively discussed.