NOx Sensor Constructed from Conductive Metal–Organic Framework and Graphene for Airway Inflammation Screening

The detection of nitric oxide in human exhaled breath (EB) has received wide attention due to its close relationship with respiratory tract inflammation. Herein, a ppb-level NOx chemiresistive sensor was prepared by assembling graphene oxide (GO) with a conductive π–d conjugated metal–organic framework Co3(HITP)2 (HITP = 2,3,6,7,10,11-hexaiminotriphenylene) in the presence of poly(dimethyldiallylammonium chloride) (PDDA). The construction of a gas sensor chip was achieved by drop-casting the GO/PDDA/Co3(HITP)2 composite onto ITO-PET interdigital electrodes, followed by in situ reduction of GO to reduced graphene oxide (rGO) in hydrazine hydrate vapor. Compared with bare rGO, the nanocomposite shows significantly improved sensitivity and selectivity for NOx among various gas analytes owing to its folded and porous structure as well as its numerous active sites. The limit of detection (LOD) for NO and NO2 can reach as low as 11.2 and 6.8 ppb, respectively, and the response/recovery time to 200 ppb NO is 24/41 s. These results indicate that rGO/PDDA/Co3(HITP)2 can achieve a sensitive and fast response toward NOx at room temperature (RT). Additionally, good repeatability and long-term stability were observed. Furthermore, the sensor shows improved humidity tolerance owing to the presence of hydrophobic benzene rings in Co3(HITP)2. To demonstrate its ability in EB detection, EB samples collected from healthy individuals were spiked with a certain amount of NO to simulate the EB of respiratory inflammatory patients. The sensor can successfully distinguish healthy people from the simulated patients. Furthermore, in real clinical sample detection, the sensor can further differentiate acute respiratory inflammatory patients from the chronic ones.