Nitric oxide is an endogenous biological signaling molecule, and the corresponding fractional exhaled NO serves as an important indicator in clinical diagnostics and therapeutic applications. However, achieving accurate and rapid monitoring of ppb-level fractional exhaled nitric oxide (FeNO) at room temperature remains a significant challenge. Herein, ultrathin porphyrin metal–organic framework (MOF) sheets are selected to assemble with supramolecularly functionalized graphene sheets through hydrogen bonding and electrostatic interaction with 6 nm thickness. The resulting porphyrin MOF/graphene sheet-on-sheet nanohybrid is designed as a chemiresistive NO sensor which exhibits superior gas sensing performance at room temperature including an ultralow practical limit of detection (Ra/Rg = 1.047, 5 ppb NO), reliable repeatability, excellent selectivity against other exhaled gases, and relative long-term stability. Mechanism study indicates that the prominent NO sensing performance is attributed to the ordered framework of active sites of ferric-pyrrole (Fe─N4) sites and less than 10 nm thick sheet-on-sheet heterojunction structure in the nanohybrid. The potential clinical utility of the obtained sensors is validated by exhalation tests toward exhalation samples from healthy individuals and asthma patients, respectively. This work provides an effective strategy of developing MOF-based room temperature ppb-level chemiresistive NO sensors for practical FeNO monitoring.

For the practical diagnosis of inflammatory respiratory diseases, achieving sensitive and rapid NO sensing at the parts per billion level, all at room temperature, is of great significance. Herein, we developed a chemiresistor gas sensor with a sheet-on-sheet structure composed of an amorphous Cu-hemin MOF with reduced graphene oxide (rGO) nanosheets. The SEM images show that the Cu-hemin MOF/rGO composite exhibits a two-dimensional sheet-like structure. Due to its nanosized architecture, the Cu-hemin MOF exhibits a significant number of active sites for efficient NO detection. The Cu-hemin MOF/rGO composite material exhibited excellent NO sensing performance, including high sensitivity (Ra/Rg = 1.06, 50 ppb), reliable repeatability, high selectivity, and fast response/recovery (43 s/367 s, 10 ppm). The mechanism study revealed that the formation of the MOF altered the hemin dimer’s structure, resulting in the release of additional Fe(III)–N4 active sites and improved sensitivity. Moreover, the incorporation of rGO significantly boosted the conductivity of Cu-hemin MOFs. Using this two-dimensional sheet-like material, a mask-type sensor was also prepared and verified to be effective as a flexible and wearable sensing device for parts per billion level exhaled NO detection.