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.

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.

The development of materials toward ppb-level nitric oxide (NO) sensing at room temperature remains in high demand for the monitoring of respiratory inflammatory diseases. In order to find an iron-containing molecule without steric hindrance to combine with graphene for room temperature NO gas sensing, here a supramolecular assembly of ferrocene (Fc) and reduced graphene oxide (rGO) was designed and prepared for NO sensing. The assembly of Fc/rGO was characterized using FT-IR, TEM, and XPS measurements. The Fc/rGO-based sensors exhibited superior NO sensing properties at room temperature including high response (R_a/R_g = 1.73, 1 ppm), high selectivity against other exhaled gases, reliable repeatability and stability (less than 4 % decrease after 40 days). A practical limit of detection (LOD) of 200 ppb was achieved. The theoretical simulation demonstrates that ferrocene is assembled via π-π interaction with rGO in edge-to-face configuration which provides relatively lower energy than face-to-face configuration does for the whole assembly. It was first verified that the enhanced adsorption capacity and the charge transfer between NO and Fc/rGO would result in improvement of the assembly's sensitivity toward NO after ferrocene was assembled with graphene. This work provides a fresh approach of anchoring iron on graphene for gas sensing via supramolecular methods.

Fractional exhaled nitric oxide (FeNO) can be used to describe inflammatory processes in the respiratory tract. Directly detecting ppb-level nitric oxide (NO) with chemiresistive sensors at room temperature faces the challenges of simultaneously obtaining high sensitivity and high stability for sensors. We aimed to improve the stability and sensitivity of NO sensors. We assembled sheet-like porphyrin-based MOF DLS-2D-Co-TCPP(Fe) with 5-aminonaphthalene-1-sulfonic acid-rGO (ANS-rGO) nanosheets through coordination interactions. In this way, we offered a room-temperature NO-sensing hybrid, DLS-2D-Co-TCPP(Fe)/ANS-rGO, with a sheet-on-sheet (SOS) architectural heterojunction. The DLS-2D-Co-TCPP(Fe)/ANS-rGO-based sensor demonstrated superior NO-sensing performance, including high sensitivity (R_a/R_g = 1.33, 250 ppb), reliable repeatability, high selectivity, and fast response/recovery (150 s/185 s, 1 ppm) at a sensing concentration from 100 ppb to 10 ppm at room temperature. The obtained sensor showed high stability, retaining >85% of its initial response after 60 days. Designing NO-responsive Fe-N₄ active units containing MOF nanosheets, along with constructing a heterojunction with an SOS architecture to facilitate carrier migration, collaboratively dominated the superior performance of synthesized NO sensors. This work provides a strategy for designing SOS architectural heterojunctions to obtain high-performance MOF-based gas-sensing materials.