Due to rapid urbanization worldwide, monitoring the concentration of nitrogen dioxide (NO2), which causes cardiovascular and respiratory diseases, has attracted considerable attention. Developing real-time sensors to detect parts-per-billion (ppb)-level NO2 remains challenging due to limited sensitivity, response, and recovery characteristics. Herein, we report a hybrid structure of Cu3HHTP2, 2D semiconducting metal-organic frameworks (MOFs), and laser-induced graphene (LIG) for high-performance NO2 sensing. The unique hierarchical pore architecture of LIG@Cu3HHTP2 promotes mass transport of gas molecules and takes full advantage of the large surface area and porosity of MOFs, enabling highly rapid and sensitive responses to NO2. Consequently, LIG@Cu3HHTP2 shows one of the fastest responses and lowest limit of detection at room temperature compared with state-of-the-art NO2 sensors. Additionally, by employing LIG as a growth platform, flexibility and patterning strategies are achieved, which are the main challenges for MOF-based electronic devices. These results provide key insight into applying MOFtronics as high-performance healthcare devices.
Cu3HHTP2 MOF formation by an LbL process
Cu3HHTP2 MOF was grown on UV-LIG using the LbL process. The PI substrate patterned with LIG was alternatively soaked in an ethanolic solution of 1 mM copper acetate and 0.1 mM 2,3,6,7,10,11-hexahydroxytriphenylene with retention times of 20 and 40 min, respectively. After each soaking cycle, the substrate was washed with ethanol to remove the residual reactants. The trigonal HHTP linker binds to the square planar Cu2+ ions to form an extended two-dimensional hexagonal layer in the ab plane. Through repeated LbL cycles, MOFs are stacked along the c-axis with a 1D open channel. The number of soaking cycles was 8, and the process was accurately automated by using a rotary dip coater (Nadetech ND-R Rotary Dip Coater). The optimization process of the soaking cycles is described in detail in Supplementary Fig. 3. Then, LIG@Cu3HHTP2 was rinsed with acetone and isopropyl alcohol and dried in a vacuum oven (65 °C, overnight).