Safeguarding a livable environment from air pollution is a global challenge due to the rapid pace of urbanization. In particular, the WHO has set the exposure limit for NO₂ to ~5 ppb due to its detrimental effects on human health, such as cardiovascular deaths and even degenerative brain diseases¹. Accordingly, there is an urgent need to develop technology to monitor NO₂ at the ppb level in real time to provide personalized pollutant information. However, previous NO₂ sensing materials (metal oxides, transition metal dichalcogenides, and carbon-based nanomaterials) have limitations in sensitivity, response time, and high-temperature operation². To overcome these obstacles, researchers have recently explored the use of semiconducting MOFs as highly sensitive NO₂ sensing material, which has a designable topology, record-breaking large surface area, and uniform pore size distribution³.

Here, we introduce LIG as a growth platform for Cu₃HHTP₂ 2D semiconducting MOFs to accomplish real-time monitoring of ppb-level NO₂ (Fig. 1). LIG is an emerging 3D macroporous material that can be produced by direct laser irradiation of various polymers and organic substrates⁴. Through the combination of Cu₃HHTP₂ and LIG (denoted LIG@Cu₃HHTP₂), we demonstrated a synergistic effect that could not be achieved when MOFs were used alone.

Figure 1 Fabrication of LIG@Cu3HHTP2. a Schematic of LIG@Cu₃HHTP₂ processing. Irradiation by the 355 nm laser directly converted PI to LIG. Subsequently, Cu₃HHTP₂ MOF is grown on LIG by a layer-by-layer process. b Schematic of the structure of Cu3HHTP2 on LIG.

First, the nanostructured MOFs grown on LIG enabled accelerated mass transport of the exposed gas due to the lung-mimicking hierarchical macro-/microporous architecture (Fig. 2). Furthermore, the increased exposed area maximized the advantage of the MOFs, which have abundant open metal sites and edge ligands to which guest molecules can adsorb. Therefore, the LIG@Cu₃HHTP₂ structure exhibited one of the shortest response/recovery times (16 s/15 s) and lowest LoD (0.168 ppb) among state-of-the-art NO₂ sensors, even at room temperature and atmospheric conditions.

Figure 2 Effect of the hierarchical porous structure. a Comparison of the response and recovery time of LIG@Cu₃HHTP₂ hybrids and dense Cu₃HHTP₂ films toward 10 ppb NO₂. b Repeatability tests with cyclic NO₂ exposure. c Mass transport of the hierarchical porous structure compared to the dense MOF film. d Comparison with other state-of-the-art NO₂ sensing materials operating in air at room temperature in terms of the limit of detection and response time.

Second, we validated the patterning strategy of solution-based MOF growth, which is one of the most significant challenges in the fabrication of MOF-based electronic devices. Figure 3 shows the spatial mapping of the graphitic G bands and metal-ligand vibration peaks only in the laser-irradiated region, which means selective growth of Cu₃HHTP₂ on LIG. The rGO-like chemical environment of LIG provides abundant defect sites and dangling hydroxyl functional groups, which provide an ideal platform for MOF nucleation by anchoring metal ions.

Figure 3. Vibrational spectroscopy analysis of LIG@Cu₃HHTP₂. Raman mapping of graphitic G bands and the M-O vibration mode after the formation of Cu₃HHTP₂ in a locally laser-irradiated region, displaying the selective growth of MOF on LIG. The inset shows an optical microscopic image of the same position.

 Finally, MOF-based electronic devices, mostly limited to rigid substrates, could be applied to lightweight and flexible substrates through formation on LIG. The sensor endured 10,000 repetitive bending cycles even at a harsh radius of curvature of 2.5 mm (Fig. 4). Therefore, we demonstrated a unique strategy for applying MOFs as personalized wearable sensors. These findings will help guide applications in MOFtronics, which remain at the laboratory level, as a high-performance sensor that could be implemented in the real world.

Figure 4. Stress analysis and flexibility tests. a Simulation of the stress distribution of the film by finite element analysis when the radius of curvature is 2.5 mm and the thickness is 25 μm. b The fatigue test shows no variation in resistance during the cyclic bending process (n = 0, 1000, 5000, and 10000, when the radius of curvature is 2.5 mm and the thickness is 25 μm)

To learn more, please check out our publication “Semiconducting MOFs on ultraviolet laser-induced graphene with a hierarchical pore architecture for NO2 monitoring” in Nature Communications: https://doi.org/10.1038/s41467-023-38918-3

References

1. World Health Organization. WHO Global Air Quality Guidelines: Particulate Matter (PM2. 5 and PM10), Ozone, Nitrogen Dioxide, Sulfur Dioxide and Carbon Monoxide (World Health Organization, Geneva, Switzerland 2021).
2. Kumar, S. et al. A review on 2D transition metal di-chalcogenides and metal oxide nanostructures based NO2 gas sensors. Materials Science in Semiconductor Processing 107, (2020).
3. Allendorf, M. D. et al. Electronic Devices Using Open Framework Materials. Chem Rev 120, 8581-8640 (2020).
4. Vivaldi, F. M. et al. Three-Dimensional (3D) Laser-Induced Graphene: Structure, Properties, and Application to Chemical Sensing. ACS Appl Mater Interfaces 13, 30245-30260 (2021).