(Invited) ppb-Level NO2 Monitoring Using Lung-Inspired MOF on Graphene Hybrids Sensor
Authors: LIM Hyeongtae , Kwon Hyeokjin , Kang Hongki , Jang Jae Eun , Kwon Hyuk-Jun 
Published: December 2024

NO2 is associated with 1.8% of all cardiovascular mortality and even the progression of neurodegenerative diseases1. To safeguard against this threat, the WHO has defined a NO2 exposure limit at ~5 ppb in its Global Air Quality Guidelines2. Consequently, it is essential to continuously and accurately monitor NO2 at ppb levels to provide personalized pollutant data. Ideal NO2 sensors should fulfil specific requirements: the ability to detect ppb-level NO2 in real-time, simplicity in operation (without external thermal- or photo excitation), cost efficiency, and wearability (lightness and flexibility). However, conventional materials such as metal oxides, transition metal dichalcogenides, or carbon derivatives often fail to meet these criteria due to their low sensitivity, slow response/recovery time, and high operating temperature3.

Metal-organic frameworks (MOFs), comprising inorganic building units coordinated with organic linkers, are compelling opportunities to develop highly sensitive gas sensors. With their customizable topology, exceptional surface area, and uniform nanopore distribution4, MOFs are becoming increasingly attractive for their use in electrical sensory platforms owing to superior gas adsorption and surface reaction capabilities. Specifically, the semiconducting Cu3HHTP2 MOF has shown superior sensitivity and selectivity toward NO2 due to its strong redox activity. Nevertheless, practical challenges remained with using MOFs for NO2 sensors, such as limited mass flow, fabricating technology, and flexibility.

In this work, we propose the use of laser-induced graphene (LIG) as a suitable platform for growing Cu3HHTP2 MOF to facilitate real-time monitoring of ppb-level NO2. LIG, an emerging 3D macroporous material, can be produced via direct laser irradiation of various polymer substrates5. By combining Cu3HHTP2 and LIG (resulting in LIG@Cu3HHTP2), we have observed synergistic effects that are not achievable with MOF alone (Fig. 1-4). Firstly, the nanostructured MOFs synthesized on the LIG platform accelerated the mass transport of exposed gases, which is attributable to their hierarchical macro-/microporous architecture. Moreover, the expanded exposure surface area leveraged the intrinsic advantage of MOFs, characterized by their abundant open metal sites and edge ligands for molecular adsorption. The human lung is an excellent example of a hierarchical pore architecture, enabling rapid transport of gas molecules over a large surface area. The LIG@Cu3HHTP2 showcased one of the most rapid response/recovery times (16 s/15 s) and the lowest limit of detection (LoD, 0.168 ppb) amongst cutting-edge NO2 sensors operating at room temperature and atmospheric conditions (Fig. 5-8). In addition, the selectivity toward NO2 over interference gases was demonstrated as a result of charge carrier transfer and size discrimination effect of MOF (Fig. 9). Secondly, we authenticated a patterning strategy for solution-based MOF growth, addressing one of the pivotal challenges in fabricating MOF-based electronic devices (Fig. 4). The LIG provides plentiful defect sites and dangling hydroxyl functional groups, forming an optimal platform for MOF nucleation. Also, in terms of device fabrication, laser processing can enable rapid, energy-efficient programmable micropatterning. Lastly, whereas traditional MOF-based electronic devices have predominantly been restricted to rigid substrates, our approach enabled their integration onto flexible and lightweight substrates via formation on LIG (Fig. 10 and 11). The flexibility of the resultant sensor was evaluated through computational modelling and real cyclic bending tests, with resilience observed across 10,000 iterative bending cycles, even under a stringent 2.5 mm curvature radius.

Consequently, we successfully developed a LIG@Cu3HHTP2 hybrid capable of monitoring NO2 at the ppb level in real-time. These results will inform the advancement of MOFtronics applications, currently confined to lab-scale explorations, facilitating their transition towards real-world implementation as high-efficiency sensors.

This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Ministry of Education (2020R1A6A1A03040516).

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F. M. Vivaldi, et al., ACS Appl. Mater. Interfaces. 13(26), 2021.

Figure 1