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Polyethylene imine functionalized porous carbon framework for selective nitrogen dioxide sensing with smartphone communication
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Xiaxia Xing, Xiaoyu Chen, Zhenxu Li, Xinhua Zhao, Yingying Tian, Xiaoyan Lang, Dachi Yang*
Chinese Chemical Letters | 2024, 35(9) : 109230
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Chinese Chemical Letters | 2024, 35(9): 109230
• Communication •
Polyethylene imine functionalized porous carbon framework for selective nitrogen dioxide sensing with smartphone communication
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Xiaxia Xing, Xiaoyu Chen, Zhenxu Li, Xinhua Zhao, Yingying Tian, Xiaoyan Lang, Dachi Yang*
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  • Tianjin Key Laboratory of Optoelectronic Sensor and Sensing Network Technology, Engineering Research Center of Thin Film Optoelectronics Technology, Ministry of Education and Department of Electronics, College of Electronic Information and Optical Engineering, Nankai University, Tianjin 300350, China
Published: 2024-09-15 doi: 10.1016/j.cclet.2023.109230
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Highly selective and remotely communicable nitrogen dioxide (NO2) sensing may contribute to future Internet of Things in environmental monitoring. However, room-temperature NO2 sensing materials such as carbon materials is still less than satisfactory due to their insensitive interaction with target gas. Here, polyethylene imine functionalized three-dimensional (3D) carbon framework (PEI/C framework) has been developed for enhanced selective NO2 sensing, via combined template synthesis and subsequent doping. Typically, the 3D PEI/C framework is observed porous shape with irregular coating. Beneficially, the response of C framework to NO2 increases while those of interfering gases decrease after being functionalized with PEI. Remarkably, the sensor prototypes show a 100 ppb-concentration detection limit at room temperature. Theoretically, such excellent NO2 sensing is attributed to the large specific surface ratio of porous 3D PEI/C framework, in which PEI serves as an active layer for target NO2, while a passivated one for interfering gases. Practically, such PEI/C framework sensor prototype is simulated for NO2 sensing device and communicated with a smartphone, showing great potential in future intelligent environmental monitoring.

PEI/C framework  /  Nitrogen dioxide sensing  /  Enhanced selectivity  /  Smartphone communication  /  Practical monitoring simulation
Xiaxia Xing, Xiaoyu Chen, Zhenxu Li, Xinhua Zhao, Yingying Tian, Xiaoyan Lang, Dachi Yang. Polyethylene imine functionalized porous carbon framework for selective nitrogen dioxide sensing with smartphone communication[J]. Chinese Chemical Letters, 2024 , 35 (9) : 109230 - . DOI: 10.1016/j.cclet.2023.109230
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Nitrogen dioxide (NO2) with toxicity and corrosiveness may cause photochemical smog and acid rain, seriously damaging the environment and threatening human health [1–3]. Specifically, NO2 harms human respiratory tract, leading to chest tightness, dyspnea and even death [4,5]. Accordingly, the time-weighted average (TWA) exposure limit of NO2 is 1 ppm and the threshold limit value is 3 ppm [6], in which selective detection and remote monitoring of low-concentration NO2 are critically required.
The chemiresistive gas sensors are extensively utilized for detecting NO2 due to their excellent portability [7] and sensing materials as a core component draw increasing attention. Among various NO2 sensing materials, semiconducting metal oxides (SMOs) such as ZnO nanowalls [8], porous In2O3 nanosheets [9], Ni-doped SnO2 nanofiber array [10], Fe-doped WO3 films [11] and Fe2O3/BiVO4 composites [12] have been investigated by exploring nanostructure, doping and constructing heterojunctions. Recently, two-dimensional transition metal dichalcogenide (TMDs) such as WS2 [13], MoS2 [14] and WSe2/WS2 [15] have been developed for room-temperature NO2 sensing. Although great progress has been made, the power consumption of SMOs-built sensors and the response speed of TMDs need further improving. Especially, the above-reported strategies on boosting selectivity will simultaneously improve the responses of both target gas and interfering ones, which could lead to a decrease in interfering selectivity.
Carbon materials such as graphene and its composites show great application potential in room-temperature NO2 sensing owing to their highly electrical conductivity and large specific surface areas [16,17]. However, the absorption of gas molecules on pristine carbon materials is determined to their weak van der Waals interaction, limiting their usage in highly selective and low-detection-limit gas sensing [18]. Actually, functionalized carbon materials with sensing dopants or nanostructures have been demonstrated to improve adsorption capabilities [19,20]. Nevertheless, it needs further exploration for a NO2 sensing that could simultaneously possess high selectivity, low detection limit and fast response.
Ideally, the carbon materials with a large specific surface area are decorated by polymers accompanied with functional groups, which may be devoted to specific identification of target gas such as NO2 while suppressing other interfering gases, however, few has been reported. In this study, a three-dimensional (3D) carbon framework functionalized with polyethylenimine (PEI/C framework, Figs. 1a and b) has been developed for selective NO2 sensing. Moreover, the detection limit of PEI/C framework was found as low as 100 ppb NO2. Here, PEI/C framework and their-built sensor prototypes will be taken as examples in the following description.
In Figs. 1a1 and a2, the morphology of C framework was observed by scanning electron microscopy (SEM) and show porous 3D structure with smooth surface. Comparatively, the irregular coating was seen over the surface of 3D framework after introducing PEI in Figs. 1b1 and b2 and Fig. S1 (Supporting information). The various PEI/C frameworks are obtained by feeding various concentration of PEI in the precursor, however, it should be illustrated that the PEI content on C framework cannot be tuned. To further confirm the doping of PEI, X-ray photoelectron spectroscopy (XPS) was conducted. In Fig. 1c, the survey spectra of C framework show the peaks of C and O elements, while that of PEI/C framework present additional N element, which is the representative element of PEI and indicate the decoration of PEI on the C framework. Meanwhile, the high-resolution N 1s and C 1s spectra in Fig. 1d and Fig. S2a (Supporting information) is verified as well. In Fig. S2b (Supporting information), the high-resolution O 1s spectrum is deconvoluted into adsorbed oxygen (Oads) and hydroxyl groups, in which Oads of PEI/C framework possess higher proportion than that of C framework, contributing to the improved NO2 sensing performance [21]. Moreover, the Fourier transform infrared spectroscopy (FT-IR) was performed in Fig. 1e, in which the FT-IR spectrum of PEI/C framework exhibits additional peaks at 2928 and 2837 cm−1 originating from PEI compared with that of C framework [22]. As a result, all the above characterizations reveal that the PEI is doped into C framework.
The PEI content-dependent NO2 sensing was firstly evaluated at room temperature, the sensing resistance curves of pristine C framework and C framework functionalized with various content of PEI were shown in Fig. 2a, in which the pristine C framework presents poor response and recovery especially at higher concentration of 10 ppm NO2. Remarkably, the response and recovery could be improved once introducing PEI, and the response value depend on the content of PEI on the C framework, especially the 10 mmol/L PEI/C framework exhibit the highest response to NO2 in Fig. 2b and is thus taken as sample in the following demonstration. In Fig. 2c, the lower concentration of NO2 was evaluated and the lowest detected content can reach ~100 ppb. Further, the response/recovery time of PEI/C framework to 5 ppm and 1 ppm NO2 are evaluated of ~ 16.8 s/98.8 s and 33.5 s/208.7 s in Fig. 2d, respectively. Comparatively, the NO2 sensing performances of various sensing materials at room temperature were summarized in Table S1 (Supporting information) [21,23–29], the PEI/C framework simultaneously presents advantages in low detection limit and fast response and recovery (Fig. 2e). In addition, the repetitive and long-term stability are crucial parameters and were thus investigated. In Fig. 2f, the PEI/C framework presents better repeatability than that of C framework by cyclically testing 1 ppm NO2, and the almost identical responses of PEI/C framework were summarized in Fig. 2g. Also, the 1 ppm NO2 sensing performance of the same PEI/C framework sensor prototype during 40 days were evaluated and compared in Fig. 2h, which suggests excellent repeatability and reproducibility. It should be illustrated that the sluggish sensing response and recovery after 40 days may be attributed to the humidity and the interference from other molecules to PEI/C framework [30].
Selectivity is a vital parameter in sensing performance, which was evaluated by exposing the C framework and PEI/C framework to various gases with 100 ppm of ethanol, acetone, H2 and CO, 10 ppm NH3, 1 ppm of H2S and NO2 in Fig. S3 (Supporting information), respectively. The sensing response values were summarized in Fig. 3a, in which the pristine C framework possesses indistinguishable response to NH3, H2S and NO2. Comparatively, PEI/C framework selectively increases the response of target NO2 and decreases those of interfering gases such as NH3 and H2S. Further, we elaborately performed the NO2 sensing evaluation of PEI/C framework after exposing it to the main interfering gases of NH3 and H2S in Fig. 3b, showing excellent anti-interference ability. Meanwhile, the principal component analysis (PCA) was carried out to distinguish the target NO2 from interfering NH3 and H2S (Fig. 3c), which further verifies the excellent selective identification of PEI/C framework to NO2. The detailed data of the above PCA are analyzed in Tables S2 and S3 (Supporting information). Additionally, the sensing performance of PEI/C framework to 1 ppm NO2 at various humidity was studied in Fig. S4 (Supporting information) and the responses were compared in Fig. 3d, and one can see that the high environment humidity may affect NO2 sensing. To further evaluate the reproducibility of the PEI/C framework sensor prototype in practical application, the NO2 sensor device was constructed in Fig. S5 (Supporting information). When sensor module is powered by laptop, the acquired NO2 sensing curve not only can be visualized on the smartphone but also on the laptop (Fig. 4a). Correspondingly, the cycling 1 ppm NO2 sensing data was recorded on the laptop (Fig. 4a1) and smartphone (Fig. 4a2), respectively, showing excellent reliability of our simulated sensing device.
To understand the sensing mechanism of the PEI/C framework, the role of the pristine C framework was first investigated. In Fig. 2a, the C framework shows decreased sensing resistance when it was exposed to oxidizing NO2 gas, which indicates the C framework possesses p-type semiconductor characteristic. Such phenomenon is verified by the Mott–Schottky (M–S) plot with a negative slope in Fig. S6 (Supporting information), depicting a typical p-type semiconductor nature (hole as the main carrier). Accordingly, the NO2 sensing mechanism of C framework is diagramed in Fig. 4b. In air, the O2 capture electrons from C framework to form adsorbed O2− (Eq. 1), increasing the surface hole concentration of C framework thus decreasing the resistance. In NO2, the NO2 will extract electrons from adsorbed oxygen ions on the surface of C framework (Eq. 2). Also, NO2 directly capture more electrons from C framework (Eq. 3) to form adsorbed NO2− due to the stronger electron affinity of NO2 than that of O2 [27,31]. Both the above reactions generate more holes on p-type C framework, the NO2 sensing with decreased resistance was observed.
Similarly, the NO2 sensing mechanism of PEI/C framework is shown in Fig. 4c. It should be noted that the response of C framework to NO2 increases while those of interfering gases decrease after being functionalized with PEI (Fig. 3a). Such phenomenon is entitled as "PEI serves as an active layer for target NO2 while a passivated one for interfering gases", suggesting a boosted selectivity. Theoretically, PEI possess rich amine groups with electron-donating property serving as n-dopants to the p-type C framework [32,33], which is confirmed by the increased initial resistance of C framework after doping PEI (Fig. 2a). Therefore, the electrons transfer from the PEI/C framework surface to the NO2 molecules due to the stronger electron affinity of NO2 [27]. In this case, the PEI polymer acts as an active intermediate layer [34], promoting NO2 sensing with fast response and recovery. On the contrary, if the PEI/C framework was exposed to the interfering gases such as H2S and NH3, which will react with pre-adsorbed oxygen species (O2−) to release electrons into PEI/C framework. However, the C framework has pre-received rich electrons from the donor of PEI, which may cause insensitivity to the extra electrons and thus decreased response to the interfering gases. As can be interpreted that PEI acts as a passivated layer for the interfering gases. Accordingly, the PEI/C framework selectively enhances the sensing response to target NO2, while decreasing the ones to those of interfering gases.
The NO2 sensing was simulated by building the sensing device integrated PEI/C framework, communicating with a smartphone. Specifically, the PEI/C framework sensor prototype and temperature-humidity (temp.-hum.) sensor are connected to the microcontroller NodeMCU (ESP8266), which conducts Wireless Fidelity (Wi-Fi) communication between the sensors and smartphone, and the corresponding block diagram is schematized in Fig. 5a. Meanwhile, the NodeMCU performs the program to process the sensing data from PEI/C framework sensor prototype and temp.-hum. sensor, and to control the NO2 alarming, the flow chart of the main program is shown in Fig. 5b. Notably, the NO2 sensor prototype on the sensor module and the displayed information on the smartphone are marked in Fig. 5c. Experimentally, the alarming was simulated in the supplementary video, when 3 ppm NO2 was injected and the sensing voltage reach alarm threshold, the smartphone shows red "AlARMING!" word (Fig. 5d). When the sensor module is placed in the air, the green "Monitoring" display on smartphone (Fig. 5c). Remarkably, as above presented sensor module powered by battery, the environmental temp.-hum. and NO2 sensing data can be remotely monitored and updated in real-time by the smartphone.
To summarize, the enhanced selective NO2 sensing has been developed with PEI/C framework via combined template synthesis and subsequent doping. Experimentally, the 3D PEI/C framework with porous morphology and irregular coating are observed. Theoretically, the PEI serves as an active layer for target NO2 while a passivated layer for interfering gases, and the PEI/C framework possesses a large specific surface ratio (637 m2/g, Fig. S7a in Supporting information). Beneficially, the sensor prototypes with PEI/C framework show excellent selectivity to NO2, in which the response of C framework to NO2 increases while those of interfering gases decrease after being functionalized with PEI. Moreover, the sensor prototypes present low detection limit of ~100 ppb NO2 and a fast response time of 16.8 s to 5 ppm NO2. As an example of application, the PEI/C framework has been integrated into a sensing device that communicates with a smartphone to simulate monitoring and alarming NO2, which is potential in future intelligent sensing of Internet of Things.
Declaration of competing interest
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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
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This work was financially supported by the National Natural Science Foundation of China (No. 52072184) and Tianjin Research Innovation Project for Postgraduate Students (General Project, No. 2022BKY035).
Supplementary materials
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Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2023.109230.
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Year 2024 volume 35 Issue 9
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doi: 10.1016/j.cclet.2023.109230
  • Receive Date：2023-07-18
  • Online Date：2025-11-20
  • Published：2024-09-15
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  • Received：2023-07-18
  • Revised：2023-09-06
  • Accepted：2023-10-23
Affiliations
    Tianjin Key Laboratory of Optoelectronic Sensor and Sensing Network Technology, Engineering Research Center of Thin Film Optoelectronics Technology, Ministry of Education and Department of Electronics, College of Electronic Information and Optical Engineering, Nankai University, Tianjin 300350, China
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表12种不同金属材料的力学参数
科
Family		 属数
Number of
genus		 种数
Number of
species		 占总种数比例
Percentage of
total species (%)		 属
Genus		 种数
Number of
species		 占总种数比例
Percentage of total
species (%)
鹅膏菌科Amanitaceae 2 11 5.26 鹅膏菌属 Amanita 10 4.78
小菇科 Mycenaceae 2 12 5.74 丝盖伞属 Inocybe 5 2.39
多孔菌科 Polyporaceae 8 14 6.70 蜡蘑属 Laccaria 5 2.39
红菇科 Russulaceae 3 23 11.00 小皮伞属 Marasmius 6 2.87
小菇属 Mycena 11 5.26
光柄菇属 Pluteus 5 2.39
红菇属 Russula 17 8.13
栓菌属 Trametes 5 2.39
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