Abstract:
Introduction
Diabetes is a global chronic disease arising from the metabolic disturbances. Detection of acetone in human respiration offers a non-invasive and painless approach for diabetes diagnosis. With increasing vehicles ownership and emission, exhaust from automobiles, i.e. nitrogen dioxide (NO₂) and its derivatives, imposes huge threaten on the air quality and public health. However, a majority of the gas sensors demand additional energy supply and undoubtedly increase the energy consumption of the whole device, inhibiting the portability and mobility of the device.
A bionic alveolus-shaped triboelectric gas sensor (ATGS) has been designed and fabricated for spontaneously NO₂ detection and respiratory monitoring at room temperature. Furthermore, a sensing modeling was proposed by combining the thermodynamic analysis and finite element calculation together with phase-field simulation. This work not only provides a facile, low-cost and portable approach for active gas sensing and real-time physiological assessment, but also proposes a theoretical modeling for self-powered gas detection.

Device fabrication
The configuration of ATGS is based on contact-separate mode TENG. A layer of acrylic with the dimension of 40mm×40mm×1mm was tailored by laser etching machine, together with a layer of latex with the dimension of 40mm×40mm×0.2mm was cleaned by ethanol and deionized water. Then copper electrode with a thickness of 200nm was coated on the one side of the tailored acrylic sheet by thermal evaporation, followed by deposition of prepared suspensions as the gas sensitive material via gas spraying. Subsequently, the WO₃/copper coated acrylic sheet was etched through laser etching machine to create a central hole with diameter of 4.0 mm. Then, the latex layer was attached to WO₃/copper coated acrylic sheet to form the gas test chamber. Epoxy resin was used to seal the edges between latex film and acrylic sheet to ensure the gas tightness. A plastic tube was integrated as the inlet and outlet of the target gas into the sensor.

Method
The internal combustion engines burning fossil fuels generate and release huge amount of nitrogen dioxide, which is large threat to environment and public health, as shown in Figure 1a. To address the concern, we designed an alveolus-shaped triboelectric gas sensor (ATGS) for both nitrogen dioxide detection and personal breath behavior monitoring, as the configuration shown in Figure 1b. It was vertically laminated with latex, sensitive material, copper electrode and a plastic air conduit serving as the gas conducting channel. Here, latex film is selected as the contacting layer for its good stretchability and strong electron affinity. Figure 1c shows the photograph of the as-fabricated AIMS, where the gas inflation will hold up the latex membrane to form a tent while the deflation will shrink the tent. During this processing, the latex forms a cycle of contact and separation with the bottom substrate material, pumping the electrons to flow back and forth between the copper electrode and the ground. The working principle and finite element calculation of the as-prepared AIMS is elucidated in Fig. 1d and 1f.

Results and Conclusions
To explore the gas sensing capability of the as-fabricated ATGS, the output voltage versus NO₂ gas concentrations from 0 ppm to 100 ppm were investigated. Compared with 0.01 g and 0.1 g alkali treatments, the device with 0.02 g NaOH treatment revealed a much better sensitivity and linearity, as shown in Fig. 2a, where a response of 452.44% was achieved when being exposed to 100 ppm NO₂. In addition, as presented in Fig. 2b, a linearity of 0.976 is observed for the AIMS with 0.02 g NaOH treatment, demonstrating its capability of actively detecting nitrogen dioxide in a wide concentration range. The ATGS also demonstrated capability in distinguishing diverse breathing patterns without any power supply (Fig. 2c). In addition, the NO₂ response is at least 20 times higher than other gases, implying a good selectivity (Fig. 2d)
A phase-field simulation and finite element calculation were implemented to numerically verify the permittivity effect on the electric field and polarization of sensitive film. As shown in Fig. 3k, the polarization increases with increasing relative dielectric constant, while the electric field follows an opposite trend. Given the depolarization field is proportional to the polarization once the device is fixed, the increase of permittivity facilitates the depolarization of the sensitive layer, which is in agreement with the aforementioned theoretical derivation and experimental data.

REFERENCES
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