A Room Temperature-Operating Acetone Gas Sensor Based on the Triboelectric Effect

Abstract

Introduction The exhaled acetone has been proved as a typical biomarker for Type-I diabetes. Normally, the acetone concentration from the exhale breath of healthy body is lower than 0.8 ppm, while the patient’s breath contains more than 1.8 ppm acetone gas [1]. Therefore, developing a reliable and accurate acetone gas sensor is of great significance. Meanwhile, traditional acetone gas sensors depend on the output power supplies, combining them with self-powered devices can fully utilize the energy of the external environment, which guarantee the normal operation of the sensors under the statues of power failure.Currently, the metal oxide semiconductor (MOS)-based acetone sensors are extensively researched owing to their high response and fast response-recovery times. However, the excessive power consumption restrains their further application. Recently, chitosan (CTS) has been proved to be an acetone sensing material working at room temperature [2]. For enhancing the gas sensing performance, fictionizing chitosan with other materials might be an effective way. Therefore, the chitosan (CTS)/zinc oxide (ZnO) bilayer sensing film based TENG as a self-powered gas sensor has been designed for detecting acetone at room temperature. Results indicated that the CTS/ZnO bilayer sensing film exhibited greater response of acetone than the CTS monolayer sensing film. Experimental ZnO were prepared by a hydrothermal method. Specifically, 3 mmol zinc acetate dihydrate and 5 mmol hexamethylenetetramine were dissolved in 30 mL of deionized water and ultrasonically treated for 30 min to form a clear solution. Next, the solution was transferred into an autoclave, and subsequently heated up to 150℃ for 210 min before cooling naturally. After that, the mixtures were centrifugated and washed by ethanol and deionized water for several times, and dried at 80℃ in an oven to obtain the ZnO.The detailed fabrication process of the self-powered gas sensor (Fig. 1a) are demonstrated as follows: First, 0.1 mm thickness of the flexible polyethylene terephthalate (PET) with the size of 3 cm×3 cm was covered by a gold electrode layer via thermal evaporation. Then, 1mL of CTS solution (4 mg/mL, dissolved in 2 vol.% of acetic acid) and 1.5 mL of ZnO aqueous solution (2 mg/mL) were deposited onto the PET substrate with gold electrode layer via spray coating method, respectively. Meanwhile, the PDMS friction layer was transferred onto the gold electrode layer mentioned above. Finally, the PET substrate with the gas-sensing materials was fixed onto a flexible silica gel film as the upper layer, while the PDMS layer was fixed at the bottom of an acrylic test chamber. The whole device was driven by the periodic force exerted on the silica gel film.All the output signals generated by the sensor were measured by a Keithley 6514 electrometer at room temperature (~25℃). By mixing a standard dry acetone gas (100 ppm, dry air was used as the balance gas) with dry air in proportion with the help of two mass flow controllers (MFC 300), the acetone gas with different concentration was generated and introduced into the test chamber. The total flow rate was kept at 200 sccm for unifying the gas-sensing measurement. The response value was calculated as follows: Response (%) = (Vpp-g−Vpp-a)/ Vpp-a×100%, where Vpp-g and Vpp-a are the peak-peak output voltage exposed in air and tested gas, respectively. Results and Conclusions Fig. 1b illustrates the output voltage of the sensor with the exposure to dry air and different concentration of acetone gas at room temperature. The output voltages increased with the increasing acetone concentration in the range of 2-50 ppm. Meanwhile, the CTS/ZnO sensor is of good linear response at 2-10 ppm (R2=0.9872) and 10-50 ppm acetone (R2=0.9030), of which the responses are 6.46-17.29% and 17.29-32.09%, respectively. Based on the above phenomenon, a possible sensing mechanism has been established: After the introduction of acetone gas, the nucleophilic addition reaction between the CTS and acetone would destroy the hydrogen bond interaction between CTS and ZnO (Eqs. (1)), leading to the interruption of electrical pathways for the effective electrons transfer [2], resulting in the decrease of surface conductivity and eventually increase the output voltage [3].CTS-NH2+CH3COCH3=CTS-N=C(CH3)2+H2O(1)In summary, we have successfully fabricated a novel triboelectric gas sensor based on CTS/ZnO bilayer membrane. After testing procedure, a possible sensing mechanism has been established. The CTS/ZnO gas sensor has a great potential in the applications of acetone detection without additional power supplies, and provides a useful strategy for the researches of novel acetone sensors.