Ten ��L of the saturated solution were coated onto the interdigitated electrodes by drop casting and dried in air at 353 K for 15 min. For the SnO2 sensor fabrication, 2.0 g SnO2 was dissolved in 25 mL DIW and the mixture was ultrasonicated for 1h to give a homogenous saturated solution at 298 K, after which 10 ��L of the prepared SnO2 solution was also coated onto the interdigitated electrodes by drop coating and then dried in air at 353 K for 15 min. All experiments were conducted at ambient conditions with a temperature of 24.5 �� 0.5 ��C and relative humidity of 45 �� 5%.2.3. Characterization and Gas Sensing Measurement SystemThe morphologies of SnO2-PDDAC and SnO2 films were characterized by a SEM (XL30S-FEG, FEI, The Netherlands) equipped with an EDX detector (EDAX Instruments, USA).

The mean grain size was analyzed by X-ray diffraction (Bruker D8 Focus, Germany). The detailed characterization of the SnO2-PDDAC sample was carried out by TEM (JEM-1011, JEOL, Japan). FT-IR (Excalibur 3100, Varian, USA) was used to characterize the components of each film. The gas sensing tests were performed by the gas sensing measurement system (NSSRL-811, Kena Smart Instruments, Wuhan, China), as shown in Figure 1. As the figure shows, two mass flow controllers (MFCs) were used to control the flow rate of synthetic air (dry air), the carrier gas, and ethanol, the target gas, respectively. The gases were purchased from the Beijing Tai Long Electron Technology Co. Ltd., Beijing, China. The carrier gas and target gas were mixed in the mixing chamber and then were introduced to the testing chamber.

A PC was connected to the testing circuit to monitor and record the resistance of the sensor. The temperature and humidity of the testing room were controlled by a central air conditioner. The gas sensing measurement was conducted by exposing the sensor in ethanol for 10 min and air for 10 min, respectively. The flow rate of the gas is 500 mL/min, and the volume of the chamber is 275 mL.Figure 1.Gas sensing measurement system.3.?Result and Discussion3.1. Ethanol Sensing TestsThe sensor response (S) was defined as:S=(Rgas?Rair)��100/Rair=��R��100/Rair(1)In Equation (1), Rgas and Rair are the electrical resistance when exposed to ethanol and air, respectively. The gas sensing properties of SnO2-PDDAC and SnO2 sensors at room temperature were both tested.

Figure 2(a) shows the typical response of SnO2-PDDAC, SnO2 and PDDAC sensors to 150 ppm ethanol. The response of the SnO2 and PDDAC sensors to 150 ppm ethanol was used as the reference. The response time is defined as the time of the sensor needs to reach 90% of the equilibrium value after the injection Cilengitide of the test gas.Figure 2.(a) Response of the SnO2�CPDDAC and SnO2 sensor to 150 ppm ethanol at room temperature; (b) The response time of the SnO2-PDDAC and SnO2 sensors to 150 ppm ethanol.