Air pollution is a global issue that seriously impacts humans and the environment. According to the World Health Organization (WHO) 2016 air quality model reports, more than 90% of the world’s population breathes air that is polluted beyond the limits specified by WHO [1
]. The most common pollutants in air are carbon monoxide (CO), sulfur dioxide (SO2
), ozone (O3
), particulate matter (PM), and volatile organic compounds (VOCs). Specifically, CO is generated when carbon-containing materials are burnt. A major source of CO is the combustion of fossil fuels in vehicles [2
]. Additionally, SO2
is produced mainly by the oxidation of sulfur-containing materials. Combustion of fossil fuels in power plants and refinery facilities is a major source of SO2
. Bad O3
is associated with chemical reactions between oxides of nitrogen (NOx
) and VOCs. It is produced when pollutants are emitted by cars, power plants, industrial boilers, and refinery plants in the presence of sunlight [1
]. Man-made sources of PM2.5 and PM0.1 particles dominate the total concentration of pollutants. Emissions of PM2.5 and PM0.1 particles can be ascribed mainly to vehicular exhaust, road dust, and forest fires [3
]. Principal sources of VOCs include paints, paint strippers and other solvents, wood preservatives, aerosol sprays, cleansers and disinfectants, moth repellents and air fresheners, and building materials and furnishings [5
Furthermore, VOCs are typically defined as compounds with an initial boiling point that is less than or equal to 250 °C at the standard atmospheric pressure of 101.3 kPa. Various toxic VOCs exert a toll on the environment and cause respiratory diseases. Based on their chemical structures, VOCs are categorized into several types, including alkanes, aromatic hydrocarbons, olefins, halogenated hydrocarbons, esters, aldehydes, and ketones. Individuals facing long-term exposure to 100 ppb formaldehyde could develop nasal cancer [6
]. Benzene may also cause acute myeloid leukemia (acute non-lymphocytic leukemia) [7
]. A few studies provide strong evidence that toluene affects the central nervous system (CNS) [8
]. To evaluate the concentration of toxic VOCs, several precision methods and instruments have been designed and examined. They include gas chromatography (GC) [9
], high-pressure liquid chromatography (LC) [10
], the gas chromatography–mass spectrometry (GC–MS) coupled method [11
], ion mobility spectroscopy [12
], atomic emission detection (AED) [13
], and Fourier transform infrared (FTIR) spectroscopy [14
]. Most of these methods provide high sensitivity and high reliability. Their major disadvantages include high costs, time-consuming processes, and requirement for advanced techniques.
Wearable devices with integrated wireless technology have the potential to integrate an end-user with the Internet of Things, and offer healthcare and long-term, real-time information for personal measurement [15
]. The features of wearable devices include light weight, low cost, highly integrated sensors, and wireless telecommunication. Several different types of miniaturized gas sensors have been investigated, such as electrochemical sensors [16
], electrochemical sensors of surface acoustic wave (SAW) devices [17
], quartz crystal microbalances (QCM) [19
], and chemiresistive gas sensors [21
]. Polymer/MWCNT composite sensors correspond to chemiresistive-type gas sensors owing to their unique electrical, physical, and chemical properties that facilitate the development of sensitive devices for use in wearable gas-sensing applications. Gas sensors using conducting polymers as the sensing layer show excellent response in wearable device applications at room temperature [27
]. However, the range of ambient temperature varies greatly. For instance, the difference in ambient temperature between summer and winter is 20–30 °C, and the climate patterns of the northern and the southern hemispheres are opposite. The influence of ambient temperature restricts the application of a wearable polymer-based gas sensor system. The effects of temperature on polymer/MWCNT composite gas sensors have been investigated in our previous study [30
]. To solve this problem, thermal treatment is a straightforward method used to decrease the influence of ambient temperature on polymer/MWCNT composite gas sensors. In previous studies, three different polymers, namely, polyethylene oxide (PEO), ethylcellulose (EC) and polyvinylpyrrolidone (PVP), were selected to manufacture a flexible polymer/MWCNT composite sensing films for gas sensor array that was exposed to 1.5% ethanol at different operating temperatures. The response of each polymer/MWCNT composite film indicated that higher operating temperature could mitigate the influence of ambient temperature but reduce the response. Review of the data from the previous experiments led to a program to improve the flexible polymer/MWCNT composite gas sensor array for the possibility of wearable device application. Improvement considerations include the selection of polymers for high gas selectivity, the suitable operating temperature to immune ambient temperature influence, and methods of data transmission.
In this study, poly (α-methylstyrene) (PMS) was conducted primarily on increasing gas selectivity in the sensor array. The suitable operating temperature was considered with power consumption and heating–cooling profile. This study also focused on the development of a stand-alone wearable and wireless gas-sensing system based on a Bluetooth module. The polymer/MWCNT composite gas sensor was accompanied the smartphone applications were programmed in the Android environment. To verify the feasibility of recognizing the selectivity of different gases, the sensor array was exposed to various gases, including ammonia, nitrogen dioxide, and toluene vapors. The resulting sensor array response patterns show that the system has good selectivity to the target gas. The real-time sensing and to display the sensor response were demonstrated on a smartphone.
2. Materials and Methods
The sensing film was prepared drop-casting to form the bilayer sensor structure of the gas sensor array. Four different polymers were selected, namely, ethylcellulose (200697, SIGMA-ALDRICH, St. Louis, MO, USA), polyethylene oxide (43678, Alfa Aesar, Haverhill, MA, USA), poly(α-methylstyrene) (81516, SIGMA-ALDRICH, St. Louis, MO, USA), and polyvinylpyrrolidone (PVP 10, SIGMA-ALDRICH, St. Louis, MO, USA). Figure 1
a–d show the morphology of a selected polymer that was examined using a scanning electron microscope (SEM) (NOVA NANO SEM 450, FEI Co., Hillsboro, OR, USA) operated at an accelerating voltage of 10 kV.
The MWCNTs used in this study have outer diameters ranging from 2 to 6 nm and lengths of 10 to 12 μm, and they were purchased from XinNano Materials, Inc, Taoyuan, Taiwan. Figure 2
shows the FTIR spectra of the MWCNTs. The nature of the chemical bonds formed was recorded via FTIR spectra (PERKIN ELMER Spectrum GX, Waltham, MA, USA) in the range of 4000–400 cm−1
for investigation purposes. Figure 2
shows the characteristic peaks of MWNT at 1488.2 cm−1
(–COOH), 1635.6 cm−1
(C=C), and 2735.1 cm−1
(bonded OH in carboxylic acid).
The drop-casting method was implemented to fabricate the polymer/MWCNT composite films. The fabrication process has been described in a preliminary study [31
], and it is presented briefly here as follows:
Approximately 2 μL of MWCNT (60 ppm) is first dropped on the interdigitated electrode by using a microjet.
The solvent is evaporated, and the MWCNT film is furnished for 6 h at 70 °C.
Approximately 2 μL solution of the selected polymers is dropped on the MWCNT layer by using a microjet.
The solvent is evaporated completely, and the selected polymer film is furnished for 24 h at 60 °C.
After performing the aforementioned casting steps, the resistance of each sensor is confirmed to limit the value within 1–50 kΩ.