1. Introduction
Gas detection has wide applications in environmental monitoring, security, industrial quality control, etc. Ammonia (NH
3) is a major industrial chemical product in the world while classified as
dangerous for the environment [
1,
2,
3,
4]. Exposure to high concentrations of ammonia could be lethal, while in low concentrations, it can cause coughing, nose, and throat irritation in the short term. Before designing a toxic gas sensor, several key aspects should be considered such as sensitivity, the minimum concentration of target gases they can detect, response speed, reversibility, energy consumption, and fabrication costs [
3,
5,
6,
7]. The ammonia concentration varies from one industry to another. The level of atmospheric ammonia at industrial parks can go as high as 150 ppb [
8]. Per the US CDC (Centers for Disease Control and Prevention) guidelines, the recommended exposure limit (REL) specified by the National Institute for Occupational Safety and Health (NIOSH) is 25 ppm for an eight-hour total weight average (TWA). NIOSH specifies the immediately dangerous to life or health concentration (IDLH) at 500 ppm [
9].
Continuous gas detection in the industrial environment has many technological challenges where the sensor is exposed to harsh conditions including solar radiation, temperature variations, humidity, and aerosols that may reduce sensor accuracy [
5,
10,
11,
12]. The presence of particle pollution (aerosol) has a high impact on the sensor performance deterioration especially for prolonged sensing operation, even in locations deemed to be convenient. The accumulation of particles and dust on the sensing module of different detectors such as optical detectors decreases its sensitivity and eventually results in sensor failure. Dust concentration and the exposure time are equally critical when dealing with prolonged sensing in particle polluted environment, and the challenges are more pronounced in an arid climate, which is categorized as a harsh environment with high dust concentration. As an example, the daily average of PM10 concentration (particulate matter that is 10 micrograms per cubic meter or less in diameter) in Kuwait during 2015 showed a high level of 2800 µg/m
3 [
11,
12,
13]. Even though the size of the dust aerosol particles varies from place to place, a detailed study by Al-Attar et al. showed that the particle pollution in Kuwait transported from the Arabian desert has a mean range of 3 to 6 µm [
14]. Very few studies have explored technological solutions to minimize the undesirable effects of aerosol on gas sensors in the past few decades [
10,
15]. This paper addresses this gap by presenting a reconfigurable and modular platform for toxic gas detection that encompasses a microfluidic-filtration module.
Particulate filters are widely used to overcome particle pollution and to protect the sensor. However, higher energy is needed to draw air through the filter and the need for regular replacement of the filter. In addition, these filters could decrease the vapor concentration in the sampled air due to the adsorption of organic gas into the filter, resulting in low sensitivity and a long response time [
10]. An alternative approach is to use a fluidic particle separator along the flow path. Such a separator is free from maintenance and could be designed as an add-on module. Passive inertia-based microfluidic particle separators are the focal point of many research groups for applications in biomedical and life science due to their low manufacturing cost, simplicity, ease of integration, and reliability [
16,
17].
With current advancements in microfabrication technologies, applications of microfluidics and microelectromechanical systems (MEMS) are rapidly expanding and gaining importance in sensing and biosensing applications. Microfluidic devices have a great potential for the detection of gas, liquid, and solid species and impurities [
5]. The integration of microfluidic devices with other sensing technologies enables reliable and autonomous sensing platforms with high sensitivity, selectivity, and reproducibility [
18,
19,
20]. Microwave resonator sensors offer attractive and non-contact solutions for the real-time monitoring of many events and properties in pipes and tubes where the need for the bulky detection systems and external characterization tools is eliminated [
21,
22]. Their working principle is based on the interaction between resonator electric fields and material properties in the sensor near surroundings. Materials’ dielectric properties such as permittivity and conductivity affect the electric field and consequently alter the resonant frequency and amplitude of the resonator. These changes could be used as a signature to detect materials [
18,
22]. Recently, microwave planar resonators have shown promising results for sensing applications due to their simplified manufacturing process and ease of integration with other technologies [
18,
22]. Compared to 3D resonators, although planar microwave resonators offer many advantages for sensing applications, they still suffer from low selectivity and sensitivity. There have been challenges for fabrication processes for making three-dimensional (3D) split-ring resonators (SRRs). With the development of 3D metal-printing techniques, manufacturing rectangular waveguide corners with higher accuracy is easier than machining [
23,
24]. The coupling of microfluidics with microwave sensing creates new opportunities for real-time detection and the characterization of single particles or cells [
18].
Selective porous nanomaterials can provide effective and reversible gas adsorption for selective sensing and protect the surface of the sensing site against harsh environment and contaminated particles [
25,
26]. The incorporation of these nanomaterials with microwave sensing enables the real-time detection of specific gases. For such an integration, microwave resonators should be isolated from the sensing materials via an interface such as thin quartz or a borosilicate glass layer to enhance the detection sensitivity while protecting the electrodes from a direct contact with the contaminated medium. Microwave sensors enable real-time calibration (within a few seconds) that considerably improves the accuracy and repeatability of sensing and therefore improves quantitative gas toxic content analysis [
27,
28].
Zarifi et al. [
22] applied zeolite 13x combined with a planar microwave resonator sensor to investigate the real-time detection of CO
2 and CH
4. The sensing was performed by monitoring the zeolite 13x permittivity change due to the adsorption of the gases. Li et al. [
4] have extensively investigated the effect of ammonia adsorption on the permittivity of zeolite Y in the millimeter-wave region of 60–90 GHz. Their experimental data have been used in the current work to simulate the performance of the proposed 3D split resonator. An extrapolation approach employed to find the corresponding permittivity values for the radio frequency (centimeter-wave wavelength) range.
In this work, we have proposed and successfully showed the proof of principle of a sensor for the continuous measurement of toxic gas in a harsh environment with particle pollution through a simulation approach. The presented platform has a microfluidic module for the filtration of intake gas, an adsorption module, and a low-frequency 3D microwave resonator for the detection of adsorbed toxic gas.