## 1 Introduction  
Radio frequency identification (RFID) is a modern technology used to wirelessly transmit and receive data. It is widely employed in a variety of Internet of Things (IoT) applications such as automatic identification, tracking, access control, security surveillance, supply chain, healthcare, and sensing applications [1; 2; 3]. A combination of RFID tags and sensing components can finally provide identification and sensing functions with a wireless and contactless data transmission. Passive RFID sensors, the latest technology for wireless monitoring, take advantage of new and interesting solutions because the sensors are simple structures that are easy to integrate and cheap to produce. Nevertheless, their silicon-free counterparts used for chipless sensing have also attracted particular attention because these sensors can be completely printed, which has the advantage of ultra-low manufacturing costs, thereby promoting the development of new IoT applications [4; 5]. The chipless sensor is essentially composed of two elements: the radiator (scatterer) and the transducer, both of which are related to the sensitivity and readability of the sensor. Because the electrical permittivity of the sensitive layer or part of the sensor is sensitive to the environmental parameter being monitored, the information can be processed by monitoring the resonant frequency shift from the backscattered electromagnetic response of the sensor [6]. Due to the limited sensor life, power, and cost constraints of traditional wired and battery-powered sensors [7], realizing sensors based on wireless power transfer (WPT) has been a challenge. It can present a promising solution that has the potential to revolutionize the future world of rich IoT sensing. The WPT represents an effective method of delivering power wirelessly from a source to an appliance [8; 9; 10]. Thanks to the wide coverage and easy mobility of RFID readers, the measurement of \"things\" marked by RFID sensors is no longer limited to specific locations. Therefore, the information-reception process of the RFID sensors becomes far more flexible and convenient, and its applications are often extended to a wider range of fields [11]. Recently, the need for sensors to monitor environmental parameters in modern homes, vehicles, and industries has attracted a lot of interest. In the literature, several designs have been proposed for the sensing of temperature and humidity [12; 13], gas concentration [14; 15; 16], and movement [17]. The availability of high-performing, reliable, and low-cost sensors is essential to facilitate the development of these applications (detection of gas, temperature, etc.) on a large scale and in a safe manner. This paper focuses on passive gas sensors. Previous research has already dealt with the use of radio frequency (RF) technologies for the implementation of innovative gas sensors. In [18], gas detection was realized using microwave transmission lines. In this case, the response of the dielectric material under the application of a microwave signal and in the presence of gas was used as a microwave transducer. Other microwave structures have also been utilized in gas detection. In [19], the dielectric response of carbon nanotubes was used in a microwave resonator to indicate the presence of gases and determine their concentration. The dielectric response affects the spectrum of the device, which can be tracked in order to identify the type and concentration of gases to be detected. [PERSON] et al. proposed in [20] an environmental gas sensor using a microwave resonator based on substrate-integrated waveguide; the sensor function was implemented by functionalizing a specific region inside the resonator. Other approaches such as the use of a waveguide and printed LC (electric circuit consisting of an inductor L and a capacitor C) resonators and dielectric resonator-based filters have also been employed to design gas sensors, as described in [21].