## 1 Toward Flexible Gas Sensors Era  
A human being collects information about the surrounding environment through its senses resulting in an emotional or intellectual behavior. In a similar way, a sensor elaborates the signals perceived by a sensing element responding with analytical data. Sensors have become an indispensable expansion of our senses and of our action opportunities by collecting information otherwise not available. The term \"sensor\" started to gain currency during the 1970s, identifying a transducer (or a device) that detects and converts events or changes in its environment into data directly observed or processed. The measured quantity and the provided output data can be of different nature (chemical, electrical, magnetic, mechanical, thermal, optical, etc.) making the sensors an essential tool in scientific applications, in industrial field as well as in everyday life. Over the years, the development of numerous types of sensors ranging from industrial process control to healthcare or medical diagnosis is so increased that the first decade of the 21 st century has been defined as \"the sensor decade\" [1]. Figure 1 shows the trend of documents number related to the sensor topic available on Scopus database [2] from the year 1950. It clearly highlights the fast growth of published documents per year, from 2000 to the present day. Nowadays, the development of sensors is certainly supported by technological improvements, in term of introduction of new nanostructured materials, new organic materials, new fabrication processes, miniaturization and the potentiality of micro- and nano-electronics and it is further enhanced by the huge market demand. Indeed, the sensors are crucial elements in \"Internet of Things (IoT)\". IoT is a network that collects, communicates and shares data from and between smart objects which in turn interact with the environment and people [3]. This powerful network rapidly advances, promoting the implementation of sensors in a large number of our everyday life objects, becoming them smart. In IoT framework, smart objects are coupled with radio frequency identification (RFID) smart tags, that include sensors and they are able to sense, monitor, and adapt to their environment. IntelligentRFID tags have the aim to combine sensing, computation and communication into a single, small and versatile device [4]. Sensors play also a key role in development and implementation of robots, automation and control systems in the factory field. By equipping robotic devices with sensors, robotic machines have become increasingly capable of performing complex and more accurate tasks, allowing manufacturers to increase efficiency, productivity, and profitability [5]. Another promising driver of sensors need during the next years is the development of \"digital twin\" (digital representation of a real-world entity or system such as industrial machines, humans or cities). The digital twin is linked in near real time to its corresponding real twin equipped with suitable sensors that monitor the real counterpart and its environment. The twins are used to understand the state of the analyzed system, its response to changes and the way to improve the operation of the real system [6]. The class of gas sensors represents one of the most diffused sensors group due to the variety of structures, materials and working principles available to realize them. In addition, they present many advantages such as working in real time, to be easy implemented and managed, to have long lifetime and low cost [7]. For these reasons, they are suitable to be used in many fields of applications, mainly related to the human health and security, as for instance:  
Air quality monitoring (indoor and outdoor) [8; 9];  
Vehicle emission monitoring [10; 11];  
Alarming of leakage of toxic and hazardous gases [12];  
Personal healthcare [13];  
Medical diagnosis [14];  
Food quality monitoring [15; 16],  
Agricultural and farming emission monitoring [4; 17].