## 4 Functional Materials
In [87], a comprehensive survey of current CNTs-based gas sensing technology is presented. The literature reports also various gas sensing application in flexible form. As an example, in [88] the fabrication of an inkjet printed CNT based sensor for DMMP (dimethyl methylphosphonate) detection is reported. It was achieved a sensitivity of 20% on 10 ppm of DMMP vapor. In [89] a flexible and reliable chemiresistor-type NO\({}_{2}\) gas sensor based on single-walled carbon nanotubes (SWNTs) on polytetrafluoroethylene (PTFE) membrane filter substrate is described. In [36] fully printed CNT network gas sensors on flexible substrates such as polyimide (PI) and polyethylene terephthalate (PET) have been used for ammonia and nitrogen dioxide detection in air at low ppm concentrations. Graphene and graphene oxideGraphene is a two-dimensional crystalline material with excellent properties like large specific surface area, high conductivity, and high Young's modulus [90]. The main characteristic is that all atoms of a graphene layer can be considered as surface atoms, so being all able of adsorbing gas molecules; in such a way a very large surface area is available for the sensing mechanism. Moreover, the interaction between graphene layers and the adsorbates molecules can be of different intensity, being of van der Waals type or covalent bonding. It is also characterized by an extremely small change in the resistance due to very small concentration of gas adsorption achieving gas detection down to the molecule level. A more interesting material is graphene oxide (GO) resulting of chemical exfoliation and oxidizing of layered crystalline graphite (natural or artificial). In specific conditions of graphite oxidizing, the resulted GO maintains 2D structure, in which layers of carbon atoms are covered by oxygen-containing functional groups. GO can be obtained also by chemical synthesis in form of single layer or multilayer structure. Starting from GO, by a reduction processes (thermal, chemical, etc.) it can be obtained the reduced graphene oxide (rGO). The rGO generally contains defects due to its synthesis process. GO and rGO properties are extremely different from those of graphene, making both of them interesting for gas and chemical sensing applications. As example, the oxygenated functional groups confer hydrophilic nature to GO, highly sensitive to water molecules and therefore employed to prepare humidity sensors. The oxygenated functional groups also offer different possibilities for the surface functionalization with noble metals, metal oxides and conductive  
Figure 8: (**a**) response curve of the sensor to gradually increased humidity levels, ranging from 0 up to 72%; (**b**) sensor response as function of the relative humidity; (**c**) response and recovery time of the sensor as function of the relative humidity. [57]. polymers [91]. An example of flexible ammonia sensor based on rGO and nano-Ag ink deposited through inkjet printing to a PET substrate is reported in [50] and shown in Figure 9.  
The sensor was tested to 15 ppm, 50 ppm, 100 ppm, and 200 ppm of NH\({}_{3}\) at room temperature showing responses of 4.25%, 6.1%, 10.08%, and 14.7%, respectively (see Figure 10). _Conducting polymers_--Are easily synthetized by chemical and electrochemical processes with very low cost. From 1977, when it was found that polyacetylene became a conductive material through a suitable doping process, they have been used as functional material for gas sensing [92]. Due to the increase of the charge carriers (polarons), the polymer conductivity grows and makes it a good functional material for gas sensing at room temperature. In a conducting polymer based chemoresistive gas sensor, a change of the target gas concentration results in a polymer conductivity variation. The most used conducting polymers, also in flexible printed gas sensor development [82; 83], are polyaniline, polypyrrole, poly(3,4-ethylenedioxythiophene) (PEDOT), and PEDOT doped with polystyrene sulfonated acid (PEDOT/PSS) [40; 49; 52].