# Breath Analysis: A Promising Tool for Disease Diagnosis--The Role of Sensors
## 2 The Role of Nanomaterials
### D-Materials  
2D-materials (e.g., graphene, MoS\({}_{2}\), MoSe\({}_{2}\), WSe\({}_{2}\), and NbSe\({}_{2}\)) have been investigated for gas sensor development. Their main advantage is low-power sensor operation at RT, while they are characterized by unique electrical properties [13] and large surface-to-volume ratio [13; 67]. The latter renders 2D materials excellent candidates for gas sensing applications [67]. Gas analyte adsorption at multiple active sites, on the edge and surface defects of the material, changes its electronic properties. Similarly to CNTs charge-transfer interactions comprise the basis of the gas sensing mechanism [13]. Gas adsorption changes the resistivity of the sensor, depending on the type of gas (reducing/oxidizing) and semiconducting 2D-material (p-/n-type) [67].  
Transition metal dichalcogenides and graphene in particular are extensively used in gas sensing systems, facilitating disease diagnosis via breath analysis [7; 53; 67]. Transition metal dichalcogenides (TMDs) (MX\({}_{2}\), M: transition metal, X: S, Se, or Te) such as MoS\({}_{2}\), MoSe\({}_{2}\), WSe\({}_{2}\), NbSe\({}_{2}\), [13] WS\({}_{2}\), SnS\({}_{2}\), and SnSe\({}_{2}\)[68], possess great structural and optical features [69] and tunable semiconducting properties [68] that render them promising materials for e-Noses. TMDs are able to create flexible structures and to operate at low temperatures; however, the increased response/recovery times and the inability to fully recover after analyte exposure are important limitations [68]. To cope with these issues, catalytic MNPs which also improve selectivity can be employed [68; 69].  
Graphene is characterized by exceptional electrical, thermal and mechanical properties, with low resistivity of 10\({}^{-6}\)\(\Omega\)-cm at RT and large surface-area of 2630 m\({}^{2}\)/g (higher than CNTs, 1000 m\({}^{2}\)/g [53]) being the most promising 2D material for gas sensing. Graphene can also behave as a p-type semiconductor while chemical functionalization can be easily achieved [7; 68]. Remarkably, graphene has been used for selective rather than cross-reactive applications. Graphene derivatives have also been used for gas sensing applications. Graphene oxide (GO) is characterized by a lower cost of production, however its application in electronic devices is hindered by reduced conductivity. Reduced graphene oxide (RGO) on the other hand exhibits tunable conductivity along with greater gas responses than graphene since the presence of oxygen functional groups enhances gas adsorption [67; 68]. It is noteworthy that the use of RGO-based gas sensors needs further investigation as the underlying gas-oxygen groups interactions are yet to be explained [53].  
2D materials are usually combined with various nanomaterials, for enhanced sensing [7]. The poor sensing performance of pristine 2D materials is primarily attributed to the weak interactions between the adsorbed molecules and the sensing layer [13]. Graphene and RGO combined with MOS [57; 70] and MNPs [59] may serve in the development of gas sensing diagnostic tools.