# Breath Analysis: A Promising Tool for Disease Diagnosis--The Role of Sensors
## 3 Types of Nanomaterial-Based Sensors in Breath Analysis
### Field-Effect Transistors (FET)
#### 3.2.2 Cross-Reactive Applications  
NWs-FET sensors comprise the most common type used for cross-reactive VOCs detection, including FETs based on SiNWs and MOS NWs (e.g., SnO\({}_{2}\), ZnO, In\({}_{2}\)O\({}_{3}\)) [63; 142]. Using pristine SiNWs, it is possible to discriminate VOCs with high dielectric constants, such as methanol, ethanol, and 2-propanol, with 96% accuracy and acetone, ethanol, and water with 100% accuracy by using pattern recognition methods [158]. As far as MOS NWs are concerned, n-type MOS have been mainly studied. SnO\({}_{2}\) NWs, e.g., have been used for acetone, ethanol, and methyl ethyl ketone discrimination; the modulation of gate voltage as well as of the operating temperature, permitted the adjustment of sensor response and selectivity [159]. p-Type MOS NWs are also reported to achieve effective VOC detection, such as CuO NWs for NO\({}_{2}\) and ethanol detection, with ethanol response being reversible by temperature increase due to oxidation towards CO\({}_{2}\) and water and electron transfer from water to CuO NWs [160]. Complementary-MOS based sensors have also been investigated, along with pattern recognition methods, especially for acetone, acetic acid ethanol, propanol, butanol, and hexanol discrimination [161]. In this case, only one sensor can be used, rather than a sensor array, as selectivity can be achieved by alternating the drain-source and gate potential and without any further modification [161]. Figure 8: Schematic representation and SEM images of (**a**) carboxylated SWCNTs aligned between Au electrodes and (**b**) Au-decorated SWCNTs, as sensing films for gas sensors (FETs), for enhanced H\({}_{2}\)S sensing. Au NPs play a crucial role in device-performance by modulating the mobility after the gaseous-molecule interactions. Reprinted with permission from Ref. [156]. Copyright © 2011 Wiley-VCH Verlag GmbH & Co. KgaA, Weinheim. Concerning FET arrays based on different nanomaterials, n-type semiconducting In\({}_{2}\)O\({}_{3}\), SnO\({}_{2}\), and ZnO NWs combined with a SWCNT-FET have successfully discriminated H\({}_{2}\), NO\({}_{2}\), and ethanol [162]. The 4-sensor array was tested at both 200 \({}^{\circ}\)C and RT as well as at different analyte concentrations, while SWCNT-FET incorporation improved ethanol and H\({}_{2}\) overlapping [162]. Remarkably, molecular functionalization has been reported, specifically by [PERSON] et al., for particularly promising FET applications. Molecularly functionalized random CNTs networks have been used for nonpolar and polar VOCs detection. As representative LC biomarkers, decane and 1,2,4-trimethylbenzene have been effectively detected by molecularly modified CNTs [163]. Notably, it has been observed that CNT-functionalization determined the semiconducting character of the material, with tricosane-CNTs leading to a p-type and pentadecane/dioctyl phthalate-CNTs to a n-type behavior thus affecting signal responses [163]. More interestingly, molecularly functionalized SiNWs have been used for cross-reactive FET sensors, targeting exhaled volatile organic biomarkers not only in synthetic but also in real-world breath samples; the discrimination of GCa patients from HC [125] as well as the discrimination of patients with asthma/COPD, LC and GCa (Figure 9) with remarkably increased accuracies [43] (Table 2) are noteworthy examples. As is to be expected, chemical functionalization of different nanomaterials allows for the detection of both polar and non-polar VOCs. Figure 9: Schematic of (**a**) surface modification of SiNW FET sensors (SSM, single-step modification; TSM, two-step modification); (**b**) sensor exposure to synthetic samples of potential organic volatile biomarkers for each disease (asthma/COPD, LC, GCa); (**c**) sensor exposure to real breath samples of patients with selected diseases, in comparison to HC; (**d**) ANN analysis representation. Increased accuracies were obtained for the discrimination of GCa vs. LC and LC vs. asthma and COPD patients. Reprinted with permission from Ref. [43].