# Breath Analysis: A Promising Tool for Disease Diagnosis--The Role of Sensors
## 3 Types of Nanomaterial-Based Sensors in Breath Analysis
### Chemiresistors
#### 3.1.2 Cross-Reactive Applications  
Apart from selective applications, chemiresistors have also been extensively investigated for the development of sensor arrays and cross-reactive devices. MOS have been used as components of such sensing systems. Arrays of commercial MOS sensors (TGS), have been used by [PERSON] et al. [86, 87] and [PERSON] et al. [88] for the differentiation of LC [86, 87, 88] and COPD patients from HC, with high accuracies (Table 2). Interestingly, LC diagnosis using an array of TGS sensors has been reported in the literature; notably, 85 real-world breath samples of LC patients have been used, aiming not only at the discrimination between LC and HC but also at before- and after-surgery patients [89]. The aforementioned system could potentially comprise a promising diagnostic and prognostic tool after LC resection surgery, since it was shown that HC and operated patients' breath-prints converged over time Table 2[89]. It is noteworthy that commercial MOS-based chemiresistive e-Noses have been developed and studied for breath analysis and diagnosis. \"Common Invent e-Nose\", for instance, has been used for asthma phenotyping and exacerbation prediction [90], while \"SpiroNose\" [91] and \"AeeNoose\" [92] have been used for asthma, COPD and LC patients and COPD, LC patients and HC differentiation, respectively. \"DiagNose\", composed of 12 different doped MOS-sensors, comprises another promising example since it managed to differentiate 10 individuals through their breath samples [93]. Self-developed MOS-chemiresistors, in the form of pellets, based on a mixture of SnO\({}_{2}\) and ZnO nano-powders (9:1) have been effectively used for asthma diagnosis; however, a small number of subjects was included [94]. The combination of various materials for sensor-array development was also reported; an on-chip array of parallel ZnO, Pd, and polypyrrole single NWs for instance has been used for BC biomarkers, permitting increased responsiveness and sensitivity as well as total discrimination of the volatile biomarkers (Table 2) [95]. However, hybrid materials comprise the most extensively used materials for chemiresistive sensor arrays. Noble metal-functionalized MOS have been broadly studied. Halitosis diagnosis using an array of pristine and Pt NPs-decorated WO\({}_{3}\) macroporous NFs comprises a representative example (Table 2); increased responses towards H\({}_{2}\)S were achieved by the 0.042 wt% Pt NPs-WO\({}_{3}\) NFs [96]. PCA successfully discriminated H\({}_{2}\)S, acetone, toluene, and methyl mercaptan vapors, while real-breath samples of healthy controls (HC) and simulated halitosis breath were effectively classified [96]. Similarly, an array of pristine Au NPs (1/3 nm) decorated with vertically ordered hematite NTs, has been studied for acetone detection; the desired analyte led to far greater responses, while the studied vapors (acetone, ethanol, CO, H\({}_{2}\), NH\({}_{3}\), toluene, and benzene) were differentiated using PCA [97]. The theoretical acetone LOD of 304 ppt renders Au NPs-Fe\({}_{2}\)O\({}_{3}\) NTs-based sensor particularly promising for diabetes diagnosis. Cross-sensitivity towards NO and toluene using pristine WO\({}_{3}\) NTs (with increased selectivity for NO) and catalyst (Pt and Pd) decorated WO\({}_{3}\) NTs (more selective for toluene) has been also achieved, holding great promise for asthma and LC diagnosis via breath analysis [56]. The development of halitosis and diabetes diagnostic sensors has been attempted by the same group using cross-sensitive, pristine, thin-walled, and Pt NPs-decorated WO\({}_{3}\) Hemi tubes, with increased sensitivity towards H\({}_{2}\)S and acetone respectively [98]. More recently, a 2 \(\times\) 4 array of sensors based on pristine Pt, Pd, or Au NPs decorated In\({}_{2}\)O\({}_{3}\) and WO\({}_{3}\) nano-rods (NRs), was investigated for the detection of acetone, NO, and H\({}_{2}\)S as diabetes, asthma, and halitosis biomarkers.