# Breath Analysis: A Promising Tool for Disease Diagnosis--The Role of Sensors
## 3 Types of Nanomaterial-Based Sensors in Breath Analysis
### Chemiresistors
#### 3.1.1 Selective Applications  
Concerning single analyte detection, nanomaterial-based sensing films such as MOS, CNTs and hybrid materials are commonly used. MOS-based chemiresistors, in particular, (e.g., WO\({}_{3}\)[55], SnO\({}_{2}\), or Cu\({}_{2}\)O [36]) are common in the detection of oxidizing/reducing gases [75]. [PERSON] et al. [55] used a chemiresistive porous thin film composed of villi-like WO\({}_{3}\) structures and achieved selective and sensitive detection of the asthma biomarker NO in presence of ethanol, acetone, NH\({}_{3}\) and CO (80% relative humidity (RH)) at 150-250 \({}^{\circ}\)C, with a LOD of 88 ppt which is far lower than exhaled NO. Selective arrays of MOS sensors have been also used; highly porous Pt, Si, Pd, or Ti doped SnO\({}_{2}\) NPs have selectively detected low levels of formaldehyde (3 ppb) in 90% RH, as well as in synthetic breath  
Figure 4: Schematic representation of (**A**) a chemiresistor under a constant bias and with an overlying metallic or semiconducting sensing layer which acts as the sensitive gas-sensing layer; (**B**) a field-effect transistor where the conductivity of the channel is sensitive to gas-analytes exposure; (**C**) an electrochemical sensor (potentiometric, amperometric, or conductometric) composed of a working (sensing) electrode on which the analyte reacts (redox reaction), a counter electrode (with respect to which electrical signal is measured) and a reference electrode of “reference” potential. Reprinted with permission from Ref. [13] Copyright © 2021 Wiley-VCH GmbH. samples containing acetone, ethanol and NH\({}_{3}\), aiming at LC diagnosis [76]. Concerning CNT-based chemiresistors, NO\({}_{2}\) selective detection has been reported, using a random or an aligned network of SWCNT, with the former achieving a lower LOD in the range of ppt [77]. Ethanol detection by vertically-aligned CNTs has also been achieved [78]. Among hybrid materials, MNPs-functionalized nanomaterials are commonly investigated. Highly porous thin-wall SnO\({}_{2}\) fibers of hierarchical structure composed of Pt NPs-decorated SnO\({}_{2}\) NTs, have been investigated for acetone and toluene detection as diabetes and LC biomarkers respectively; increased sensor responses for both analytes were permitted, due to MNPs-functionalization [58]. More recently, Pt-decorated Zn\({}_{2}\)SnO\({}_{4}\) hollow octahedra were used for enhanced selective detection of acetone, in ppb levels, adequate for breath analysis applications and diabetes diagnosis [79]. In another application, Au NPs-decorated SWCNTs were used for selective detection of H\({}_{2}\)S (halitosis biomarker) in the presence of NH\({}_{3}\) and NO, with a LOD of 3 ppb [80]. Au NPs functionalization permitted increased sensitivity for the desired analyte [80]. Other types of combinations of nanomaterials, including MOS, CNTs, or 2D materials, have been also studied. The respective halitosis and diabetes biomarkers H\({}_{2}\)S and acetone for example, have been detected using rGO nanosheets-functionalized SnO\({}_{2}\) NFs with increased selectivity and sensitivity [81] as well as using GO-WO\({}_{3}\) and thin-layer graphite-WO\({}_{3}\) Hemi tubes in the presence of CO, NH\({}_{3}\), NO, ethanol, and pentane; high responsiveness and sensitivity and a lower LOD for H\({}_{2}\)S were achieved in the latter study [57]. The combination of graphene derivatives with conducting polymers was also reported; porous and mechanically improved films composed of GO and polypyrene for instance, have been used for rapid toluene detection [82]. In the field of CNTs-based hybrid materials, SWCNTs were recently combined with porphyrin-based NFs to form a nano-composite of 1D nano-architecture that permitted the selective detection of H\({}_{2}\)O\({}_{2}\), in the simultaneous presence of various gases and VOCs. The enhanced sensitivity, reproducibility, and responsiveness, rendered the sensing film promising for cancer, traumatic brain injury, chronic obstructive pulmonary disease (COPD), and asthma diagnosis [83].