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The analysis of exhaled breath is important for health vs. disease diagnostics. Diagnosis and monitoring methods based on metal-oxide gas sensors are highly attractive, owing to their portability and ease of operation. These methods are non-invasive and affordable, and allow early disease diagnosis. Metal-oxide gas sensors are heat-resistant and can easily be combined with various nanostructures. We review concurrent research into metal-oxide gas sensors used to detect various biomarkers (e.g., acetone, nitric oxide, hydrogen sulfide, and ammonia) in the exhaled breath of humans. We also review efforts to improve the properties of metal-oxide gas sensors, especially structures, morphologies, the control of impurities in semiconductor materials, and changes in the specific surface area of metal-oxide materials that have been used in gas sensors to analyze exhaled breath. Various approaches to the humidity resistance characteristics and nonlinear responses of metal-oxide semiconductor gas sensors are addressed and should be applicable in real life.

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A combined MER (mediated electrochemical reduction) and MEO (mediated electrochemical oxidation) approach was examined for the first time for the efficient removal of NO by electro-scrubbing. The generation of a mediator (Ni(I) from Ni(II)(CN) 4 2− in 9 M KOH) by electrochemical reduction was identified by the changes in the ORP value and potentiometric titration. The Ni(I) formation found to be 9% (4.3 mM) in 2 h electrolysis, which is decreased to 1.83 mM (4%) during addition of NO demonstrates NO removal occurred. The MER of NO at the cathodic half-cell by electro-scrubbing revealed the formation of NH 3 and N 2 . The reductive removal efficiency of NO was almost 100% up to a gas flow rate of 0.25 g min −1 . At the anodic half-cell, the Co(III) mediator from Co(II)SO 4 in 5 M H 2 SO 4 was generated electrochemically with concomitant conversion of NO to NO 3 − in the solution phase with a removal efficiency of 28%. The combination of MEO and MER with electro-scrubbing demonstrates 97% removal efficiency of NO up to 0.5 g min −1 . The combined removal approach is more suitable for high concentrated NO with energy efficiency of 0. Abundantly generated nitric oxide (NO) gas that includes power plant and steel plant etc., is decomposed by many technologies, such as wet scrubbing, catalytic oxidation, and selective catalytic reduction (SCR).1-3 The reduction of NO leads a combination of cationic intermediates and products, N 2 O, NH 2 OH, N 2 , and NH 3 , in the solution phase. [4][5][6] The reduction products varied depending on the catalyst and experimental conditions: on TiO 2 in the colloid solutions to produce NH 3 ; 4 in heterogeneous catalytic reduction process with NH 3 as the reactant produced N 2 ; 7,8 and N 2 O formed on boron-doped graphene method.9 At the same time, the oxidation of NO leads to the anionic products of NO 2 and NO 3 − . 10,11 Heterogeneous catalytic oxidation at the gas phase produced NO 2 , 12 but solution phase oxidation by ozone and H 2 O 2 produced the NO 3 − product. 1,13 Because of the catalytic reaction, NO is solubilized in an aqueous solution, otherwise it less soluble or insoluble in an aqueous solution.14 Using this approach, many oxidant-15,16 and reductant-17 containing wet-scrubbing processes have been adopted in NO removal. Note that the above methods require a continuous catalyst or reactant feed to make the process sustainable.An electron considered to be a green catalyst and can be feed continuously to regenerate the spent catalyst sustainably. Using this approach, electrogenerated Ce(IV) and Mn(III) mediators have been used to oxidize NOx to NO 3 − in HNO 3 medium. 18,19 In a similar method, electrogenerated Ag(II) with a wet scrubber in the name of 'electro-scrubbing' have used NOx oxidation to NO 2 in a single stage scrubber and NO 3 − in a two stage scrubber column. 20 In the presence of SO 2 in NO oxidation using the electrogenerated Ag(II) mediator in highly concentrated nitric acid have shown NO 2 with 60 to 80% of NO 3 − formation. [21][22][23] Note that unless...