Table 2.

Summary of colorimetric sensing of breath biomarkers.

Breath biomarkers	Clinical correlation	Related diseases	Sensing probes	Substrates	Humidity concerns
Oxygen (O2)	Oxygen consumption related to metabolism.	Oxygen uptake, metabolic rate.95	Lignin95	Paper95	The sensor is capable of selectively detecting oxygen in human breath at high humidity (100%).95
Carbone dioxide (CO2)	CO2 production related to metabolism.	COPD (chronic obstructive pulmonary disease), asthma,96 and capnography.46, 97	pH indicators, such as mcresol purple,98 cresol red,98 phenol red,46 and thymol blue.96	Hydrophobic plastic film (ethyl cellulose) incorporated with a phase-transfer agent (tetraoctyl ammonium hydroxide)46	The colorimetric CO2 sensor showed stable performance at 37°C and 100% humidity.46
Acetone	Breath acetone is correlated with blood glucose (BG) and blood ketone levels.99, 100	Diabetes99, 100	Resorcinol35, 101	Nafion membrane35, 101	The incorporated organic acids played an important role in mitigating the humidity interference to the Nafion membrane catalysts in humid breath environment.35
Nitric oxide (NO)	NO level is elevated in asthma with inflammatory.	Asthma102, 103	Cobalt tetraphenylpor phyrin (CoTPP)104 and phenylenediam ine derivatives.105	Polystyrene (PSt), ethylcellulose (EC), polycyclohexyl methacrylate (PCHMA),104 and microporous cellulose membrane.105	The hydrophobic polymer matrix significantly improved the humidity resistance.104 A Nafion tubing was integrated to avoid the humidity interference.105
Hydrogen sulfide (H2S)	An endogenous gaseous signaling molecule and potential therapeutic agent.106	Bad breath107	Lead(II) acetate,10 silver nanoparticles,108 and copper(II) complex.107	Polyacrylonitrile nanofibers,10 nylon membrane,108 and Prozorb substrate.107	Sensor can detect sub-ppm H2S gas in high relative humidity (~90% RH).10 The sensitivity of the sensor is retained in 50–96% RH, but decreased at low humidity level, e.g. 20% RH.108
Ammonia (NH3)	Associated with blood urea nitrogen	Renal failure109–111	pH indicators109,110	Hydrogel film (with the combination of polyvinyl alcohol, titanium oxide nanoparticles, and hydroquinone)109	The hydrogel film with the combination of other chemicals provides a versatile matrix to minimize the environmental interference.109
VOCs	Volatile organic compounds (VOCs) related to metabolic changes within cancer cells.	Lung cancer8,91, 92	Metalloporphyr ins, pH indicators, and dyes with large permanent dipoles.8, 91, 92	Poly(vinylidene fluoride) membrane92	Sensor is stable against humidity because of the high hydrophobicity of polyvinylidene fluoride.92