Humidity-Assisted Highly Selective Ammonia Sensors Based on Nonconjugated Proton-Conductive Polymers
Authors: Zhao Hongran , Liu Lichao , Dai Jianxun , Fei Teng , Zhang Tong 
Published: May 2020

Introduction
Analysis of exhaled human breath has great significance for the early non-invasive diagnosis [1]. At present, gas chromatography and mass spectrometry, which can accurately analyze gas composition, are the most common methods used in exhaled breath analysis. However, these methods remain high cost, complex and require the preconcentration of the exhaled breath; thus, the advantages of exhaled breath analysis in medical diagnostics have yet to be realized [2]. Gas sensors are considered as the most promising candidate for exhaled breath analysis due to the advantages of high sensitivity, fast response, low-cost and integratable [3]. However, exhaled breath is a complex environment that is highly humidified (~100% relative humidity (RH)) and is composed of thousands of gases [4]. Poor selectivity and strong humidity influence are two bottlenecks which limit the application of recent gas sensors in exhaled breath analysis.
In this work, instead of seeking to avoid the influence of high humidity, a novel method, which utilizes the conductivity change caused by the adsorption, dissolution, ionization and migration processes of ammonia in water to build a proton-conductive gas sensor, is presented for realizing highly sensitive and selective ammonia detection in a humid atmosphere. The mechanism is similar to the humidity sensing of impedance-type devices based on an ionogel. Base on this mechanism, water becomes the functional material that absorbs and ionizes ammonia. To detect trace-level ammonia, a miniscule water layer is required; however, such a water layer is unstable. Hence, a sensitive film that can collect water molecules from the atmosphere and create an aqueous solution-like environment is needed. Compared to the majority of rigid materials, flexible polymers are good at providing a stable aqueous solution-like environment. In the field of gas sensors, semiconductor materials such as metal oxides, conjugated polymers and reduced graphene oxide present clear responses to most of active gases via catalytic redox reactions, which dose not promote a high selectivity. Hence, nonconjugated hydrophilic polymers, which exhibit an excellent capacity for water adsorption and conduct only via the proton conductive channel formed by the adsorbed water, were considered to be the optimal choice to build a highly selective poroton-conductive ammonia sensor of ammonia in a humid atmosphere. This method provides a new potential approach for the analysis of ammonia in exhaled breath.

References

[1] L. Pauling, A. Robinson, R. Teranishi, P. Cary, Quantitative analysis of urine vapor and breath by gas-liquid partition chromatography, Proc. Natl. Acad. Sci. USA 68 (1971), 2374–2376. doi: 10.1073/pnas.68.10.2374
[2] B. Buszewski, M. Kesy, T. Ligor, A. Amann, Human exhaled air analytics: biomarkers of diseases. Biomed. Chromatogr, 21 (2007), 553–566. doi: 10.1002/bmc.835
[3] M. Righettoni, A. Tricoli, S. Pratsinis, Si:WO3 sensors for highly selective detection of acetone for easy diagnosis of diabetes by breath analysis, Anal. Chem. 82 (2010), 3581–3587. doi: 10.1021/ac902695n
[4] A. Jones, Effects of temperature and humidity of inhaled air on the concentration of ethanol in a man's exhaled breath, Clin. Sci. 63 (1982), 441–445. doi: 10.1042/cs0630441
Figure 1 (a) The dynamic current-time curves of PVA, PVP and PAM sensors during the process of sensors switch back and forth between dry air (11% RH) and humid air (97% RH) with increasing concentration of ammonia. (b) Selectivity of the PVP sensor in 97% RH. (c) The response characteristic of the PVP sensor to normal exhaled breath and exhaled breath with additional ammonia (n = 5).

Figure 1