Humidity-activated ammonia sensor with excellent selectivity for exhaled breath analysis

Recent studies have reported the use of hydrogels as the sensitive material for gas-sensing applications such as NO2, NH3, CO2, and O2.

“Recent studies have reported the use of hydrogels as the sensitive material for gas-sensing applications, such as NO2, NH3, CO2, and O2”

At high humidity, the hydrophilic hydrogel-sensitive membrane adsorbs more water and is activated; water self-ionizes into H+ and OH−, with some H+ combining with water to form H3O+ under the grotto mechanism, and the membrane exhibits ionic conductivity.

“At high-humidity values (Fig. 7i), the hydrophilic hydrogel-sensitive membrane adsorbs more water molecules and is activated. The water molecules are self-ionized into H+ and OH−, with some of the H+ ions combining with other water molecules to form H3O+ under the grotto mechanism (Eq. 1). In addition, the sensitive membrane exhibits ionic conductivity.”

At low humidity (10-50% RH), PAA/GA hydrogel-sensitive film had no proton-conducting channels and its impedance did not change significantly even with NH3 introduction.

“At low-humidity values (10–50% RH; Fig. 7g), no proton-conducting channels were noted in the hydrogel-sensitive film, and its impedance did not change significantly even with the introduction of NH3”

At moderate humidity, hydrophilic groups in the hydrogel absorb water molecules to form a hydrogen bonding network and proton conduction pathways; introduced NH3 combines with water molecules through hydrogen bonding and is adsorbed on the sensitive membrane, affecting device impedance.

“At moderate humidity values (Fig. 7h), hydrophilic groups, such as carboxyl, amino, and amide groups, absorb water molecules to form a hydrogen bonding network and proton conduction pathways. The introduced NH3 can combine with water molecules through hydrogen bonding, which are adsorbed on the sensitive membrane, thereby affecting the impedance of the device”

When NH3 is introduced at high humidity, the hydrogel-sensitive membrane takes up a large amount of NH3 by acid-base adsorption; H2O reacts with dissolved NH3 to form NH4+, and carboxyl groups in GA react with NH3 to form carboxylate and ionize into NH4+ and RCOO−, decreasing resistance and increasing conductivity.

“When NH3 is introduced, the hydrogel-sensitive membrane takes up a large amount of NH3 by acid–base adsorption. On the one hand, H2O reacts with the dissolved NH3 to form NH4+ under a proton transfer reaction (Eq. 2). Furthermore, the carboxyl group in GA reacts with NH3 to form carboxylate and ionizes into NH4+ and RCOO− in the liquid phase (Eq. 3). Under the combined effect of multiple”

The PAA/GA hydrogel has better sensing characteristics for NH3 in high humidity environments, especially its selectivity.

“The PAA/GA hydrogel has better sensing characteristics for NH3 in high humidity environments, especially its selectivity (Fig. 9b).”

The sensor was able to accurately identify the presence of 5 ppm NH3 in exhaled gas, indicating potential for healthcare applications.

“In addition, they made a test on human exhalation, the sensor was able to accurately identify the presence of 5 ppm NH3 in the exhaled gas, which means that the sensor has the potential to be used in healthcare applications.”

The sensor has a response of 9.2 to 50 ppm NH3 at room temperature with 80% RH.

“The sensor has a response of 9.2 to 50 ppm NH3 at room temperature with 80% RH.”

Liu et al. [72] developed an environmentally friendly and non-toxic biomass hydrogel-based NH3 sensor employing PAA and GA as sensing materials based on a humidity-activated mechanism at room temperature.

“Liu et al. [72] developed an environmentally friendly and non-toxic biomass hydrogel-based NH3 sensor employing PAA and GA as sensing materials based on a humidity-activated mechanism at room temperature.”