In another study by Krishnan et al.

In another study by Krishnan et al. To detect and measure ammonia gas from exhaled breath VOCs in renal patients, an array of hydrogel-based colorimetric sensors was performed, wherein they created a new hydrogel thin film as a substrate, via incorporating PVA with TiO₂ nanoparticles and hydroquinone HQ, in addition to sol-gel formulated pH indicator sensor array for the real-time quantification and increase the response to ammonia gas. After the production of sol-gel formulations, they are cast in an array on a hydrogel film. For fabrication of the hydrogel film, commercially available PET transparent films through using O2 plasma treatment have been used as a substrate. also in this study, glutaraldehyde was added as a crosslinker to prevent swelling and shrinkage of the hydrogel. Then TiO₂ and HQ nanoparticles were added to the hydrogel at optimized concentrations to increase the resulting color change. 9 dyes of pH indicator have been used which were created through sol-gel technique and eventually drop cast on the hydrogel. It was determined that TiO₂ NP and HQ also improved the sensitivity of the colorimetric sensor array. in this study, Ammonia can be detected with a detection limit of 0. 3 ppm, which is much lower than the allowable limit. It was also found that within the range of 0. 5 to 10 ppm there's a linear relationship for ammonia. This approach minimizes the effect of environmental factors and succeeds in detecting various concentrations of ammonia along with other relevant breath analytes[350].