## 4 Discussion
Bromocresol green was chosen among 17 potential indicators for the development of this vaporochromic sensor due to its superior performance against the analytes tested. However, all possible dye/target combinations were not tested; thus, it is possible that higher sensitivity or faster response times could be obtained by dyes or target chemicals that were not tested herein. For example, in 2021, another vapor sensor was designed specifically for the detection of ammonia gas and was able to obtain an LOD of 2.0 ppb\({}_{v}\) with a reaction time of 3 min [23]; however, unlike the current developed cross-contamination sensor, it was found to not be reusable. Moreover, [PERSON] and [PERSON] (2018) also created a colorimetric ammonia gas sensor with an LOD near 10 ppm and gave a color change for ammonia within 10 s when concentrations were over 20 ppm [37]. Again, this color change was irreversible. More recently, in 2023, [PERSON] et al. developed a fluorescent ammonia gas sensor to monitor chicken freshness in real time but could only monitor the fluorescent signal every 24 h [38]. In general, there have been many studies that have worked to build sensors capable of detecting ammonia for their own different purposes that may or may not also be applicable to our described application; however, the sensor described in this paper was designed solely as a prototype tool for the canine handler community which required a low-cost, robust, and reusable sensor. This developed sensor was designed as a general alert system for canine handlers to become aware of when cross-contamination may be occurring within their training materials. In its application, the target chemical is included inside the closed vessel and stored with the same containment parameters as the other items within the container. The vaporochromic sensor is also included in the container alongside the target chemical. In theory, the containment of the stored materials may fail in extreme circumstances involving high temperatures or rough handling. This would induce a color change in the sensor as a response to the vapor of the target chemical, and thereby alert the end-user that their materials have leaked due to a potential containment failure. It is only intended to give a color response when the chosen method of containment (i.e., mason jars, plastic bag, etc.) fails uniformly due to a systematic event. The sensor was not designed for specific detection of the training aid odors themselves. Thus, this vaporochromic sensor will not alert if the containment for one of the training aids is defective, but the rest are functional. Overall, it was observed that the cases manufactured by Ray Allen were not compatible with this sensor and its intended application, while the Nanuk and Pelican brands (the latter of which is more widely used) were. Additionally, the sensor must be stored in the dark to prevent any degradation of the product's effectiveness. The sensors were also found to be functional in high-temperature environments (relative to ambient car cabin temperatures). Future directions of this study should expand the performance testing of the sensor and involve testing the sensor in other commonly used containers such as odor-proof bags, training aid delivery devices (TADDs), aluminum bags, metal containers, plastic bags, etc. There is also room for improvement in the end-user means of determining the sensor response. The final color response may be interpreted differently by each individual due to bias, colorblindness, or other reasons. As such, future studies should also look to create a computer/phone interface that would allow for a more unbiased determination of the color response to help overcome these difficulties and give definitive notification of cross-contamination.