Colorimetric array indicator for NH3 and CO2 detection

NH3 and CO2 are often involved in the spoilage of foods. In this study, a colorimetric array indicator was evaluated for the detection of NH3 and CO2 using pH-sensitive dyes (dimethyl yellow, methyl red, chlorophenol red, methyl orange, phenol red, thymol blue, m-cresol purple) encapsulated within ethylcellulose matrices. The pH dyes were doped either with hydrochloric acid or tetrabutylammonium hydroxide to manipulate the colorimetric responses. The sensing dyes were applied onto a plastic substrate to produce array indicators consisting of 4 × 4 elements. Digital images of the indicator array, before and after the exposure to NH3, CO2 or NH3/CO2 mixtures, were taken to monitor the change in color for each of the array elements. Unique ΔRGB patterns for NH3, CO2, and their mixture were observed. At 4 °C, principal component analysis discriminated samples into two clusters that were higher and lower than 15 μg/mL NH3 respectively. For CO2, at both 4 and 25 °C, the array indicator discriminated air samples with lower than 5% CO2 from those higher than 20% CO2. For NH3/CO2 mixtures, the array indicator differentiated air samples of high NH3/low CO2 and low NH3/high CO2 combinations. These results show that the array indicator may be useful for intelligent packaging application of food products.