Smart color-changing paper packaging sensors with pH sensitive chromophores based on azo-anthraquinone reactive dyes

In this paper, two novel pH sensitive reactive dyes with intramolecular color matching based on the combination of anthraquinone and azo chromophores were design and synthesized. The incorporation of azo chromophore into anthraquinone structures enlarged visible absorption wavelength. Smart color-changing paper packaging sensors with pH sensitive chromophores were developed by printing patterns. Two reactive dyes on paper cellulose fibers could not be dissolved by environmental solvents due to covalent bonds. The printed patterns showed an obvious, sensitive and timely color change from green to purple for the paper printed with D-1, and from red to green for the paper printed with D-2 under different pH conditions. The maximum wavelengths of the color-changing samples shifted from 460 nm to 520 nm for D-1 and from 502 nm to 420 nm for D-2. The freshness experiment of cooked crabs indicated that cooked fresh crabs began to deteriorate after 1.5 h, and completely deteriorated after 4 h. The results showed that the printed patterns as the smart color-changing packaging sensors could be used in freshness judgment of the food. The color-changing mechanism of the printed patterns was revealed by 1H NMR and FT-IR spectra. The smart packaging sensors have potential application in the rapid identification of food freshness during delivery or before eating.