Novel copper-based metal–organic skeleton smart tags that respond to ammonia for real-time visual freshness monitoring of shrimp

In recent years, an amount of attention has been paid to natural pigment-based pH/ammonia-responsive colorimetric smart labels in the field of real-time visual food freshness monitoring. However, the drawbacks of natural pigments, such as complicated extraction processes, poor photostability, and easy inactivation, have severely limited their application in smart packaging. In this research, natural pigments were replaced with copper-based metal–organic frameworks (Cu-MOF) to serve as ammonia-responsive indicators. It attained the benefits of excellent mechanical qualities, high photostability, and greenness. The changes of ΔE values of the smart label composite films of starch/polyvinyl alcohol (PVA)/Cu-MOF were less than 4.8 after being placed in a constant temperature and humidity environment for 40 d, indicating that they have excellent color stability. Within an atmosphere of 0.8 mM ammonia, the composite film can produce a significant color change from teal green to deep blue in 1 min, indicating that the film has ammonia sensitivity. The composite film, in addition, successfully monitored the changes in shrimp freshness at temperatures of 4 °C and 28 °C, effectively prolonging the shelf life of the shrimp. Therefore, starch/ PVA/Cu-MOF composite films have good prospects for smart and active packaging applications.