An intelligent cellulose pad loaded with copper nanoparticles for real-time freshness monitoring of beef

This study aimed to develop a colorimetric indicator using copper nanoparticles (CuNPs) to identify the spoilage of packaged beef. CuNPs-based colorimetric indicators were prepared with cellulose papers impregnated with CuNPs. The stability test showed that the color parameters of the CuNPs-based indicator had no significant color changes during 30 d of storage. The prepared indicator had an excellent and selective sensitivity to H₂S gas produced during beef spoilage. The prepared indicator was used in the beef package to monitor its shelf life. The CuNPs-based colorimetric indicator showed noticeable color changes from white to brown due to the reaction of CuNPs with sulfide gases and the production of Cu₂S and CuS compounds, confirmed by XPS analysis. The pH and total viable count results indicated a perfect correlation between the colorimetric indicator and the qualitative parameters determining spoilage. The results of the color change of the indicator used in the damaged package showed that not only was the color change of the indicator irreversible, but it also clearly showed the stage of spoilage of the beef during its storage. The CuNPs-based colorimetric indicator provides a promising approach in intelligent packaging systems of meat products to monitor their freshness with the naked eye.