A colour-coded smart tag using nanorods could be used to tell if food is still fresh, according to researchers.
The tags contain tiny metallic nanorods that can have different colors: red, orange, yellow, green, blue and violet.
In one configuration red, or reddish orange, would mean fresh.
The color-changing food deterioration tags were presented at the 247th National Meeting & Exposition of the American Chemical Society (ACS).
The tags, which are about the size of a kernel of corn, would appear in various color codes on packaging.
Although the nanorods are made of gold and silver, a tag would still be very inexpensive, and all the chemicals cost much less than one cent, claim the researchers.
Chao Zhang, Ph.D., the lead researcher of the study, said the tag has a gel-like consistency and can be programmed to mimic almost all ambient-temperature deterioration processes in foods.
"Over time, the tag changes its color to orange, yellow and later green, which indicates the food is spoiled," he said.
The technique has been patented in China and preliminary results have been published in ACS Nano .
Wageningen UR – head of the EU FUSIONS project - is part of a group which is nearing commercial production of a small chip to go into packaging to measure the actual deterioration rate of food.
The colors signify a range between 100% fresh and 100% spoiled.
If the label says that the product should remain fresh for 14 days under refrigeration, but the tag is orange, it means that the product is only roughly half as fresh.
This allows the consumer to know the product is edible for another seven days if kept refrigerated, Zhang explained.
“The gold nanorods we used are inherently red, which dictates the initial tag color," he said.
"Silver chloride and vitamin C are also in the tags, reacting slowly and controllably. Over time, the metallic silver gradually deposits on each gold nanorod, forming a silver shell layer.
“That changes the particle's chemical composition and shape, so the tag color now would be different.
“Therefore, as the silver layer thickens over time, the tag color evolves from the initial red to orange, yellow, and green, and even blue and violet."
The researchers developed and tested the tags using E.coli in milk as a reference model.
"We successfully synchronized, at multiple temperatures, the chemical evolution process in the smart tag with microbial growth processes in the milk,” said Zhang.
He added that the next step is to contact manufacturers and explain how the tag would be useful for them and their customers.
The study was supported by National Natural Science Foundation of China, Hong Kong Research Grants Council and National Basic Research Program of China.