Angew.Chem.Int.Ed.http://doi.org/f27ccx(2015)

Credit: © 2015 WILEY

For the consumer, ignoring a food's sell-by date can result in a supermarket bargain or an upset stomach, but monitoring food spoilage is a serious issue for the industry. As well as preventing unfit goods entering the market, identification of which items are spoilt also allows food to be sold that might otherwise have been needlessly thrown away. A key identifier of spoilt fish and meat is the generation of particularly odorous molecules, such as putrescine and cadaverine, through bacterial decomposition. A host of spectroscopic methods are able to detect these biogenic amines, but most require sophisticated equipment and a trained technician. In contrast, 'chemiresistors' can quantitatively indicate the presence of certain gases simply through an induced change in their electrical conductivity.

Now, Timothy Swager and colleagues from Massachusetts Institute of Technology, have fabricated a chemiresistive carbon nanotube–metalloporphyrin composite that is able to detect biogenic amines at sub-ppm concentrations. Chemiresistivity is a well-known property of carbon nanotubes, but here Swager and colleagues significantly increased their sensitivity by non-covalently functionalizing them, with electron deficient cobalt porphyrins — such species adhere well to the walls of carbon nanotubes and are known to have an affinity for amines. It seems that binding of amines to the cobalt centre elicits an increase in the resistivity of the underlying carbon nanotube, which is indicative of the level of exposure.

The team successfully demonstrated that they could monitor the spoilage of samples of salmon, cod, chicken and pork left out of the fridge over a four-day period. Stability under ambient conditions for an extended period of time, coupled with low-cost and high portability, renders these devices promising for commercial implementation. For example, by integrating the devices into food packaging, the foods viability could be quickly read by an automated process, without having to open the container or see the food.