Electron density of cobalt porphyrin complexes key to meat freshness sensor

Cheap chemical sensors that can detect compounds given off by rotting meat have for several years suggested a better way to monitor food freshness. Now, a US group has taken a step towards that goal through careful manipulation of the electron density of cobalt porphyrin complexes non-covalently bound to carbon nanotubes.

Use-by dates are notoriously conservative. They help to keep us safe, but strict adherence inevitably wastes a lot of perfectly good food. Waste might be greatly reduced with direct monitoring of the deterioration of meat via evolved gases, particularly biogenic amines (BAs), such as putrescine and cadaverine. Chemi-resistors sensitive to these BAs would give an electronic indication of the freshness. If they were sufficiently small and cheap, they could even be integrated into food packaging to give data in real time.

To test this idea, Timothy Swager and his group at MIT in Boston, US, made sensors from carbon nanotubes functionalised with various cobalt porphyrin complexes and tested them with ammonia, the simplest amine. They tested two ligands: meso-tetra-phenyl-porphyrinato (TPP) and the meso-tetrakis-(penta-fluorophenyl)-porphyrinato (TPFPP); two oxidation states, Co(II) and Co(III); and three counter-ions, ClO4-, BF4- and Cl-. The best results were obtained with the more electron-withdrawing ligand, TPP; one of the more weakly co-ordinating counter-ions, ClO4-; and the higher oxidation state of Co(III) – suggesting high sensitivity to BAs can be achieved with a low electron density Co centre.


The sensors consist of carbon nanotubes functionalised with cobalt porphyrin complexes

The sensors showed a detection limit below 0.5 ppm for ammonia. Additionally, they were able to distinctly differentiate ammonia from 14 other chemical vapours with a wide range of functional groups. To simulate real meat deterioration, the group tried putrescine and cadaverine, which the sensors responded to strongly. They then measured total volatile basic nitrogen (TVBN) readings from samples of cod, salmon, chicken and pork – some refrigerated, some not – over four days. The results matched well the expected deterioration for these foods.

‘The central challenge in chemical sensors is selectivity,’ says Swager. His group has created systems that are highly selective for amines by taking a more ‘chemical’ approach compared with others in the field, who have taken a more ‘instrumentation–device’ approach. ‘I believe that putting the technology in the materials holds the most promise,’ he adds.

Nanotechnology could provide powerful tools for the food production and distribution industries, says Fausto Gardini, a microbiologist at the University of Bologna, Italy, who has studied the production of BAs from various foods. But because of the ‘intrinsic characteristics’ of these industries, costs must be kept low for innovations to deliver commercial uptake. Further trials to assess economic sustainability, as well as tests with real food matrices, which are typically much more complex than model systems, would be needed. ‘The potential is high but there is a lot of work to do,’ he says.