Sensor that sniffs like a dog

Scientists in Korea have developed a biosensor for assessing food quality that mimics the way receptors in a canine nose respond to smells.

Tai Hyun Park and Seunghun Hong and colleagues from Seoul National University made a device that detects hexanal - a volatile compound produced when food is past its best.

A current method to detect compounds emitted by oxidised food is chromatography, but it isn’t portable and it involves a complicated pre-treatment process. To overcome these obstacles, scientists have moved on to semiconductor-based, olfactory cell-based and olfactory receptor protein-based sensors, but they are not as capable as an animal olfactory system in terms of selectivity and sensitivity.

In an animal’s nose, specific odorants bind to the corresponding olfactory receptors with high selectivity, generating a chemical signal. That signal is converted into an electrical signal and then amplified through a pathway in an olfactory sensory neuron (a nerve cell that processes and transmits information by electrical and chemical signalling). 

To mimic this process, Park and Hong’s device consists of nanovesicles made from cells expressing canine olfactory receptors specific for hexanal. The team immobilised the vesicles onto a carbon nanotube transistor (CNT) sensor and added a drop of solution containing Ca²⁺ ions. Then, they applied an electrical current to the transistor. When they introduced hexanal to the device, the hexanal bound to the vesicles, causing an influx of Ca²⁺ into the vesicles. ’This increases the nanovesicles’ potential in the vicinity of the CNT,’ says Hong. ’Since the conductance of a CNT channel is affected by the potential, we can detect hexanal by measuring the conductance change of the channel.’ 

The team tested the device with spoiled milk and found that the conductance changed measurably and the response increased as more days went by. They also found that the sensor could detect hexanal down to 1fM, even when it was mixed with its analogues pentanal, heptanal and octanal.

’I find the result obtained on real food samples quite exciting,’ says Jasmina Vidic, who works on bioelectronic nose devices at the National Institute of Agricultural Research in Jouy-en-Josas, France. She adds that using microelectrodes with immobilised whole cells expressing an odorant receptor is not a new idea,’but instead of using whole cells, Park and Hong have disintegrated cells expressing the canine receptor and immobilised them on functionalised nanotubes. This has certainly increased the sensibility of the sensor.’  

Hong’s future plan is to develop a complete bioelectronic nose by fabricating an array of sensors with diverse olfactory nanovesicles in a single chip. He says that another potential use for the sensor is to screen for lung cancer using blood samples. However, he points out that ’many molecules in the blood may affect the conductance of CNT so reducing the effect of non-specific binding to CNT is a challenge’.

Elinor Richards