By coating polymer nanotubes with proteins found in human taste buds, researchers in South Korea have made an electronic tongue that can taste bitter compounds (Nano Lett., DOI: 10.1021/nl3038147). The device senses two chemicals at femtomolar levels, making it 100,000 times more sensitive at detecting these molecules than previous devices.
Research groups around the world want to develop electronic taste sensors to replace human tasters in quality screening of foods and beverages, to reduce the cost and time of the process. To date, electronic tongues have relied on lipids, polymers, and dyes that can bind to certain taste molecules and then produce an electrical or color signal. However, these sensors are neither as sensitive nor as selective as the human tongue, says Tai Hyun Park, a chemical and biological engineering professor at Seoul National University. In particular, these devices cannot identify trace amounts of molecules in complex mixtures.
To make a better sensor, Park, Jyongsik Jang, and their colleagues used a natural taste receptor found on the human tongue. When certain bitter compounds bind to this protein on taste buds, the protein produces electrical signals that the brain can read.
The researchers anchored the proteins to nanotubes made of the conducting polymer polypyrrole through amine groups on the protein. Then they deposited the nanotubes on silver and platinum electrodes to make transistors.
The researchers tested their device’s response to four bitter compounds: phenylthiocarbamide, propylthiouracil, goitrin, and isothiocyanate. When these compounds bound to the protein-coated nanotubes, the researchers noted, the current through the transistors changed. For solutions of phenylthiocarbamide and propylthiouracil in buffer, the researchers could detect concentrations of 1 and 10 femtomolar, respectively. The device could sense goitrin and isothiocyanate, which are found in cruciferous vegetables, at picomolar concentrations in samples taken from vegetables such as cabbage, broccoli, and kale.
The team also tested the sensor’s response to mixtures of bitter, sweet, and umami (or savory) flavor molecules. The device responded only when the bitter compounds were present in the mixtures, even at femtomolar concentrations. Park says that the researchers are now trying to make sensors for sweet and umami tastes by using human taste receptors that respond to those flavors.
Adding biological taste receptors to an electronic taste sensor is novel, says Kiyoshi Toko of Kyushu University, in Japan, who developed the first electronic tongues, in 1990. The new sensor mimics the ability of biological tissue to pick out taste compounds from a mixture, says David R. Walt at Tufts University. But both Toko and Walt point out that the protein receptors could denature over time, making the device less sensitive. The experts say that the Korean team would have to make the protein receptors more stable before the device could find use in the food industry.