Issue Date: July 25, 2011
The (Synthetic) Nose Knows
By tethering mouse proteins responsible for detecting odor molecules to carbon nanotube transistors, researchers have built a synthetic system that can “smell” chemical vapors (ACS Nano, DOI: 10.1021/nn200489j).
The researchers, led by physics professor A. T. Charlie Johnson of the University of Pennsylvania, are the first “to demonstrate that reconstituted olfactory receptors can be used to recognize molecules and transduce their presence on nanotube sensors,” says Michael S. Strano, a chemical engineering professor at MIT who develops nanosensors. “What’s exciting is that these receptors are membrane proteins and typically very difficult to place correctly at a sensor interface.”
Johnson and coworkers embedded the olfactory receptors in mimics of cell membranes before attaching them to the nanotubes. They exposed the devices to vapor streams containing different odorant molecules and measured the current through the nanotube transistor. The signal disappeared when the odorants were removed.
Johnson and coworkers used two types of membrane mimics, the surfactant digitonin and lipid-protein particles known as nanodiscs. Both systems responded comparably to vapors, but the nanodiscs extended the sensors’ shelf life. The digitonin sensors worked for about five days, but the nanodisc devices remained stable for more than two months.
Because membrane proteins are difficult to express and purify, Johnson and his team used only three mouse olfactory receptors. “If you want something like a nose in biology, you need a couple hundred different olfactory receptor proteins,” Johnson says.
Olfactory receptors are a subset of a larger class of membrane proteins known as G-protein-coupled receptors (GPCRs). Johnson would like to develop sensors that can convert the binding of other GPCRs, many of which are drug targets, into signals.
“I suspect the method would be generic to this entire receptor class,” says David R. Walt, a chemistry professor at Tufts University who also develops chemical sensors. Still, he says, “there remains a lot of work to be done before one can expect such systems to be stable for longer periods of time and to achieve the sensitivity of the mammalian nose.”
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