Liquid crystals formed from molecules weakly tethered to a nanotextured surface could form the basis of highly sensitive, wearable sensors to detect personal exposure to certain synthetic organic chemicals, researchers report in the 17 August 2001 issue of the journal Science. Such sensors could be used to detect environmental exposure to residential pesticides, monitor chemical markers of food spoilage, and could possibly serve some day as sensors for certain toxic nerve gases such as sarin, according to Science co-author Nicholas L. Abbott of the University of Wisconsin, Madison.
Techniques to detect such compounds already exist, but these methods (like mass spectrometry) are mostly confined to the laboratory--too bulky and complex to provide real-time, portable detection.
"These lab methods are extremely sensitive, but they're never going to be the basis for measuring personal exposure. We wanted to create something that could be worn like a badge, like those worn to detect radiation," says Abbott.
The sensor devised by Abbott and co-author Rahul R. Shah of the 3M Corporation (formerly of University of Wisconsin) consists of an ultrathin gold film with nanoscale corrugation. The surface of the gold film is then dotted with protruding chemical receptors that weakly anchor liquid crystal in a well-defined orientation along the film's surface.
When these receptors are exposed to the specific chemical that is the object of detection, however, they bond more strongly with that target chemical than they do with the liquid crystal. In effect, the target chemical muscles in on the liquid crystal's territory, shoving it away from the receptors. This displaces the liquid crystal into a new orientation that is controlled by the underlying surface texture, and the new orientation is visible to the naked eye as a change in the sensor's color or brightness.
On a surface with carboxylic acid receptors, for instance, exposure to a vapor of the chemical hexylamine caused the liquid crystal to shift from an orientation perpendicular to the gold film's corrugations to an orientation that was parallel with the corrugations. The two orientations are visually distinct, report Shah and Abbott.
The sensors are extremely sensitive--triggered by exposure to parts-per-billion vapor concentrations--and quick, taking only seconds to complete detection and to reset after being removed from exposure, say the Science researchers.
By pairing particular liquid crystals and receptors with specific chemical properties, the sensors can be tailored to detect specific compounds. Multiple receptor/liquid crystal combinations can be patterned over the sensor's surface, allowing simultaneous detection of several different chemicals.
The "competitive binding" mechanism also allows the sensor to tolerate non-targeted compounds, such as water, which can interfere with detection in other types of sensors, says Abbott. In this case, the non-target forms an even weaker bond with the receptor than the liquid crystal, and is unable to dislodge it.
Shah and Abbott's sensors can be used to measure cumulative exposure over time, by measuring the spread of the targeted chemical across the liquid crystal, and the sensor surfaces can also be designed to trigger an instantaneous response upon exposure.
The researchers hope that their sensors will prove useful in food safety applications, such as monitoring levels of putrescine and cadaverine, the smelly compounds produced by rotting fish and meat, as well as sensors to detect exposure to pesticides such as diazinon and parathion.
This research was supported in part by the National Science Foundation and the Office of Naval Research.
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