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.
Such sensors could be used to detect environmental exposure to residential pesticides, monitor chemical markers of food spoilage, and could one day serve as sensors for certain toxic nerve gases such as sarin, according to Nicholas L. Abbott of the University of Wisconsin, Madison.
Techniques to detect such compounds already exist, but these methods, such as mass spectrometry, are mostly confined to the laboratory because they are 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,’ said Abbott.
The sensor devised by Abbott and Rahul R. Shah of the 3M Corporation 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 they bond more strongly with that target chemical than they do with the liquid crystal.
Essentially, the target chemical forces its way in on the liquid crystal’s territory, moving 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 colour or brightness.
On a surface with carboxylic acid receptors exposure to a vapour 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 reported to be extremely sensitive – triggered by exposure to parts-per-billion vapour concentrations – and quick, taking only seconds to complete detection and to reset after being removed from exposure, said the 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 is also said to allow the sensor to tolerate non-targeted compounds, such as water, which can interfere with detection in other types of sensors, said 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.