A MODIFIED liquid-crystal display could quickly detect bioterror agents like anthrax or plague bacterium. And because it is based on such a widespread technology, its inventors say the detector could be cheap enough to use across large, vulnerable networks such as postal services.
Attacks such as the anthrax letters sent in the US mail in 2001 have made technologies for detecting bioagents a priority. To give adequate warning, a sensing system must rapidly detect very low concentrations of pathogens, but until now no detector has been able to combine speed with sensitivity. For instance, complex procedures such as amplifying DNA sequences can take hours.
Christopher Woolverton at Kent State University in Ohio wondered if he could harness the instant on-off response of liquid crystal displays to detect bugs much faster. After all, he reasoned, the numbers on a cellphone or calculator appear instantly because the liquid crystals rapidly change their orientation in response to the voltage when you press a button. Could pathogens also trigger this change of orientation, thus revealing their presence?
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They can indeed, Woolverton told a meeting of the American Society for Microbiology in New Orleans last month. He and his team have developed a liquid crystal detector in which the crystals change state in response to a specific pathogen. This changes the display from dark to light, indicating the pathogen’s presence.
Conventional LCDs are made of liquid crystal molecules sandwiched between two polarised glass plates whose polarisations are at right angles to each other. In their natural, twisted state, the liquid crystals rotate the light polarised by the first plate so that it can pass through the other, making the display light. But when a voltage is applied to the crystals, they untwist – leaving the polarisation of the light unchanged, so that the second plate blocks it.
But for their device, Woolverton’s team chose a type of liquid crystal that is naturally untwisted, so that the glass/crystal sandwich normally blocks light. He dosed this liquid crystal layer with antibodies that bind to a specific bacterium, Bacillus atrophaeus, a harmless relative of anthrax. When the team injected the bacteria into the crystals, the antibodies bound with them to form globular clumps that completely disrupted the ordered structure of the liquid crystal (see Diagram). This scattered and repolarised the light in different directions, allowing some of it through within seconds of adding the bacteria. The device is so sensitive it will give a signal with only four bacterial cells present.
Woolverton says the device could be adapted to detect different bacteria, such as the one that causes plague, simply by substituting the right antibodies. And if he can refine the technology, the device should be inexpensive to produce because LCDs are so cheap.
David Cullen, an expert in biosensing at Cranfield University in Bedfordshire, UK, says Woolverton’s challenge will be to turn the sensor into a device that will work in the field. He predicts that concentrating the bacteria and feeding them into the liquid crystal will not be easy. “How does one quickly get the sample into the liquid crystal layer,” he asks.
