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Liquid crystals are the core of new, relatively inexpensive sensors that are engineered on a nanoscale to provide a sensitive approach for detecting microbial pathogens or environmental toxins. Readout is rapid and sidesteps the need to culture microorganisms or to rely on additional instruments. A prototype device is being evaluated for its ability to detect West Nile virus, according to researchers at the University of Wisconsin (UW) in Madison, and a nearby company, who are developing and commercializing this technology.

            The unexpected response of liquid crystals to microorganisms and other biological molecules is the critical component of this new device, according to UW chemical engineer Nick Abbott. Members of his lab group discovered that various entities, including microorganisms, can reorient liquid crystals to change their appearance, similar to what happens in a computer screen when they are subject to changes in electric fields.

            To pursue this phenomenon, Abbott began collaborating with UW virologist Barbara Israel and veterinary ophthalmologist Chris Murphy. In April 2000, they formed a new company, Platypus Technologies, to harness liquid crystals in analytic devices. “Our technology involves highly specialized surfaces to detect target molecules in complex solutions,” says Israel, who also serves as chief operating officer of the company. The company name was chosen to allude to the natural sensitivity of the platypus bill, which contains specialized receptors for detecting prey in muddy waters.

            The development team finds that a variety of materials can be used to construct liquid crystal sensors. Originally, they coated glass slides with gold films that are engineered to contain nanoscale channels and a layer of receptor molecules, such as antibodies. Liquid crystals align with the receptors along nanoscale surface features. Then, when a target, such as a microbe, specifically binds the receptors, it displaces the liquid crystals and changes their appearance--such as making them look darker. “It takes only seconds for the alignment to change when the target is present,” Israel says. Recognizing that change requires no special training or equipment.

            Subsequently, the team at Platyplus learned how to substitute inexpensive polyurethane for costly gold film to serve as a substrate on which to embed the antibody-liquid crystal complexes. “Polyurethane is very easily molded with defined nanoscale features,” Israel says. However, skill is required to prepare, or “functionalize,” the polyurethane surface with an attached receptor in a way that allows liquid crystals to align in an orderly fashion.

            The prototype liquid crystal sensor device now being developed is designed for detecting West Nile virus, and is being supported in part through a small business innovative research grant from the National Institute of Allergy and Infectious Diseases. One such sensor is being designed to detect the virus itself, while another will detect antibodies that infected individuals produce in response to viral infections. The latter detector provides a means of estimating the prevalence of infections among particular population groups, including humans, birds, and horses.

            Current methods for detecting this virus rely on an enzyme-linked immunosorbent assay (ELISA)-format test, which uses serum samples from potentially infected individuals and relies on species-specific secondary antibodies to identify virus-positive samples. Considering that more than 150 bird species in North America now carry West Nile virus, obtaining species-specific secondary antibodies to survey different bird species becomes impractical. “These reagents just aren't available, and laboratories must resort to time-consuming plaque-reduction neutralization assays,” Israel says. However, she adds, “We can detect antibodies to West Nile virus regardless of the species of the host animal.”

            In a collaborative effort with investigators from the National Wildlife Health Center in Madison, the sensor is being evaluated through tests of specimens from virus-infected crows. So far, the sensor is proving resistant to nonspecific binding.

            Similar liquid crystal devices could be adapted to monitoring environmental toxins or other pathogens, including those that cause food poisoning or might be used as biowarfare agents. A 1- by 2-inch surface can be functionalized to detect 20 different agents. Platypus is also designing liquid crystal sensors to detect the pathogen responsible for plague, with support from the U.S. Army, and agricultural pesticides, with funding from the National Institute of Environmental Health Sciences.

 

Carol Potera

Carol Potera is a freelance writer in Great Falls, Mont.

 

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