A group at the University of Alberta is using a unique thermo-sensitive polymer to create devices that could one day provide quick, visual detection of disease biomarkers.

Michael Serpe, a chemistry professor at U of A, has been working with poly (N-isopropylacrylamide) (pNIPAm) since graduate school. “You can imagine it as a spaghetti strand floating in water,” says Serpe. “When you heat it up, it transitions into a very dense globular structure, a ball.” But temperature isn’t the only way to achieve this conformational change. During his PhD, Serpe showed that positively charged linear polymers can interact with pNIPAm-based microgels and cause them to contract. The more polymer you add, the smaller the microgel gets.

A few years ago, Serpe’s group discovered that by sandwiching pNIPAm-based microgels between two layers of reflective material — in their case, gold — you can create an optical device called an etalon that reflects only certain frequencies of light. As the microgel layer swells and shrinks, the changing distance between the mirrors alters the colour of reflected light. And since the amount of swelling and shrinking depends on the concentration of charged polymer added, the exact colour observed is a measure of how much polymer is there.

In a recently published paper in Biosensors and Bioelectronics, the team exploited these effects for biosensing. They developed a charged linear polymer that binds to streptavidin, a common protein. A set amount of this polymer is mixed with a sample of streptavidin, after which the bound complexes are separated from solution. The leftover, unbound polymer is then added to the etalons, where the corresponding colour change indicates how much polymer is left, and in turn, how much streptavidin there was to bind with. “We’re now expanding this, looking at detecting biomarkers for disease as well as specific sequences of DNA,” says Serpe. “I think we’re going to make a big impact.”