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New technology could offer


Specialised droplets interact with bacteria and can be analysed using a smartphone


he foodborne pathogen Escherichia coli O157 causes an estimated

73,000 illnesses and 60 deaths every year in the USA. Better safety tests could help avoid some of the illnesses caused by this strain of E. coli and other harmful bacteria, according to MIT researchers who have come up with a possible new solution. Te new MIT test is based

on a novel type of liquid droplet that can bind to bacterial proteins. Tis interaction, which can be detected by either the naked eye or a smartphone, could offer a much faster and cheaper alternative to existing food safety tests. “It’s a brand new way to

do sensing,” says Timothy Swager, the John D. MacArthur Professor of Chemistry at MIT and the senior author of the study. “What we have here is something that can be massively cheaper, with low entry costs.”

Detecting bacteria Two years ago, Swager’s lab developed a way to easily make complex droplets, including droplets called Janus emulsions. Tese Janus droplets consist of two equally sized hemispheres, one made of a fluorocarbon and one made of a hydrocarbon. Fluorocarbon is denser than hydrocarbon, so when the droplets sit on a surface, the fluorocarbon half is always at the bottom. Te researchers decided to

explore using these droplets as sensors because of their unique optical properties. In their natural state, the Janus droplets are transparent when viewed from above, but they appear opaque if viewed from the side, because of the way that light bends as it travels through the droplets. To turn the droplets into sensors, the researchers designed a surfactant molecule containing mannose sugar to self-assemble at the hydrocarbon–water interface, which makes up the top half of the droplet surface. Tese molecules can bind to a protein called lectin, which is found on the surface of some strains of E. coli. When E. coli is present, the droplets attach to the proteins and become clumped together. Tis knocks the particles off balance, so

that light hitting them scatters in many directions, and the droplets become opaque when viewed from above. “We’re using the native

molecular recognition that these pathogens use. Tey recognise each other with these weak carbohydrate-lectin binding schemes.” Swager says. “We took advantage of the droplets’ multivalency to increase the binding affinity, and this is something that is very different to what other sensors are using.” To demonstrate how these

droplets could be used for sensing, the researchers placed them into a Petri dish atop a QR code that can be scanned with a smartphone. When E. coli are present, the droplets clump together and the QR code can’t be read.

Chad Mirkin, a professor of chemistry at Northwestern

University and director of the International Institute for Nanotechnology, described the particles as “a powerful new class of assays.” “Tey are elegantly simple but rely on clever new approaches to making and manipulating emulsions,” says Mirkin, who was not involved in the research. “Tis proof-of-concept demonstration in detecting foodborne pathogens is compelling, as they constitute a major class of analytes that defines an unmet need in the biosensor community.”

When E. coli is present, the droplets attach to the proteins and become clumped together

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