Researchers develop new technique for rapid food testing

The new safety test for foodborne pathogens is based on a novel type of liquid droplet that can bind to bacterial proteins

The new test could be read on a smartphone. Image Jose-Luis Olivares/MIT (droplet images courtesy of Qifan Zhang)

Researchers at MIT are working on a test based on a novel type of liquid droplet that can bind to bacterial proteins. Discovered by manipulating emulsions, the droplets interact in a way that can be detected either by the naked eye or a smartphone, which could be developed into a 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.”

Qifan Zhang, an MIT graduate student, is the lead author of the paper1, which appears in the journal ACS Central Science. Other authors are Suchol Savagatrup, an MIT postdoc; Peter Seeberger, director of the Max Planck Institute of Colloids and Interfaces in Germany; and Paulina Kaplonek, a graduate student at the Max Planck Institute.

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

In their natural state, the Janus droplets are transparent when viewed from above. However, they appear opaque if viewed from the side because of the way that light bends as it travels through the droplets. Their unique optical properties led the researchers to explore using these droplets as sensors.

Janus droplets viewed from above (left). After the droplets encounter their target, a bacterial protein, they clump together (right). Image courtesy of Qifan Zhang/MIT

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. These 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 clump together. This 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. They 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 on top of a two-dimensional barcode that can be scanned with a smartphone. When E. coli bacteria are present, the droplets clump together and the code cannot 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.”

“They are elegantly simple but rely on clever new approaches to making and manipulating emulsions,” says Mirkin, who was not involved in the research. “This 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.”

Rapid inexpensive testing

Current food safety testing often involves placing food samples in a culture dish to see if harmful bacterial colonies form, but that process takes two to three days. More rapid techniques based on bacterial DNA amplification or antibody-bacteria interactions are expensive and require special instruments.

The MIT team hopes to adapt its new technology into arrays of small wells, each containing droplets customised to detect a different pathogen and linked to a different two-dimensional barcode. This could enable rapid, inexpensive detection of contamination using only a smartphone.

“The great advantage of our device is you don’t need specialised instruments and technical training in order to do this,” Zhang says. “That can enable people from the factory, before shipping the food, to scan and test it to make sure it is safe.”

The researchers are now working on optimising the food sample preparation so they can be placed into the wells with the droplets. They also plan to create droplets customised with more complex sugars that would bind to different bacterial proteins. In this paper, the researchers used a sugar that binds to a nonpathogenic type of E. coli, but they expect that they could adapt the sensor to other bacterial strains.

They hope to launch a company to commercialise the technology within the next year and a half.

1. Q. Zhang, S. Savagatrup, P. Kaplonek, et al, ACS Cent. Sci., 2017, 3 (4), 309–313