E. coli Testing: Using Nanotechnology and Liquid Droplet Tests to Detect Pathogens

The next big step, Dr. Banerjee says, is to convert the nanosensor technology into a chip device that will make it quick, easy, and inexpensive for consumers to determine outside of the lab if food or water is contaminated with pathogens. “That will take a bit more time and collaborations with engineers, but we believe the efforts will have long term benefits for the greater society,” she emphasizes. “Besides handy use in retail settings for grocery shoppers, potential global applications include food manufacturing and water analysis in developing countries.”

Liquid Droplet Test

Researchers at Massachusetts Institute of Technology (MIT) in Cambridge have developed a new test for E. coli based on a novel type of liquid droplet that can bind to bacterial proteins.

This interaction can be detected by either the naked eye or a smartphone, according to Timothy Swager, PhD, 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 faster and massively cheaper that traditional pathogen tests, with low entry costs,” he says.

In 2015, Dr. Swager’s lab developed a way to easily make complex droplets, including droplets called Janus emulsions. “These Janus droplets consist of two equally sized hemispheres, one made of a fluorocarbon and one made of a hydrocarbon,” Dr. Swager explains. “Fluorocarbon is denser than hydrocarbon, so when the droplets sit on a surface, the fluorocarbon half is always at the bottom.”

Two years later, Dr. Swager, his colleagues, and students 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,” Dr. Swager relates.

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 lectin proteins, which are found on the surface of some strains of E. coli,” Dr. Swager points out. “When E. coli is present, the droplets attach to the proteins and become clumped 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.”

Dr. Swager says his team is using the native molecular recognition that these pathogens use. “They recognize each other with these weak carbohydrate-lectin binding schemes,” he notes. “We took advantage of the droplets’ multivalency to increase the binding affinity, and this is something that is very different than what other sensors are using.”

To demonstrate how these droplets could be used for sensing, the MIT 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,” Dr. Swager relates.

The MIT team hopes to adapt its new technology into arrays of small wells, each containing droplets customized to detect a different pathogen and linked to a different QR code. “This could enable rapid, inexpensive detection of food contamination in most any venue using only a smartphone,” Dr. Swager emphasizes.

The MIT researchers are now working on optimizing the food sample preparation so they can be placed into the wells with the droplets. “We also plan to create droplets customized with more complex sugars that would bind to different bacterial proteins,” Dr. Swager says.

In their initial work, the team used a sugar that binds to a nonpathogenic type of E. coli, but they foresee adapting the sensor to other strains of E. coli and other pathogenic bacteria, Dr. Swager mentions. Another step would be to make really selective droplets to catch different bacteria, based on the sugar one puts on them.