Researchers detect fluoride in water with new test

The simple colour change test is the first to use artificial cell sensors to detect environmental contaminants.

Environmental contaminants like fluoride, lead and pesticides exist all around and even within us. While researchers have simple ways to measure concentrations of such pollutants inside lab environments, levels are much more challenging to test in the field. That’s because they require costly specialised equipment.

Recent efforts in synthetic biology have leveraged cellular biosensors to detect and report environmental contaminants in a cost-effective and field-deployable manner. Even as progress is being made, scientists have needed help to answer the question of protecting sensor components from substances that naturally exist in extracted samples.

A cross-disciplinary team of synthetic biologists at Northwestern University is developing a sensor platform that will be able to detect a range of environmental and biological targets in real-world samples. Using an established riboswitch to build a biosensor for fluoride, the team found they could protect the sensor and operate it similarly to how cells do by encapsulating the sensor inside a fatty membrane.

In a new paper published on January 4 in the journal Science Advances, researchers demonstrated that by modifying the makeup and penetrability of the lipid bilayer membrane, they could further tune and control their sensor performance.

“So much data is being generated, and a lot of it is being driven by health apps like smartwatches,” said Julius Lucks, a chemical and biological engineering professor at Northwestern’s McCormick School of Engineering. “We can sense our heartbeat, our temperature, but if you think about it, we have no way to sense chemical things. We live in an information age, but our information is minuscule. Chemical sensing opens enormous dimensions of information you can tap into.”

Fluoride colour change test is simple and efficient

Lucks’ lab has advanced the field’s understanding of molecular systems that respond to environmental changes by studying RNA and its role in cells. It also looks at how RNA is used by cells to sense changes in their environment; and how these concepts can be used within cell-free systems to monitor the environment for health and sustainability.

Cell-free synthetic biology, in which engineered biomolecular systems activate biological machinery rather than living cells, is compelling because it is efficient, versatile and low-cost. Lucks designed a riboswitch sensor using bacterial cell extracts to power gene expression reactions (including fluorescent RNA or protein that lights up in response to contaminants) that produce visual outputs cheaply and within minutes.

Neha Kamat, an assistant professor of biomedical engineering, initially met Lucks at their faculty orientation. She was interested in his desire to expand access to information. Kamat wondered if she could improve Lucks’ test tube system using a vesicle, a membrane with two layers.

“They’re using RNA and its associated machinery to sense molecules in real water samples and generate meaningful outputs,” Kamat said. “My lab works extensively with the lipids commonly used to encapsulate mRNA for drug delivery, to use these compartments to build more cell-like structures. We thought we could protect Julius’ switches and allow them to work in samples dirty with other contaminants like a cell can.”

Other researchers have tried to place a sensor inside a membrane. The switch stopped working correctly and produced a much smaller signal because it was challenging to fit everything within the small container and then scale it up. To overcome this, the team modified the genetic output in the sensor to amplify and colour it so it’s visible and “you don’t need a fancy detector to do it,” said Lucks.

New approach to fluoride tests can be used anywhere

Encapsulation and protection are essential to the sensor to make it function in native environments, like a wastewater channel with lots of other contaminants to erode the switch. This would be an example of “distributed sensing,” which could aid in fields from agriculture to human health.

The group came together more officially after receiving Northwestern’s Chemistry of Life Processes Institute’s (CLP) Cornew Innovation Award by pitching their “potentially disruptive” idea to the CLP’s advisory board. The team earned seed funding to get their idea off the ground.

Lucks calls this project a “jumping off point” from which they can embed sensors into more materials, including “smart” materials that can change properties, as in biology.

“As synthetic biologists, one of our major themes is identifying challenges and looking to nature,” Lucks said. “What is it doing already? Can we build off that and make it do more to meet our needs?”

Fluoride became an obvious choice because a natural RNA molecule senses it, allowing the team to design a more straightforward mechanism. But in the future, Kamat and Lucks have high ambitions about where the use of the sensors can expand.

For example, the sensors could flow through the human body to detect small molecules and biomarkers before the sensor is retrieved through urine or another passive method. It could also detect nitrate levels in soil and aid in monitoring run-off. Beyond that, Lucks and Kamat are excited to see uses within materials science, such as soft robotics, thinking about how to build something akin to a butterfly that smells through its feet.