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Streamlining Food Safety Testing with Rapid Test Kits

Graphene-based sensors have the potential to simplify testing for foodborne pathogens

by
Andrea Tolu

Andrea Tolu is a freelance writer and ghostwriter for the food and life sciences industries. He can be reached at andrea@andreatolu.com.

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For food manufacturers, detecting foodborne pathogens in products and processing environments is a process that requires laboratory equipment and trained technicians. This may soon change, thanks to rapid test kits that target Salmonella, Listeria, and E. coli—the three foodborne pathogens that cause most recalls—not only detecting their presence but also measuring their concentration.

“The sensors for these tests work similarly to a glucose monitor where, with the help of enzymes, glucose generates an electrical signal, which is then converted into a numeric value indicating its concentration,” explains Jonathan Claussen, PhD, associate professor of mechanical engineering at Iowa State University, who is developing a prototype of these sensors with the help of an interdisciplinary team of postdocs, graduate, and undergraduate students.

The advantage of these tests is that they could be done right at the food processing facility without needing to send samples to a laboratory and wait for the results. The sample preparation process designed by Claussen and his team doesn’t involve any plate counting or DNA isolation: “It would be similar to preparing a sample for a COVID-19 test, although the volume would be larger, about ten milliliters,” says Claussen. “If the swab is taken directly from the surface of food processing equipment, the kit would come with pre-loaded solvent and preservatives. But it would also be possible to sample carcass rinse in a meat processing facility directly and with no solvent at all.”

To convert chemical impulses into electrical ones, different biorecognition agents are used: “These could be enzymes, like in glucose sensors, DNA molecules, or antibodies,” says Claussen. “Some of these agents are more selective than others towards pathogens. Certain antibodies, for example, have evolved to bind to specific strains of Salmonella or Listeria. By using the right biorecognition agents, we can make tests suitable for different pathogens.”

The challenge of low-cost electrodes

The biorecognition agent for the pathogen is applied directly on electrodes made with graphene, a nanomaterial consisting of a monolayer of carbon atoms held together by very strong bonds in a hexagonal arrangement. What makes graphene ideal for the sensor's electrodes is its high electrical conductivity, much higher than copper. But its properties don't end there: it's also highly thermally conductive, extremely resistant, flexible, and light. (An example often used to give an idea of its strength is that an invisible hammock made with graphene could hold a cat.)

After graphene was first isolated from graphite in 2004 by Andre Geim and Konstantin Novoselov (who both won the Nobel Prize six years later for that discovery), it was hailed as a revolutionary material with potential groundbreaking applications in aviation, medicine, and electronics. Twenty years later, however, the graphene revolution hasn’t happened yet. What held it back was mainly the high cost of producing it.

What makes graphene ideal for the sensor's electrodes is its high electrical conductivity, much higher than copper. But its properties don't end there: it's also highly thermally conductive, extremely resistant, flexible, and light.”

The typical way to synthesize graphene, Claussen explains, is to mine graphite and use chemical exfoliation to extract carbon layers from it. However, the process can be expensive and harsh on the environment. “We’re currently working with some researchers on a more environmentally friendly method to use bio-derived graphene from prairie grasses, in particular Miscanthus, which is very common in the middle part of the United States and has the right lignin-to-cellulose ratio,” he says.

The result of this synthesis process, explains Claussen, are graphene flakes, which are then mixed in a solution with a binder and solvents. The final step is to add the solution to an inkjet printer or—the more scalable options—a screen or a graver printer and print the electrodes. 

“Printing the electrodes in a continuous layer allows us to achieve the circuit design that we need,” says Claussen. “It also prevents them from flaking and dispersing graphene nanoparticles into the environment.”

An alternative approach that Claussen’s team is experimenting with is synthesizing graphene from polyamide, an insulating material commonly used in electronics: “With this technique, we heat up polyamide with a carbon dioxide laser—the type commonly found in mechanical workshops—and convert carbon atoms into graphene through a process called carbon hybridization. Instead of printing the electrodes, we etch them out of the polymer in the pattern that we want. After that, we cut them and add the biorecognition agent.”

Positive defects

As Claussen explains, these alternative ways to obtain graphene are lower cost than those of conventional precious metals used in electrochemical sensors such as gold or platinum. However, what also helps make these production techniques cost-effective is that the graphene doesn’t need to be high-grade to provide the desired conductivity for the electrodes.

“If you’re trying to use graphene to make a transistor or a computer chip, you might need a pristine monolayer, but we don't need that level of precision for our sensors to work,” says Claussen. “Some of the graphene flakes we obtain are single layer, while some might be double or triple layer. They’re all connected through a binder, so they’re still very electrically conductive.”

Indeed, a less-than-perfect monolayer of carbon atoms is beneficial: “For the type of electrochemical sensing we’re doing, it’s actually more advantageous to have a less pristine graphene. By using multi-layered graphene, we can adjust different elements, such as the surface wettability or the number of breaks in carbon bonds,” says Claussen. “The surface we end up with has a micro or nanostructure with a lot of roughness. While that would normally be considered a defect, it actually improves the charge transport from the liquid sample to the surface of the graphene, giving us a sensor with lower detection limits for the pathogen that we're trying to measure.”

The advantage of these tests is that they could be done right at the food processing facility without needing to send samples to a laboratory and wait for the results.

Claussen’s goal is not just to develop a proof of concept for graphene-based sensors but to bring them to market: “Our sensors are more amenable towards scaling up, which puts us slightly more ahead of the game towards commercialization. Before they can enter the market, however, they will have to be tested and approved by a third-party agency. Overall, we're probably looking at three to five years before they’re fully approved,” says Claussen.

In the meantime, Claussen and his team are working on improving usability: “We’re trying to make testing protocols clear, looking at every detail, such as how to prepare the sample, how to shake the vial, and for how long. If you are a researcher, you generally don't think about these things, but if your goal is to make the test commercially viable, you need to make it easy to use and understandable; otherwise, there could be noncompliance issues or false positives.”

Food safety and beyond

This is not Claussen’s first breakthrough with this nanomaterial. In 2020 he co-authored a paper that described how graphene electrodes coupled with antibodies were able to detect histamine in fish down to 3.41 parts per million, a much lower concentration of 50 parts per million I set by the FDA. 

However, the scope of Claussen’s research is not limited to food safety: “Graphene is really easy to functionalize, meaning we have a wide choice of biorecognition agents we can put onto it. This allows us to use it for a lot of different agricultural biosensors going into farm fields to monitor and map out nitrogen, fertilizer ions, or pesticide concentrations as they change with heavy rain events or with the different compositions of soil. This way, we can give farmers tools to know where they can use less of these chemicals. In the biomedical health space, we also work on applications for cancer diagnostics and on wearable noninvasive devices to monitor hydration or fatigue levels through analytes in the sweat or saliva.”