A Nose for Sustainability

Electronic noses, also known as e-noses, are devices that mimic the olfactory system of humans and animals. These devices are designed to detect and identify complex molecules, similar to how the human nose detects and distinguishes these molecules to recognize scents. E-noses are typically made up of a combination of sensors, software, and artificial intelligence algorithms that work together to analyze and classify odors.

E-noses have a wide range of applications in various industries, including healthcare, food and beverage, and agriculture. Volatile organic compounds in the breath serve as indicators of conditions such as kidney disease and asthma. In the food and beverage industry, e-noses are used to analyze the aroma of products such as wine, coffee, and cheese to ensure consistency in quality and to detect any potential contamination. E-noses can also be used in agriculture to detect pests and diseases in crops, helping farmers take proactive measures to prevent crop damage.

To fabricate tiny sensors that are highly performant, researchers have been turning more and more to nanowire-based technologies. By attaching specific functional groups to the ends of the nanowires, it is possible to create sensors that bind very tightly to a molecule of interest. While this approach works very well, fabrication of these components poses many challenges and comes with a number of risks, as well.

The chemical processes traditionally used to attach functional groups to nanowires are complex, which makes it difficult to make these connections at scale. Moreover, synthetic nanowire fabrication, whether the are silicon nanowires or carbon nanotubes, require the use of highly toxic chemicals that raise serious sustainability concerns. As if that were not bad enough, carbon nanotubes are also toxic and carcinogenic.

Sure, accurate e-noses can do a lot of good, but considering the negative impacts of the fabrication process and the nanotubes themselves, are they doing more harm than good? We may not need to ask that question in the future, thanks to the work of researchers at the University of Massachusetts Amherst. They have developed a process to create non-toxic, electrically conductive nanowires. They also demonstrated that they can attach proteins to these nanowires that are able to bind molecules with a high level of specificity.

Rather than rely on an artificial process, the team leveraged the pilin-based protein nanowires that are naturally produced by a bacteria called Geobacter sulfurreducens. These nanowires have a diameter of three nanometers and are naturally conductive. This might be the perfect solution, except G. sulfurreducens needs very specific conditions to thrive, and is very difficult to grow in culture. o create a more robust platform, the researchers spliced the pilin gene into the genome of Escherichia coli, the most widespread bacteria in the world.

This only left the problem of attaching functional groups to the nanowires to bind molecules of interest. Since they were already splicing DNA into the E. coli genome, they decided that the best approach was to modify the inserted DNA to also encode a protein to serve as the functional group. By going in this direction, they were able to turn the bacterial cells into tiny factories by co-opting their normal protein synthesis machinery to produce exactly what they needed.

The genetically modified nanowires were built into sensors and put to the test in a series of validation experiments. It was found that they were 100 times more responsive to ammonia than other current approaches. Another sensor was created to detect acetic acid, and this was found to be four times more responsive than traditional methods.

These tests look like they were just the beginning. The researchers note that their methods could be used to recognize hundreds of different chemicals. Such a device could have a huge impact in healthcare and beyond.