March 2, 2023 | Researchers at University of Massachusetts, Amherst, have created bacteria-generated sensing devices capable of identifying real-time changes in various chemical concentrations. They published their work in Biosensors and Bioelectronics (DOI: 10.1016/j.bios.2023.115147).
Nanowires—nano-scale filaments—have been used to fabricate electronic sensors for years, many created from silicon, carbon, gold, and other materials. But carbon and silicon nanowire fabrication is technically complex, expensive, and includes toxic steps. Derek Lovley, senior author on the paper, and his group have been exploring protein nanowires generated by microorganisms as a more sustainable, biodegradable alternative.
“The protein nanowires are a natural product of a microbe known as Geobacter that lives in soil and mud, and it uses them to make electrical connections to other microbes or to minerals,” Lovley explained to Diagnostics World. “In the last couple of years, we’ve been developing those nanowires as a sustainable electronic material, and we’ve had a couple previous studies using those nanowires to fabricate devices that, for example, can produce electricity from the humidity in atmosphere or the sweat from your skin to power wearable sensors.”
Geobacter’s pilin protein nanowires measure only 3 nm in diameter, and simple changes to the pilin gene sequence can modify the structure of the nanowire. In this case, the team modified the pilin gene to add peptides to enable selective sensing of analytes and introduced the modified Geobacter gene to grow the nanowires on an E. Coli substrate.
“Geobacter is an interesting microbe, but it’s finicky,” Lovley said. “It’s an anaerobe; it grows slowly. E. Coli can be grown at the benchtop and is used for mass production of different commodities.” Modifying the pilin gene meant the team could naturally incorporate specific peptides. “We don’t have to do any post-production fabrication. It’s naturally part of the wire,” he said.
The researchers specifically focused on detecting ammonia and acetic acid analytes, which are present in the breath of patients with kidney disease and asthma, respectively, so they added peptides to the nanowires that would bind to either ammonia or acetic acid. They chose these targets for the proof-of-concept study, Lovley said, because the peptides to bind ammonia and acetic acid were already identified.
Once the nanowires were produced, a network of wires was added to an electronic sensor, spanning the distance between two electrodes. The two electrodes were attached to a semiconductor characterization system and one volt of current was passed across the electrodes along with a steady stream of air which included various gasses to detect. The sensor compared the current generated when just air passed by to the difference in voltage when various gasses wafted by.
And it worked. They found that current output increased as ammonia concentration in the air increased. “We saw pretty close to a linear response with the concentration, and it got down to quite a low detection limit,” Lovley said.
The system proved about 100 times more sensitive than controls created with nanowires with no ammonia peptides added, and the effect was fleeting—a good thing. The ammonia blowing across the sensor was bound, changed the current, and was quickly released. In fact, the sensor response was stable for the 30-day evaluation.
“The response to ammonia was rapid and the electrical signal quickly returned to baseline as the air flow flushed the ammonia from the sensing chamber,” the authors wrote. “Thus, the sensor is capable of detecting dynamic changes in ammonia concentrations over time.” Other gasses added to the air flow—ethanol and acetone—created no change in the current, demonstrating a specificity to ammonia.
The acetic acid sensing device—set up the same way—generated results four times more sensitive than unmodified nanowires. “Although the relative increase in current output achieved with the acetic acid ligand modification was smaller than that with the ammonia-specific ligand, the results do further demonstrate that nanowires can be customized to improve sensor response,” the authors write.
In fact, both sensors—for ammonia and acetic acid—performed better than previous nanowire sensors fabricated out of various materials including gold, silicon, carbon, and more. This is very good news for the pilin-based nanowires, the authors say.
They expect that microbially-produced nanowires could be created to specifically detect many different gases of biomedical, environmental, or practical importance, and those individual wire sensors could be combined.
“It’s just a question of designing the wire for each thing,” Lovley said. “These wires are so small you [can] pack a lot of different wires into one very small sensor. You could monitor a whole suite of health-related parameters with a device with a combination of different types of wires in them.”
Lovley also sees a future for the sensor to detect non-volatile substances. “We do believe we could expand this, now, onto soluble molecules: for example, having a wearable patch that detects metabolites that show up in sweat,” he said.