Researchers at the University of Massachusetts (UMass) Amherst report that they have developed a novel bioelectronic ammonia gas sensor. The sensor uses electric-charge-conducting protein nanowires derived from the bacterium Geobacter to provide biomaterials for electrical devices. The microbes grow hair-like protein filaments that work as nanoscale “wires” to transfer charges for their nourishment and to communicate with other bacteria.

The team published its study “Bioelectronic protein nanowire sensors for ammonia detection” in NanoResearch.

“Electronic sensors based on biomaterials can lead to novel green technologies that are low cost, renewable, and eco-friendly. Here we demonstrate bioelectronic ammonia sensors made from protein nanowires harvested from the microorganism Geobacter sulfurreducens. The nanowire sensor responds to a broad range of ammonia concentrations (10 to 106 ppb), which covers the range relevant for industrial, environmental, and biomedical applications,” write the investigators.

“The sensor also demonstrates high selectivity to ammonia compared to moisture and other common gases found in human breath. These results provide a proof-of-concept demonstration for developing protein nanowire based gas sensors for applications in industry, agriculture, environmental monitoring, and healthcare.”

First author and biomedical engineering doctoral student Alexander Smith, with his advisor Jun Yao, PhD, and Derek Lovley, PhD, say they designed this first sensor to measure ammonia because that gas is important to biomedicine, agriculture, and the environment. For example, in humans, ammonia on the breath may signal disease, while in poultry farming, the gas must be closely monitored and controlled for bird health and comfort and to avoid feed imbalances and production losses.

“This sensor allows you to do high-precision sensing; it’s much better than previous electronic sensors,” according to Yao. “Every time I do a new experiment, I’m pleasantly surprised. We didn’t expect them to work as well as they have. I really think they could have a real positive impact on the world,” adds Smith.

Smith notes that existing electronic sensors often have either limited or low sensitivity, and they are prone to interference from other gases. In addition to superior function and low cost, he adds, “our sensors are biodegradable, so they do not produce electronic waste, and they are produced sustainably by bacteria using renewable feedstocks without the need for toxic chemicals.”

Smith conducted the experiments over the past 18 months as part of his PhD work. It was known from Lovley’s earlier studies that the protein nanowires’ conductivity changed in response to pH of solution around the protein nanowires. This moved the researchers to test the idea that they could be highly responsive to molecule binding for biosensing. “If you expose them to a chemical, the properties change and you can measure the response,” points out Smith.

When he exposed the nanowires to ammonia, “the response was really noticeable and significant,” Smith says. “Early on, we found we could tune the sensors in a way that shows this significant response. They are really sensitive to ammonia and much less to other compounds, so the sensors can be very specific.”

Lovley adds, that the “very stable” nanowires last a long time, the sensor functions consistently and robustly after months of use and work so well “it is remarkable.”

Previously, the team has reported using protein nanowires to harvest energy from humidity and applying them as memristors for biological computing.

Smith, who calls himself “entrepreneurial,” won first place in UMass Amherst’s 2018 Innovation Challenge for the startup business plan for the company he formed with Yao and Lovley, e-Biologics. The researchers have followed up with a patent application, fundraising, business development and research and development plans.

“This work is the first proof-of-concept for the nanowire sensor,” says Smith. “Once we get back in the lab, we’ll develop sensors for other compounds. We are working on tuning them for an array of other compounds.”

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