Microbial bioelectronic sensors use living bacteria that can create an electrical signal in response to the presence of a target substance, or analyte. These types of sensors offer many advantages over other types of biosensors based on proteins and enzymes: The bacteria can perform multiple functions, survive in a variety of environments and even grow and regenerate for potential long-term use.
However, building devices using living bacteria poses several challenges. The mediators some bacteria use to send and receive electrons, creating the electric signal, can be swept away from the sensor by liquid environments researchers would want to monitor, like wastewater. Some mediators are toxic to humans or the environment. Rice University researcher Rafael Verduzco developed a safe bioelectronic sensor that allows for effective electronic communication even in liquid environments. The study was recently published in the journal Advanced Materials.
"This system uses a naturally occurring polymer chitosan, which is found in the hard outer shells of crustaceans. In our system, the chitosan also acts kind of like a shell to keep the bacteria from escaping. It is also modified to have anchor points the mediators can attach to, which are critical to transport electrons," said Verduzco, corresponding author on the paper and the A.J. Hartsook Professor of Chemical and Biomolecular Engineering. "This material provides a flexible way to encapsulate the bacteria and enhance electronic signals. Since it's based on a low-cost and renewable polymer, we think it has great potential for real-world applications."
To arrive at this system, doctoral student and first author Xinyuan Zuo had the idea to develop a hydrogel that traps the bacteria near the electrode. Hydrogels are soft, flexible gels that are made from a mixture of porous and permeable solids. The hydrogel allows the media, with its potential analytes, to flow in and around the trapped bacteria without releasing bacteria into the liquid. However, the researchers also needed a way to transport electrons between the bacteria and the electrode.
"We realized we could solve both of those problems by using a redox-active polymer, which would allow us to effectively capture the electrical signal from the bacteria and pass it along to the electrode," Zuo said.
A redox-active polymer, here used as an anchor point for the mediators, is a material that can both accept an electron from another source and pass the electron on. This redox property allows electrons to flow along the polymers until they reach the electrode and are read as electricity. The researchers developed a redox polymer based on chitosan, a versatile biopolymer that can be used to make a hydrogel and can also be modified to attach mediators for transporting electrons. By attaching these redox mediators and forming a hydrogel around the bacteria, they produced a living, biohybrid electronic material. They found that this living hydrogel could generate a stable electronic current when placed on an electrode.
The researchers built a version that tested milk for the presence of sakacin P, an antimicrobial often used as a preservative. To produce the signal, they used an engineered version of L. plantarum, a probiotic bacteria commonly found in fermented dairy foods, that would produce a small amount of electricity when it came into contact with sakacin P. The hydrogel, with its bacteria, was attached to an electrode and placed into milk. After just a few hours, the electrode registered an electrical signal — the bacteria were responding to the presence of sakacin P in the milk. Like the polymer, L. plantarum is safe for use in the environment and in foods, opening up a wide variety of uses. And the hydrogel can be used with other bacteria as well.
"There are a large number of bacteria that are electroactive, and therefore there's huge potential for these types of living microbial devices," Verduzco said. "This hydrogel provides a way to communicate electronically with these bacteria, enabling living bioelectronic devices that can be used for sensing, chemical production or isolation or destruction of harmful chemicals."
This work was supported by the Army Research Office (W911NF-22-1-0239), the Cancer Prevention and Research Institute of Texas (RR190063), the National Science Foundation (EFMA-2226374) and the Welch Foundation for Chemical Research (C-2124).
