A hydrogel made from crustacean shells lets common food bacteria generate electrical signals 15 times more efficiently, opening the door to reusable living sensors for food safety.
(Nanowerk Spotlight) Microbial bioelectronics aims to harness living bacteria as functional components in devices that generate power, detect chemicals, or synthesize valuable compounds, with obvious appeal for environmental monitoring and medical diagnostics. But the field faces a stubborn constraint: almost all research depends on just two bacterial species.
Shewanella oneidensis and Geobacter sulfurreducens dominate because they can transfer electrons efficiently to external surfaces. Nearly every microbial fuel cell, biosensor, and bioelectrochemical reactor in the scientific literature relies on one of these organisms. Both, however, metabolize limited food sources, tolerate environmental stress poorly, and inhabit ecological niches far removed from settings where bioelectronic devices might prove most valuable: hospitals, food processing plants, agricultural fields, the human gut. Thousands of bacterial species with more desirable traits remain essentially locked out of the technology.
Gram-positive bacteria represent an enormous untapped resource. Many carry regulatory approval for use in food and medicine. Lactiplantibacillus plantarum, a cornerstone of the fermented food industry, exemplifies the potential: it thrives in diverse environments, colonizes human and plant microbiomes, and poses no safety concerns. But Gram-positive bacteria share a fundamental structural barrier, a thick cell wall that wraps each organism like insulation around a wire, blocking electron transfer to electrodes.
Scientists have attempted workarounds using small redox-active molecules that accept electrons from bacteria and ferry them to electrodes. These molecular intermediaries function adequately, but they leak into surrounding environments, wash away during operation, and often poison the cells they’re meant to assist.
The hybrid material combines chitosan, a biocompatible polymer derived from crustacean shells and already widely used in biomedical applications, with naphthoquinone molecules chemically bonded to the polymer backbone. This modified chitosan forms a hydrogel that physically traps bacteria near an electrode surface while tethered quinone groups mediate electron transfer. Because the mediators remain fixed in place, they create a stable interface between living cells and electronics without escaping into the environment.
The synthesis required only mild conditions. The team grafted naphthoquinone units onto chitosan in acetic acid solution, achieving roughly 49% substitution, meaning nearly half of chitosan’s repeating molecular units carried a quinone group. Adding sodium tripolyphosphate cross-linked the polymer chains into a soft, water-swollen gel with a stiffness of approximately 10³ pascals.
Schematic of the NQ-Chit (naphthoquinone units grafted onto chitosan) hydrogel encapsulating L. plantarum. Quinone groups on the NQ-Chit hydrogel mediate electron transport from L. plantarum to the electrode. By immobilizing microorganisms and redox mediators near the electrode, the NQ-Chit hydrogel enhances current flux, decreases environmental impact, and enables reuse and biosensing. (Image: Reproduced with permission from Wiley-VCH Verlag)
To construct living devices, the scientists mixed this polymer solution with L. plantarum cells, deposited the mixture onto carbon felt electrodes, and triggered cross-linking on the spot. Bacteria became embedded within a three-dimensional redox-active matrix pressed against the electrode surface.
The performance gains were substantial. Electrochemical measurements revealed that the quinone-chitosan hydrogel containing L. plantarum generated a peak current density of 588 mA m⁻², settling to approximately 307 mA m⁻² over 48 hours. This represented a 15.6-fold increase compared to bacteria encapsulated in plain chitosan without quinone groups.
Encapsulation mattered as much as chemistry. When researchers coated electrodes with quinone-chitosan but left bacteria floating freely in surrounding liquid, current density dropped to just 43.55 mA m⁻². Trapping cells within the gel improved contact between bacteria, mediator groups, and electrode, and all three components require proximity for efficient electron transfer.
The system sustained continuous current for at least nine days while maintaining bacterial viability comparable to unencapsulated cells. An unexpected bonus emerged: quinone molecules typically degrade under repeated oxidation-reduction cycles, but living bacteria appeared to stabilize them. The cells continuously converted oxidized quinones back to their reduced form before degradation reactions could proceed, effectively recycling the mediators.
The team identified a bacterial enzyme called NADH:quinone oxidoreductase as the driver of electron transfer. When they tested a strain lacking this enzyme, current dropped dramatically. The relationship between quinone concentration and current output followed Michaelis-Menten kinetics, the same mathematical pattern that describes how enzymes process their target molecules. This confirmed that bacterial metabolism, not electron movement through the polymer, limited overall performance.
Containment proved equally impressive. After 48 hours, bacterial leakage into surrounding liquid dropped by two to five orders of magnitude compared to unencapsulated systems. Electron microscopy revealed dense bacterial populations confined within the hydrogel matrix.
Repeated use posed no problem. Over four complete medium replacements during 57 hours of testing, encapsulated systems maintained stable output. Control systems using free-floating mediator molecules lost performance with each exchange as the small compounds washed away.
The platform demonstrated broad compatibility across different organisms and chemistries. Two additional bacterial species showed enhanced electron transfer when encapsulated: Lactococcus lactis, another food-safe Gram-positive organism, and the Gram-negative model bacterium Escherichia coli. A different quinone variant, 1,4-naphthoquinone, grafted onto chitosan performed comparably to the original formulation.
A proof-of-concept biosensor illustrated why this matters for real-world applications. The researchers engineered L. plantarum to activate its electron-transfer machinery only when exposed to sakacin P, an antimicrobial peptide used as a food preservative. Encapsulated in the hydrogel and immersed in milk samples, the modified bacteria detected sakacin P within 1.7 hours with 95% confidence, converting a chemical signal into an electrical readout. Because genetic circuits can be reprogrammed to respond to different targets, this approach could enable sensors for pathogens, toxins, or other substances relevant to food safety.
This work removes several barriers that have constrained microbial bioelectronics. Tethering mediator molecules to a polymer network eliminates contamination concerns and enables reuse. Encapsulating bacteria in a biocompatible gel maximizes contact with both mediators and electrodes while preserving cell health. The modular architecture, allowing interchangeable bacteria, interchangeable quinones, and programmable genetic circuits, opens applications in environmental monitoring, food safety testing, and sustainable chemical production.
By demonstrating that safe, food-grade bacteria can reliably interface with electronic systems, this research points toward a future where microbial bioelectronics no longer depends on a handful of specialist organisms.
For authors and communications departmentsclick to open
Lay summary
Prefilled posts
Nanowerk Newsletter
Get our Nanotechnology Spotlight updates to your inbox!
Thank you!
You have successfully joined our subscriber list.
Become a Spotlight guest author! Join our large and growing group of guest contributors. Have you just published a scientific paper or have other exciting developments to share with the nanotechnology community? Here is how to publish on nanowerk.com.
