A biohybrid system combines polymer nanosheets with E. coli to improve hydrogen production by capturing near-infrared light from the solar spectrum.
(Nanowerk Spotlight) The search for viable solar hydrogen production has repeatedly run up against the same set of limitations: narrow spectral absorption, poor quantum efficiencies, and the incompatibility of many promising materials with biological systems. Traditional approaches, including the use of photosynthetic microbes or inorganic semiconductor hybrids, have achieved only modest success due to their inability to harness the full solar spectrum. One particular gap has been the underuse of near-infrared (NIR) light, which accounts for a substantial portion of solar radiation but remains largely inaccessible to conventional systems.
Using bacteria for hydrogen (H₂) production is relevant because microbial systems offer unique advantages over purely chemical or photophysical methods. Bacteria operate under mild, energy-efficient conditions, often at room temperature and ambient pressure, avoiding the intensive heat or electricity required by conventional hydrogen production. They serve as self-replicating catalysts, capable of sustaining their activity without constant material replacement.
Many species can metabolize low-value or waste organic feedstocks, turning agricultural runoff, wastewater, or glycerol into usable energy. This dual function—energy conversion and waste remediation—offers a compelling route for sustainable hydrogen production.
Bacteria also possess flexible and tunable metabolic networks that can support various redox processes. These can be redirected, either through genetic engineering or by interfacing with light-absorbing materials, to favor hydrogen-producing pathways. Altogether, microbial systems present a versatile, scalable, and potentially carbon-neutral platform for solar-to-hydrogen conversion.
Certain species, including Escherichia coli, possess native metabolic pathways that can generate hydrogen, though usually in small quantities. These biological processes can be enhanced or redirected by interfacing bacteria with external materials that deliver photo-generated electrons. As such, bacterial systems provide a flexible, scalable, and potentially sustainable route to hydrogen generation, especially when integrated into light-responsive platforms.
Metal-based semiconductors such as cadmium sulfide or copper oxides have been used as light harvesters in hybrid systems, but they often suffer from toxicity and limited spectral range. Their application poses risks to microbial viability and environmental sustainability, making them less suitable for large-scale biological integration. Efforts to expand spectral response and minimize cytotoxicity have led to interest in organic alternatives, particularly conjugated polymers.
These organic semiconductors offer tunable optical properties, chemical versatility, and compatibility with living cells. While conjugated polymers have been studied in other contexts such as bioimaging and drug delivery, their use as photosensitizers in microbial hydrogen production remains underexplored.
Researchers from the University of Science and Technology of China have developed a semi-artificial photosynthetic system that addresses these challenges by combining non-photosynthetic, non-genetically modified Escherichia coli with a custom-designed conjugated polymer. The polymer, called PyTT-tBAL-HAB, is built from phenazine-based units known for their redox activity. When exfoliated into two-dimensional nanosheets, PyTT-tBAL-HAB exhibits a narrow bandgap of 1.16 electronvolts, allowing it to absorb light efficiently in the NIR range. Its metal-free nature and chemical structure support high biocompatibility, enabling stable integration with E. coli without compromising cell health.
Pseudocolor-enhanced SEM image of the photosynthetic biohybrid deposited on the AAO template (scale bar: 1 μm). E. coli were colored in red, and the nanosheet was colored in cyan. (Image: Adapte with permission from Wiley-VCH Verlag)
Because both the nanosheets and bacterial surfaces carry negative charges, direct assembly was initially hindered by electrostatic repulsion. To overcome this, researchers modified the bacterial membrane using polyethyleneimine (PEI), a positively charged polymer. This adjustment reversed the surface charge of E. coli, allowing the negatively charged PyTT-tBAL-HAB nanosheets to attach securely. The resulting hybrid formed through spontaneous self-assembly and remained viable over extended periods, suggesting that the interface is stable and minimally disruptive to cell physiology.
Under simulated solar light, the hybrid system demonstrated enhanced hydrogen production compared to unmodified bacteria. In tests under visible light, hydrogen generation increased by 1.89 times. When exposed to NIR light, production rose by a factor of 1.96. The hybrid achieved a quantum efficiency of 18.36% at 940 nanometers, a wavelength previously inaccessible to similar systems. Importantly, this performance was not driven by thermal effects, as temperature controls ruled out photothermal contributions. The increased hydrogen yield stemmed directly from improved photoelectron generation and transfer.
Mechanistic investigations confirmed that the PyTT-tBAL-HAB nanosheets serve as effective photosensitizers. Photoluminescence measurements showed quenching of emission signals in the hybrid, indicating that electrons generated by light exposure were rapidly transferred to the bacterial cells rather than lost through recombination. Time-resolved spectroscopy revealed shortened carrier lifetimes in the hybrid compared to the standalone polymer, further supporting efficient electron injection. Electrochemical impedance testing showed reduced charge-transfer resistance, highlighting the improved interface conductivity between the polymer and bacterial membranes.
The biological response to this integration was also examined. Gene expression analysis using RNA sequencing revealed that exposure to the nanosheets led to the upregulation of pathways related to hydrogen metabolism, including those encoding hydrogenase enzymes and redox-active cofactors. Specifically, genes associated with glycolysis and pyruvate formate lyase pathways showed increased activity, indicating a metabolic shift toward enhanced formate production and subsequent hydrogen evolution. At the same time, competing pathways involved in lactate synthesis and long-chain fatty acid metabolism were downregulated. This redistribution of cellular resources helped direct more energy and electrons toward hydrogen output.
The system demonstrated stability over multiple days of operation, maintaining consistent hydrogen production and structural integrity. The polymer nanosheets resisted degradation under light exposure and in the presence of bacterial enzymes, which is essential for continuous operation. Furthermore, bacterial viability remained high, suggesting that the polymer provided a protective barrier without interfering with essential cellular functions.
By facilitating NIR light absorption, efficient charge transfer, and metabolic reprogramming without relying on genetic modification or toxic components, this study presents a significant step toward practical solar-driven hydrogen production in biohybrid systems. The approach combines precise materials design with a biological interface strategy that preserves cell function while enhancing energy conversion. As the field moves toward sustainable and decentralized hydrogen sources, systems like this could help bridge the gap between laboratory demonstrations and real-world implementation.
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