Self-Assembled Protein Nanofibril-Liquid Metal Gallium Biocomposites

Introduction:
Liquid metal gallium (Ga) has unique properties like low toxicity, fluidity, and high electrical conductivity, making it promising for advanced materials. However, the integration of Ga with biomacromolecules remains underexplored, presenting a gap in advancing these composites in various applications. Recent studies show that various proteins can self-assemble into β-sheet-rich nanofibrils, which impart materials with enhanced tensile strength, continuity, and stability, making them ideal for biocomposites synthesis.
Here, we develop a composite film integrating Ga droplets with self-assembled protein nanofibrils. Soy protein isolate (SPI) is induced to undergo self-assembly with Ga droplets via sonication, creating a biocomposite with both high conductivity and strong mechanical properties. The improved electron transfer-ability of the biocomposite enhances the gas-sensing responsivity compared to only Ga droplets or only nanofibrils. Moreover, the electrical conductivity provided by sintered Ga droplets imparts the protein-based film with electrical stimulation capability, promoting the release of Ga ions and thereby enhancing its antibacterial effects.
Results:
We synthesize SPI-Ga biocomposite films by adopting in situ self-assembly of SPI nanofibrils with an aqueous dispersion of liquid Ga droplets. During the sonication process, the SPI unfolds and partially hydrolyzes, allowing it more accessible for forming new intermolecular interactions. Upon cooling, these SPI particles subsequently self-assemble into nanofibrils around Ga droplets. Transmission electron microscopy confirms that SPI nanofibrils tightly enveloped the Ga droplets, while X-ray photoelectron spectroscopy shows a significant reduction in Ga oxidation within the SPI matrix compared to Ga alone. Importantly, Fourier-transform infrared spectroscopy and circular dichroism analyses indicate that the β-sheet conformation of SPI nanofibrils remains intact with 32 wt.% Ga droplets.
The SPI-Ga biocomposites exhibit significantly improved conductivity, reaching 3.20 × 10^-2 S/m with 32 wt% Ga content, whereas pure SPI films or Ga droplets with oxide layers are non-conductive. This enhancement is due to the reduced oxidation of Ga facilitated by the self-assembled SPI nanofibrils and the interconnected conductive pathways formed by the Ga droplets. The mechanical properties of the biocomposite are also improved due to the high strength of the β-sheet-rich structure in the SPI nanofibrils.
Inspired by heme-based carbon monoxide (CO) sensors, we hypothesize that Ga droplets, with their accessible redox-active surfaces, can serve as efficient gas-reactive materials when combined with SPI nanofibrils. Our experiment confirms that the SPI-Ga biocomposite films have enhanced, reversible, and selective responses to CO compared to pure SPI or Ga films. The ordered alignment of SPI nanofibrils facilitates efficient electron transfer between the embedded Ga droplets and CO molecules, boosting sensitivity and stability for CO detection.
Bacterial adhesion remains a persistent challenge for protein-based materials used in biocompatible coatings. We evaluate the antibacterial activity of our SPI-Ga films under electrical stimulation. Colony-forming unit assays show that the SPI-Ga biocomposite effectively reduces the proliferation of Escherichia coli by over 99% with applied electrical stimulation. This antibacterial efficacy is attributed to the increased concentration of Ga ions in the solution, as confirmed by inductively coupled plasma mass spectrometry.
Overall, this study introduces possibilities for utilizing liquid metals with self-assembled protein nanofibrils to create functional biocomposite materials with gas sensing and antibacterial properties and provides fundamental insights into the interaction between liquid metal entities and proteins.