[Image above] Scheme of silk fibroin (SF) assembly on highly oriented pyrolytic graphite (HOPG) characterized by in-situ atomic force microscopy (AFM). Credit: Shi et al., Science Advances (CC BY 4.0)
Silk, a natural protein-based material used for thousands of years as a luxurious fabric, is becoming a promising material for next-generation flexible electronics and transistors for hybrid applications.
Natural silk fibers consist of two main protein components: fibroin (72–81%) and sericin (19–28%). The fibroin component has excellent biocompatibility and biodegradability, along with other attractive properties including high optical transmittance, mechanical durability, lightweight, and ease of processing.
However, natural fibroin has a disordered structure, resembling spaghetti, which makes it unable to modulate electronic signals uniformly or accurately. Fortunately, by depositing fibroin on surfaces with which the protein has some affinity, studies have shown that unique, highly ordered 2D phases of fibroin can form.
In a recent open-access study, researchers led by Pacific Northwest National Laboratory investigated the possibility of growing ordered fibroin films on graphene, a van der Waals material.
Van der Waals materials consist of two-dimensional layers held together by weak intermolecular forces. This structure allows for easy exfoliation and stacking of different 2D materials to form heterostructures with unique properties.
Researchers have extensively investigated the use of van der Waals materials for thin-film electronics. The possibility of using silk to improve film performance has been briefly explored, but more research is needed to develop strategies that “address challenges associated with the mismatch between the soft SF [silk fibroin] proteins and the hard, planar substrates,” the authors of the recent open-access paper write.
In their study, the authors dissolved lyophilized fibroin powder in deionized water, resulting in a disordered solution with a high content of random coils and negligible beta sheet conformation. But after incubating the solution with highly oriented pyrolytic graphite, fully ordered monolayers of beta sheet lamellae (layers) successfully formed.
The researchers determined that the new 2D crystalline phase forms when the solution’s fibroin concentration is above a minimum value needed to drive nucleation but below the concentration at which fibroin begins to assemble before incubation.
They then used a variety of techniques—atomic force microscopy, synchrotron-based nanoscale Fourier transform infrared spectroscopy, and molecular dynamics—to show that assembly of the ordered structure proceeds along two distinct pathways:
- At low fibroin concentrations (0.01 to 0.08 µg/ml), direct epitaxial growth occurs.
- At high fibroin concentrations (>0.1 µg/ml), a two-step process occurs . This process involves the conversion of a metastable phase consisting of unfolded fibroin molecules into the structured film through folding and reorganization.
Regarding the second pathway, the folding transition was found to be rate limiting because the fibroin concentration had little or no effect on the growth rate of formed lamellae. Instead, the growth rate was determined by the transformation rate. The speed remained the same even when the concentration increased threefold. Initial formation of the unstructured film was rapid, covering the entire surface at a higher concentration before any ordered structures appeared.
Based on these results, the authors believe highly ordered 2D fibroin layers can be created on multiple van der Waals materials, thus providing “an unexplored strategy for both extending and improving the performance of silk in electronic and optical applications.”
The open-access paper, published in Science Advances, is “Two-dimensional silk” (DOI: 10.1126/sciadv.ado4142).
