Facile production of natural silk nanofibers for electronic device applications
Graphical abstract
Natural silk nanofibers/carbon nanotubes composite films with high electroconductivity and flexibility are prepared based on facile deconstruction of the silk fibroin fibers.
Introduction
With the ever-rising need for smart-wearable electronics, a great of attention were attached to the flexible, sustainable, and environmentally friendly materials, which hold great potential applications in next-generation bioelectronics, including electronic circuits, energy transfer, and electric-biological devices as an implantation [[1], [2], [3]]. Thus, the natural fibers draw great attention to more and more researchers due to its good performance mentioned above. Recently, the conductive materials based on cellulose nanofibers (CNFs) were widely reported owing to its ease of processing, good flexibility, light weight, and low cost [[4], [5], [6]]. Yet, the CNFs were still limited by insufficient biocompatibility and complex processing in their further utilities.
Silks as an ancient high-performance material offered lasting inspirations and attractive for developing flexible, sustainable, and biological energy storage devices or electrical transistors [[7], [8], [9]]. Although regenerated silk-based electronic devices performed preferable biocompatibility and sustainability, their natural superior mechanical, biological features, and fiber properties were sacrificed inevitably during regenerated process of silk fibroin materials, especially in large surface area and fiber materials [10,11]. Generally, the preparation of silk nanofibers (SNFs) depends on “bottom-up” and “top-down” strategies [12,13]. Self-assembly and electrospinning are the most common ways to obtain SNFs based on “bottom-up”, whereas, an obvious deteriorated performance of SNFs is exposed in mechanical properties due to the destruction of natural silk fibrillar structure during the regeneration process. These high-order hierarchical structures are considered as the main reason for outstanding mechanical property of natural silk [14]. Recently, the researchers tend to directly isolate natural SNFs based on “top-down” technique, which preserved hierarchical structures and outstanding mechanical properties of natural silks [15]. The ultrasonication [16], ionic liquid [17], formic acid-Ca2+ [18], and hexafluoroisopropanol [15] were used in exfoliation of SNFs from natural silks. However, the barriers are still subsistent, including low production, wild preparation process, and toxic solvent residue.
As a unique fibrous protein biopolymer, the silk fibers with typical hierarchical structures are assembled by binding forces, including hydrogen bonds [19,20], hydrophobic segments [21], ionic bonds [22], and Van der Waals' force [23]. Specifically, silk is considered as a semi-crystalline polymer with highly oriented antiparallel β-sheet nanocrystals, a stable structure supported by intra-sheet hydrogen bond and inter-sheet side chain interactions [24,25]. Although the hydrogen bonds are weak interactions, they act cooperatively to achieve high strength and dissipating energy through the stick-slip mechanism [26], contributing to the stability of silk secondary structure. Another important binding force in the silk is hydrophilic-hydrophobic interaction. The silk molecular chain consists of highly repetitive core sequence with plenty of hydrophobic segments, which constructed by N- and C- terminal domains in the internal modules (N- and C- around 130 and 100 amino acids, respectively), and the hydrophobic and hydrophilic form a robust network [27]. Besides, the ionic bond and Van der Waals' force are ubiquitous and valuable interactions that contribute to silk aggregation, especially in tertiary and quaternary structures of silk protein [28]. Therefore, it is key point that how to establish an effective approach to weaken these binding forces for promoting SNFs isolation.
In this study, we established a facile and benign strategy to weak binding forces among silk hierarchical structures for preparation of natural SNFs firstly, which retaining natural silk structures and properties partly with mild chemical treatment, low energy consumption and high yield (~87%) (Table S1, see the Supplementary Information). Herein, due to its excellent electrical conductivity, high surface area, and low density [29,30], carbon nanotube (CNTs) were selected to integrate with natural SNFs to explore the flexible electronic devices. The SNFs-CNTs composite films were fabricated via vacuum filtration with lightweight, high flexibility, and good conductivity. The surface morphology, chemical components, mechanical properties, and electrical-thermal conductivity of the SNFs-CNTs films were analyzed, and its potential as electronic-storage devices was evaluated.
Section snippets
Preparation of natural silk nanofibers
As the previous study [31], Bombyx mori (B. mori) silk fibers were boiled three times in 0.06% (w/v) Na2CO3 solution for 30 min to remove sericin. After that, the degummed silk fibers were washed with distilled water thoroughly and dried in the oven at 60 °C overnight. The natural SNFs were fabricated by the established procedure described as the following process (Fig. 1A). Briefly, the degummed silk fibers were immersed into a ternary solution of CaCl2: CH3CH2OH: H2O (1:2:8 M ratio) with bath
Morphology of natural silk nanofibers
In order to fabricate natural SNFs, Fig. 1A summarized the facile and effective route to prepare natural SNFs. First, the degummed silk fibroin fibers have a relatively larger diameters (~12 μm, Fig. S1 (see the Supplementary Information)) after swelling treated for 6 h (Fig. 2A, E), and maintain a smooth surface of natural silk fibers. The clearly changes took place in the diameter distribution and surface morphology of the silk fibers after mechanical-shearing action, and most partially of
Conclusions
In conclusion, we reported a facile, environment friendly, high-efficiency strategy for the large-scale production of natural SNFs by combining mild physical and chemical treatments. More importantly, the SNFs with diameters ~200 nm and contour length of 10–30 μm were successfully prepared. Based on the good dispersion in water and the inherited hierarchical structure and superior physical properties from natural silk fibers, the pure SNFs and SNFs-CNTs composite films were easily fabricated by
Declaration of competing interest
The authors declare no competing financial interest.
Acknowledgments
This work was supported by the Natural Science Foundation of Hubei Province, China (2018CFB663), National Natural Science Foundation of China (51803154, 51403163, and 31600774).
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