Feeding silkworms with HPMC dispersed MoO2 NPs: An efficient strategy to enhance the supercapacitance performance of carbonized silk
Graphical abstract
Introduction
Flexible electronics is an exciting emerging technological arena, enabling platforms for many types of wearable devices [1]. Among the various materials used in manufacturing flexible electronics, silkworm silk, spun by Bombyx mori, has attracted significant attention. Although recent research has shown that in vivo feeding of silkworms with functional nanomaterial-loaded mulberry leaves [2,3] enables the production of silkworm silk fibers with enhanced electrical properties [4,5], their performance is limited by the poor conductivity of the natural silk. Carbonized silkworm silk maintains excellent flexibility and conductivity, and therefore has been extensively studied as an appealing material in the construction of flexible electronics [6,7]. In our previous study [8], molybdenum dioxide nanoparticles (MoO2 NPs) were successfully introduced into spun silk through feeding silkworms with mulberry leaves coated with MoO2 NPs. The study broadened the possibility of utilizing carbonized silk in energy storage devices. However, it was found that the introduced MoO2 NPs suffered from aggregation which hinders surface-redox reactions and lowers the efficacy of MoO2 NPs in enhancing the capacitance of carbonized silk [[9], [10], [11]]. Therefore, increasing the level of dispersion of the MoO2 NPs would be beneficial for enhancing their pseudocapacitive behavior and give rise to higher energy storage kinetics [11].
Hydroxypropyl methyl cellulose (HPMC) is a cellulose derivative. It has been approved as a food additive by the Food and Agriculture Organization of the United Nations and by the World Health Organization (FAO/WHO) [12]. HPMC solutions are odorless and tasteless. It is widely used as a dispersant, a film forming agent, and an emulsifier, to name a few, in food and industrial products and systems [13]. In recent years, HPMC has also been used to increase the water dispersibility of additives that were sprayed on mulberry leaves in order to produce functionalized silk fibers [[14], [15], [16]]. However, none of these studies considered whether HPMC ensures the separation of nanoparticles once they enter the silk gland and are encapsulated within silk fibers.
In this work, 0.5% (w/v) HPMC solution was utilized to disperse MoO2 NPs and the mulberry leaves were soaked with this mixture to ensure that the MoO2 NPs were evenly distributed on the mulberry leaves used for feeding the silkworms. We explored and found that the use of HPMC significantly reduces the level of aggregation observed for MoO2 NPs encapsulated within the as-spun silk. Furthermore, it was found that the carbonized silk maintains its fibrous morphology and exhibits enhanced specific capacitance. For the first time, we investigated the impact of HPMC on silkworm growth, nanoparticle feed efficiency, silk production and several properties of the resulting silk.
Section snippets
Impact of feeding methods on larvae growth and the spun silk
Here, we compare results for silkworms which were fed using MoO2 NP suspensions prepared via two different approaches, then loaded onto mulberry leaves. The first feeding approach, the focus of this paper, was described in the methods section of supplementary material. The second feeding approach was HPMC-free and has been described previously [8]. In that previous study, different amounts of MoO2 NPs were first dispersed in DI-H2O and then sprayed on the surface of the mulberry leaves. These
Conclusion
In this study, a 0.5% HPMC solution was employed to disperse the MoO2 NPs fed to silkworms, in order to avoid aggregation of the MoO2 NPs incorporated into silk at high MoO2 NP feeding dosages. Such agglomeration was observed in our previous study [8]. The experimental results confirmed that HPMC, serving as the dispersant, could ensure the MoO2 NPs ultimately incorporated into the silk remain well separated. Our study reveals that feeding HPMC to silkworms did exhibit adverse effects on the
Credit author contribution statement
Jianwei Liang: Investigation, Conceptualization, Methodology, Writing – Original Draft. Xiaoning Zhang: Conceptualization, Methodology, Writing – Original Draft, Resources. Yansong Ji: Investigation, Formal analysis. Zhenyu Chen: Investigation. Micheal L Norton: Writing – Review & Editing. Yixuan Wang: Investigation. Chi Yan: Investigation. Xi Zheng: Resources. Yong Zhu: Resources. Guotao Cheng: Formal analysis.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was funded by Venture & Innovation Support Program for Chongqing Overseas Returnees [grant number cx2019098]; the Fundamental Research Funds for the Central Universities [grant number SWU117036]; the 2020 National Training Program of Innovation and Entrepreneurship for Undergraduates in Southwest University [grant number 202010635088]. And this research was partially supported by the National Science Foundation under Award No. OIA-1458952. We specially thank Mr. David Neff from
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