Decay behavior and stability of free radicals of silk fibroin with alkali/urea pretreatment induced by electron beam irradiation
Introduction
Silk fibroin, derived from Bombyx mori cocoons, has been widely studied and used for material applications as a protein polymer. Silk fibroin has remarkable mechanical properties when formed into different materials, demonstrates biocompatibility, and controllable biodegradability [1]. And, it can be modified by various methods to meet different environmental needs, including solution blending, chemical-modification, microwave treatment, and radiation modification [2], [3], [4], [5], [6]. Among them, the radiation-induced grafting technique is considered to be one of the meaningful ways for silk fibroin modification. There has been a lot of research on silk fibroin irradiation [7], [8], [9]. The properties of the free radicals (such as yield and stability) induced by irradiation will directly affect the grafting efficiency. It is an important method to improve the performance of silk fibroin. Generally, the yield of free radicals of silk fibroin could restrict the grafting rate. To improve the grafting rate, the yield of free radicals induced by irradiation increased with absorbed dose. However, with the absorbed dose development, the surface of the silk fibroin turned yellow, the cracks occurred, and the molecular chain was broken [10], [11]. That means the mechanical properties of silk-based materials decreased [12]. Besides, the grafting degree of pre-irradiated silk fibroin may decrease with the storage time after irradiation, whose grafting degree would decrease by 50% in 30.0 min, owing to the rapid quenching of free radicals by oxygen and moisture when exposed to air [8]. Thus, increasing the yield and stability of free radicals may be more conducive to irradiation-induced grafting process. Furthermore, there is also controversy existed about the attribution of free radicals induced by irradiation [13], [14]. Therefore, it is necessary to further study the properties of free radicals and provide certain experimental basis for their assignation.
The free radicals are related to space and crystal structure of silk fibroin [15]. To transfer the crystalline structure and improve the yield and stability of free radicals, the silk fibroin was pretreated by the combination of the alkali/urea system and the rapid freezing in liquid nitrogen. The pretreated silk fibroin was dried in a vacuum and then irradiated at different doses at room temperature. The crystal and chemical structure changes were studied via FT-IR and XRD, respectively. Synchrotron radiation small angle X-ray scattering (SR-SAXS) was used to characterize the spatial structure changes. The free radicals formation, decay, and stability were characterized by electron spin resonance (ESR). (The higher free radicals concentration of 8.04*1019 spins/g was obtained at low dose of 25 kGy. The stability of the free radicals also enhanced and the half-life time was almost 10 times longer. This work can ameliorate the conditions and efficiency of the silk irradiation grafting process, and it may inspire designing structures to regulate free radicals formation.
Section snippets
Materials
Cocoons from B. Mori silkworms were purchased from the Gui Sheng cocoon silk industry and trade co. LTD. The Na2CO3, NaOH and urea were supplied by Sinopharm chemical reagent Co. Ltd.
Silk fibroin pretreatment and irradiation
Silk fibroin was prepared according to the earlier literature [17]. Cocoons were boiled for 25.0 min in an aqueous solution containing 0.02 M Na2CO3 and then rinsed thoroughly with deionized water to remove sericin and residual Na2CO3. The mixed solutions based on NaOH (7.0 g)/urea (9.0 g)/H2O (84.0 g) and silk
Structural transformation
The structural changes of unpretreated (S0) and pretreated (Sp0) silk fibroin were characterized by FT-IR spectra in Fig. 1 (a). The silk fibroin has important characteristic information at amide-I band (1705 cm−1–1595 cm−1) and amide-II (1595 cm−1–1465 cm−1) [17], which are a typical polypeptides and proteins conformational characterization [18], [19]. The peak located at 1700 cm−1of unpretreated (S0) and pretreated (Sp0) silk fibroin assigned to stretching vibration of CO bond, and the peak
Discussion
The results indicate that the yield and stability of free radicals can be significantly improved after the pretreatment. Usually, the stability of free radicals is closely related to the crystalline structure of the polymers including the crystalline degree and the crystallites size. Their persistence mainly depends on the steric protection of the unpaired electron [30]. Moreover, free radicals migration would also decrease radical stability [30]. The crystal structure could also promote the
Conclusion
After pretreatment, the crystalline structure transformed and the spatial structure shrank promoted the yield increase of free radicals induced by silk fibroin irradiation. And the stability of the free radicals formed by EB irradiation enhanced greatly. The transformation mechanism is the molecular chain folding caused by hydrogen bonds formation. Furthermore, the debate about the attribution of free radicals irradiated by silk fibroin provides certain experimental data. This work can
CRediT authorship contribution statement
Hao Zhang: Data curation, Writing - original draft. Feng Tian: Software, Data curation, Writing - original draft. Haitao Lin: Supervision, Resources. Rongfang Shen: Software. Weihua Liu: Writing - review & editing, Formal analysis. Yuying Huang: Conceptualization, Validation, Project administration. Zhongfeng Tang: Conceptualization, Validation, Writing - review & editing.
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.
Acknowledgments
This work was supported by National Key R&D Program of China (No. 2017YFA0403000), National Natural Science Foundation of China (51772308, 51763001, 51963002), and Qinghai Major Science and Technology Projects (No. 2017-GX-A3).
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First author: Hao Zhang and Feng Tian