Effects Of Metal Nanoparticles On The Structures And Mechanical Properties Of Silk Fiber

Silk,known as the queen of fiber,has been employed for many years in textile in China.Silk fabric is very popular in many overseas markets due to its excellent mechanical properties,luster,smoothness and comfort properties,and thus,the Silk Road has been well developed for thousands of years,making possible the dialogue between Chinese and European civilizations.On the other hand,silk possesses attractive biodegradability and biocompatibility,and it has been widely used in biomedical applications such as drug delivery and tissue engineering.In order to meet some specific requirements across different fields,silk with enhanced mechanical properties is always required.New strategies include gene transfection,variations in spinning process and chemical or physical modifications of the silk.However,these strategies are difficult for commercialization.As a result,seeking an effective and economic way to enhance silk is vitally important.Noticeably,the in vivo modification method via feeding for silkworm silk appears a green,sustainable and promising route for commercial production in future.Metal nanoparticles have been widely used in our daily life because of their unique properties,and they may be benefit to animals in some feeding conditions.Therefore,in this research,five metal nanoparticles(Cu,Fe,Zn,ZnO and TiO2)are selected to investage the effects on the silkworms and the morphology,structures,and mechanical properties of the resulting silk.The details are listed as below:1.In order to study the biological effects of the feeding metal nanoparticles,the average larval body mass,cocooning rate,and tissue damage of silkworms were firstly tested.The result showed that feeding Cu,Fe,and TiO2 appeared to be non-harmful to the growth of silkworms under this intake condition,as there were no significant weight and cocooning rate differences among Cu,Fe,TiO2 and control groups,and there was no dead silkworm in all testing groups treated with these three nanoparticles.Zn and ZnO nanoparticles were potent harmful to silkworms that acted on the food intake,digestion,transportation,and production of silk.Owing to the high mortality,the cocooning rates of Zn and ZnO groups were 10% and 15%,respectively.Additionally,severe tissue damage of silkworms could be observed in Zn and ZnO groups by histological method.2.The effects of metal nanoparticle on silk fibroin's mechanical properties were also examined,and results demonstrated that the diameter and mechanical properties of silk were both remarkably improved after feeding silkworms with Cu,Fe and TiO2 nanoparticles,respectively.The optimal feeding method was that Cu nanoparticles were employed as foodadditives with a feeding concentration at 100 mg/group,and the resulting silk exhibited good tensile strength of 360 MPa and reached a strain of 38%,which were 87% and 36%higher than those of the natural silk.Although Zn and ZnO nanoparticles exhibited highly toxic for silkworms,the silk obtained from Zn and ZnO groups showed similar mechanical properties to those of control group.As a result,we deeply believed that Cu nanoparticles might be a better food additive in comparsion to other metal nanoparticles.3.Combined techniques,such as SEM,FTIR,XRD,and TGA,were used to characterize the detailed changes in the structure,morphology and thermal stability of the silk obtained from Cu,Fe and TiO2 groups.SEM results showed that the testing groups silk exhibited debris on the surface,and this might originate from the protein reconstruction which must be related to the presence of the metal additives.Results from FTIR,XRD,and TGA confirmed that the intrinsic features of the silk were inherited nearly completely with the addition of metal nanoparticles.The deconvolution of FTIR spectra in amide I band of testing groups silk were measured,and the results demonstrated that silk obtained from the testing groups had higher random coil/?-helix content and lower ?-sheet and ?-turn content than control silk.In a word,the feeding metal nanoparticles,Cu,Fe,and TiO2,hindered the conformation transition from random coil/?-helix to ?-sheet of silk fibroin.4.In order to obtain more details about the feeding metal nanoparticles in silk,ICP-MS,TEM and TEM-SAED were employed.Results showed that the feeding metal nanoparticles could be accumulated in the silk glands and midgut of silkworm,especially in poster silk gland,and the content was gradually increased.From the TEM images of tissue biopsies,it could be observed that Cu,Fe,and TiO2 were still remained at nanoscale in the silk glands and midgut.Finally,Fe2O3 and CuO particles at nanoscale were clearly observed in the residues of silk after decomposition,which might have resulted from the oxidation of the silk containing Cu and Fe nanoparticles.It inferred that the feeding metal nanoparticles entered the hemolymph from the alimentary canal of the silkworms,then went into the silk glands,and finally ended up in the silk and presented themselves as an effective additive for the current composite.