Processing and characterization of powdered silk micro- and nanofibers by ultrasonication

•In this article, powdered micro- and nanofibers were obtained by ultrasonication.•The ultrasonication method which we introduce is simple, efficient and industrial.•The diameter of the powdered microfibers was approximately 5–10 μm.•The diameter of the powdered nanofibers was approximately 30–120 nm.•The ε-amino group content of the powdered-fibers increased significantly.

Silk derived from Bombyx mori silkworm cocoons was degummed in an aqueous sodium carbonate solution, and the resulting silk fibroin fibers were placed in an acidic aqueous solution and were treated with ultrasonication to obtain powdered micro- and nanofibers. The morphologies and spectral characteristics of these powdered silk fibers were investigated in detail. The shape, surface and structural features of the powdered fibers were affected by the ultrasonic power and media. Increasing the acidity of the ultrasonic solution and increasing the ultrasonic power increased the fiber breakage speed, resulting in shorter fiber lengths. Powdered microfibers could not be obtained in a formic acid solution, while powdered nanofibers whose diameter below 1 μm were obtained in a combined formic acid and hydrochloric acid ultrasonication solution. Observation via SEM and optical microscopy revealed that the microfiber diameters were approximately 5–10 μm, and those of the nanofibers were approximately 30–120 nm. The analysis of laser sizer showed that the microfiber sizes ranged mainly from 20 to 100 μm. FT-IR and XRD spectra demonstrated that the relative amount of β-sheets increased after the ultrasonic treatment. The ε-amino group content on the surface of the micro- and nanofibers increased significantly. These studies provide reliable methods for the preparation of nano-scale silk fibroin fibers by ultrasonication and open new avenues for the development of powdered silk fibers as advanced functional biomaterials.