Preparation of natural amphoteric silk nanofibers by acid hydrolysis

Direct extraction of silk nanofibers (SNs) from natural silk fibers was developed via a low-intensity ultrasonic-assisted sulfuric acid hydrolysis process. The effects of sulfuric acid (H2SO4) amount, hydrolysis time and temperature were investigated. SNs with a length and width of 306 ± 107 nm and 4-18 nm, respectively, were obtained with a yield of 64.54%. Furthermore, unlike for the silk nanofibers formed from a regeneration process, the original crystal structure of the silk was maintained. The aqueous dispersion of SNs was steady for at least 30 days at pH 3 and 7-10 with no aggregation. Furthermore, the as-prepared SNs were amphoteric due to the intrinsic characteristics of the protein that endow them with the ability to mix well with either positively or negatively charged polymers, possibly contributing to the field of high-tech silk materials in the future.