Ag/silk fibroin nanofibers: Effect of fibroin morphology on Ag+ release and antibacterial activity

• Fibroin nanofibers are potential carrier for the controlled delivery drug/biocides.
• Fibroin morphology; random coil or β sheets affects the release of Ag+.
• Amount of Ag+ released from random coil was higher than β sheet morphology.
• P. aeruginosa is more susceptible to released Ag+ than S. aureus and S. epidermidis.

Silk based nanofibers are potential carrier systems for the controlled delivery of antibiotics and other antimicrobial agents. For silk based biomaterials, silk morphology; random coil (Silk I) or β sheets (Silk II) is an important factor affecting their mechanical, thermal and hydrolytic stability and also release of drug/biocides. In this study, Ag/fibroin composite nanofibers were produced in the form of both random coil (Silk I) and β sheet (Silk II) morphologies via glutaraldehyde vapor and methanol post-treatments, respectively and effect of fibroin morphology on Ag ion release and concomitant antibacterial activity was investigated. Ag nanoparticles were synthesized in fibroin nanofibers with diameter of 200–600 nm, by reduction of Ag ions to Ag0. It was proven that random coil (Silk I) and β sheet (Silk II) morphologies cause significant differences on Ag+ release profiles, thermal properties and hydrolytic stability of nanofibers. According to the kinetics data, Ag+ was released by the first order kinetics for both random coil and β sheet (Silk II) morphologies during the first 8 h. However, due to crystalline structure of the fibers, the cumulative release of silver ions from β sheet structure (Silk II) was lower than random coil (Silk I) structure. Furthermore, according to the disc diffusion test results for both nanofiber structures, containing 1% (w/v) of AgNO3, clear zones of inhibition were observed against Staphylococcus aureus, Staphylococcus epidermidis and Pseudomonas aeruginosa.

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