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 beta 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 13 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 Ag-0. It was proven that random coil (Silk I) and beta 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 beta sheet (Silk II) morphologies during the first 8 h. However, due to crystalline structure of the fibers, the cumulative release of silver ions from beta 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. (C) 2015 Elsevier Ltd. All rights reserved.