Silk Layering As Studiedwith Neutron Reflectivity.

Neutron reflectivity (NR) measurements of ultrathin surfacefilms(below 30 nm) composed of Bombyx morisilk fibroin protein in combination with atomic force microscopyand ellipsometry were used to reveal the internal structural organizationin both dry and swollen states. Reconstituted aqueous silk solutiondeposited on a silicon substrate using the spin-assisted layer-by-layer(SA-LbL) technique resulted in a monolayer silk film composed of randomnanofibrils with constant scattering length density (SLD). However,a vertically segregated ordering with two different regions has beenobserved in dry, thicker, seven-layer SA-LbL silk films. The verticalsegregation of silk multilayer films indicates the presence of a differentsecondary structure of silk in direct contact with the silicon oxidesurface (first 6 nm). The layered structure can be attributed to interfacialβ-sheet crystallization and the formation of well-developednanofibrillar nanoporous morphology for the initially deposited silksurface layers with the preservation of less dense, random coil secondarystructure for the layers that follow. This segregated structure ofsolid silk films defines their complex nonuniform behavior in theD2O environment with thicker silk films undergoing delaminationduring swelling. For a silk monolayer with an initial thickness of6 nm, we observed the increase in the effective thickness by 60% combinedwith surprising decrease in density. Considering the nanoporous morphologyof the hydrophobic silk layer, we suggested that the apparent increasein its thickness in liquid environment is caused by the air nanobubbletrapping phenomenon at the liquid–solid interface. [ABSTRACT FROM AUTHOR]