Ag nanoparticles with a gradient structure prepared by diffusion method on a biomembrane

Silver (Ag) nanoparticles (NPs) with a gradient structure were successfully immobilized on a natural eggshell membrane (ESM) using a diffusion-controlled method. The ESM had a cross-linked network fibrous structure. The exposed functional group on the surface of the fiber can act as reactive sites for the preparation of metal NPs. Silver ions anchored on fibers of ESM were reduced by borohydride (BH₄ ⁻) ions to form silver NPs. The ESM acted as a stabilizer of silver NPs. The synthesized silver NPs@ESM composites were characterized by field-emission scanning electronic microscopy, X-ray diffraction and ultraviolet–visible diffuse reflectance spectroscopy in detail. It was found that silver NPs immobilized on the ESM exhibited a gradient distribution of size and density along the direction perpendicular to the ESM surface. This phenomenon may be attributed to the difference in diffusion velocity between silver and borohydride ions flowing through the ESM. This diffusion-controlled method may be expanded to prepare other metals or compounds with a gradient structure. Additionally, silver NPs@ESM composites can be recycled and exhibited good catalytic activity.