Synthesis of gold nanoparticles using silk sericin as a green reducing and capping agent

[Display omitted] •Green synthesis Gold nanoparticles (AuNPs) using hydrophilic silk sericin.•Sericin acted as a reducing agent and capping agent in the formation of AuNPs.•The hydroxyl and carboxyl groups of sericin mainly participated in AuNPs synthesis. Various natural materials were used recently for the synthesis of gold nanoparticles (AuNPs). These natural materials are attracting attention because they are more environmentally friendly than conventional chemicals used for AuNP synthesis. Silk sericin (SS) is a protein that is removed during the silk degumming process, and it has been studied for the synthesis of several inorganic materials, such as hydroxyapatite and silica. The present study synthesized AuNPs using SS. To optimize the conditions for the synthesis of AuNPs, the effects of reaction pH, SS concentration, reaction time and temperature during the formation of AuNPs were monitored using visual observation, a UV–Vis spectrophotometer, and energy-filtered transmission electron microscopy (EF-TEM) analysis. When the concentration of HAuCl4 was 1 mM, the optimal conditions for synthesizing AuNPs were an SS concentration of 1 mg/ml, a reaction pH of 7, a reaction temperature of 70 °C and a reaction time of 12 h. The AuNPs prepared under the optimal conditions had a size of approximately 17 nm, and the particles were well dispersed. When ethanol was added, the secondary structure of SS transitioned to β-sheets, the size of the gold nanoparticles increased to approximately 20 nm, and the amount of AuNPs increased. Amino acids bearing a hydroxyl group or negative charge were effective in the synthesis of AuNPs, and SS has a high content of these amino acids. Therefore, SS reduced gold ions and induced the synthesis of AuNPs. High-resolution TEM (HR-TEM), energy-dispersive X-ray spectroscopy (EDS), and thermogravimetric analysis (TGA) revealed that SS remained in a form that capped the synthesized AuNPs.