Control of gas permeability by transforming the molecular structure of silk fibroin in multilayered nanocoatings for CO₂ adsorptive separation

Carbon dioxide (CO₂) is considered one of the causes of global warming because anthropogenic CO₂ concentration in the atmosphere has greatly increased. The objective of this work was to design silk fibroin (SF) and graphene oxide (GO) membranes with very high CO₂ gas capture characteristics and high gas selectivity toward nitrogen rather than CO₂ gas, and to demonstrate the effect on transforming the SF secondary structure at the molecular level. GO-NH₃⁺/SF multilayer nanocoatings were fabricated by automatic spray-assisted layer-by-layer (LbL) assembly with physical molecular interaction between GO-NH₃⁺ and SF. We demonstrated transformation of the SF secondary structures in GO-NH₃⁺/SF LbL-assembled nanocoatings with MeOH solvent treatment. After MeOH treatment, the SF secondary structures in the GO-NH₃⁺/SF LbL-assembled nanocoatings preferred β-sheets to random coils. This phenomenon affects the internal amine groups and CO₂ transport. The MeOH-treated GO-NH₃⁺/SF LbL-assembled nanocoatings have high β-sheet content, and the gas permeability of the nanocoatings was quite different. Before the predominance of the β-sheet, CO₂ gas permeated twice as fast as nitrogen (N₂) gas in the GO-NH₃⁺/SF LbL-assembled nanocoatings. On the other hand, CO₂ gas was transmitted 10-times slower than N₂ gas in MeOH-treated GO-NH₃⁺/SF LbL-assembled nanocoatings. In other words, we successfully fabricated a CO₂ adsorptive separation membrane with GO-NH₃⁺ and SF.