Green modification of graphene oxide nanosheets under specific pH conditions

Green surface modification of graphene oxide (GO) using lysine (Lys) under different pH conditions was studied, aiming to activate the specific functional group of the amino acid structure: carboxylic acid (pH 2), the primary amine of alpha-carbon (pH 9.8) and, the primary amine of the lateral chain and alpha-carbon (pH 12), promoting three interaction mechanisms between Lys and the oxygenated groups of GO nanosheets. Chemical analyses suggest that at pH 2, ester bonds are formed on the GO's surface from its interaction with Lys, while at pH 9.8, the formation of C-N bonds takes place. However, at pH 14, chemical reduction of GO nanosheets is induced. Density functional calculations predicted possible binding modes and reaction pathways for forming ester (at pH 2) and C-N bonds (at pH 9.8) on basal planes of GO nanosheets and gave deep insights into the electronic structure of reduced GO materials. The chemical changes generated at the surface of GO modified the crystalline structure and the thermal properties of GO nanosheets. Our results suggest that controlled chemical functionalization/ reduction of GO nanosheets under environmental conditions can be performed under specific pH. However, the promotion of those functional groups benefits the potential used in biological applications.