Green synthesis of biocompatiable chitosan–graphene oxide hybrid nanosheet by ultrasonication method

•Biocompatiable chitosan–graphene oxide hybrid was prepared using ultrasound.•The synthesis process is green and simple without organic solvent and catalyst.•Roles of ultrasound and supercritical CO2 in forming amidation were explained. Ultrasound-induced synthesis of chitosan-modified nano-scale graphene oxide (CS-NGO) hybrid nanosheets, which has great potential pharmaceutical applications, in supercritical CO2 without catalyst was presented for the first time. The preparation process does not require organic solvent and post-processing, and CO2 easily escapes from the product. The morphology and structure of the CS-NGO, characterized using scanning electron microscopy, transmission electron microscopy, infrared spectroscopy, X-ray photoelectron spectroscopy and thermogravimetric analysis, confirms that it was combined via the amide linkage, and had excellent dispersibility and stability toward acidic and physiological aqueous solution, which implies that it could be used as a drug-carrier. The sonication power played a crucial role in inducing forming amidation, and the conversion rate increased with the sonication time. The mechanism of this reaction was explained.