One-pot preparation of zwitterionic graphene nanosheets with exceptional redispersibility and its application in pickering emulsions

Using biomass materials as exfoliating agents to produce graphene is a promising strategy due to their biocompatibility, reproducibility and sustainability. However, it's still a challenge to prepare high-concentration graphene dispersion using biomass exfoliating agents. Here, we present a green and facile method to prepare few layered graphene nanosheets in aqueous solution by one-pot ball milling in the presence of protein, casein (CN). The results show that CN protein could be acted as an efficient exfoliating and stabilizing agent, producing water-soluble graphene nanosheets with a concentration up to 2.14 mg ml(-1). As CN protein contains both amino and carboxyl groups, thus the prepared graphene nanosheets decorated with CN protein (GN-CN) show zwitterionically charged character and thereby can be stably dispersed under both acidic and alkaline conditions. Moreover, the oven-dried GN-CN hybrid exhibits exceptional redispersibility in water with a concentration as high as 100 mg ml(-1) and even more. Benefiting from the zwitterionic character, GN-CN shows the Pickering emulsion effect over a wide pH range. As a proof of concept, the application of the zwitterionic GN-CN as a Pickering emulsifier to prepare microcapsule phase change composite material has been successfully demonstrated. (C) 2019 Elsevier Ltd. All rights reserved.